NCAT(1) Ncat Reference Guide NCAT(1)
ncat - Concatenate and redirect sockets
SYNOPSIS top
ncat [OPTIONS...] [hostname] [port]
DESCRIPTION top
Ncat is a feature-packed networking utility which reads and writes
data across networks from the command line. Ncat was written for the
Nmap Project and is the culmination of the currently splintered
family of Netcat incarnations. It is designed to be a reliable
back-end tool to instantly provide network connectivity to other
applications and users. Ncat will not only work with IPv4 and IPv6
but provides the user with a virtually limitless number of potential
uses.
Among Ncat's vast number of features there is the ability to chain
Ncats together; redirection of TCP, UDP, and SCTP ports to other
sites; SSL support; and proxy connections via SOCKS4 or HTTP proxies
(with optional proxy authentication as well). Some general principles
apply to most applications and thus give you the capability of
instantly adding networking support to software that would normally
never support it.
OPTIONS SUMMARY top
Ncat 7.50SVN ( https://nmap.org/ncat )
Usage: ncat [options] [hostname] [port]
Options taking a time assume seconds. Append 'ms' for milliseconds,
's' for seconds, 'm' for minutes, or 'h' for hours (e.g. 500ms).
-4 Use IPv4 only
-6 Use IPv6 only
-U, --unixsock Use Unix domain sockets only
-C, --crlf Use CRLF for EOL sequence
-c, --sh-exec <command> Executes the given command via /bin/sh
-e, --exec <command> Executes the given command
--lua-exec <filename> Executes the given Lua script
-g hop1[,hop2,...] Loose source routing hop points (8 max)
-G <n> Loose source routing hop pointer (4, 8, 12, ...)
-m, --max-conns <n> Maximum <n> simultaneous connections
-h, --help Display this help screen
-d, --delay <time> Wait between read/writes
-o, --output <filename> Dump session data to a file
-x, --hex-dump <filename> Dump session data as hex to a file
-i, --idle-timeout <time> Idle read/write timeout
-p, --source-port port Specify source port to use
-s, --source addr Specify source address to use (doesn't affect -l)
-l, --listen Bind and listen for incoming connections
-k, --keep-open Accept multiple connections in listen mode
-n, --nodns Do not resolve hostnames via DNS
-t, --telnet Answer Telnet negotiations
-u, --udp Use UDP instead of default TCP
--sctp Use SCTP instead of default TCP
-v, --verbose Set verbosity level (can be used several times)
-w, --wait <time> Connect timeout
-z Zero-I/O mode, report connection status only
--append-output Append rather than clobber specified output files
--send-only Only send data, ignoring received; quit on EOF
--recv-only Only receive data, never send anything
--allow Allow only given hosts to connect to Ncat
--allowfile A file of hosts allowed to connect to Ncat
--deny Deny given hosts from connecting to Ncat
--denyfile A file of hosts denied from connecting to Ncat
--broker Enable Ncat's connection brokering mode
--chat Start a simple Ncat chat server
--proxy <addr[:port]> Specify address of host to proxy through
--proxy-type <type> Specify proxy type ("http" or "socks4" or "socks5")
--proxy-auth <auth> Authenticate with HTTP or SOCKS proxy server
--ssl Connect or listen with SSL
--ssl-cert Specify SSL certificate file (PEM) for listening
--ssl-key Specify SSL private key (PEM) for listening
--ssl-verify Verify trust and domain name of certificates
--ssl-trustfile PEM file containing trusted SSL certificates
--ssl-ciphers Cipherlist containing SSL ciphers to use
--version Display Ncat's version information and exit
See the ncat(1) manpage for full options, descriptions and usage examples
CONNECT MODE AND LISTEN MODE top
Ncat operates in one of two primary modes: connect mode and listen
mode. Other modes, such as the HTTP proxy server, act as special
cases of these two. In connect mode, Ncat works as a client. In
listen mode it is a server.
In connect mode, the hostname and port arguments tell what to connect
to. hostname is required, and may be a hostname or IP address. If
port is supplied, it must be a decimal port number. If omitted, it
defaults to 31337.
In listen mode, hostname and port control the address the server will
bind to. Both arguments are optional in listen mode. If hostname is
omitted, it defaults to listening on all available addresses over
IPv4 and IPv6. If port is omitted, it defaults to 31337.
PROTOCOL OPTIONS top
-4 (IPv4 only)
Force the use of IPv4 only.
-6 (IPv6 only)
Force the use of IPv6 only.
-U, --unixsock (Use Unix domain sockets)
Use Unix domain sockets rather than network sockets. This option
may be used on its own for stream sockets, or combined with --udp
for datagram sockets. A description of -U mode is in the section
called “UNIX DOMAIN SOCKETS”.
-u, --udp (Use UDP)
Use UDP for the connection (the default is TCP).
