NMAP(1) Nmap Reference Guide NMAP(1)
nmap - Network exploration tool and security / port scanner
SYNOPSIS top
nmap [Scan Type...] [Options] {target specification}
DESCRIPTION top
Nmap (“Network Mapper”) is an open source tool for network
exploration and security auditing. It was designed to rapidly scan
large networks, although it works fine against single hosts. Nmap
uses raw IP packets in novel ways to determine what hosts are
available on the network, what services (application name and
version) those hosts are offering, what operating systems (and OS
versions) they are running, what type of packet filters/firewalls are
in use, and dozens of other characteristics. While Nmap is commonly
used for security audits, many systems and network administrators
find it useful for routine tasks such as network inventory, managing
service upgrade schedules, and monitoring host or service uptime.
The output from Nmap is a list of scanned targets, with supplemental
information on each depending on the options used. Key among that
information is the “interesting ports table”. That table lists the
port number and protocol, service name, and state. The state is
either open, filtered, closed, or unfiltered. Open means that an
application on the target machine is listening for
connections/packets on that port. Filtered means that a firewall,
filter, or other network obstacle is blocking the port so that Nmap
cannot tell whether it is open or closed. Closed ports have no
application listening on them, though they could open up at any time.
Ports are classified as unfiltered when they are responsive to Nmap's
probes, but Nmap cannot determine whether they are open or closed.
Nmap reports the state combinations open|filtered and closed|filtered
when it cannot determine which of the two states describe a port. The
port table may also include software version details when version
detection has been requested. When an IP protocol scan is requested
(-sO), Nmap provides information on supported IP protocols rather
than listening ports.
In addition to the interesting ports table, Nmap can provide further
information on targets, including reverse DNS names, operating system
guesses, device types, and MAC addresses.
A typical Nmap scan is shown in Example 1. The only Nmap arguments
used in this example are -A, to enable OS and version detection,
script scanning, and traceroute; -T4 for faster execution; and then
the hostname.
Example 1. A representative Nmap scan
# nmap -A -T4 scanme.nmap.org
Nmap scan report for scanme.nmap.org (74.207.244.221)
Host is up (0.029s latency).
rDNS record for 74.207.244.221: li86-221.members.linode.com
Not shown: 995 closed ports
PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 5.3p1 Debian 3ubuntu7 (protocol 2.0)
| ssh-hostkey: 1024 8d:60:f1:7c:ca:b7:3d:0a:d6:67:54:9d:69:d9:b9:dd (DSA)
|_2048 79:f8:09:ac:d4:e2:32:42:10:49:d3:bd:20:82:85:ec (RSA)
80/tcp open http Apache httpd 2.2.14 ((Ubuntu))
|_http-title: Go ahead and ScanMe!
646/tcp filtered ldp
1720/tcp filtered H.323/Q.931
9929/tcp open nping-echo Nping echo
Device type: general purpose
Running: Linux 2.6.X
OS CPE: cpe:/o:linux:linux_kernel:2.6.39
OS details: Linux 2.6.39
Network Distance: 11 hops
Service Info: OS: Linux; CPE: cpe:/o:linux:kernel
TRACEROUTE (using port 53/tcp)
HOP RTT ADDRESS
[Cut first 10 hops for brevity]
11 17.65 ms li86-221.members.linode.com (74.207.244.221)
Nmap done: 1 IP address (1 host up) scanned in 14.40 seconds
The newest version of Nmap can be obtained from https://nmap.org . The
newest version of this man page is available at
https://nmap.org/book/man.html . It is also included as a chapter of
Nmap Network Scanning: The Official Nmap Project Guide to Network
Discovery and Security Scanning (see https://nmap.org/book/ ).
OPTIONS SUMMARY top
This options summary is printed when Nmap is run with no arguments,
and the latest version is always available at
https://svn.nmap.org/nmap/docs/nmap.usage.txt . It helps people
remember the most common options, but is no substitute for the
in-depth documentation in the rest of this manual. Some obscure
options aren't even included here.
Nmap 7.50SVN ( https://nmap.org )
Usage: nmap [Scan Type(s)] [Options] {target specification}
TARGET SPECIFICATION:
Can pass hostnames, IP addresses, networks, etc.
Ex: scanme.nmap.org, microsoft.com/24, 192.168.0.1; 10.0.0-255.1-254
-iL <inputfilename>: Input from list of hosts/networks
-iR <num hosts>: Choose random targets
--exclude <host1[,host2][,host3],...>: Exclude hosts/networks
--excludefile <exclude_file>: Exclude list from file
HOST DISCOVERY:
-sL: List Scan - simply list targets to scan
-sn: Ping Scan - disable port scan
-Pn: Treat all hosts as online -- skip host discovery
-PS/PA/PU/PY[portlist]: TCP SYN/ACK, UDP or SCTP discovery to given ports
-PE/PP/PM: ICMP echo, timestamp, and netmask request discovery probes
-PO[protocol list]: IP Protocol Ping
-n/-R: Never do DNS resolution/Always resolve [default: sometimes]
--dns-servers <serv1[,serv2],...>: Specify custom DNS servers
--system-dns: Use OS's DNS resolver
--traceroute: Trace hop path to each host
SCAN TECHNIQUES:
-sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans
-sU: UDP Scan
-sN/sF/sX: TCP Null, FIN, and Xmas scans
--scanflags <flags>: Customize TCP scan flags
-sI <zombie host[:probeport]>: Idle scan
-sY/sZ: SCTP INIT/COOKIE-ECHO scans
-sO: IP protocol scan
-b <FTP relay host>: FTP bounce scan
PORT SPECIFICATION AND SCAN ORDER:
-p <port ranges>: Only scan specified ports
Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080,S:9
--exclude-ports <port ranges>: Exclude the specified ports from scanning
-F: Fast mode - Scan fewer ports than the default scan
-r: Scan ports consecutively - don't randomize
--top-ports <number>: Scan <number> most common ports
--port-ratio <ratio>: Scan ports more common than <ratio>
SERVICE/VERSION DETECTION:
-sV: Probe open ports to determine service/version info
--version-intensity <level>: Set from 0 (light) to 9 (try all probes)
--version-light: Limit to most likely probes (intensity 2)
--version-all: Try every single probe (intensity 9)
--version-trace: Show detailed version scan activity (for debugging)
SCRIPT SCAN:
-sC: equivalent to --script=default
--script=<Lua scripts>: <Lua scripts> is a comma separated list of
directories, script-files or script-categories
--script-args=<n1=v1,[n2=v2,...]>: provide arguments to scripts
--script-args-file=filename: provide NSE script args in a file
--script-trace: Show all data sent and received
--script-updatedb: Update the script database.
--script-help=<Lua scripts>: Show help about scripts.
<Lua scripts> is a comma-separated list of script-files or
script-categories.
OS DETECTION:
-O: Enable OS detection
--osscan-limit: Limit OS detection to promising targets
--osscan-guess: Guess OS more aggressively
TIMING AND PERFORMANCE:
Options which take <time> are in seconds, or append 'ms' (milliseconds),
's' (seconds), 'm' (minutes), or 'h' (hours) to the value (e.g. 30m).
-T<0-5>: Set timing template (higher is faster)
--min-hostgroup/max-hostgroup <size>: Parallel host scan group sizes
--min-parallelism/max-parallelism <numprobes>: Probe parallelization
--min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout <time>: Specifies
probe round trip time.
--max-retries <tries>: Caps number of port scan probe retransmissions.
--host-timeout <time>: Give up on target after this long
--scan-delay/--max-scan-delay <time>: Adjust delay between probes
--min-rate <number>: Send packets no slower than <number> per second
--max-rate <number>: Send packets no faster than <number> per second
FIREWALL/IDS EVASION AND SPOOFING:
-f; --mtu <val>: fragment packets (optionally w/given MTU)
-D <decoy1,decoy2[,ME],...>: Cloak a scan with decoys
-S <IP_Address>: Spoof source address
-e <iface>: Use specified interface
-g/--source-port <portnum>: Use given port number
--proxies <url1,[url2],...>: Relay connections through HTTP/SOCKS4 proxies
--data <hex string>: Append a custom payload to sent packets
--data-string <string>: Append a custom ASCII string to sent packets
--data-length <num>: Append random data to sent packets
--ip-options <options>: Send packets with specified ip options
--ttl <val>: Set IP time-to-live field
--spoof-mac <mac address/prefix/vendor name>: Spoof your MAC address
--badsum: Send packets with a bogus TCP/UDP/SCTP checksum
OUTPUT:
-oN/-oX/-oS/-oG <file>: Output scan in normal, XML, s|<rIpt kIddi3,
and Grepable format, respectively, to the given filename.
-oA <basename>: Output in the three major formats at once
-v: Increase verbosity level (use -vv or more for greater effect)
-d: Increase debugging level (use -dd or more for greater effect)
--reason: Display the reason a port is in a particular state
--open: Only show open (or possibly open) ports
--packet-trace: Show all packets sent and received
--iflist: Print host interfaces and routes (for debugging)
--append-output: Append to rather than clobber specified output files
--resume <filename>: Resume an aborted scan
--stylesheet <path/URL>: XSL stylesheet to transform XML output to HTML
--webxml: Reference stylesheet from Nmap.Org for more portable XML
--no-stylesheet: Prevent associating of XSL stylesheet w/XML output
MISC:
-6: Enable IPv6 scanning
-A: Enable OS detection, version detection, script scanning, and traceroute
--datadir <dirname>: Specify custom Nmap data file location
--send-eth/--send-ip: Send using raw ethernet frames or IP packets
--privileged: Assume that the user is fully privileged
--unprivileged: Assume the user lacks raw socket privileges
-V: Print version number
-h: Print this help summary page.
EXAMPLES:
nmap -v -A scanme.nmap.org
nmap -v -sn 192.168.0.0/16 10.0.0.0/8
nmap -v -iR 10000 -Pn -p 80
SEE THE MAN PAGE (https://nmap.org/book/man.html) FOR MORE OPTIONS AND EXAMPLES
TARGET SPECIFICATION top
Everything on the Nmap command-line that isn't an option (or option
argument) is treated as a target host specification. The simplest
case is to specify a target IP address or hostname for scanning.
Sometimes you wish to scan a whole network of adjacent hosts. For
this, Nmap supports CIDR-style addressing. You can append /numbits to
an IPv4 address or hostname and Nmap will scan every IP address for
which the first numbits are the same as for the reference IP or
hostname given. For example, 192.168.10.0/24 would scan the 256 hosts
between 192.168.10.0 (binary: 11000000 10101000 00001010 00000000)
and 192.168.10.255 (binary: 11000000 10101000 00001010 11111111),
inclusive. 192.168.10.40/24 would scan exactly the same targets.
Given that the host scanme.nmap.org is at the IP address
64.13.134.52, the specification scanme.nmap.org/16 would scan the
65,536 IP addresses between 64.13.0.0 and 64.13.255.255. The smallest
allowed value is /0, which targets the whole Internet. The largest
value is /32, which scans just the named host or IP address because
all address bits are fixed.
CIDR notation is short but not always flexible enough. For example,
you might want to scan 192.168.0.0/16 but skip any IPs ending with .0
or .255 because they may be used as subnet network and broadcast
addresses. Nmap supports this through octet range addressing. Rather
than specify a normal IP address, you can specify a comma-separated
list of numbers or ranges for each octet. For example,
192.168.0-255.1-254 will skip all addresses in the range that end in
.0 or .255, and 192.168.3-5,7.1 will scan the four addresses
192.168.3.1, 192.168.4.1, 192.168.5.1, and 192.168.7.1. Either side
of a range may be omitted; the default values are 0 on the left and
255 on the right. Using - by itself is the same as 0-255, but
remember to use 0- in the first octet so the target specification
doesn't look like a command-line option. Ranges need not be limited
to the final octets: the specifier 0-255.0-255.13.37 will perform an
Internet-wide scan for all IP addresses ending in 13.37. This sort of
broad sampling can be useful for Internet surveys and research.
IPv6 addresses can be specified by their fully qualified IPv6 address
or hostname or with CIDR notation for subnets. Octet ranges aren't
yet supported for IPv6.
IPv6 addresses with non-global scope need to have a zone ID suffix.
On Unix systems, this is a percent sign followed by an interface
name; a complete address might be fe80::a8bb:ccff:fedd:eeff%eth0. On
Windows, use an interface index number in place of an interface name:
fe80::a8bb:ccff:fedd:eeff%1. You can see a list of interface indexes
by running the command netsh.exe interface ipv6 show interface.
Nmap accepts multiple host specifications on the command line, and
they don't need to be the same type. The command nmap scanme.nmap.org
192.168.0.0/8 10.0.0,1,3-7.- does what you would expect.
While targets are usually specified on the command lines, the
following options are also available to control target selection:
-iL inputfilename (Input from list)
Reads target specifications from inputfilename. Passing a huge
list of hosts is often awkward on the command line, yet it is a
common desire. For example, your DHCP server might export a list
of 10,000 current leases that you wish to scan. Or maybe you want
to scan all IP addresses except for those to locate hosts using
unauthorized static IP addresses. Simply generate the list of
hosts to scan and pass that filename to Nmap as an argument to
the -iL option. Entries can be in any of the formats accepted by
Nmap on the command line (IP address, hostname, CIDR, IPv6, or
octet ranges). Each entry must be separated by one or more
spaces, tabs, or newlines. You can specify a hyphen (-) as the
filename if you want Nmap to read hosts from standard input
rather than an actual file.
The input file may contain comments that start with # and extend
to the end of the line.
-iR num hosts (Choose random targets)
For Internet-wide surveys and other research, you may want to
choose targets at random. The num hosts argument tells Nmap how
many IPs to generate. Undesirable IPs such as those in certain
private, multicast, or unallocated address ranges are
automatically skipped. The argument 0 can be specified for a
never-ending scan. Keep in mind that some network administrators
bristle at unauthorized scans of their networks and may complain.
Use this option at your own risk! If you find yourself really
bored one rainy afternoon, try the command nmap -Pn -sS -p 80 -iR
0 --open to locate random web servers for browsing.
--exclude host1[,host2[,...]] (Exclude hosts/networks)
Specifies a comma-separated list of targets to be excluded from
the scan even if they are part of the overall network range you
specify. The list you pass in uses normal Nmap syntax, so it can
include hostnames, CIDR netblocks, octet ranges, etc. This can be
useful when the network you wish to scan includes untouchable
mission-critical servers, systems that are known to react
adversely to port scans, or subnets administered by other people.
--excludefile exclude_file (Exclude list from file)
This offers the same functionality as the --exclude option,
except that the excluded targets are provided in a newline-,
space-, or tab-delimited exclude_file rather than on the command
line.
The exclude file may contain comments that start with # and
extend to the end of the line.
HOST DISCOVERY top
One of the very first steps in any network reconnaissance mission is
to reduce a (sometimes huge) set of IP ranges into a list of active
or interesting hosts. Scanning every port of every single IP address
is slow and usually unnecessary. Of course what makes a host
interesting depends greatly on the scan purposes. Network
administrators may only be interested in hosts running a certain
service, while security auditors may care about every single device
with an IP address. An administrator may be comfortable using just an
ICMP ping to locate hosts on his internal network, while an external
penetration tester may use a diverse set of dozens of probes in an
attempt to evade firewall restrictions.
Because host discovery needs are so diverse, Nmap offers a wide
variety of options for customizing the techniques used. Host
discovery is sometimes called ping scan, but it goes well beyond the
simple ICMP echo request packets associated with the ubiquitous ping
tool. Users can skip the ping step entirely with a list scan (-sL) or
by disabling ping (-Pn), or engage the network with arbitrary
combinations of multi-port TCP SYN/ACK, UDP, SCTP INIT and ICMP
probes. The goal of these probes is to solicit responses which
demonstrate that an IP address is actually active (is being used by a
host or network device). On many networks, only a small percentage of
IP addresses are active at any given time. This is particularly
common with private address space such as 10.0.0.0/8. That network
has 16 million IPs, but I have seen it used by companies with less
than a thousand machines. Host discovery can find those machines in a
sparsely allocated sea of IP addresses.
If no host discovery options are given, Nmap sends an ICMP echo
request, a TCP SYN packet to port 443, a TCP ACK packet to port 80,
and an ICMP timestamp request. (For IPv6, the ICMP timestamp request
is omitted because it is not part of ICMPv6.) These defaults are
equivalent to the -PE -PS443 -PA80 -PP options. The exceptions to
this are the ARP (for IPv4) and Neighbor Discovery (for IPv6) scans
which are used for any targets on a local ethernet network. For
unprivileged Unix shell users, the default probes are a SYN packet to
ports 80 and 443 using the connect system call. This host discovery
is often sufficient when scanning local networks, but a more
comprehensive set of discovery probes is recommended for security
auditing.
The -P* options (which select ping types) can be combined. You can
increase your odds of penetrating strict firewalls by sending many
probe types using different TCP ports/flags and ICMP codes. Also note
that ARP/Neighbor Discovery (-PR) is done by default against targets
on a local ethernet network even if you specify other -P* options,
because it is almost always faster and more effective.
