NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | NOTES | BUGS | SEE ALSO | COLOPHON
RAW(7) Linux Programmer's Manual RAW(7)
raw, SOCK_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.
Only processes with an effective user ID of 0 or the CAP_NET_RAW capability
are allowed to open raw sockets.
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
RFC 1700 assigned 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. Receiving 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 non-zero destination
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.
In Linux 2.2, all IP header fields and options can be set using IP socket
options. This means raw sockets are usually only needed 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 handlers (e.g.,
kernel protocol modules).
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 should be always set to 0 (see BUGS).
For incoming packets, sin_port is set to the protocol of the packet. See the
<netinet/in.h> include file for valid IP protocols.
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.
Errors originating from the network are only passed to the user when the
socket is connected or the IP_RECVERR flag is enabled. For connected sockets,
only EMSGSIZE and EPROTO are passed for compatibility. 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> include file for the IP protocol.
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-11-20 RAW(7)