--sctp (Use SCTP)
Use SCTP for the connection (the default is TCP). SCTP support is
implemented in TCP-compatible mode.
CONNECT MODE OPTIONS top
-g hop1[,hop2,...] (Loose source routing)
Sets hops for IPv4 loose source routing. You can use -g once with
a comma-separated list of hops, use -g multiple times with single
hops to build the list, or combine the two. Hops can be given as
IP addresses or hostnames.
-G ptr (Set source routing pointer)
Sets the IPv4 source route “pointer” for use with -g. The
argument must be a multiple of 4 and no more than 28. Not all
operating systems support setting this pointer to anything other
than four.
-p port, --source-port port (Specify source port)
Set the port number for Ncat to bind to.
-s host, --source host (Specify source address)
Set the address for Ncat to bind to.
LISTEN MODE OPTIONS top
See the section called “ACCESS CONTROL OPTIONS” for information on
limiting the hosts that may connect to the listening Ncat process.
-l, --listen (Listen for connections)
Listen for connections rather than connecting to a remote machine
-m numconns, --max-conns numconns (Specify maximum number of
connections)
The maximum number of simultaneous connections accepted by an
Ncat instance. 100 is the default (60 on Windows).
-k, --keep-open (Accept multiple connections)
Normally a listening server accepts only one connection and then
quits when the connection is closed. This option makes it accept
multiple simultaneous connections and wait for more connections
after they have all been closed. It must be combined with
--listen. In this mode there is no way for Ncat to know when its
network input is finished, so it will keep running until
interrupted. This also means that it will never close its output
stream, so any program reading from Ncat and looking for
end-of-file will also hang.
--broker (Connection brokering)
Allow multiple parties to connect to a centralised Ncat server
and communicate with each other. Ncat can broker communication
between systems that are behind a NAT or otherwise unable to
directly connect. This option is used in conjunction with
--listen, which causes the --listen port to have broker mode
enabled.
--chat (Ad-hoc “chat server”)
The --chat option enables chat mode, intended for the exchange of
text between several users. In chat mode, connection brokering is
turned on. Ncat prefixes each message received with an ID before
relaying it to the other connections. The ID is unique for each
connected client. This helps distinguish who sent what.
Additionally, non-printing characters such as control characters
are escaped to keep them from doing damage to a terminal.
SSL OPTIONS top
--ssl (Use SSL)
In connect mode, this option transparently negotiates an SSL
session with an SSL server to securely encrypt the connection.
This is particularly handy for talking to SSL enabled HTTP
servers, etc.
In server mode, this option listens for incoming SSL connections,
rather than plain untunneled traffic.
--ssl-verify (Verify server certificates)
In client mode, --ssl-verify is like --ssl except that it also
requires verification of the server certificate. Ncat comes with
a default set of trusted certificates in the file ca-bundle.crt.
Some operating systems provide a default list of trusted
certificates; these will also be used if available. Use
--ssl-trustfile to give a custom list. Use -v one or more times
to get details about verification failures. Ncat does not check
for revoked certificates.
This option has no effect in server mode.
--ssl-cert certfile.pem (Specify SSL certificate)
This option gives the location of a PEM-encoded certificate files
used to authenticate the server (in listen mode) or the client
(in connect mode). Use it in combination with --ssl-key.
--ssl-key keyfile.pem (Specify SSL private key)
This option gives the location of the PEM-encoded private key
file that goes with the certificate named with --ssl-cert.
--ssl-trustfile cert.pem (List trusted certificates)
This option sets a list of certificates that are trusted for
purposes of certificate verification. It has no effect unless
combined with --ssl-verify. The argument to this option is the
name of a PEM file containing trusted certificates. Typically,
the file will contain certificates of certification authorities,
though it may also contain server certificates directly. When
this option is used, Ncat does not use its default certificates.
--ssl-ciphers cipherlist (Specify SSL ciphersuites)
This option sets the list of ciphersuites that Ncat will use when
connecting to servers or when accepting SSL connections from
clients. The syntax is described in the OpenSSL ciphers(1) man
page, and defaults to ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH
PROXY OPTIONS top
--proxy host[:port] (Specify proxy address)
Requests proxying through host:port, using the protocol specified
by --proxy-type.
If no port is specified, the proxy protocol's well-known port is
used (1080 for SOCKS and 3128 for HTTP). However, when specifying
an IPv6 HTTP proxy server using the IP address rather than the
hostname, the port number MUST be specified as well. If the proxy
requires authentication, use --proxy-auth.
--proxy-type proto (Specify proxy protocol)
In connect mode, this option requests the protocol proto to
connect through the proxy host specified by --proxy. In listen
mode, this option has Ncat act as a proxy server using the
specified protocol.