By default, Nmap does host discovery and then performs a port scan
against each host it determines is online. This is true even if you
specify non-default host discovery types such as UDP probes (-PU).
Read about the -sn option to learn how to perform only host
discovery, or use -Pn to skip host discovery and port scan all target
hosts. The following options control host discovery:
-sL (List Scan)
The list scan is a degenerate form of host discovery that simply
lists each host of the network(s) specified, without sending any
packets to the target hosts. By default, Nmap still does
reverse-DNS resolution on the hosts to learn their names. It is
often surprising how much useful information simple hostnames
give out. For example, fw.chi is the name of one company's
Chicago firewall. Nmap also reports the total number of IP
addresses at the end. The list scan is a good sanity check to
ensure that you have proper IP addresses for your targets. If the
hosts sport domain names you do not recognize, it is worth
investigating further to prevent scanning the wrong company's
network.
Since the idea is to simply print a list of target hosts, options
for higher level functionality such as port scanning, OS
detection, or ping scanning cannot be combined with this. If you
wish to disable ping scanning while still performing such higher
level functionality, read up on the -Pn (skip ping) option.
-sn (No port scan)
This option tells Nmap not to do a port scan after host
discovery, and only print out the available hosts that responded
to the host discovery probes. This is often known as a “ping
scan”, but you can also request that traceroute and NSE host
scripts be run. This is by default one step more intrusive than
the list scan, and can often be used for the same purposes. It
allows light reconnaissance of a target network without
attracting much attention. Knowing how many hosts are up is more
valuable to attackers than the list provided by list scan of
every single IP and host name.
Systems administrators often find this option valuable as well.
It can easily be used to count available machines on a network or
monitor server availability. This is often called a ping sweep,
and is more reliable than pinging the broadcast address because
many hosts do not reply to broadcast queries.
The default host discovery done with -sn consists of an ICMP echo
request, TCP SYN to port 443, TCP ACK to port 80, and an ICMP
timestamp request by default. When executed by an unprivileged
user, only SYN packets are sent (using a connect call) to ports
80 and 443 on the target. When a privileged user tries to scan
targets on a local ethernet network, ARP requests are used unless
--send-ip was specified. The -sn option can be combined with any
of the discovery probe types (the -P* options, excluding -Pn) for
greater flexibility. If any of those probe type and port number
options are used, the default probes are overridden. When strict
firewalls are in place between the source host running Nmap and
the target network, using those advanced techniques is
recommended. Otherwise hosts could be missed when the firewall
drops probes or their responses.
In previous releases of Nmap, -sn was known as -sP.
-Pn (No ping)
This option skips the Nmap discovery stage altogether. Normally,
Nmap uses this stage to determine active machines for heavier
scanning. By default, Nmap only performs heavy probing such as
port scans, version detection, or OS detection against hosts that
are found to be up. Disabling host discovery with -Pn causes Nmap
to attempt the requested scanning functions against every target
IP address specified. So if a class B target address space (/16)
is specified on the command line, all 65,536 IP addresses are
scanned. Proper host discovery is skipped as with the list scan,
but instead of stopping and printing the target list, Nmap
continues to perform requested functions as if each target IP is
active. To skip ping scan and port scan, while still allowing NSE
to run, use the two options -Pn -sn together.
For machines on a local ethernet network, ARP scanning will still
be performed (unless --disable-arp-ping or --send-ip is
specified) because Nmap needs MAC addresses to further scan
target hosts. In previous versions of Nmap, -Pn was -P0 and -PN.
-PS port list (TCP SYN Ping)
This option sends an empty TCP packet with the SYN flag set. The
default destination port is 80 (configurable at compile time by
changing DEFAULT_TCP_PROBE_PORT_SPEC in nmap.h). Alternate ports
can be specified as a parameter. The syntax is the same as for
the -p except that port type specifiers like T: are not allowed.
Examples are -PS22 and -PS22-25,80,113,1050,35000. Note that
there can be no space between -PS and the port list. If multiple
probes are specified they will be sent in parallel.
The SYN flag suggests to the remote system that you are
attempting to establish a connection. Normally the destination
port will be closed, and a RST (reset) packet sent back. If the
port happens to be open, the target will take the second step of
a TCP three-way-handshake by responding with a SYN/ACK TCP
packet. The machine running Nmap then tears down the nascent
connection by responding with a RST rather than sending an ACK
packet which would complete the three-way-handshake and establish
a full connection. The RST packet is sent by the kernel of the
machine running Nmap in response to the unexpected SYN/ACK, not
by Nmap itself.
Nmap does not care whether the port is open or closed. Either the
RST or SYN/ACK response discussed previously tell Nmap that the
host is available and responsive.
On Unix boxes, only the privileged user root is generally able to
send and receive raw TCP packets. For unprivileged users, a
workaround is automatically employed whereby the connect system
call is initiated against each target port. This has the effect
of sending a SYN packet to the target host, in an attempt to
establish a connection. If connect returns with a quick success
or an ECONNREFUSED failure, the underlying TCP stack must have
received a SYN/ACK or RST and the host is marked available. If
the connection attempt is left hanging until a timeout is
reached, the host is marked as down.
-PA port list (TCP ACK Ping)
The TCP ACK ping is quite similar to the just-discussed SYN ping.
The difference, as you could likely guess, is that the TCP ACK
flag is set instead of the SYN flag. Such an ACK packet purports
to be acknowledging data over an established TCP connection, but
no such connection exists. So remote hosts should always respond
with a RST packet, disclosing their existence in the process.
The -PA option uses the same default port as the SYN probe (80)
and can also take a list of destination ports in the same format.
If an unprivileged user tries this, the connect workaround
discussed previously is used. This workaround is imperfect
because connect is actually sending a SYN packet rather than an
ACK.
The reason for offering both SYN and ACK ping probes is to
maximize the chances of bypassing firewalls. Many administrators
configure routers and other simple firewalls to block incoming
SYN packets except for those destined for public services like
the company web site or mail server. This prevents other incoming
connections to the organization, while allowing users to make
unobstructed outgoing connections to the Internet. This
non-stateful approach takes up few resources on the
firewall/router and is widely supported by hardware and software
filters. The Linux Netfilter/iptables firewall software offers
the --syn convenience option to implement this stateless
approach. When stateless firewall rules such as this are in
place, SYN ping probes (-PS) are likely to be blocked when sent
to closed target ports. In such cases, the ACK probe shines as it
cuts right through these rules.
Another common type of firewall uses stateful rules that drop
unexpected packets. This feature was initially found mostly on
high-end firewalls, though it has become much more common over
the years. The Linux Netfilter/iptables system supports this
through the --state option, which categorizes packets based on
connection state. A SYN probe is more likely to work against such
a system, as unexpected ACK packets are generally recognized as
bogus and dropped. A solution to this quandary is to send both
SYN and ACK probes by specifying -PS and -PA.
-PU port list (UDP Ping)
Another host discovery option is the UDP ping, which sends a UDP
packet to the given ports. For most ports, the packet will be
empty, though some use a protocol-specific payload that is more
likely to elicit a response. The payload database is described
at https://nmap.org/book/nmap-payloads.html . --data,
--data-string, and --data-length options.
The port list takes the same format as with the previously
discussed -PS and -PA options. If no ports are specified, the
default is 40125. This default can be configured at compile-time
by changing DEFAULT_UDP_PROBE_PORT_SPEC in nmap.h. A highly
uncommon port is used by default because sending to open ports is
often undesirable for this particular scan type.
Upon hitting a closed port on the target machine, the UDP probe
should elicit an ICMP port unreachable packet in return. This
signifies to Nmap that the machine is up and available. Many
other types of ICMP errors, such as host/network unreachables or
TTL exceeded are indicative of a down or unreachable host. A lack
of response is also interpreted this way. If an open port is
reached, most services simply ignore the empty packet and fail to
return any response. This is why the default probe port is 40125,
which is highly unlikely to be in use. A few services, such as
the Character Generator (chargen) protocol, will respond to an
empty UDP packet, and thus disclose to Nmap that the machine is
available.
The primary advantage of this scan type is that it bypasses
firewalls and filters that only screen TCP. For example, I once
owned a Linksys BEFW11S4 wireless broadband router. The external
interface of this device filtered all TCP ports by default, but
UDP probes would still elicit port unreachable messages and thus
give away the device.
-PY port list (SCTP INIT Ping)
This option sends an SCTP packet containing a minimal INIT chunk.
The default destination port is 80 (configurable at compile time
by changing DEFAULT_SCTP_PROBE_PORT_SPEC in nmap.h). Alternate
ports can be specified as a parameter. The syntax is the same as
for the -p except that port type specifiers like S: are not
allowed. Examples are -PY22 and -PY22,80,179,5060. Note that
there can be no space between -PY and the port list. If multiple
probes are specified they will be sent in parallel.
The INIT chunk suggests to the remote system that you are
attempting to establish an association. Normally the destination
port will be closed, and an ABORT chunk will be sent back. If the
port happens to be open, the target will take the second step of
an SCTP four-way-handshake by responding with an INIT-ACK chunk.
If the machine running Nmap has a functional SCTP stack, then it
tears down the nascent association by responding with an ABORT
chunk rather than sending a COOKIE-ECHO chunk which would be the
next step in the four-way-handshake. The ABORT packet is sent by
the kernel of the machine running Nmap in response to the
unexpected INIT-ACK, not by Nmap itself.
Nmap does not care whether the port is open or closed. Either the
ABORT or INIT-ACK response discussed previously tell Nmap that
the host is available and responsive.
On Unix boxes, only the privileged user root is generally able to
send and receive raw SCTP packets. Using SCTP INIT Pings is
currently not possible for unprivileged users.
-PE; -PP; -PM (ICMP Ping Types)
In addition to the unusual TCP, UDP and SCTP host discovery types
discussed previously, Nmap can send the standard packets sent by
the ubiquitous ping program. Nmap sends an ICMP type 8 (echo
request) packet to the target IP addresses, expecting a type 0
(echo reply) in return from available hosts. Unfortunately for
network explorers, many hosts and firewalls now block these
packets, rather than responding as required by RFC 1122[2]. For
this reason, ICMP-only scans are rarely reliable enough against
unknown targets over the Internet. But for system administrators
monitoring an internal network, they can be a practical and
efficient approach. Use the -PE option to enable this echo
request behavior.
While echo request is the standard ICMP ping query, Nmap does not
stop there. The ICMP standards (RFC 792[3] and RFC 950[4] ) also
specify timestamp request, information request, and address mask
request packets as codes 13, 15, and 17, respectively. While the
ostensible purpose for these queries is to learn information such
as address masks and current times, they can easily be used for
host discovery. A system that replies is up and available. Nmap
does not currently implement information request packets, as they
are not widely supported. RFC 1122 insists that “a host SHOULD
NOT implement these messages”. Timestamp and address mask queries
can be sent with the -PP and -PM options, respectively. A
timestamp reply (ICMP code 14) or address mask reply (code 18)
discloses that the host is available. These two queries can be
valuable when administrators specifically block echo request
packets while forgetting that other ICMP queries can be used for
the same purpose.
-PO protocol list (IP Protocol Ping)
One of the newer host discovery options is the IP protocol ping,
which sends IP packets with the specified protocol number set in
their IP header. The protocol list takes the same format as do
port lists in the previously discussed TCP, UDP and SCTP host
discovery options. If no protocols are specified, the default is
to send multiple IP packets for ICMP (protocol 1), IGMP (protocol
2), and IP-in-IP (protocol 4). The default protocols can be
configured at compile-time by changing
DEFAULT_PROTO_PROBE_PORT_SPEC in nmap.h. Note that for the ICMP,
IGMP, TCP (protocol 6), UDP (protocol 17) and SCTP (protocol
132), the packets are sent with the proper protocol headers while
other protocols are sent with no additional data beyond the IP
header (unless any of --data, --data-string, or --data-length
options are specified).
This host discovery method looks for either responses using the
same protocol as a probe, or ICMP protocol unreachable messages
which signify that the given protocol isn't supported on the
destination host. Either type of response signifies that the
target host is alive.
-PR (ARP Ping)
One of the most common Nmap usage scenarios is to scan an
ethernet LAN. On most LANs, especially those using private
address ranges specified by RFC 1918[5], the vast majority of IP
addresses are unused at any given time. When Nmap tries to send a
raw IP packet such as an ICMP echo request, the operating system
must determine the destination hardware (ARP) address
corresponding to the target IP so that it can properly address
the ethernet frame. This is often slow and problematic, since
operating systems weren't written with the expectation that they
would need to do millions of ARP requests against unavailable
hosts in a short time period.
ARP scan puts Nmap and its optimized algorithms in charge of ARP
requests. And if it gets a response back, Nmap doesn't even need
to worry about the IP-based ping packets since it already knows
the host is up. This makes ARP scan much faster and more reliable
than IP-based scans. So it is done by default when scanning
ethernet hosts that Nmap detects are on a local ethernet network.
Even if different ping types (such as -PE or -PS) are specified,
Nmap uses ARP instead for any of the targets which are on the
same LAN. If you absolutely don't want to do an ARP scan, specify
--disable-arp-ping.
For IPv6 (-6 option), -PR uses ICMPv6 Neighbor Discovery instead
of ARP. Neighbor Discovery, defined in RFC 4861, can be seen as
the IPv6 equivalent of ARP.
--disable-arp-ping (No ARP or ND Ping)
Nmap normally does ARP or IPv6 Neighbor Discovery (ND) discovery
of locally connected ethernet hosts, even if other host discovery
options such as -Pn or -PE are used. To disable this implicit
behavior, use the --disable-arp-ping option.
The default behavior is normally faster, but this option is
useful on networks using proxy ARP, in which a router
speculatively replies to all ARP requests, making every target
appear to be up according to ARP scan.
--traceroute (Trace path to host)
Traceroutes are performed post-scan using information from the
scan results to determine the port and protocol most likely to
reach the target. It works with all scan types except connect
scans (-sT) and idle scans (-sI). All traces use Nmap's dynamic
timing model and are performed in parallel.
Traceroute works by sending packets with a low TTL (time-to-live)
in an attempt to elicit ICMP Time Exceeded messages from
intermediate hops between the scanner and the target host.
Standard traceroute implementations start with a TTL of 1 and
increment the TTL until the destination host is reached. Nmap's
traceroute starts with a high TTL and then decrements the TTL
until it reaches zero. Doing it backwards lets Nmap employ clever
caching algorithms to speed up traces over multiple hosts. On
average Nmap sends 5–10 fewer packets per host, depending on
network conditions. If a single subnet is being scanned (i.e.
192.168.0.0/24) Nmap may only have to send two packets to most
hosts.
-n (No DNS resolution)
Tells Nmap to never do reverse DNS resolution on the active IP
addresses it finds. Since DNS can be slow even with Nmap's
built-in parallel stub resolver, this option can slash scanning
times.
-R (DNS resolution for all targets)
Tells Nmap to always do reverse DNS resolution on the target IP
addresses. Normally reverse DNS is only performed against
responsive (online) hosts.
--system-dns (Use system DNS resolver)
By default, Nmap resolves IP addresses by sending queries
directly to the name servers configured on your host and then
listening for responses. Many requests (often dozens) are
performed in parallel to improve performance. Specify this option
to use your system resolver instead (one IP at a time via the
getnameinfo call). This is slower and rarely useful unless you
find a bug in the Nmap parallel resolver (please let us know if
you do). The system resolver is always used for IPv6 scans.
--dns-servers server1[,server2[,...]] (Servers to use for reverse
DNS queries)
By default, Nmap determines your DNS servers (for rDNS
resolution) from your resolv.conf file (Unix) or the Registry
(Win32). Alternatively, you may use this option to specify
alternate servers. This option is not honored if you are using
--system-dns or an IPv6 scan. Using multiple DNS servers is often
faster, especially if you choose authoritative servers for your
target IP space. This option can also improve stealth, as your
requests can be bounced off just about any recursive DNS server
on the Internet.
This option also comes in handy when scanning private networks.
Sometimes only a few name servers provide proper rDNS
information, and you may not even know where they are. You can
scan the network for port 53 (perhaps with version detection),
then try Nmap list scans (-sL) specifying each name server one at
a time with --dns-servers until you find one which works.
This option might not be honored if the DNS response exceeds the
size of a UDP packet. In such a situation our DNS resolver will
make the best effort to extract a response from the truncated
packet, and if not successful it will fall back to using the
system resolver. Also, responses that contain CNAME aliases will
fall back to the system resolver.
PORT SCANNING BASICS top
While Nmap has grown in functionality over the years, it began as an
efficient port scanner, and that remains its core function. The
simple command nmap target scans 1,000 TCP ports on the host target.
While many port scanners have traditionally lumped all ports into the
open or closed states, Nmap is much more granular. It divides ports
into six states: open, closed, filtered, unfiltered, open|filtered,
or closed|filtered.
These states are not intrinsic properties of the port itself, but
describe how Nmap sees them. For example, an Nmap scan from the same
network as the target may show port 135/tcp as open, while a scan at
the same time with the same options from across the Internet might
show that port as filtered.
