|
NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | NOTES | BUGS | SEE ALSO | COLOPHON |
RAW(7) Linux Programmer's Manual RAW(7)
raw - Linux IPv4 raw sockets
#include <sys/socket.h>
#include <netinet/in.h>
raw_socket = socket(AF_INET, SOCK_RAW, int protocol);
Raw sockets allow new IPv4 protocols to be implemented in user space.
A raw socket receives or sends the raw datagram not including link
level headers.
The IPv4 layer generates an IP header when sending a packet unless
the IP_HDRINCL socket option is enabled on the socket. When it is
enabled, the packet must contain an IP header. For receiving, the IP
header is always included in the packet.
In order to create a raw socket, a process must have the CAP_NET_RAW
capability in the user namespace that governs its network namespace.
All packets or errors matching the protocol number specified for the
raw socket are passed to this socket. For a list of the allowed
protocols, see the IANA list of assigned protocol numbers at
⟨http://www.iana.org/assignments/protocol-numbers/⟩ and
getprotobyname(3).
A protocol of IPPROTO_RAW implies enabled IP_HDRINCL and is able to
send any IP protocol that is specified in the passed header. Receiv‐
ing of all IP protocols via IPPROTO_RAW is not possible using raw
sockets.
┌───────────────────────────────────────────────────┐
│IP Header fields modified on sending by IP_HDRINCL │
├──────────────────────┬────────────────────────────┤
│IP Checksum │ Always filled in │
├──────────────────────┼────────────────────────────┤
│Source Address │ Filled in when zero │
├──────────────────────┼────────────────────────────┤
│Packet ID │ Filled in when zero │
├──────────────────────┼────────────────────────────┤
│Total Length │ Always filled in │
└──────────────────────┴────────────────────────────┘
If IP_HDRINCL is specified and the IP header has a nonzero destina‐
tion address, then the destination address of the socket is used to
route the packet. When MSG_DONTROUTE is specified, the destination
address should refer to a local interface, otherwise a routing table
lookup is done anyway but gatewayed routes are ignored.
If IP_HDRINCL isn't set, then IP header options can be set on raw
sockets with setsockopt(2); see ip(7) for more information.
Starting with Linux 2.2, all IP header fields and options can be set
using IP socket options. This means raw sockets are usually needed
only for new protocols or protocols with no user interface (like
ICMP).
When a packet is received, it is passed to any raw sockets which have
been bound to its protocol before it is passed to other protocol han‐
dlers (e.g., kernel protocol modules).
Address format
For sending and receiving datagrams (sendto(2), recvfrom(2), and sim‐
ilar), raw sockets use the standard sockaddr_in address structure
defined in ip(7). The sin_port field could be used to specify the IP
protocol number, but it is ignored for sending in Linux 2.2 and
later, and should be always set to 0 (see BUGS). For incoming pack‐
ets, sin_port is set to zero.
Socket options
Raw socket options can be set with setsockopt(2) and read with
getsockopt(2) by passing the IPPROTO_RAW family flag.
ICMP_FILTER
Enable a special filter for raw sockets bound to the
IPPROTO_ICMP protocol. The value has a bit set for each ICMP
message type which should be filtered out. The default is to
filter no ICMP messages.
In addition, all ip(7) IPPROTO_IP socket options valid for datagram
sockets are supported.
Error handling
Errors originating from the network are passed to the user only when
the socket is connected or the IP_RECVERR flag is enabled. For con‐
nected sockets, only EMSGSIZE and EPROTO are passed for compatibil‐
ity. With IP_RECVERR, all network errors are saved in the error
queue.
EACCES User tried to send to a broadcast address without having the
broadcast flag set on the socket.
EFAULT An invalid memory address was supplied.
EINVAL Invalid argument.
EMSGSIZE
Packet too big. Either Path MTU Discovery is enabled (the
IP_MTU_DISCOVER socket flag) or the packet size exceeds the
maximum allowed IPv4 packet size of 64KB.
EOPNOTSUPP
Invalid flag has been passed to a socket call (like MSG_OOB).
EPERM The user doesn't have permission to open raw sockets. Only
processes with an effective user ID of 0 or the CAP_NET_RAW
attribute may do that.
EPROTO An ICMP error has arrived reporting a parameter problem.
IP_RECVERR and ICMP_FILTER are new in Linux 2.2. They are Linux
extensions and should not be used in portable programs.
Linux 2.0 enabled some bug-to-bug compatibility with BSD in the raw
socket code when the SO_BSDCOMPAT socket option was set; since Linux
2.2, this option no longer has that effect.
By default, raw sockets do path MTU (Maximum Transmission Unit)
discovery. This means the kernel will keep track of the MTU to a
specific target IP address and return EMSGSIZE when a raw packet
write exceeds it. When this happens, the application should decrease
the packet size. Path MTU discovery can be also turned off using the
IP_MTU_DISCOVER socket option or the
/proc/sys/net/ipv4/ip_no_pmtu_disc file, see ip(7) for details. When
turned off, raw sockets will fragment outgoing packets that exceed
the interface MTU. However, disabling it is not recommended for
performance and reliability reasons.
A raw socket can be bound to a specific local address using the
bind(2) call. If it isn't bound, all packets with the specified IP
protocol are received. In addition, a raw socket can be bound to a
specific network device using SO_BINDTODEVICE; see socket(7).
An IPPROTO_RAW socket is send only. If you really want to receive
all IP packets, use a packet(7) socket with the ETH_P_IP protocol.
Note that packet sockets don't reassemble IP fragments, unlike raw
sockets.
If you want to receive all ICMP packets for a datagram socket, it is
often better to use IP_RECVERR on that particular socket; see ip(7).
Raw sockets may tap all IP protocols in Linux, even protocols like
ICMP or TCP which have a protocol module in the kernel. In this
case, the packets are passed to both the kernel module and the raw
socket(s). This should not be relied upon in portable programs, many
other BSD socket implementation have limitations here.
Linux never changes headers passed from the user (except for filling
in some zeroed fields as described for IP_HDRINCL). This differs
from many other implementations of raw sockets.
Raw sockets are generally rather unportable and should be avoided in
programs intended to be portable.
Sending on raw sockets should take the IP protocol from sin_port;
this ability was lost in Linux 2.2. The workaround is to use
IP_HDRINCL.
Transparent proxy extensions are not described.
When the IP_HDRINCL option is set, datagrams will not be fragmented
and are limited to the interface MTU.
Setting the IP protocol for sending in sin_port got lost in Linux
2.2. The protocol that the socket was bound to or that was specified
in the initial socket(2) call is always used.
recvmsg(2), sendmsg(2), capabilities(7), ip(7), socket(7)
RFC 1191 for path MTU discovery. RFC 791 and the <linux/ip.h> header
file for the IP protocol.
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-10-08 RAW(7)
Pages that refer to this page: icmp(7), ip(7), packet(7), udp(7)