The currently available protocols in connect mode are http
(CONNECT) and socks4 (SOCKSv4). The only server currently
supported is http. If this option is not used, the default
protocol is http.
--proxy-auth user[:pass] (Specify proxy credentials)
In connect mode, gives the credentials that will be used to
connect to the proxy server. In listen mode, gives the
credentials that will be required of connecting clients. For use
with --proxy-type http, the form should be user:pass. For
--proxy-type socks4, it should be a username only.
COMMAND EXECUTION OPTIONS top
-e command, --exec command (Execute command)
Execute the specified command after a connection has been
established. The command must be specified as a full pathname.
All input from the remote client will be sent to the application
and responses sent back to the remote client over the socket,
thus making your command-line application interactive over a
socket. Combined with --keep-open, Ncat will handle multiple
simultaneous connections to your specified port/application like
inetd. Ncat will only accept a maximum, definable, number of
simultaneous connections controlled by the -m option. By default
this is set to 100 (60 on Windows).
-c command, --sh-exec command (Execute command via sh)
Same as -e, except it tries to execute the command via /bin/sh.
This means you don't have to specify the full path for the
command, and shell facilities like environment variables are
available.
--lua-exec file (Execute a .lua script)
Runs the specified file as a Lua script after a connection has
been established, using a built-in interpreter. Both the script's
standard input and the standard output are redirected to the
connection data streams.
All exec options add the following variables to the child's
environment:
NCAT_REMOTE_ADDR, NCAT_REMOTE_PORT
The IP address and port number of the remote host. In connect
mode, it's the target's address; in listen mode, it's the
client's address.
NCAT_LOCAL_ADDR, NCAT_LOCAL_PORT
The IP address and port number of the local end of the
connection.
NCAT_PROTO
The protocol in use: one of TCP, UDP, and SCTP.
ACCESS CONTROL OPTIONS top
--allow host[,host,...] (Allow connections)
The list of hosts specified will be the only hosts allowed to
connect to the Ncat process. All other connection attempts will
be disconnected. In case of a conflict between --allow and
--deny, --allow takes precedence. Host specifications follow the
same syntax used by Nmap.
--allowfile file (Allow connections from file)
This has the same functionality as --allow, except that the
allowed hosts are provided in a new-line delimited allow file,
rather than directly on the command line.
--deny host[,host,...] (Deny connections)
Issue Ncat with a list of hosts that will not be allowed to
connect to the listening Ncat process. Specified hosts will have
their session silently terminated if they try to connect. In case
of a conflict between --allow and --deny, --allow takes
precedence. Host specifications follow the same syntax used by
Nmap.
--denyfile file (Deny connections from file)
This is the same functionality as --deny, except that excluded
hosts are provided in a new-line delimited deny file, rather than
directly on the command line.
TIMING OPTIONS top
These 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.
-d time, --delay time (Specify line delay)
Set the delay interval for lines sent. This effectively limits
the number of lines that Ncat will send in the specified period.
This may be useful for low-bandwidth sites, or have other uses
such as coping with annoying iptables --limit options.
-i time, --idle-timeout time (Specify idle timeout)
Set a fixed timeout for idle connections. If the idle timeout is
reached, the connection is terminated.
-w time, --wait time (Specify connect timeout)
Set a fixed timeout for connection attempts.
OUTPUT OPTIONS top
-o file, --output file (Save session data)
Dump session data to a file
-x file, --hex-dump file (Save session data in hex)
Dump session data in hex to a file.
--append-output (Append output)
Issue Ncat with --append-ouput along with -o and/or -x and it
will append the resulted output rather than truncating the
specified output files.
-v, --verbose (Be verbose)
Issue Ncat with -v and it will be verbose and display all kinds
of useful connection based information. Use more than once (-vv,
-vvv...) for greater verbosity.
MISC OPTIONS top
-C, --crlf (Use CRLF as EOL)
This option tells Ncat to convert LF line endings to CRLF when
taking input from standard input. This is useful for talking to
some stringent servers directly from a terminal in one of the
many common plain-text protocols that use CRLF for end-of-line.
-h, --help (Help screen)
Displays a short help screen with common options and parameters,
and then exits.
--recv-only (Only receive data)
If this option is passed, Ncat will only receive data and will
not try to send anything.
--send-only (Only send data)
If this option is passed, then Ncat will only send data and will
ignore anything received. This option also causes Ncat to close
the network connection and terminate after EOF is received on
standard input.
--no-shutdown (Do not shutdown into half-duplex mode)
If this option is passed, Ncat will not invoke shutdown on a
socket aftering seeing EOF on stdin. This is provided for
backward-compatibility with OpenBSD netcat, which exhibits this
behavior when executed with its '-d' option.
-t, --telnet (Answer Telnet negotiations)
Handle DO/DONT WILL/WONT Telnet negotiations. This makes it
possible to script Telnet sessions with Ncat.