The six port states recognized by Nmap
open
An application is actively accepting TCP connections, UDP
datagrams or SCTP associations on this port. Finding these is
often the primary goal of port scanning. Security-minded people
know that each open port is an avenue for attack. Attackers and
pen-testers want to exploit the open ports, while administrators
try to close or protect them with firewalls without thwarting
legitimate users. Open ports are also interesting for
non-security scans because they show services available for use
on the network.
closed
A closed port is accessible (it receives and responds to Nmap
probe packets), but there is no application listening on it. They
can be helpful in showing that a host is up on an IP address
(host discovery, or ping scanning), and as part of OS detection.
Because closed ports are reachable, it may be worth scanning
later in case some open up. Administrators may want to consider
blocking such ports with a firewall. Then they would appear in
the filtered state, discussed next.
filtered
Nmap cannot determine whether the port is open because packet
filtering prevents its probes from reaching the port. The
filtering could be from a dedicated firewall device, router
rules, or host-based firewall software. These ports frustrate
attackers because they provide so little information. Sometimes
they respond with ICMP error messages such as type 3 code 13
(destination unreachable: communication administratively
prohibited), but filters that simply drop probes without
responding are far more common. This forces Nmap to retry several
times just in case the probe was dropped due to network
congestion rather than filtering. This slows down the scan
dramatically.
unfiltered
The unfiltered state means that a port is accessible, but Nmap is
unable to determine whether it is open or closed. Only the ACK
scan, which is used to map firewall rulesets, classifies ports
into this state. Scanning unfiltered ports with other scan types
such as Window scan, SYN scan, or FIN scan, may help resolve
whether the port is open.
open|filtered
Nmap places ports in this state when it is unable to determine
whether a port is open or filtered. This occurs for scan types in
which open ports give no response. The lack of response could
also mean that a packet filter dropped the probe or any response
it elicited. So Nmap does not know for sure whether the port is
open or being filtered. The UDP, IP protocol, FIN, NULL, and Xmas
scans classify ports this way.
closed|filtered
This state is used when Nmap is unable to determine whether a
port is closed or filtered. It is only used for the IP ID idle
scan.
PORT SCANNING TECHNIQUES top
As a novice performing automotive repair, I can struggle for hours
trying to fit my rudimentary tools (hammer, duct tape, wrench, etc.)
to the task at hand. When I fail miserably and tow my jalopy to a
real mechanic, he invariably fishes around in a huge tool chest until
pulling out the perfect gizmo which makes the job seem effortless.
The art of port scanning is similar. Experts understand the dozens of
scan techniques and choose the appropriate one (or combination) for a
given task. Inexperienced users and script kiddies, on the other
hand, try to solve every problem with the default SYN scan. Since
Nmap is free, the only barrier to port scanning mastery is knowledge.
That certainly beats the automotive world, where it may take great
skill to determine that you need a strut spring compressor, then you
still have to pay thousands of dollars for it.
Most of the scan types are only available to privileged users. This
is because they send and receive raw packets, which requires root
access on Unix systems. Using an administrator account on Windows is
recommended, though Nmap sometimes works for unprivileged users on
that platform when WinPcap has already been loaded into the OS.
Requiring root privileges was a serious limitation when Nmap was
released in 1997, as many users only had access to shared shell
accounts. Now, the world is different. Computers are cheaper, far
more people have always-on direct Internet access, and desktop Unix
systems (including Linux and Mac OS X) are prevalent. A Windows
version of Nmap is now available, allowing it to run on even more
desktops. For all these reasons, users have less need to run Nmap
from limited shared shell accounts. This is fortunate, as the
privileged options make Nmap far more powerful and flexible.
While Nmap attempts to produce accurate results, keep in mind that
all of its insights are based on packets returned by the target
machines (or firewalls in front of them). Such hosts may be
untrustworthy and send responses intended to confuse or mislead Nmap.
Much more common are non-RFC-compliant hosts that do not respond as
they should to Nmap probes. FIN, NULL, and Xmas scans are
particularly susceptible to this problem. Such issues are specific to
certain scan types and so are discussed in the individual scan type
entries.
This section documents the dozen or so port scan techniques supported
by Nmap. Only one method may be used at a time, except that UDP scan
(-sU) and any one of the SCTP scan types (-sY, -sZ) may be combined
with any one of the TCP scan types. As a memory aid, port scan type
options are of the form -sC, where C is a prominent character in the
scan name, usually the first. The one exception to this is the
deprecated FTP bounce scan (-b). By default, Nmap performs a SYN
Scan, though it substitutes a connect scan if the user does not have
proper privileges to send raw packets (requires root access on Unix).
Of the scans listed in this section, unprivileged users can only
execute connect and FTP bounce scans.
-sS (TCP SYN scan)
SYN scan is the default and most popular scan option for good
reasons. It can be performed quickly, scanning thousands of ports
per second on a fast network not hampered by restrictive
firewalls. It is also relatively unobtrusive and stealthy since
it never completes TCP connections. SYN scan works against any
compliant TCP stack rather than depending on idiosyncrasies of
specific platforms as Nmap's FIN/NULL/Xmas, Maimon and idle scans
do. It also allows clear, reliable differentiation between the
open, closed, and filtered states.
This technique is often referred to as half-open scanning,
because you don't open a full TCP connection. You send a SYN
packet, as if you are going to open a real connection and then
wait for a response. A SYN/ACK indicates the port is listening
(open), while a RST (reset) is indicative of a non-listener. If
no response is received after several retransmissions, the port
is marked as filtered. The port is also marked filtered if an
ICMP unreachable error (type 3, code 0, 1, 2, 3, 9, 10, or 13) is
received. The port is also considered open if a SYN packet
(without the ACK flag) is received in response. This can be due
to an extremely rare TCP feature known as a simultaneous open or
split handshake connection (see
https://nmap.org/misc/split-handshake.pdf ).
-sT (TCP connect scan)
TCP connect scan is the default TCP scan type when SYN scan is
not an option. This is the case when a user does not have raw
packet privileges. Instead of writing raw packets as most other
scan types do, Nmap asks the underlying operating system to
establish a connection with the target machine and port by
issuing the connect system call. This is the same high-level
system call that web browsers, P2P clients, and most other
network-enabled applications use to establish a connection. It is
part of a programming interface known as the Berkeley Sockets
API. Rather than read raw packet responses off the wire, Nmap
uses this API to obtain status information on each connection
attempt.
When SYN scan is available, it is usually a better choice. Nmap
has less control over the high level connect call than with raw
packets, making it less efficient. The system call completes
connections to open target ports rather than performing the
half-open reset that SYN scan does. Not only does this take
longer and require more packets to obtain the same information,
but target machines are more likely to log the connection. A
decent IDS will catch either, but most machines have no such
alarm system. Many services on your average Unix system will add
a note to syslog, and sometimes a cryptic error message, when
Nmap connects and then closes the connection without sending
data. Truly pathetic services crash when this happens, though
that is uncommon. An administrator who sees a bunch of connection
attempts in her logs from a single system should know that she
has been connect scanned.
-sU (UDP scans)
While most popular services on the Internet run over the TCP
protocol, UDP[6] services are widely deployed. DNS, SNMP, and
DHCP (registered ports 53, 161/162, and 67/68) are three of the
most common. Because UDP scanning is generally slower and more
difficult than TCP, some security auditors ignore these ports.
This is a mistake, as exploitable UDP services are quite common
and attackers certainly don't ignore the whole protocol.
Fortunately, Nmap can help inventory UDP ports.
UDP scan is activated with the -sU option. It can be combined
with a TCP scan type such as SYN scan (-sS) to check both
protocols during the same run.
UDP scan works by sending a UDP packet to every targeted port.
For some common ports such as 53 and 161, a protocol-specific
payload is sent to increase response rate, but for most ports the
packet is empty unless the --data, --data-string, or
--data-length options are specified. If an ICMP port unreachable
error (type 3, code 3) is returned, the port is closed. Other
ICMP unreachable errors (type 3, codes 0, 1, 2, 9, 10, or 13)
mark the port as filtered. Occasionally, a service will respond
with a UDP packet, proving that it is open. If no response is
received after retransmissions, the port is classified as
open|filtered. This means that the port could be open, or perhaps
packet filters are blocking the communication. Version detection
(-sV) can be used to help differentiate the truly open ports from
the filtered ones.
A big challenge with UDP scanning is doing it quickly. Open and
filtered ports rarely send any response, leaving Nmap to time out
and then conduct retransmissions just in case the probe or
response were lost. Closed ports are often an even bigger
problem. They usually send back an ICMP port unreachable error.
But unlike the RST packets sent by closed TCP ports in response
to a SYN or connect scan, many hosts rate limit ICMP port
unreachable messages by default. Linux and Solaris are
particularly strict about this. For example, the Linux 2.4.20
kernel limits destination unreachable messages to one per second
(in net/ipv4/icmp.c).
Nmap detects rate limiting and slows down accordingly to avoid
flooding the network with useless packets that the target machine
will drop. Unfortunately, a Linux-style limit of one packet per
second makes a 65,536-port scan take more than 18 hours. Ideas
for speeding your UDP scans up include scanning more hosts in
parallel, doing a quick scan of just the popular ports first,
scanning from behind the firewall, and using --host-timeout to
skip slow hosts.
-sY (SCTP INIT scan)
SCTP[7] is a relatively new alternative to the TCP and UDP
protocols, combining most characteristics of TCP and UDP, and
also adding new features like multi-homing and multi-streaming.
It is mostly being used for SS7/SIGTRAN related services but has
the potential to be used for other applications as well. SCTP
INIT scan is the SCTP equivalent of a TCP SYN scan. It can be
performed quickly, scanning thousands of ports per second on a
fast network not hampered by restrictive firewalls. Like SYN
scan, INIT scan is relatively unobtrusive and stealthy, since it
never completes SCTP associations. It also allows clear, reliable
differentiation between the open, closed, and filtered states.
This technique is often referred to as half-open scanning,
because you don't open a full SCTP association. You send an INIT
chunk, as if you are going to open a real association and then
wait for a response. An INIT-ACK chunk indicates the port is
listening (open), while an ABORT chunk is indicative of a
non-listener. If no response is received after several
retransmissions, the port is marked as filtered. The port is also
marked filtered if an ICMP unreachable error (type 3, code 0, 1,
2, 3, 9, 10, or 13) is received.
-sN; -sF; -sX (TCP NULL, FIN, and Xmas scans)
These three scan types (even more are possible with the
--scanflags option described in the next section) exploit a
subtle loophole in the TCP RFC[8] to differentiate between open
and closed ports. Page 65 of RFC 793 says that “if the
[destination] port state is CLOSED .... an incoming segment not
containing a RST causes a RST to be sent in response.” Then the
next page discusses packets sent to open ports without the SYN,
RST, or ACK bits set, stating that: “you are unlikely to get
here, but if you do, drop the segment, and return.”
When scanning systems compliant with this RFC text, any packet
not containing SYN, RST, or ACK bits will result in a returned
RST if the port is closed and no response at all if the port is
open. As long as none of those three bits are included, any
combination of the other three (FIN, PSH, and URG) are OK. Nmap
exploits this with three scan types:
Null scan (-sN)
Does not set any bits (TCP flag header is 0)
FIN scan (-sF)
Sets just the TCP FIN bit.
Xmas scan (-sX)
Sets the FIN, PSH, and URG flags, lighting the packet up like
a Christmas tree.
These three scan types are exactly the same in behavior except
for the TCP flags set in probe packets. If a RST packet is
received, the port is considered closed, while no response means
it is open|filtered. The port is marked filtered if an ICMP
unreachable error (type 3, code 0, 1, 2, 3, 9, 10, or 13) is
received.
The key advantage to these scan types is that they can sneak
through certain non-stateful firewalls and packet filtering
routers. Another advantage is that these scan types are a little
more stealthy than even a SYN scan. Don't count on this though—
most modern IDS products can be configured to detect them. The
big downside is that not all systems follow RFC 793 to the
letter. A number of systems send RST responses to the probes
regardless of whether the port is open or not. This causes all of
the ports to be labeled closed. Major operating systems that do
this are Microsoft Windows, many Cisco devices, BSDI, and IBM
OS/400. This scan does work against most Unix-based systems
though. Another downside of these scans is that they can't
distinguish open ports from certain filtered ones, leaving you
with the response open|filtered.
-sA (TCP ACK scan)
This scan is different than the others discussed so far in that
it never determines open (or even open|filtered) ports. It is
used to map out firewall rulesets, determining whether they are
stateful or not and which ports are filtered.
The ACK scan probe packet has only the ACK flag set (unless you
use --scanflags). When scanning unfiltered systems, open and
closed ports will both return a RST packet. Nmap then labels them
as unfiltered, meaning that they are reachable by the ACK packet,
but whether they are open or closed is undetermined. Ports that
don't respond, or send certain ICMP error messages back (type 3,
code 0, 1, 2, 3, 9, 10, or 13), are labeled filtered.
-sW (TCP Window scan)
Window scan is exactly the same as ACK scan except that it
exploits an implementation detail of certain systems to
differentiate open ports from closed ones, rather than always
printing unfiltered when a RST is returned. It does this by
examining the TCP Window field of the RST packets returned. On
some systems, open ports use a positive window size (even for RST
packets) while closed ones have a zero window. So instead of
always listing a port as unfiltered when it receives a RST back,
Window scan lists the port as open or closed if the TCP Window
value in that reset is positive or zero, respectively.
This scan relies on an implementation detail of a minority of
systems out on the Internet, so you can't always trust it.
Systems that don't support it will usually return all ports
closed. Of course, it is possible that the machine really has no
open ports. If most scanned ports are closed but a few common
port numbers (such as 22, 25, 53) are filtered, the system is
most likely susceptible. Occasionally, systems will even show the
exact opposite behavior. If your scan shows 1,000 open ports and
three closed or filtered ports, then those three may very well be
the truly open ones.
-sM (TCP Maimon scan)
The Maimon scan is named after its discoverer, Uriel Maimon. He
described the technique in Phrack Magazine issue #49 (November
1996). Nmap, which included this technique, was released two
issues later. This technique is exactly the same as NULL, FIN,
and Xmas scans, except that the probe is FIN/ACK. According to
RFC 793[8] (TCP), a RST packet should be generated in response to
such a probe whether the port is open or closed. However, Uriel
noticed that many BSD-derived systems simply drop the packet if
the port is open.
--scanflags (Custom TCP scan)
Truly advanced Nmap users need not limit themselves to the canned
scan types offered. The --scanflags option allows you to design
your own scan by specifying arbitrary TCP flags. Let your
creative juices flow, while evading intrusion detection systems
whose vendors simply paged through the Nmap man page adding
specific rules!
The --scanflags argument can be a numerical flag value such as 9
(PSH and FIN), but using symbolic names is easier. Just mash
together any combination of URG, ACK, PSH, RST, SYN, and FIN. For
example, --scanflags URGACKPSHRSTSYNFIN sets everything, though
it's not very useful for scanning. The order these are specified
in is irrelevant.
In addition to specifying the desired flags, you can specify a
TCP scan type (such as -sA or -sF). That base type tells Nmap how
to interpret responses. For example, a SYN scan considers
no-response to indicate a filtered port, while a FIN scan treats
the same as open|filtered. Nmap will behave the same way it does
for the base scan type, except that it will use the TCP flags you
specify instead. If you don't specify a base type, SYN scan is
used.
-sZ (SCTP COOKIE ECHO scan)
SCTP COOKIE ECHO scan is a more advanced SCTP scan. It takes
advantage of the fact that SCTP implementations should silently
drop packets containing COOKIE ECHO chunks on open ports, but
send an ABORT if the port is closed. The advantage of this scan
type is that it is not as obvious a port scan than an INIT scan.
Also, there may be non-stateful firewall rulesets blocking INIT
chunks, but not COOKIE ECHO chunks. Don't be fooled into thinking
that this will make a port scan invisible; a good IDS will be
able to detect SCTP COOKIE ECHO scans too. The downside is that
SCTP COOKIE ECHO scans cannot differentiate between open and
filtered ports, leaving you with the state open|filtered in both
cases.
-sI zombie host[:probeport] (idle scan)
This advanced scan method allows for a truly blind TCP port scan
of the target (meaning no packets are sent to the target from
your real IP address). Instead, a unique side-channel attack
exploits predictable IP fragmentation ID sequence generation on
the zombie host to glean information about the open ports on the
target. IDS systems will display the scan as coming from the
zombie machine you specify (which must be up and meet certain
criteria). This fascinating scan type is too complex to fully
describe in this reference guide, so I wrote and posted an
informal paper with full details at
https://nmap.org/book/idlescan.html .
Besides being extraordinarily stealthy (due to its blind nature),
this scan type permits mapping out IP-based trust relationships
between machines. The port listing shows open ports from the
perspective of the zombie host. So you can try scanning a target
using various zombies that you think might be trusted (via
router/packet filter rules).
You can add a colon followed by a port number to the zombie host
if you wish to probe a particular port on the zombie for IP ID
changes. Otherwise Nmap will use the port it uses by default for
TCP pings (80).
-sO (IP protocol scan)
IP protocol scan allows you to determine which IP protocols (TCP,
ICMP, IGMP, etc.) are supported by target machines. This isn't
technically a port scan, since it cycles through IP protocol
numbers rather than TCP or UDP port numbers. Yet it still uses
the -p option to select scanned protocol numbers, reports its
results within the normal port table format, and even uses the
same underlying scan engine as the true port scanning methods. So
it is close enough to a port scan that it belongs here.
Besides being useful in its own right, protocol scan demonstrates
the power of open-source software. While the fundamental idea is
pretty simple, I had not thought to add it nor received any
requests for such functionality. Then in the summer of 2000,
Gerhard Rieger conceived the idea, wrote an excellent patch
implementing it, and sent it to the announce mailing list (then
called nmap-hackers). I incorporated that patch into the Nmap
tree and released a new version the next day. Few pieces of
commercial software have users enthusiastic enough to design and
contribute their own improvements!
Protocol scan works in a similar fashion to UDP scan. Instead of
iterating through the port number field of a UDP packet, it sends
IP packet headers and iterates through the eight-bit IP protocol
field. The headers are usually empty, containing no data and not
even the proper header for the claimed protocol. The exceptions
are TCP, UDP, ICMP, SCTP, and IGMP. A proper protocol header for
those is included since some systems won't send them otherwise
and because Nmap already has functions to create them. Instead of
watching for ICMP port unreachable messages, protocol scan is on
the lookout for ICMP protocol unreachable messages. If Nmap
receives any response in any protocol from the target host, Nmap
marks that protocol as open. An ICMP protocol unreachable error
(type 3, code 2) causes the protocol to be marked as closed while
port unreachable (type 3, code 3) marks the protocol open. Other
ICMP unreachable errors (type 3, code 0, 1, 9, 10, or 13) cause
the protocol to be marked filtered (though they prove that ICMP
is open at the same time). If no response is received after
retransmissions, the protocol is marked open|filtered
-b FTP relay host (FTP bounce scan)
An interesting feature of the FTP protocol (RFC 959[9]) is
support for so-called proxy FTP connections. This allows a user
to connect to one FTP server, then ask that files be sent to a
third-party server. Such a feature is ripe for abuse on many
levels, so most servers have ceased supporting it. One of the
abuses this feature allows is causing the FTP server to port scan
other hosts. Simply ask the FTP server to send a file to each
interesting port of a target host in turn. The error message will
describe whether the port is open or not. This is a good way to
bypass firewalls because organizational FTP servers are often
placed where they have more access to other internal hosts than
any old Internet host would. Nmap supports FTP bounce scan with
the -b option. It takes an argument of the form
username:password@server:port. Server is the name or IP address
of a vulnerable FTP server. As with a normal URL, you may omit
username:password, in which case anonymous login credentials
(user: anonymous password:-wwwuser@) are used. The port number
(and preceding colon) may be omitted as well, in which case the
default FTP port (21) on server is used.
This vulnerability was widespread in 1997 when Nmap was released,
but has largely been fixed. Vulnerable servers are still around,
so it is worth trying when all else fails. If bypassing a
firewall is your goal, scan the target network for port 21 (or
even for any FTP services if you scan all ports with version
detection) and use the ftp-bounce NSE script. Nmap will tell you
whether the host is vulnerable or not. If you are just trying to
cover your tracks, you don't need to (and, in fact, shouldn't)
limit yourself to hosts on the target network. Before you go
scanning random Internet addresses for vulnerable FTP servers,
consider that sysadmins may not appreciate you abusing their
servers in this way.
PORT SPECIFICATION AND SCAN ORDER top
In addition to all of the scan methods discussed previously, Nmap
offers options for specifying which ports are scanned and whether the
scan order is randomized or sequential. By default, Nmap scans the
most common 1,000 ports for each protocol.
-p port ranges (Only scan specified ports)
This option specifies which ports you want to scan and overrides
the default. Individual port numbers are OK, as are ranges
separated by a hyphen (e.g. 1-1023). The beginning and/or end
values of a range may be omitted, causing Nmap to use 1 and
65535, respectively. So you can specify -p- to scan ports from 1
through 65535. Scanning port zero is allowed if you specify it
explicitly. For IP protocol scanning (-sO), this option specifies
the protocol numbers you wish to scan for (0–255).
When scanning a combination of protocols (e.g. TCP and UDP), you
can specify a particular protocol by preceding the port numbers
by T: for TCP, U: for UDP, S: for SCTP, or P: for IP Protocol.
The qualifier lasts until you specify another qualifier. For
example, the argument -p U:53,111,137,T:21-25,80,139,8080 would
scan UDP ports 53, 111,and 137, as well as the listed TCP ports.
Note that to scan both UDP and TCP, you have to specify -sU and
at least one TCP scan type (such as -sS, -sF, or -sT). If no
protocol qualifier is given, the port numbers are added to all
protocol lists. Ports can also be specified by name according to
what the port is referred to in the nmap-services. You can even
use the wildcards * and ? with the names. For example, to scan
FTP and all ports whose names begin with “http”, use -p
ftp,http*. Be careful about shell expansions and quote the
argument to -p if unsure.
Ranges of ports can be surrounded by square brackets to indicate
ports inside that range that appear in nmap-services. For
example, the following will scan all ports in nmap-services equal
to or below 1024: -p [-1024]. Be careful with shell expansions
and quote the argument to -p if unsure.
--exclude-ports port ranges (Exclude the specified ports from
scanning)
This option specifies which ports you do want Nmap to exclude
from scanning. The port ranges are specified similar to -p. For
IP protocol scanning (-sO), this option specifies the protocol
numbers you wish to exclude (0–255).
When ports are asked to be excluded, they are excluded from all
types of scans (i.e. they will not be scanned under any
circumstances). This also includes the discovery phase.
-F (Fast (limited port) scan)
Specifies that you wish to scan fewer ports than the default.
Normally Nmap scans the most common 1,000 ports for each scanned
protocol. With -F, this is reduced to 100.
Nmap needs an nmap-services file with frequency information in
order to know which ports are the most common. If port frequency
information isn't available, perhaps because of the use of a
custom nmap-services file, Nmap scans all named ports plus ports
1-1024. In that case, -F means to scan only ports that are named
in the services file.
-r (Don't randomize ports)
By default, Nmap randomizes the scanned port order (except that
certain commonly accessible ports are moved near the beginning
for efficiency reasons). This randomization is normally
desirable, but you can specify -r for sequential (sorted from
lowest to highest) port scanning instead.
--port-ratio ratio<decimal number between 0 and 1>
Scans all ports in nmap-services file with a ratio greater than
the one given. ratio must be between 0.0 and 1.0.
--top-ports n
Scans the n highest-ratio ports found in nmap-services file after
excluding all ports specified by --exclude-ports. n must be 1 or
greater.
SERVICE AND VERSION DETECTION top
Point Nmap at a remote machine and it might tell you that ports
25/tcp, 80/tcp, and 53/udp are open. Using its nmap-services database
of about 2,200 well-known services, Nmap would report that those
ports probably correspond to a mail server (SMTP), web server (HTTP),
and name server (DNS) respectively. This lookup is usually accurate—
the vast majority of daemons listening on TCP port 25 are, in fact,
mail servers. However, you should not bet your security on this!
People can and do run services on strange ports.
Even if Nmap is right, and the hypothetical server above is running
SMTP, HTTP, and DNS servers, that is not a lot of information. When
doing vulnerability assessments (or even simple network inventories)
of your companies or clients, you really want to know which mail and
DNS servers and versions are running. Having an accurate version
number helps dramatically in determining which exploits a server is
vulnerable to. Version detection helps you obtain this information.
After TCP and/or UDP ports are discovered using one of the other scan
methods, version detection interrogates those ports to determine more
about what is actually running. The nmap-service-probes database
contains probes for querying various services and match expressions
to recognize and parse responses. Nmap tries to determine the service
protocol (e.g. FTP, SSH, Telnet, HTTP), the application name (e.g.
ISC BIND, Apache httpd, Solaris telnetd), the version number,
hostname, device type (e.g. printer, router), the OS family (e.g.
Windows, Linux). When possible, Nmap also gets the Common Platform
Enumeration (CPE) representation of this information. Sometimes
miscellaneous details like whether an X server is open to
connections, the SSH protocol version, or the KaZaA user name, are
available. Of course, most services don't provide all of this
information. If Nmap was compiled with OpenSSL support, it will
connect to SSL servers to deduce the service listening behind that
encryption layer. Some UDP ports are left in the open|filtered state
after a UDP port scan is unable to determine whether the port is open
or filtered. Version detection will try to elicit a response from
these ports (just as it does with open ports), and change the state
to open if it succeeds. open|filtered TCP ports are treated the same
way. Note that the Nmap -A option enables version detection among
other things. A paper documenting the workings, usage, and
customization of version detection is available at
https://nmap.org/book/vscan.html .
When RPC services are discovered, the Nmap RPC grinder is
automatically used to determine the RPC program and version numbers.
It takes all the TCP/UDP ports detected as RPC and floods them with
SunRPC program NULL commands in an attempt to determine whether they
are RPC ports, and if so, what program and version number they serve
up. Thus you can effectively obtain the same info as rpcinfo -p even
if the target's portmapper is behind a firewall (or protected by TCP
wrappers). Decoys do not currently work with RPC scan.
When Nmap receives responses from a service but cannot match them to
its database, it prints out a special fingerprint and a URL for you
to submit if to if you know for sure what is running on the port.
Please take a couple minutes to make the submission so that your find
can benefit everyone. Thanks to these submissions, Nmap has about
6,500 pattern matches for more than 650 protocols such as SMTP, FTP,
HTTP, etc.
Version detection is enabled and controlled with the following
options:
-sV (Version detection)
Enables version detection, as discussed above. Alternatively, you
can use -A, which enables version detection among other things.
-sR is an alias for -sV. Prior to March 2011, it was used to
active the RPC grinder separately from version detection, but now
these options are always combined.
--allports (Don't exclude any ports from version detection)
By default, Nmap version detection skips TCP port 9100 because
some printers simply print anything sent to that port, leading to
dozens of pages of HTTP GET requests, binary SSL session
requests, etc. This behavior can be changed by modifying or
removing the Exclude directive in nmap-service-probes, or you can
specify --allports to scan all ports regardless of any Exclude
directive.
--version-intensity intensity (Set version scan intensity)
When performing a version scan (-sV), Nmap sends a series of
probes, each of which is assigned a rarity value between one and
nine. The lower-numbered probes are effective against a wide
variety of common services, while the higher-numbered ones are
rarely useful. The intensity level specifies which probes should
be applied. The higher the number, the more likely it is the
service will be correctly identified. However, high intensity
scans take longer. The intensity must be between 0 and 9. The
default is 7. When a probe is registered to the target port via
the nmap-service-probesports directive, that probe is tried
regardless of intensity level. This ensures that the DNS probes
will always be attempted against any open port 53, the SSL probe
will be done against 443, etc.
--version-light (Enable light mode)
This is a convenience alias for --version-intensity 2. This light
mode makes version scanning much faster, but it is slightly less
likely to identify services.
--version-all (Try every single probe)
An alias for --version-intensity 9, ensuring that every single
probe is attempted against each port.
--version-trace (Trace version scan activity)
This causes Nmap to print out extensive debugging info about what
version scanning is doing. It is a subset of what you get with
--packet-trace.
OS DETECTION top
One of Nmap's best-known features is remote OS detection using TCP/IP
stack fingerprinting. Nmap sends a series of TCP and UDP packets to
the remote host and examines practically every bit in the responses.
After performing dozens of tests such as TCP ISN sampling, TCP
options support and ordering, IP ID sampling, and the initial window
size check, Nmap compares the results to its nmap-os-db database of
more than 2,600 known OS fingerprints and prints out the OS details
if there is a match. Each fingerprint includes a freeform textual
description of the OS, and a classification which provides the vendor
name (e.g. Sun), underlying OS (e.g. Solaris), OS generation (e.g.
10), and device type (general purpose, router, switch, game console,
etc). Most fingerprints also have a Common Platform Enumeration (CPE)
representation, like cpe:/o:linux:linux_kernel:2.6.
If Nmap is unable to guess the OS of a machine, and conditions are
good (e.g. at least one open port and one closed port were found),
Nmap will provide a URL you can use to submit the fingerprint if you
know (for sure) the OS running on the machine. By doing this you
contribute to the pool of operating systems known to Nmap and thus it
will be more accurate for everyone.
OS detection enables some other tests which make use of information
that is gathered during the process anyway. One of these is TCP
Sequence Predictability Classification. This measures approximately
how hard it is to establish a forged TCP connection against the
remote host. It is useful for exploiting source-IP based trust
relationships (rlogin, firewall filters, etc) or for hiding the
source of an attack. This sort of spoofing is rarely performed any
more, but many machines are still vulnerable to it. The actual
difficulty number is based on statistical sampling and may fluctuate.
It is generally better to use the English classification such as
“worthy challenge” or “trivial joke”. This is only reported in normal
output in verbose (-v) mode. When verbose mode is enabled along with
-O, IP ID sequence generation is also reported. Most machines are in
the “incremental” class, which means that they increment the ID field
in the IP header for each packet they send. This makes them
vulnerable to several advanced information gathering and spoofing
attacks.
Another bit of extra information enabled by OS detection is a guess
at a target's uptime. This uses the TCP timestamp option (RFC
1323[10]) to guess when a machine was last rebooted. The guess can be
inaccurate due to the timestamp counter not being initialized to zero
or the counter overflowing and wrapping around, so it is printed only
in verbose mode.
A paper documenting the workings, usage, and customization of OS
detection is available at https://nmap.org/book/osdetect.html .
OS detection is enabled and controlled with the following options:
-O (Enable OS detection)
Enables OS detection, as discussed above. Alternatively, you can
use -A to enable OS detection along with other things.
--osscan-limit (Limit OS detection to promising targets)
OS detection is far more effective if at least one open and one
closed TCP port are found. Set this option and Nmap will not even
try OS detection against hosts that do not meet this criteria.
This can save substantial time, particularly on -Pn scans against
many hosts. It only matters when OS detection is requested with
-O or -A.
--osscan-guess; --fuzzy (Guess OS detection results)
When Nmap is unable to detect a perfect OS match, it sometimes
offers up near-matches as possibilities. The match has to be very
close for Nmap to do this by default. Either of these
(equivalent) options make Nmap guess more aggressively. Nmap will
still tell you when an imperfect match is printed and display its
confidence level (percentage) for each guess.
--max-os-tries (Set the maximum number of OS detection tries against
a target)
When Nmap performs OS detection against a target and fails to
find a perfect match, it usually repeats the attempt. By default,
Nmap tries five times if conditions are favorable for OS
fingerprint submission, and twice when conditions aren't so good.
Specifying a lower --max-os-tries value (such as 1) speeds Nmap
up, though you miss out on retries which could potentially
identify the OS. Alternatively, a high value may be set to allow
even more retries when conditions are favorable. This is rarely
done, except to generate better fingerprints for submission and
integration into the Nmap OS database.
NMAP SCRIPTING ENGINE (NSE) top
The Nmap Scripting Engine (NSE) is one of Nmap's most powerful and
flexible features. It allows users to write (and share) simple
scripts (using the Lua programming language[11] ) to automate a wide
variety of networking tasks. Those scripts are executed in parallel
with the speed and efficiency you expect from Nmap. Users can rely on
the growing and diverse set of scripts distributed with Nmap, or
write their own to meet custom needs.
Tasks we had in mind when creating the system include network
discovery, more sophisticated version detection, vulnerability
detection. NSE can even be used for vulnerability exploitation.
To reflect those different uses and to simplify the choice of which
scripts to run, each script contains a field associating it with one
or more categories. Currently defined categories are auth, broadcast,
default. discovery, dos, exploit, external, fuzzer, intrusive,
malware, safe, version, and vuln. These are all described at
https://nmap.org/book/nse-usage.html#nse-categories .
Scripts are not run in a sandbox and thus could accidentally or
maliciously damage your system or invade your privacy. Never run
scripts from third parties unless you trust the authors or have
carefully audited the scripts yourself.
The Nmap Scripting Engine is described in detail at
https://nmap.org/book/nse.html and is controlled by the following
options:
-sC
Performs a script scan using the default set of scripts. It is
equivalent to --script=default. Some of the scripts in this
category are considered intrusive and should not be run against a
target network without permission.
--script filename|category|directory|expression[,...]
Runs a script scan using the comma-separated list of filenames,
script categories, and directories. Each element in the list may
also be a Boolean expression describing a more complex set of
scripts. Each element is interpreted first as an expression, then
as a category, and finally as a file or directory name.
There are two special features for advanced users only. One is to
prefix script names and expressions with + to force them to run
even if they normally wouldn't (e.g. the relevant service wasn't
detected on the target port). The other is that the argument all
may be used to specify every script in Nmap's database. Be
cautious with this because NSE contains dangerous scripts such as
exploits, brute force authentication crackers, and denial of
service attacks.