--version (Display version)
Displays the Ncat version number and exits.
UNIX DOMAIN SOCKETS top
The -U option (same as --unixsock) causes Ncat to use Unix domain
sockets rather than network sockets. Unix domain sockets exist as an
entry in the filesystem. You must give the name of a socket to
connect to or to listen on. For example, to make a connection,
ncat -U ~/unixsock
To listen on a socket:
ncat -l -U ~/unixsock
Listen mode will create the socket if it doesn't exist. The socket
will continue to exist after the program ends.
Both stream and datagram domain sockets are supported. Use -U on its
own for stream sockets, or combine it with --udp for datagram
sockets. Datagram sockets require a source socket to connect from. By
default, a source socket with a random filename will be created as
needed, and deleted when the program ends. Use the --source with a
path to use a source socket with a specific name.
EXAMPLES top
Connect to example.org on TCP port 8080.
ncat example.org 8080
Listen for connections on TCP port 8080.
ncat -l 8080
Redirect TCP port 8080 on the local machine to host on port 80.
ncat --sh-exec "ncat example.org 80" -l 8080 --keep-open
Bind to TCP port 8081 and attach /bin/bash for the world to access
freely.
ncat --exec "/bin/bash" -l 8081 --keep-open
Bind a shell to TCP port 8081, limit access to hosts on a local
network, and limit the maximum number of simultaneous connections to
3.
ncat --exec "/bin/bash" --max-conns 3 --allow 192.168.0.0/24 -l
8081 --keep-open
Connect to smtphost:25 through a SOCKS4 server on port 1080.
ncat --proxy socks4host --proxy-type socks4 --proxy-auth user
smtphost 25
Create an HTTP proxy server on localhost port 8888.
ncat -l --proxy-type http localhost 8888
Send a file over TCP port 9899 from host2 (client) to host1 (server).
HOST1$ ncat -l 9899 > outputfile
HOST2$ ncat HOST1 9899 < inputfile
Transfer in the other direction, turning Ncat into a “one file”
server.
HOST1$ ncat -l 9899 < inputfile
HOST2$ ncat HOST1 9899 > outputfile
EXIT CODE top
The exit code reflects whether a connection was made and completed
successfully. 0 means there was no error. 1 means there was a network
error of some kind, for example “Connection refused” or “Connection
reset”. 2 is reserved for all other errors, like an invalid option or
a nonexistent file.
Like its authors, Ncat isn't perfect. But you can help make it better
by sending bug reports or even writing patches. If Ncat 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 Googling the error message or 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 Ncat you are running and what operating system
version it is running on. Problem reports and Ncat usage questions
sent to dev@nmap.org are far more likely to be answered than those
sent to Fyodor directly.
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.
AUTHORS top
· Chris Gibson <chris@linuxops.net>
· Kris Katterjohn <katterjohn@gmail.com>
· Mixter <mixter@gmail.com>
· Fyodor <fyodor@nmap.org> (http://insecure.org )
The original Netcat was written by *Hobbit* <hobbit@avian.org>. While
Ncat isn't built on any code from the “traditional” Netcat (or any
other implementation), Ncat is most definitely based on Netcat in
spirit and functionality.
LEGAL NOTICES top
Ncat Copyright and Licensing
Ncat is (C) 2005–2012 Insecure.Com LLC. It is distributed as free and
open source software under the same license terms as our Nmap
software. Precise terms and further details are available from
https://nmap.org/man/man-legal.html .
Creative Commons License for this Ncat Guide
This Ncat Reference Guide is (C) 2005–2012 Insecure.Com LLC. It is
hereby placed under version 3.0 of the Creative Commons Attribution
License[1]. 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 Ncap 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
(none have been found so far).
Source code also allows you to port Nmap (which includes Ncat) 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 (Insecure.Com LLC) 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 in the
Nmap man page. 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.
Inappropriate Usage
Ncat 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
This product includes software developed by the Apache Software
Foundation[2]. A modified version of the Libpcap portable packet
capture library[3] is distributed along with Ncat. The Windows
version of Ncat utilized the Libpcap-derived WinPcap library[4]
instead. Certain raw networking functions use the Libdnet[5]
networking library, which was written by Dug Song. A modified
version is distributed with Ncat. Ncat can optionally link with the
OpenSSL cryptography toolkit[6] for SSL version detection support.
All of the third-party software described in this paragraph is freely
redistributable under BSD-style software licenses.
1. Creative Commons Attribution License
http://creativecommons.org/licenses/by/3.0/
2. Apache Software Foundation
http://www.apache.org
3. Libpcap portable packet capture library
http://www.tcpdump.org
4. WinPcap library
http://www.winpcap.org
5. Libdnet
http://libdnet.sourceforge.net
6. OpenSSL cryptography toolkit
http://www.openssl.org
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