File and directory names may be relative or absolute. Absolute
names are used directly. Relative paths are looked for in the
scripts of each of the following places until found: --datadir
$NMAPDIR
~/.nmap (not searched on Windows)
HOME\AppData\Roaming\nmap (only on Windows)
the directory containing the nmap executable
the directory containing the nmap executable, followed by
../share/nmap
NMAPDATADIR
the current directory.
When a directory name is given, Nmap loads every file in the
directory whose name ends with .nse. All other files are ignored and
directories are not searched recursively. When a filename is given,
it does not have to have the .nse extension; it will be added
automatically if necessary. Nmap scripts are stored in a scripts
subdirectory of the Nmap data directory by default (see
https://nmap.org/book/data-files.html ). For efficiency, scripts are
indexed in a database stored in scripts/script.db, which lists the
category or categories in which each script belongs. When referring
to scripts from script.db by name, you can use a shell-style ‘*’
wildcard.
nmap --script "http-*"
Loads all scripts whose name starts with http-, such as http-auth
and http-open-proxy. The argument to --script had to be in quotes
to protect the wildcard from the shell.
More complicated script selection can be done using the and, or, and
not operators to build Boolean expressions. The operators have the
same precedence[12] as in Lua: not is the highest, followed by and
and then or. You can alter precedence by using parentheses. Because
expressions contain space characters it is necessary to quote them.
nmap --script "not intrusive"
Loads every script except for those in the intrusive category.
nmap --script "default or safe"
This is functionally equivalent to nmap --script "default,safe".
It loads all scripts that are in the default category or the safe
category or both.
nmap --script "default and safe"
Loads those scripts that are in both the default and safe
categories.
nmap --script "(default or safe or intrusive) and not http-*"
Loads scripts in the default, safe, or intrusive categories,
except for those whose names start with http-.
--script-args n1=v1,n2={n3=v3},n4={v4,v5}
Lets you provide arguments to NSE scripts. Arguments are a
comma-separated list of name=value pairs. Names and values may be
strings not containing whitespace or the characters ‘{’, ‘}’,
‘=’, or ‘,’. To include one of these characters in a string,
enclose the string in single or double quotes. Within a quoted
string, ‘\’ escapes a quote. A backslash is only used to escape
quotation marks in this special case; in all other cases a
backslash is interpreted literally. Values may also be tables
enclosed in {}, just as in Lua. A table may contain simple string
values or more name-value pairs, including nested tables. Many
scripts qualify their arguments with the script name, as in
xmpp-info.server_name. You may use that full qualified version to
affect just the specified script, or you may pass the unqualified
version (server_name in this case) to affect all scripts using
that argument name. A script will first check for its fully
qualified argument name (the name specified in its documentation)
before it accepts an unqualified argument name. A complex example
of script arguments is --script-args
'user=foo,pass=",{}=bar",whois={whodb=nofollow+ripe},xmpp-info.server_name=localhost'.
The online NSE Documentation Portal at https://nmap.org/nsedoc/
lists the arguments that each script accepts.
--script-args-file filename
Lets you load arguments to NSE scripts from a file. Any arguments
on the command line supersede ones in the file. The file can be
an absolute path, or a path relative to Nmap's usual search path
(NMAPDIR, etc.) Arguments can be comma-separated or
newline-separated, but otherwise follow the same rules as for
--script-args, without requiring special quoting and escaping,
since they are not parsed by the shell.
--script-help filename|category|directory|expression|all[,...]
Shows help about scripts. For each script matching the given
specification, Nmap prints the script name, its categories, and
its description. The specifications are the same as those
accepted by --script; so for example if you want help about the
ftp-anon script, you would run nmap --script-help ftp-anon. In
addition to getting help for individual scripts, you can use this
as a preview of what scripts will be run for a specification, for
example with nmap --script-help default.
--script-trace
This option does what --packet-trace does, just one ISO layer
higher. If this option is specified all incoming and outgoing
communication performed by a script is printed. The displayed
information includes the communication protocol, the source, the
target and the transmitted data. If more than 5% of all
transmitted data is not printable, then the trace output is in a
hex dump format. Specifying --packet-trace enables script tracing
too.
--script-updatedb
This option updates the script database found in
scripts/script.db which is used by Nmap to determine the
available default scripts and categories. It is only necessary to
update the database if you have added or removed NSE scripts from
the default scripts directory or if you have changed the
categories of any script. This option is generally used by
itself: nmap --script-updatedb.
TIMING AND PERFORMANCE top
One of my highest Nmap development priorities has always been
performance. A default scan (nmap hostname) of a host on my local
network takes a fifth of a second. That is barely enough time to
blink, but adds up when you are scanning hundreds or thousands of
hosts. Moreover, certain scan options such as UDP scanning and
version detection can increase scan times substantially. So can
certain firewall configurations, particularly response rate limiting.
While Nmap utilizes parallelism and many advanced algorithms to
accelerate these scans, the user has ultimate control over how Nmap
runs. Expert users carefully craft Nmap commands to obtain only the
information they care about while meeting their time constraints.
Techniques for improving scan times include omitting non-critical
tests, and upgrading to the latest version of Nmap (performance
enhancements are made frequently). Optimizing timing parameters can
also make a substantial difference. Those options are listed below.
Some options accept a time parameter. This is specified in seconds by
default, though you can append ‘ms’, ‘s’, ‘m’, or ‘h’ to the value to
specify milliseconds, seconds, minutes, or hours. So the
--host-timeout arguments 900000ms, 900, 900s, and 15m all do the same
thing.
--min-hostgroup numhosts; --max-hostgroup numhosts (Adjust parallel
scan group sizes)
Nmap has the ability to port scan or version scan multiple hosts
in parallel. Nmap does this by dividing the target IP space into
groups and then scanning one group at a time. In general, larger
groups are more efficient. The downside is that host results
can't be provided until the whole group is finished. So if Nmap
started out with a group size of 50, the user would not receive
any reports (except for the updates offered in verbose mode)
until the first 50 hosts are completed.
By default, Nmap takes a compromise approach to this conflict. It
starts out with a group size as low as five so the first results
come quickly and then increases the groupsize to as high as 1024.
The exact default numbers depend on the options given. For
efficiency reasons, Nmap uses larger group sizes for UDP or
few-port TCP scans.
When a maximum group size is specified with --max-hostgroup, Nmap
will never exceed that size. Specify a minimum size with
--min-hostgroup and Nmap will try to keep group sizes above that
level. Nmap may have to use smaller groups than you specify if
there are not enough target hosts left on a given interface to
fulfill the specified minimum. Both may be set to keep the group
size within a specific range, though this is rarely desired.
These options do not have an effect during the host discovery
phase of a scan. This includes plain ping scans (-sn). Host
discovery always works in large groups of hosts to improve speed
and accuracy.
The primary use of these options is to specify a large minimum
group size so that the full scan runs more quickly. A common
choice is 256 to scan a network in Class C sized chunks. For a
scan with many ports, exceeding that number is unlikely to help
much. For scans of just a few port numbers, host group sizes of
2048 or more may be helpful.
--min-parallelism numprobes; --max-parallelism numprobes (Adjust
probe parallelization)
These options control the total number of probes that may be
outstanding for a host group. They are used for port scanning and
host discovery. By default, Nmap calculates an ever-changing
ideal parallelism based on network performance. If packets are
being dropped, Nmap slows down and allows fewer outstanding
probes. The ideal probe number slowly rises as the network proves
itself worthy. These options place minimum or maximum bounds on
that variable. By default, the ideal parallelism can drop to one
if the network proves unreliable and rise to several hundred in
perfect conditions.
The most common usage is to set --min-parallelism to a number
higher than one to speed up scans of poorly performing hosts or
networks. This is a risky option to play with, as setting it too
high may affect accuracy. Setting this also reduces Nmap's
ability to control parallelism dynamically based on network
conditions. A value of 10 might be reasonable, though I only
adjust this value as a last resort.
The --max-parallelism option is sometimes set to one to prevent
Nmap from sending more than one probe at a time to hosts. The
--scan-delay option, discussed later, is another way to do this.
--min-rtt-timeout time, --max-rtt-timeout time, --initial-rtt-timeout
time (Adjust probe timeouts)
Nmap maintains a running timeout value for determining how long
it will wait for a probe response before giving up or
retransmitting the probe. This is calculated based on the
response times of previous probes.
If the network latency shows itself to be significant and
variable, this timeout can grow to several seconds. It also
starts at a conservative (high) level and may stay that way for a
while when Nmap scans unresponsive hosts.
Specifying a lower --max-rtt-timeout and --initial-rtt-timeout
than the defaults can cut scan times significantly. This is
particularly true for pingless (-Pn) scans, and those against
heavily filtered networks. Don't get too aggressive though. The
scan can end up taking longer if you specify such a low value
that many probes are timing out and retransmitting while the
response is in transit.
If all the hosts are on a local network, 100 milliseconds
(--max-rtt-timeout 100ms) is a reasonable aggressive value. If
routing is involved, ping a host on the network first with the
ICMP ping utility, or with a custom packet crafter such as Nping
that is more likely to get through a firewall. Look at the
maximum round trip time out of ten packets or so. You might want
to double that for the --initial-rtt-timeout and triple or
quadruple it for the --max-rtt-timeout. I generally do not set
the maximum RTT below 100 ms, no matter what the ping times are.
Nor do I exceed 1000 ms.
--min-rtt-timeout is a rarely used option that could be useful
when a network is so unreliable that even Nmap's default is too
aggressive. Since Nmap only reduces the timeout down to the
minimum when the network seems to be reliable, this need is
unusual and should be reported as a bug to the nmap-dev mailing
list.
--max-retries numtries (Specify the maximum number of port scan probe
retransmissions)
When Nmap receives no response to a port scan probe, it could
mean the port is filtered. Or maybe the probe or response was
simply lost on the network. It is also possible that the target
host has rate limiting enabled that temporarily blocked the
response. So Nmap tries again by retransmitting the initial
probe. If Nmap detects poor network reliability, it may try many
more times before giving up on a port. While this benefits
accuracy, it also lengthen scan times. When performance is
critical, scans may be sped up by limiting the number of
retransmissions allowed. You can even specify --max-retries 0 to
prevent any retransmissions, though that is only recommended for
situations such as informal surveys where occasional missed ports
and hosts are acceptable.
The default (with no -T template) is to allow ten
retransmissions. If a network seems reliable and the target hosts
aren't rate limiting, Nmap usually only does one retransmission.
So most target scans aren't even affected by dropping
--max-retries to a low value such as three. Such values can
substantially speed scans of slow (rate limited) hosts. You
usually lose some information when Nmap gives up on ports early,
though that may be preferable to letting the --host-timeout
expire and losing all information about the target.
--host-timeout time (Give up on slow target hosts)
Some hosts simply take a long time to scan. This may be due to
poorly performing or unreliable networking hardware or software,
packet rate limiting, or a restrictive firewall. The slowest few
percent of the scanned hosts can eat up a majority of the scan
time. Sometimes it is best to cut your losses and skip those
hosts initially. Specify --host-timeout with the maximum amount
of time you are willing to wait. For example, specify 30m to
ensure that Nmap doesn't waste more than half an hour on a single
host. Note that Nmap may be scanning other hosts at the same time
during that half an hour, so it isn't a complete loss. A host
that times out is skipped. No port table, OS detection, or
version detection results are printed for that host.
--script-timeout time
Some scripts take long time before they complete their execution,
this can happen due to many reasons maybe some bug in script,
delay in the network or nature of the script
itself(example:http-slowloris). If you want to keep some limit on
time for which script should run then you need to specify
--script-timeout with the maximum amount of time for which script
should be run. Note that all scripts will have same timeout. Once
script gets timed out no output for that script will be shown.
Whether a script has timed out or not, can be seen in debug
output.
--scan-delay time; --max-scan-delay time (Adjust delay between
probes)
This option causes Nmap to wait at least the given amount of time
between each probe it sends to a given host. This is particularly
useful in the case of rate limiting. Solaris machines (among
many others) will usually respond to UDP scan probe packets with
only one ICMP message per second. Any more than that sent by Nmap
will be wasteful. A --scan-delay of 1s will keep Nmap at that
slow rate. Nmap tries to detect rate limiting and adjust the scan
delay accordingly, but it doesn't hurt to specify it explicitly
if you already know what rate works best.
When Nmap adjusts the scan delay upward to cope with rate
limiting, the scan slows down dramatically. The --max-scan-delay
option specifies the largest delay that Nmap will allow. A low
--max-scan-delay can speed up Nmap, but it is risky. Setting this
value too low can lead to wasteful packet retransmissions and
possible missed ports when the target implements strict rate
limiting.
Another use of --scan-delay is to evade threshold based intrusion
detection and prevention systems (IDS/IPS).
--min-rate number; --max-rate number (Directly control the scanning
rate)
Nmap's dynamic timing does a good job of finding an appropriate
speed at which to scan. Sometimes, however, you may happen to
know an appropriate scanning rate for a network, or you may have
to guarantee that a scan will be finished by a certain time. Or
perhaps you must keep Nmap from scanning too quickly. The
--min-rate and --max-rate options are designed for these
situations.
When the --min-rate option is given Nmap will do its best to send
packets as fast as or faster than the given rate. The argument is
a positive real number representing a packet rate in packets per
second. For example, specifying --min-rate 300 means that Nmap
will try to keep the sending rate at or above 300 packets per
second. Specifying a minimum rate does not keep Nmap from going
faster if conditions warrant.
Likewise, --max-rate limits a scan's sending rate to a given
maximum. Use --max-rate 100, for example, to limit sending to 100
packets per second on a fast network. Use --max-rate 0.1 for a
slow scan of one packet every ten seconds. Use --min-rate and
--max-rate together to keep the rate inside a certain range.
These two options are global, affecting an entire scan, not
individual hosts. They only affect port scans and host discovery
scans. Other features like OS detection implement their own
timing.
There are two conditions when the actual scanning rate may fall
below the requested minimum. The first is if the minimum is
faster than the fastest rate at which Nmap can send, which is
dependent on hardware. In this case Nmap will simply send packets
as fast as possible, but be aware that such high rates are likely
to cause a loss of accuracy. The second case is when Nmap has
nothing to send, for example at the end of a scan when the last
probes have been sent and Nmap is waiting for them to time out or
be responded to. It's normal to see the scanning rate drop at the
end of a scan or in between hostgroups. The sending rate may
temporarily exceed the maximum to make up for unpredictable
delays, but on average the rate will stay at or below the
maximum.
Specifying a minimum rate should be done with care. Scanning
faster than a network can support may lead to a loss of accuracy.
In some cases, using a faster rate can make a scan take longer
than it would with a slower rate. This is because Nmap's adaptive
retransmission algorithms will detect the network congestion
caused by an excessive scanning rate and increase the number of
retransmissions in order to improve accuracy. So even though
packets are sent at a higher rate, more packets are sent overall.
Cap the number of retransmissions with the --max-retries option
if you need to set an upper limit on total scan time.
--defeat-rst-ratelimit
Many hosts have long used rate limiting to reduce the number of
ICMP error messages (such as port-unreachable errors) they send.
Some systems now apply similar rate limits to the RST (reset)
packets they generate. This can slow Nmap down dramatically as it
adjusts its timing to reflect those rate limits. You can tell
Nmap to ignore those rate limits (for port scans such as SYN scan
which don't treat non-responsive ports as open) by specifying
--defeat-rst-ratelimit.
Using this option can reduce accuracy, as some ports will appear
non-responsive because Nmap didn't wait long enough for a
rate-limited RST response. With a SYN scan, the non-response
results in the port being labeled filtered rather than the closed
state we see when RST packets are received. This option is useful
when you only care about open ports, and distinguishing between
closed and filtered ports isn't worth the extra time.
--defeat-icmp-ratelimit
Similar to --defeat-rst-ratelimit, the --defeat-icmp-ratelimit
option trades accuracy for speed, increasing UDP scanning speed
against hosts that rate-limit ICMP error messages. Because this
option causes Nmap to not delay in order to receive the port
unreachable messages, a non-responsive port will be labeled
closed|filtered instead of the default open|filtered. This has
the effect of only treating ports which actually respond via UDP
as open. Since many UDP services do not respond in this way, the
chance for inaccuracy is greater with this option than with
--defeat-rst-ratelimit.
--nsock-engine epoll|kqueue|poll|select
Enforce use of a given nsock IO multiplexing engine. Only the
select(2)-based fallback engine is guaranteed to be available on
your system. Engines are named after the name of the IO
management facility they leverage. Engines currently implemented
are epoll, kqueue, poll, and select, but not all will be present
on any platform. Use nmap -V to see which engines are supported.
-T paranoid|sneaky|polite|normal|aggressive|insane (Set a timing
template)
While the fine-grained timing controls discussed in the previous
section are powerful and effective, some people find them
confusing. Moreover, choosing the appropriate values can
sometimes take more time than the scan you are trying to
optimize. So Nmap offers a simpler approach, with six timing
templates. You can specify them with the -T option and their
number (0–5) or their name. The template names are paranoid (0),
sneaky (1), polite (2), normal (3), aggressive (4), and
insane (5). The first two are for IDS evasion. Polite mode slows
down the scan to use less bandwidth and target machine resources.
Normal mode is the default and so -T3 does nothing. Aggressive
mode speeds scans up by making the assumption that you are on a
reasonably fast and reliable network. Finally insane mode assumes
that you are on an extraordinarily fast network or are willing to
sacrifice some accuracy for speed.
These templates allow the user to specify how aggressive they
wish to be, while leaving Nmap to pick the exact timing values.
The templates also make some minor speed adjustments for which
fine-grained control options do not currently exist. For example,
-T4 prohibits the dynamic scan delay from exceeding 10 ms for TCP
ports and -T5 caps that value at 5 ms. Templates can be used in
combination with fine-grained controls, and the fine-grained
controls will you specify will take precedence over the timing
template default for that parameter. I recommend using -T4 when
scanning reasonably modern and reliable networks. Keep that
option even when you add fine-grained controls so that you
benefit from those extra minor optimizations that it enables.
If you are on a decent broadband or ethernet connection, I would
recommend always using -T4. Some people love -T5 though it is too
aggressive for my taste. People sometimes specify -T2 because
they think it is less likely to crash hosts or because they
consider themselves to be polite in general. They often don't
realize just how slow -T polite really is. Their scan may take
ten times longer than a default scan. Machine crashes and
bandwidth problems are rare with the default timing options (-T3)
and so I normally recommend that for cautious scanners. Omitting
version detection is far more effective than playing with timing
values at reducing these problems.
While -T0 and -T1 may be useful for avoiding IDS alerts, they
will take an extraordinarily long time to scan thousands of
machines or ports. For such a long scan, you may prefer to set
the exact timing values you need rather than rely on the canned
-T0 and -T1 values.
The main effects of T0 are serializing the scan so only one port
is scanned at a time, and waiting five minutes between sending
each probe. T1 and T2 are similar but they only wait 15 seconds
and 0.4 seconds, respectively, between probes. T3 is Nmap's
default behavior, which includes parallelization. -T4 does the
equivalent of --max-rtt-timeout 1250ms --min-rtt-timeout 100ms
--initial-rtt-timeout 500ms --max-retries 6 and sets the maximum
TCP scan delay to 10 milliseconds. T5 does the equivalent of
--max-rtt-timeout 300ms --min-rtt-timeout 50ms
--initial-rtt-timeout 250ms --max-retries 2 --host-timeout 15m as
well as setting the maximum TCP scan delay to 5 ms.
FIREWALL/IDS EVASION AND SPOOFING top
Many Internet pioneers envisioned a global open network with a
universal IP address space allowing virtual connections between any
two nodes. This allows hosts to act as true peers, serving and
retrieving information from each other. People could access all of
their home systems from work, changing the climate control settings
or unlocking the doors for early guests. This vision of universal
connectivity has been stifled by address space shortages and security
concerns. In the early 1990s, organizations began deploying firewalls
for the express purpose of reducing connectivity. Huge networks were
cordoned off from the unfiltered Internet by application proxies,
network address translation, and packet filters. The unrestricted
flow of information gave way to tight regulation of approved
communication channels and the content that passes over them.
Network obstructions such as firewalls can make mapping a network
exceedingly difficult. It will not get any easier, as stifling casual
reconnaissance is often a key goal of implementing the devices.
Nevertheless, Nmap offers many features to help understand these
complex networks, and to verify that filters are working as intended.
It even supports mechanisms for bypassing poorly implemented
defenses. One of the best methods of understanding your network
security posture is to try to defeat it. Place yourself in the
mind-set of an attacker, and deploy techniques from this section
against your networks. Launch an FTP bounce scan, idle scan,
fragmentation attack, or try to tunnel through one of your own
proxies.
In addition to restricting network activity, companies are
increasingly monitoring traffic with intrusion detection systems
(IDS). All of the major IDSs ship with rules designed to detect Nmap
scans because scans are sometimes a precursor to attacks. Many of
these products have recently morphed into intrusion prevention
systems (IPS) that actively block traffic deemed malicious.
Unfortunately for network administrators and IDS vendors, reliably
detecting bad intentions by analyzing packet data is a tough problem.
Attackers with patience, skill, and the help of certain Nmap options
can usually pass by IDSs undetected. Meanwhile, administrators must
cope with large numbers of false positive results where innocent
activity is misdiagnosed and alerted on or blocked.
Occasionally people suggest that Nmap should not offer features for
evading firewall rules or sneaking past IDSs. They argue that these
features are just as likely to be misused by attackers as used by
administrators to enhance security. The problem with this logic is
that these methods would still be used by attackers, who would just
find other tools or patch the functionality into Nmap. Meanwhile,
administrators would find it that much harder to do their jobs.
Deploying only modern, patched FTP servers is a far more powerful
defense than trying to prevent the distribution of tools implementing
the FTP bounce attack.
There is no magic bullet (or Nmap option) for detecting and
subverting firewalls and IDS systems. It takes skill and experience.
A tutorial is beyond the scope of this reference guide, which only
lists the relevant options and describes what they do.
-f (fragment packets); --mtu (using the specified MTU)
The -f option causes the requested scan (including ping scans) to
use tiny fragmented IP packets. The idea is to split up the TCP
header over several packets to make it harder for packet filters,
intrusion detection systems, and other annoyances to detect what
you are doing. Be careful with this! Some programs have trouble
handling these tiny packets. The old-school sniffer named Sniffit
segmentation faulted immediately upon receiving the first
fragment. Specify this option once, and Nmap splits the packets
into eight bytes or less after the IP header. So a 20-byte TCP
header would be split into three packets. Two with eight bytes of
the TCP header, and one with the final four. Of course each
fragment also has an IP header. Specify -f again to use 16 bytes
per fragment (reducing the number of fragments). Or you can
specify your own offset size with the --mtu option. Don't also
specify -f if you use --mtu. The offset must be a multiple of
eight. While fragmented packets won't get by packet filters and
firewalls that queue all IP fragments, such as the
CONFIG_IP_ALWAYS_DEFRAG option in the Linux kernel, some networks
can't afford the performance hit this causes and thus leave it
disabled. Others can't enable this because fragments may take
different routes into their networks. Some source systems
defragment outgoing packets in the kernel. Linux with the
iptables connection tracking module is one such example. Do a
scan while a sniffer such as Wireshark is running to ensure that
sent packets are fragmented. If your host OS is causing problems,
try the --send-eth option to bypass the IP layer and send raw
ethernet frames.
Fragmentation is only supported for Nmap's raw packet features,
which includes TCP and UDP port scans (except connect scan and
FTP bounce scan) and OS detection. Features such as version
detection and the Nmap Scripting Engine generally don't support
fragmentation because they rely on your host's TCP stack to
communicate with target services.
-D decoy1[,decoy2][,ME][,...] (Cloak a scan with decoys)
Causes a decoy scan to be performed, which makes it appear to the
remote host that the host(s) you specify as decoys are scanning
the target network too. Thus their IDS might report 5–10 port
scans from unique IP addresses, but they won't know which IP was
scanning them and which were innocent decoys. While this can be
defeated through router path tracing, response-dropping, and
other active mechanisms, it is generally an effective technique
for hiding your IP address.
Separate each decoy host with commas, and you can optionally use
ME as one of the decoys to represent the position for your real
IP address. If you put ME in the sixth position or later, some
common port scan detectors (such as Solar Designer's excellent
Scanlogd) are unlikely to show your IP address at all. If you
don't use ME, Nmap will put you in a random position. You can
also use RND to generate a random, non-reserved IP address, or
RND:number to generate number addresses.
Note that the hosts you use as decoys should be up or you might
accidentally SYN flood your targets. Also it will be pretty easy
to determine which host is scanning if only one is actually up on
the network. You might want to use IP addresses instead of names
(so the decoy networks don't see you in their nameserver logs).
Right now random IP address generation is only supported with
IPv4
Decoys are used both in the initial ping scan (using ICMP, SYN,
ACK, or whatever) and during the actual port scanning phase.
Decoys are also used during remote OS detection (-O). Decoys do
not work with version detection or TCP connect scan. When a scan
delay is in effect, the delay is enforced between each batch of
spoofed probes, not between each individual probe. Because decoys
are sent as a batch all at once, they may temporarily violate
congestion control limits.
It is worth noting that using too many decoys may slow your scan
and potentially even make it less accurate. Also, some ISPs will
filter out your spoofed packets, but many do not restrict spoofed
IP packets at all.
-S IP_Address (Spoof source address)
In some circumstances, Nmap may not be able to determine your
source address (Nmap will tell you if this is the case). In this
situation, use -S with the IP address of the interface you wish
to send packets through.
Another possible use of this flag is to spoof the scan to make
the targets think that someone else is scanning them. Imagine a
company being repeatedly port scanned by a competitor! The -e
option and -Pn are generally required for this sort of usage.
Note that you usually won't receive reply packets back (they will
be addressed to the IP you are spoofing), so Nmap won't produce
useful reports.
-e interface (Use specified interface)
Tells Nmap what interface to send and receive packets on. Nmap
should be able to detect this automatically, but it will tell you
if it cannot.
--source-port portnumber; -g portnumber (Spoof source port number)
One surprisingly common misconfiguration is to trust traffic
based only on the source port number. It is easy to understand
how this comes about. An administrator will set up a shiny new
firewall, only to be flooded with complaints from ungrateful
users whose applications stopped working. In particular, DNS may
be broken because the UDP DNS replies from external servers can
no longer enter the network. FTP is another common example. In
active FTP transfers, the remote server tries to establish a
connection back to the client to transfer the requested file.
Secure solutions to these problems exist, often in the form of
application-level proxies or protocol-parsing firewall modules.
Unfortunately there are also easier, insecure solutions. Noting
that DNS replies come from port 53 and active FTP from port 20,
many administrators have fallen into the trap of simply allowing
incoming traffic from those ports. They often assume that no
attacker would notice and exploit such firewall holes. In other
cases, administrators consider this a short-term stop-gap measure
until they can implement a more secure solution. Then they forget
the security upgrade.
Overworked network administrators are not the only ones to fall
into this trap. Numerous products have shipped with these
insecure rules. Even Microsoft has been guilty. The IPsec filters
that shipped with Windows 2000 and Windows XP contain an implicit
rule that allows all TCP or UDP traffic from port 88 (Kerberos).
In another well-known case, versions of the Zone Alarm personal
firewall up to 2.1.25 allowed any incoming UDP packets with the
source port 53 (DNS) or 67 (DHCP).
Nmap offers the -g and --source-port options (they are
equivalent) to exploit these weaknesses. Simply provide a port
number and Nmap will send packets from that port where possible.
Most scanning operations that use raw sockets, including SYN and
UDP scans, support the option completely. The option notably
doesn't have an effect for any operations that use normal
operating system sockets, including DNS requests, TCP connect
scan, version detection, and script scanning. Setting the source
port also doesn't work for OS detection, because Nmap must use
different port numbers for certain OS detection tests to work
properly.
--data hex string (Append custom binary data to sent packets)
This option lets you include binary data as payload in sent
packets. hex string may be specified in any of the following
formats: 0xAABBCCDDEEFF..., AABBCCDDEEFF... or
\xAA\xBB\xCC\xDD\xEE\xFF.... Examples of use are --data
0xdeadbeef and --data \xCA\xFE\x09. Note that if you specify a
number like 0x00ff no byte-order conversion is performed. Make
sure you specify the information in the byte order expected by
the receiver.
--data-string string (Append custom string to sent packets)
This option lets you include a regular string as payload in sent
packets. string can contain any string. However, note that some
characters may depend on your system's locale and the receiver
may not see the same information. Also, make sure you enclose the
string in double quotes and escape any special characters from
the shell. Examples: --data-string "Scan conducted by Security
Ops, extension 7192" or --data-string "Ph34r my l33t skills".
Keep in mind that nobody is likely to actually see any comments
left by this option unless they are carefully monitoring the
network with a sniffer or custom IDS rules.
--data-length number (Append random data to sent packets)
Normally Nmap sends minimalist packets containing only a header.
So its TCP packets are generally 40 bytes and ICMP echo requests
are just 28. Some UDP ports and IP protocols get a custom payload
by default. This option tells Nmap to append the given number of
random bytes to most of the packets it sends, and not to use any
protocol-specific payloads. (Use --data-length 0 for no random or
protocol-specific payloads. OS detection (-O) packets are not
affected because accuracy there requires probe consistency, but
most pinging and portscan packets support this. It slows things
down a little, but can make a scan slightly less conspicuous.
--ip-options S|R [route]|L [route]|T|U ... ; --ip-options hex string
(Send packets with specified ip options)
The IP protocol[13] offers several options which may be placed in
packet headers. Unlike the ubiquitous TCP options, IP options are
rarely seen due to practicality and security concerns. In fact,
many Internet routers block the most dangerous options such as
source routing. Yet options can still be useful in some cases for
determining and manipulating the network route to target
machines. For example, you may be able to use the record route
option to determine a path to a target even when more traditional
traceroute-style approaches fail. Or if your packets are being
dropped by a certain firewall, you may be able to specify a
different route with the strict or loose source routing options.
The most powerful way to specify IP options is to simply pass in
values as the argument to --ip-options. Precede each hex number
with \x then the two digits. You may repeat certain characters by
following them with an asterisk and then the number of times you
wish them to repeat. For example, \x01\x07\x04\x00*36\x01 is a
hex string containing 36 NUL bytes.
Nmap also offers a shortcut mechanism for specifying options.
Simply pass the letter R, T, or U to request record-route,
record-timestamp, or both options together, respectively. Loose
or strict source routing may be specified with an L or S followed
by a space and then a space-separated list of IP addresses.
If you wish to see the options in packets sent and received,
specify --packet-trace. For more information and examples of
using IP options with Nmap, see
http://seclists.org/nmap-dev/2006/q3/52 .
--ttl value (Set IP time-to-live field)
Sets the IPv4 time-to-live field in sent packets to the given
value.
--randomize-hosts (Randomize target host order)
Tells Nmap to shuffle each group of up to 16384 hosts before it
scans them. This can make the scans less obvious to various
network monitoring systems, especially when you combine it with
slow timing options. If you want to randomize over larger group
sizes, increase PING_GROUP_SZ in nmap.h and recompile. An
alternative solution is to generate the target IP list with a
list scan (-sL -n -oN filename), randomize it with a Perl script,
then provide the whole list to Nmap with -iL.
--spoof-mac MAC address, prefix, or vendor name (Spoof MAC address)
Asks Nmap to use the given MAC address for all of the raw
ethernet frames it sends. This option implies --send-eth to
ensure that Nmap actually sends ethernet-level packets. The MAC
given can take several formats. If it is simply the number 0,
Nmap chooses a completely random MAC address for the session. If
the given string is an even number of hex digits (with the pairs
optionally separated by a colon), Nmap will use those as the MAC.
If fewer than 12 hex digits are provided, Nmap fills in the
remainder of the six bytes with random values. If the argument
isn't a zero or hex string, Nmap looks through nmap-mac-prefixes
to find a vendor name containing the given string (it is case
insensitive). If a match is found, Nmap uses the vendor's OUI
(three-byte prefix) and fills out the remaining three bytes
randomly. Valid --spoof-mac argument examples are Apple, 0,
01:02:03:04:05:06, deadbeefcafe, 0020F2, and Cisco. This option
only affects raw packet scans such as SYN scan or OS detection,
not connection-oriented features such as version detection or the
Nmap Scripting Engine.
--proxies Comma-separated list of proxy URLs (Relay TCP connections
through a chain of proxies)
Asks Nmap to establish TCP connections with a final target
through supplied chain of one or more HTTP or SOCKS4 proxies.
Proxies can help hide the true source of a scan or evade certain
firewall restrictions, but they can hamper scan performance by
increasing latency. Users may need to adjust Nmap timeouts and
other scan parameters accordingly. In particular, a lower
--max-parallelism may help because some proxies refuse to handle
as many concurrent connections as Nmap opens by default.
This option takes a list of proxies as argument, expressed as
URLs in the format proto://host:port. Use commas to separate node
URLs in a chain. No authentication is supported yet. Valid
protocols are HTTP and SOCKS4.
Warning: this feature is still under development and has
limitations. It is implemented within the nsock library and thus
has no effect on the ping, port scanning and OS discovery phases
of a scan. Only NSE and version scan benefit from this option so
far—other features may disclose your true address. SSL
connections are not yet supported, nor is proxy-side DNS
resolution (hostnames are always resolved by Nmap).
--badsum (Send packets with bogus TCP/UDP checksums)
Asks Nmap to use an invalid TCP, UDP or SCTP checksum for packets
sent to target hosts. Since virtually all host IP stacks properly
drop these packets, any responses received are likely coming from
a firewall or IDS that didn't bother to verify the checksum. For
more details on this technique, see https://nmap.org/p60-12.html
--adler32 (Use deprecated Adler32 instead of CRC32C for SCTP
checksums)
Asks Nmap to use the deprecated Adler32 algorithm for calculating
the SCTP checksum. If --adler32 is not given, CRC-32C
(Castagnoli) is used. RFC 2960[14] originally defined Adler32 as
checksum algorithm for SCTP; RFC 4960[7] later redefined the SCTP
checksums to use CRC-32C. Current SCTP implementations should be
using CRC-32C, but in order to elicit responses from old, legacy
SCTP implementations, it may be preferable to use Adler32.
OUTPUT top
Any security tool is only as useful as the output it generates.
Complex tests and algorithms are of little value if they aren't
presented in an organized and comprehensible fashion. Given the
number of ways Nmap is used by people and other software, no single
format can please everyone. So Nmap offers several formats, including
the interactive mode for humans to read directly and XML for easy
parsing by software.
In addition to offering different output formats, Nmap provides
options for controlling the verbosity of output as well as debugging
messages. Output types may be sent to standard output or to named
files, which Nmap can append to or clobber. Output files may also be
used to resume aborted scans.
Nmap makes output available in five different formats. The default is
called interactive output, and it is sent to standard output
(stdout). There is also normal output, which is similar to
interactive except that it displays less runtime information and
warnings since it is expected to be analyzed after the scan completes
rather than interactively.
XML output is one of the most important output types, as it can be
converted to HTML, easily parsed by programs such as Nmap graphical
user interfaces, or imported into databases.
The two remaining output types are the simple grepable output which
includes most information for a target host on a single line, and
sCRiPt KiDDi3 0utPUt for users who consider themselves |<-r4d.
While interactive output is the default and has no associated
command-line options, the other four format options use the same
syntax. They take one argument, which is the filename that results
should be stored in. Multiple formats may be specified, but each
format may only be specified once. For example, you may wish to save
normal output for your own review while saving XML of the same scan
for programmatic analysis. You might do this with the options -oX
myscan.xml -oN myscan.nmap. While this chapter uses the simple names
like myscan.xml for brevity, more descriptive names are generally
recommended. The names chosen are a matter of personal preference,
though I use long ones that incorporate the scan date and a word or
two describing the scan, placed in a directory named after the
company I'm scanning.
While these options save results to files, Nmap still prints
interactive output to stdout as usual. For example, the command nmap
-oX myscan.xml target prints XML to myscan.xml and fills standard
output with the same interactive results it would have printed if -oX
wasn't specified at all. You can change this by passing a hyphen
character as the argument to one of the format types. This causes
Nmap to deactivate interactive output, and instead print results in
the format you specified to the standard output stream. So the
command nmap -oX - target will send only XML output to stdout.
Serious errors may still be printed to the normal error stream,
stderr.
Unlike some Nmap arguments, the space between the logfile option flag
(such as -oX) and the filename or hyphen is mandatory. If you omit
the flags and give arguments such as -oG- or -oXscan.xml, a backwards
compatibility feature of Nmap will cause the creation of normal
format output files named G- and Xscan.xml respectively.
All of these arguments support strftime-like conversions in the
filename. %H, %M, %S, %m, %d, %y, and %Y are all exactly the same as
in strftime. %T is the same as %H%M%S, %R is the same as %H%M, and
%D is the same as %m%d%y. A % followed by any other character just
yields that character (%% gives you a percent symbol). So -oX
'scan-%T-%D.xml' will use an XML file with a name in the form of
scan-144840-121307.xml.
Nmap also offers options to control scan verbosity and to append to
output files rather than clobbering them. All of these options are
described below.
Nmap Output Formats
-oN filespec (normal output)
Requests that normal output be directed to the given filename. As
discussed above, this differs slightly from interactive output.
-oX filespec (XML output)
Requests that XML output be directed to the given filename. Nmap
includes a document type definition (DTD) which allows XML
parsers to validate Nmap XML output. While it is primarily
intended for programmatic use, it can also help humans interpret
Nmap XML output. The DTD defines the legal elements of the
format, and often enumerates the attributes and values they can
take on. The latest version is always available from
https://svn.nmap.org/nmap/docs/nmap.dtd .
XML offers a stable format that is easily parsed by software.
Free XML parsers are available for all major computer languages,
including C/C++, Perl, Python, and Java. People have even written
bindings for most of these languages to handle Nmap output and
execution specifically. Examples are Nmap::Scanner[15] and
Nmap::Parser[16] in Perl CPAN. In almost all cases that a
non-trivial application interfaces with Nmap, XML is the
preferred format.
The XML output references an XSL stylesheet which can be used to
format the results as HTML. The easiest way to use this is simply
to load the XML output in a web browser such as Firefox or IE. By
default, this will only work on the machine you ran Nmap on (or a
similarly configured one) due to the hard-coded nmap.xsl
filesystem path. Use the --webxml or --stylesheet options to
create portable XML files that render as HTML on any
web-connected machine.
-oS filespec (ScRipT KIdd|3 oUTpuT)
Script kiddie output is like interactive output, except that it
is post-processed to better suit the l33t HaXXorZ who previously
looked down on Nmap due to its consistent capitalization and
spelling. Humor impaired people should note that this option is
making fun of the script kiddies before flaming me for supposedly
“helping them”.
-oG filespec (grepable output)
This output format is covered last because it is deprecated. The
XML output format is far more powerful, and is nearly as
convenient for experienced users. XML is a standard for which
dozens of excellent parsers are available, while grepable output
is my own simple hack. XML is extensible to support new Nmap
features as they are released, while I often must omit those
features from grepable output for lack of a place to put them.
Nevertheless, grepable output is still quite popular. It is a
simple format that lists each host on one line and can be
trivially searched and parsed with standard Unix tools such as
grep, awk, cut, sed, diff, and Perl. Even I usually use it for
one-off tests done at the command line. Finding all the hosts
with the SSH port open or that are running Solaris takes only a
simple grep to identify the hosts, piped to an awk or cut command
to print the desired fields.
Grepable output consists of comments (lines starting with a pound
(#)) and target lines. A target line includes a combination of
six labeled fields, separated by tabs and followed with a colon.
The fields are Host, Ports, Protocols, Ignored State, OS, Seq
Index, IP ID, and Status.
The most important of these fields is generally Ports, which
gives details on each interesting port. It is a comma separated
list of port entries. Each port entry represents one interesting
port, and takes the form of seven slash (/) separated subfields.
Those subfields are: Port number, State, Protocol, Owner,
Service, SunRPC info, and Version info.
As with XML output, this man page does not allow for documenting
the entire format. A more detailed look at the Nmap grepable
output format is available from
https://nmap.org/book/output-formats-grepable-output.html .
-oA basename (Output to all formats)
As a convenience, you may specify -oA basename to store scan
results in normal, XML, and grepable formats at once. They are
stored in basename.nmap, basename.xml, and basename.gnmap,
respectively. As with most programs, you can prefix the filenames
with a directory path, such as ~/nmaplogs/foocorp/ on Unix or
c:\hacking\sco on Windows.
Verbosity and debugging options
-v (Increase verbosity level)
Increases the verbosity level, causing Nmap to print more
information about the scan in progress. Open ports are shown as
they are found and completion time estimates are provided when
Nmap thinks a scan will take more than a few minutes. Use it
twice or more for even greater verbosity: -vv, or give a
verbosity level directly, for example -v3.
Most changes only affect interactive output, and some also affect
normal and script kiddie output. The other output types are meant
to be processed by machines, so Nmap can give substantial detail
by default in those formats without fatiguing a human user.
However, there are a few changes in other modes where output size
can be reduced substantially by omitting some detail. For
example, a comment line in the grepable output that provides a
list of all ports scanned is only printed in verbose mode because
it can be quite long.
-d (Increase debugging level)
When even verbose mode doesn't provide sufficient data for you,
debugging is available to flood you with much more! As with the
verbosity option (-v), debugging is enabled with a command-line
flag (-d) and the debug level can be increased by specifying it
multiple times, as in -dd, or by setting a level directly. For
example, -d9 sets level nine. That is the highest effective level
and will produce thousands of lines unless you run a very simple
scan with very few ports and targets.
Debugging output is useful when a bug is suspected in Nmap, or if
you are simply confused as to what Nmap is doing and why. As this
feature is mostly intended for developers, debug lines aren't
always self-explanatory. You may get something like: Timeout
vals: srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==> srtt: 14987
rttvar: 14987 to: 100000. If you don't understand a line, your
only recourses are to ignore it, look it up in the source code,
or request help from the development list (nmap-dev). Some lines
are self explanatory, but the messages become more obscure as the
debug level is increased.
--reason (Host and port state reasons)
Shows the reason each port is set to a specific state and the
reason each host is up or down. This option displays the type of
the packet that determined a port or hosts state. For example, A
RST packet from a closed port or an echo reply from an alive
host. The information Nmap can provide is determined by the type
of scan or ping. The SYN scan and SYN ping (-sS and -PS) are very
detailed, but the TCP connect scan (-sT) is limited by the
implementation of the connect system call. This feature is
automatically enabled by the debug option (-d) and the results
are stored in XML log files even if this option is not specified.
--stats-every time (Print periodic timing stats)
Periodically prints a timing status message after each interval
of time. The time is a specification of the kind described in the
section called “TIMING AND PERFORMANCE”; so for example, use
--stats-every 10s to get a status update every 10 seconds.
Updates are printed to interactive output (the screen) and XML
output.
--packet-trace (Trace packets and data sent and received)
Causes Nmap to print a summary of every packet sent or received.
This is often used for debugging, but is also a valuable way for
new users to understand exactly what Nmap is doing under the
covers. To avoid printing thousands of lines, you may want to
specify a limited number of ports to scan, such as -p20-30. If
you only care about the goings on of the version detection
subsystem, use --version-trace instead. If you only care about
script tracing, specify --script-trace. With --packet-trace, you
get all of the above.
--open (Show only open (or possibly open) ports)
Sometimes you only care about ports you can actually connect to
(open ones), and don't want results cluttered with closed,
filtered, and closed|filtered ports. Output customization is
normally done after the scan using tools such as grep, awk, and
Perl, but this feature was added due to overwhelming requests.
Specify --open to only see hosts with at least one open,
open|filtered, or unfiltered port, and only see ports in those
states. These three states are treated just as they normally are,
which means that open|filtered and unfiltered may be condensed
into counts if there are an overwhelming number of them.
--iflist (List interfaces and routes)
Prints the interface list and system routes as detected by Nmap.
This is useful for debugging routing problems or device
mischaracterization (such as Nmap treating a PPP connection as
ethernet).
Miscellaneous output options
--append-output (Append to rather than clobber output files)
When you specify a filename to an output format flag such as -oX
or -oN, that file is overwritten by default. If you prefer to
keep the existing content of the file and append the new results,
specify the --append-output option. All output filenames
specified in that Nmap execution will then be appended to rather
than clobbered. This doesn't work well for XML (-oX) scan data as
the resultant file generally won't parse properly until you fix
it up by hand.
--resume filename (Resume aborted scan)
Some extensive Nmap runs take a very long time—on the order of
days. Such scans don't always run to completion. Restrictions may
prevent Nmap from being run during working hours, the network
could go down, the machine Nmap is running on might suffer a
planned or unplanned reboot, or Nmap itself could crash. The
administrator running Nmap could cancel it for any other reason
as well, by pressing ctrl-C. Restarting the whole scan from the
beginning may be undesirable. Fortunately, if normal (-oN) or
grepable (-oG) logs were kept, the user can ask Nmap to resume
scanning with the target it was working on when execution ceased.
Simply specify the --resume option and pass the normal/grepable
output file as its argument. No other arguments are permitted, as
Nmap parses the output file to use the same ones specified
previously. Simply call Nmap as nmap --resume logfilename. Nmap
will append new results to the data files specified in the
previous execution. Resumption does not support the XML output
format because combining the two runs into one valid XML file
would be difficult.
--stylesheet path or URL (Set XSL stylesheet to transform XML output)
Nmap ships with an XSL stylesheet named nmap.xsl for viewing or
translating XML output to HTML. The XML output includes an
xml-stylesheet directive which points to nmap.xml where it was
initially installed by Nmap. Run the XML file through an XSLT
processor such as xsltproc[17] to produce an HTML file. Directly
opening the XML file in a browser no longer works well because
modern browsers limit the locations a stylesheet may be loaded
from. If you wish to use a different stylesheet, specify it as
the argument to --stylesheet. You must pass the full pathname or
URL. One common invocation is --stylesheet
https://nmap.org/svn/docs/nmap.xsl . This tells an XSLT processor
to load the latest version of the stylesheet from Nmap.Org. The
--webxml option does the same thing with less typing and
memorization. Loading the XSL from Nmap.Org makes it easier to
view results on a machine that doesn't have Nmap (and thus
nmap.xsl) installed. So the URL is often more useful, but the
local filesystem location of nmap.xsl is used by default for
privacy reasons.
--webxml (Load stylesheet from Nmap.Org)
This is a convenience option, nothing more than an alias for
--stylesheet https://nmap.org/svn/docs/nmap.xsl.
--no-stylesheet (Omit XSL stylesheet declaration from XML)
Specify this option to prevent Nmap from associating any XSL
stylesheet with its XML output. The xml-stylesheet directive is
omitted.
MISCELLANEOUS OPTIONS top
This section describes some important (and not-so-important) options
that don't really fit anywhere else.
-6 (Enable IPv6 scanning)
Nmap has IPv6 support for its most popular features. Ping
scanning, port scanning, version detection, and the Nmap
Scripting Engine all support IPv6. The command syntax is the same
as usual except that you also add the -6 option. Of course, you
must use IPv6 syntax if you specify an address rather than a
hostname. An address might look like
3ffe:7501:4819:2000:210:f3ff:fe03:14d0, so hostnames are
recommended. The output looks the same as usual, with the IPv6
address on the “interesting ports” line being the only IPv6
giveaway.
While IPv6 hasn't exactly taken the world by storm, it gets
significant use in some (usually Asian) countries and most modern
operating systems support it. To use Nmap with IPv6, both the
source and target of your scan must be configured for IPv6. If
your ISP (like most of them) does not allocate IPv6 addresses to
you, free tunnel brokers are widely available and work fine with
Nmap. I use the free IPv6 tunnel broker service at
http://www.tunnelbroker.net . Other tunnel brokers are listed at
Wikipedia[18]. 6to4 tunnels are another popular, free approach.
On Windows, raw-socket IPv6 scans are supported only on ethernet
devices (not tunnels), and only on Windows Vista and later. Use
the --unprivileged option in other situations.
-A (Aggressive scan options)
This option enables additional advanced and aggressive options.
Presently this enables OS detection (-O), version scanning (-sV),
script scanning (-sC) and traceroute (--traceroute). More
features may be added in the future. The point is to enable a
comprehensive set of scan options without people having to
remember a large set of flags. However, because script scanning
with the default set is considered intrusive, you should not use
-A against target networks without permission. This option only
enables features, and not timing options (such as -T4) or
verbosity options (-v) that you might want as well. Options which
require privileges (e.g. root access) such as OS detection and
traceroute will only be enabled if those privileges are
available.
--datadir directoryname (Specify custom Nmap data file location)
Nmap obtains some special data at runtime in files named
nmap-service-probes, nmap-services, nmap-protocols, nmap-rpc,
nmap-mac-prefixes, and nmap-os-db. If the location of any of
these files has been specified (using the --servicedb or
--versiondb options), that location is used for that file. After
that, Nmap searches these files in the directory specified with
the --datadir option (if any). Any files not found there, are
searched for in the directory specified by the NMAPDIR
environment variable. Next comes ~/.nmap for real and effective
UIDs; or on Windows, HOME\AppData\Roaming\nmap (where HOME is the
user's home directory, like C:\Users\user). This is followed by
the location of the nmap executable and the same location with
../share/nmap appended. Then a compiled-in location such as
/usr/local/share/nmap or /usr/share/nmap.
--servicedb services file (Specify custom services file)
Asks Nmap to use the specified services file rather than the
nmap-services data file that comes with Nmap. Using this option
also causes a fast scan (-F) to be used. See the description for
--datadir for more information on Nmap's data files.
--versiondb service probes file (Specify custom service probes file)
Asks Nmap to use the specified service probes file rather than
the nmap-service-probes data file that comes with Nmap. See the
description for --datadir for more information on Nmap's data
files.
--send-eth (Use raw ethernet sending)
Asks Nmap to send packets at the raw ethernet (data link) layer
rather than the higher IP (network) layer. By default, Nmap
chooses the one which is generally best for the platform it is
running on. Raw sockets (IP layer) are generally most efficient
for Unix machines, while ethernet frames are required for Windows
operation since Microsoft disabled raw socket support. Nmap still
uses raw IP packets on Unix despite this option when there is no
other choice (such as non-ethernet connections).
--send-ip (Send at raw IP level)
Asks Nmap to send packets via raw IP sockets rather than sending
lower level ethernet frames. It is the complement to the
--send-eth option discussed previously.
--privileged (Assume that the user is fully privileged)
Tells Nmap to simply assume that it is privileged enough to
perform raw socket sends, packet sniffing, and similar operations
that usually require root privileges on Unix systems. By default
Nmap quits if such operations are requested but geteuid is not
zero. --privileged is useful with Linux kernel capabilities and
similar systems that may be configured to allow unprivileged
users to perform raw-packet scans. Be sure to provide this option
flag before any flags for options that require privileges (SYN
scan, OS detection, etc.). The NMAP_PRIVILEGED environment
variable may be set as an equivalent alternative to --privileged.
--unprivileged (Assume that the user lacks raw socket privileges)
This option is the opposite of --privileged. It tells Nmap to
treat the user as lacking network raw socket and sniffing
privileges. This is useful for testing, debugging, or when the
raw network functionality of your operating system is somehow
broken. The NMAP_UNPRIVILEGED environment variable may be set as
an equivalent alternative to --unprivileged.
--release-memory (Release memory before quitting)
This option is only useful for memory-leak debugging. It causes
Nmap to release allocated memory just before it quits so that
actual memory leaks are easier to spot. Normally Nmap skips this
as the OS does this anyway upon process termination.
-V; --version (Print version number)
Prints the Nmap version number and exits.
-h; --help (Print help summary page)
Prints a short help screen with the most common command flags.
Running Nmap without any arguments does the same thing.
RUNTIME INTERACTION top
During the execution of Nmap, all key presses are captured. This
allows you to interact with the program without aborting and
restarting it. Certain special keys will change options, while any
other keys will print out a status message telling you about the
scan. The convention is that lowercase letters increase the amount of
printing, and uppercase letters decrease the printing. You may also
press ‘?’ for help.
v / V
Increase / decrease the verbosity level
d / D
Increase / decrease the debugging Level
p / P
Turn on / off packet tracing
?
Print a runtime interaction help screen
Anything else
Print out a status message like this:
Stats: 0:00:07 elapsed; 20 hosts completed (1 up), 1 undergoing Service Scan
Service scan Timing: About 33.33% done; ETC: 20:57 (0:00:12 remaining)
EXAMPLES top
Here are some Nmap usage examples, from the simple and routine to a
little more complex and esoteric. Some actual IP addresses and domain
names are used to make things more concrete. In their place you
should substitute addresses/names from your own network. While I
don't think port scanning other networks is or should be illegal,
some network administrators don't appreciate unsolicited scanning of
their networks and may complain. Getting permission first is the best
approach.
For testing purposes, you have permission to scan the host
scanme.nmap.org. This permission only includes scanning via Nmap and
not testing exploits or denial of service attacks. To conserve
bandwidth, please do not initiate more than a dozen scans against
that host per day. If this free scanning target service is abused, it
will be taken down and Nmap will report Failed to resolve given
hostname/IP: scanme.nmap.org. These permissions also apply to the
hosts scanme2.nmap.org, scanme3.nmap.org, and so on, though those
hosts do not currently exist.
nmap -v scanme.nmap.org
This option scans all reserved TCP ports on the machine
scanme.nmap.org . The -v option enables verbose mode.
nmap -sS -O scanme.nmap.org/24
Launches a stealth SYN scan against each machine that is up out of
the 256 IPs on the class C sized network where Scanme resides. It
also tries to determine what operating system is running on each host
that is up and running. This requires root privileges because of the
SYN scan and OS detection.
nmap -sV -p 22,53,110,143,4564 198.116.0-255.1-127
Launches host enumeration and a TCP scan at the first half of each of
the 255 possible eight-bit subnets in the 198.116 class B address
space. This tests whether the systems run SSH, DNS, POP3, or IMAP on
their standard ports, or anything on port 4564. For any of these
ports found open, version detection is used to determine what
application is running.
nmap -v -iR 100000 -Pn -p 80
Asks Nmap to choose 100,000 hosts at random and scan them for web
servers (port 80). Host enumeration is disabled with -Pn since first
sending a couple probes to determine whether a host is up is wasteful
when you are only probing one port on each target host anyway.
nmap -Pn -p80 -oX logs/pb-port80scan.xml -oG logs/pb-port80scan.gnmap
216.163.128.20/20
This scans 4096 IPs for any web servers (without pinging them) and
saves the output in grepable and XML formats.
NMAP BOOK top
While this reference guide details all material Nmap options, it
can't fully demonstrate how to apply those features to quickly solve
real-world tasks. For that, we released Nmap Network Scanning: The
Official Nmap Project Guide to Network Discovery and Security
Scanning. Topics include subverting firewalls and intrusion
detection systems, optimizing Nmap performance, and automating common
networking tasks with the Nmap Scripting Engine. Hints and
instructions are provided for common Nmap tasks such as taking
network inventory, penetration testing, detecting rogue wireless
access points, and quashing network worm outbreaks. Examples and
diagrams show actual communication on the wire. More than half of the
book is available free online. See https://nmap.org/book for more
information.
Like its author, Nmap isn't perfect. But you can help make it better
by sending bug reports or even writing patches. If Nmap doesn't
behave the way you expect, first upgrade to the latest version
available from https://nmap.org . If the problem persists, do some
research to determine whether it has already been discovered and
addressed. Try searching for the error message on our search page at
http://insecure.org/search.html or at Google. Also try browsing the
nmap-dev archives at http://seclists.org/ . Read this full manual
page as well. If nothing comes of this, mail a bug report to
<dev@nmap.org>. Please include everything you have learned about the
problem, as well as what version of Nmap you are running and what
operating system version it is running on. Problem reports and Nmap
usage questions sent to <dev@nmap.org> are far more likely to be
answered than those sent to Fyodor directly. If you subscribe to the
nmap-dev list before posting, your message will bypass moderation and
get through more quickly. Subscribe at
https://nmap.org/mailman/listinfo/dev .
Code patches to fix bugs are even better than bug reports. Basic
instructions for creating patch files with your changes are available
at https://svn.nmap.org/nmap/HACKING . Patches may be sent to nmap-dev
(recommended) or to Fyodor directly.
AUTHOR top
Gordon “Fyodor” Lyon <fyodor@nmap.org> (http://insecure.org )
Hundreds of people have made valuable contributions to Nmap over the
years. These are detailed in the CHANGELOG file which is distributed
with Nmap and also available from https://nmap.org/changelog.html .
LEGAL NOTICES top
Nmap Copyright and Licensing
The Nmap Security Scanner is (C) 1996–2016 Insecure.Com LLC ("The
Nmap Project"). Nmap is also a registered trademark of the Nmap
Project. This program free software; you may redistribute and/or
modify it under the terms of the GNU General Public License as
published by the Free Software Foundation; Version 2 (“GPL”), BUT
ONLY WITH ALL OF THE CLARIFICATIONS AND EXCEPTIONS DESCRIBED HEREIN.
This guarantees your right to use, modify, and redistribute this
software under certain conditions. If you wish to embed Nmap
technology into proprietary software, we sell alternative licenses
(contact <sales@nmap.com>). Dozens of software vendors already
license Nmap technology such as host discovery, port scanning, OS
detection, version detection, and the Nmap Scripting Engine.
Note that the GPL places important restrictions on “derivative
works”, yet it does not provide a detailed definition of that term.
To avoid misunderstandings, we interpret that term as broadly as
copyright law allows. For example, we consider an application to
constitute a derivative work for the purpose of this license if it
does any of the following with any software or content covered by
this license (“Covered Software”):
· Integrates source code from Covered Software.
· Reads or includes copyrighted data files, such as Nmap's
nmap-os-db or nmap-service-probes.
· Is designed specifically to execute Covered Software and parse
the results (as opposed to typical shell or execution-menu apps,
which will execute anything you tell them to).
· Includes Covered Software in a proprietary executable installer.
The installers produced by InstallShield are an example of this.
Including Nmap with other software in compressed or archival form
does not trigger this provision, provided appropriate open source
decompression or de-archiving software is widely available for no
charge. For the purposes of this license, an installer is
considered to include Covered Software even if it actually
retrieves a copy of Covered Software from another source during
runtime (such as by downloading it from the Internet).
· Links (statically or dynamically) to a library which does any of
the above.
· Executes a helper program, module, or script to do any of the
above.
This list is not exclusive, but is meant to clarify our
interpretation of derived works with some common examples. Other
people may interpret the plain GPL differently, so we consider this a
special exception to the GPL that we apply to Covered Software. Works
which meet any of these conditions must conform to all of the terms
of this license, particularly including the GPL Section 3
requirements of providing source code and allowing free
redistribution of the work as a whole.
As another special exception to the GPL terms, the Nmap Project
grants permission to link the code of this program with any version
of the OpenSSL library which is distributed under a license identical
to that listed in the included docs/licenses/OpenSSL.txt file, and
distribute linked combinations including the two.
The Nmap Project has permission to redistribute Npcap, a packet
capturing driver and library for the Microsoft Windows platform.
Npcap is a separate work with it's own license rather than this Nmap
license. Since the Npcap license does not permit redistribution
without special permission, our Nmap Windows binary packages which
contain Npcap may not be redistributed without special permission.
Any redistribution of Covered Software, including any derived works,
must obey and carry forward all of the terms of this license,
including obeying all GPL rules and restrictions. For example, source
code of the whole work must be provided and free redistribution must
be allowed. All GPL references to "this License", are to be treated
as including the terms and conditions of this license text as well.
Because this license imposes special exceptions to the GPL, Covered
Work may not be combined (even as part of a larger work) with plain
GPL software. The terms, conditions, and exceptions of this license
must be included as well. This license is incompatible with some
other open source licenses as well. In some cases we can relicense
portions of Nmap or grant special permissions to use it in other open
source software. Please contact fyodor@nmap.org with any such
requests. Similarly, we don't incorporate incompatible open source
software into Covered Software without special permission from the
copyright holders.
If you have any questions about the licensing restrictions on using
Nmap in other works, are happy to help. As mentioned above, we also
offer alternative license to integrate Nmap into proprietary
applications and appliances. These contracts have been sold to dozens
of software vendors, and generally include a perpetual license as
well as providing for priority support and updates. They also fund
the continued development of Nmap. Please email <sales@nmap.com> for
further information.
If you have received a written license agreement or contract for
Covered Software stating terms other than these, you may choose to
use and redistribute Covered Software under those terms instead of
these.
Creative Commons License for this Nmap Guide
This Nmap Reference Guide is (C) 2005–2016 Insecure.Com LLC. It is
hereby placed under version 3.0 of the Creative Commons Attribution
License[19]. This allows you redistribute and modify the work as you
desire, as long as you credit the original source. Alternatively, you
may choose to treat this document as falling under the same license
as Nmap itself (discussed previously).
Source Code Availability and Community Contributions
Source is provided to this software because we believe users have a
right to know exactly what a program is going to do before they run
it. This also allows you to audit the software for security holes.
Source code also allows you to port Nmap to new platforms, fix bugs,
and add new features. You are highly encouraged to send your changes
to <dev@nmap.org> for possible incorporation into the main
distribution. By sending these changes to Fyodor or one of the
Insecure.Org development mailing lists, it is assumed that you are
offering the Nmap Project the unlimited, non-exclusive right to
reuse, modify, and relicense the code. Nmap will always be available
open source, but this is important because the inability to relicense
code has caused devastating problems for other Free Software projects
(such as KDE and NASM). We also occasionally relicense the code to
third parties as discussed above. If you wish to specify special
license conditions of your contributions, just say so when you send
them.
No Warranty
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License v2.0 for more details at
http://www.gnu.org/licenses/gpl-2.0.html , or in the COPYING file
included with Nmap.
It should also be noted that Nmap has occasionally been known to
crash poorly written applications, TCP/IP stacks, and even operating
systems. While this is extremely rare, it is important to keep in
mind. Nmap should never be run against mission critical systems
unless you are prepared to suffer downtime. We acknowledge here that
Nmap may crash your systems or networks and we disclaim all liability
for any damage or problems Nmap could cause.
Inappropriate Usage
Because of the slight risk of crashes and because a few black hats
like to use Nmap for reconnaissance prior to attacking systems, there
are administrators who become upset and may complain when their
system is scanned. Thus, it is often advisable to request permission
before doing even a light scan of a network.
Nmap should never be installed with special privileges (e.g. suid
root). That would open up a major security vulnerability as other
users on the system (or attackers) could use it for privilege
escalation.
Third-Party Software and Funding Notices
This product includes software developed by the Apache Software
Foundation[20]. A modified version of the Libpcap portable packet
capture library[21] is distributed along with Nmap. The Windows
version of Nmap utilized the Libpcap-derived WinPcap library[22]
instead. Regular expression support is provided by the PCRE
library[23], which is open-source software, written by Philip Hazel.
Certain raw networking functions use the Libdnet[24] networking
library, which was written by Dug Song. A modified version is
distributed with Nmap. Nmap can optionally link with the OpenSSL
cryptography toolkit[25] for SSL version detection support. The Nmap
Scripting Engine uses an embedded version of the Lua programming
language[26]. The Liblinear linear classification library[27] is
used for our IPv6 OS detection machine learning techniques[28]. All
of the third-party software described in this paragraph is freely
redistributable under BSD-style software licenses.
Binary packages for Windows and Mac OS X include support libraries
necessary to run Zenmap and Ndiff with Python and PyGTK. (Unix
platforms commonly make these libraries easy to install, so they are
not part of the packages.) A listing of these support libraries and
their licenses is included in the LICENSES files.
This software was supported in part through the Google Summer of
Code[29] and the DARPA CINDER program[30] (DARPA-BAA-10-84).
United States Export Control
Nmap only uses encryption when compiled with the optional OpenSSL
support and linked with OpenSSL. When compiled without OpenSSL
support, the Nmap Project believes that Nmap is not subject to U.S.
Export Administration Regulations (EAR)[31] export control. As such,
there is no applicable ECCN (export control classification number)
and exportation does not require any special license, permit, or
other governmental authorization.
When compiled with OpenSSL support or distributed as source code, the
Nmap Project believes that Nmap falls under U.S. ECCN 5D002[32]
(“Information Security Software”). We distribute Nmap under the TSU
exception for publicly available encryption software defined in EAR
740.13(e)[33].
1. Nmap Network Scanning: The Official Nmap Project Guide to Network
Discovery and Security Scanning
https://nmap.org/book/
2. RFC 1122
http://www.rfc-editor.org/rfc/rfc1122.txt
3. RFC 792
http://www.rfc-editor.org/rfc/rfc792.txt
4. RFC 950
http://www.rfc-editor.org/rfc/rfc950.txt
5. RFC 1918
http://www.rfc-editor.org/rfc/rfc1918.txt
6. UDP
http://www.rfc-editor.org/rfc/rfc768.txt
7. SCTP
http://www.rfc-editor.org/rfc/rfc4960.txt
8. TCP RFC
http://www.rfc-editor.org/rfc/rfc793.txt
9. RFC 959
http://www.rfc-editor.org/rfc/rfc959.txt
10. RFC 1323
http://www.rfc-editor.org/rfc/rfc1323.txt
11. Lua programming language
http://lua.org
12. precedence
http://www.lua.org/manual/5.1/manual.html#2.5.3
13. IP protocol
http://www.rfc-editor.org/rfc/rfc791.txt
14. RFC 2960
http://www.rfc-editor.org/rfc/rfc2960.txt
15. Nmap::Scanner
http://sourceforge.net/projects/nmap-scanner/
16. Nmap::Parser
http://nmapparser.wordpress.com/
17. xsltproc
http://xmlsoft.org/XSLT/
18. listed at Wikipedia
http://en.wikipedia.org/wiki/List_of_IPv6_tunnel_brokers
19. Creative Commons Attribution License
http://creativecommons.org/licenses/by/3.0/
20. Apache Software Foundation
http://www.apache.org
21. Libpcap portable packet capture library
http://www.tcpdump.org
22. WinPcap library
http://www.winpcap.org
23. PCRE library
http://www.pcre.org
24. Libdnet
http://libdnet.sourceforge.net
25. OpenSSL cryptography toolkit
http://www.openssl.org
26. Lua programming language
http://www.lua.org
27. Liblinear linear classification library
http://www.csie.ntu.edu.tw/~cjlin/liblinear/
28. IPv6 OS detection machine learning techniques
https://nmap.org/book/osdetect-guess.html#osdetect-guess-ipv6
29. Google Summer of Code
https://nmap.org/soc/
30. DARPA CINDER program
https://www.fbo.gov/index?s=opportunity&mode=form&id=585e02a51f77af5cb3c9e06b9cc82c48&tab=core&_cview=1
31. Export Administration Regulations (EAR)
http://www.access.gpo.gov/bis/ear/ear_data.html
32. 5D002
http://www.access.gpo.gov/bis/ear/pdf/ccl5-pt2.pdf
33. EAR 740.13(e)
http://www.access.gpo.gov/bis/ear/pdf/740.pdf
COLOPHON top
This page is part of the nmap (a network scanner) project.
Information about the project can be found at ⟨http://nmap.org/⟩. If
you have a bug report for this manual page, send it to dev@nmap.org.
This page was obtained from the project's upstream Subversion reposi‐
tory ⟨https://svn.nmap.org/nmap⟩ on 2017-07-05. If you discover any
rendering problems in this HTML version of the page, or you believe
there is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org