man-pages/man7/raw.7

.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
.\"
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.\" $Id: raw.7,v 1.6 1999/06/05 10:32:08 freitag Exp $
.\"
.TH RAW  7 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
raw \- Linux IPv4 raw sockets
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.B #include <netinet/in.h>
.BI "raw_socket = socket(AF_INET, SOCK_RAW, int " protocol );
.fi
.SH DESCRIPTION
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.
.PP
The IPv4 layer generates an IP header when sending a packet unless the
.B 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.
.PP
In order to create a raw socket, a process must have the
.B CAP_NET_RAW
capability in the user namespace that governs its network namespace.
.PP
All packets or errors matching the
.I 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
.UR http://www.iana.org/assignments/protocol\-numbers/
.UE
and
.BR getprotobyname (3).
.PP
A protocol of
.B IPPROTO_RAW
implies enabled
.B IP_HDRINCL
and is able to send any IP protocol that is specified in the passed
header.
Receiving of all IP protocols via
.B IPPROTO_RAW
is not possible using raw sockets.
.RS
.TS
tab(:) allbox;
c s
l l.
IP Header fields modified on sending by \fBIP_HDRINCL\fP
IP Checksum:Always filled in
Source Address:Filled in when zero
Packet ID:Filled in when zero
Total Length:Always filled in
.TE
.RE
.PP
.PP
If
.B IP_HDRINCL
is specified and the IP header has a nonzero destination address, then
the destination address of the socket is used to route the packet.
When
.B 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.
.PP
If
.B IP_HDRINCL
isn't set, then IP header options can be set on raw sockets with
.BR setsockopt (2);
see
.BR ip (7)
for more information.
.PP
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).
.PP
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).
.SS Address format
For sending and receiving datagrams
.RB ( sendto (2),
.BR recvfrom (2),
and similar),
raw sockets use the standard
.I sockaddr_in
address structure defined in
.BR ip (7).
The
.I 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 packets,
.I sin_port
.\" commit f59fc7f30b710d45aadf715460b3e60dbe9d3418
is set to zero.
.SS Socket options
Raw socket options can be set with
.BR setsockopt (2)
and read with
.BR getsockopt (2)
by passing the
.B IPPROTO_RAW
.\" Or SOL_RAW on Linux
family flag.
.TP
.B ICMP_FILTER
Enable a special filter for raw sockets bound to the
.B 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.
.PP
In addition, all
.BR ip (7)
.B IPPROTO_IP
socket options valid for datagram sockets are supported.
.SS Error handling
Errors originating from the network are passed to the user only when the
socket is connected or the
.B IP_RECVERR
flag is enabled.
For connected sockets, only
.B EMSGSIZE
and
.B EPROTO
are passed for compatibility.
With
.BR IP_RECVERR ,
all network errors are saved in the error queue.
.SH ERRORS
.TP
.B EACCES
User tried to send to a broadcast address without having the
broadcast flag set on the socket.
.TP
.B EFAULT
An invalid memory address was supplied.
.TP
.B EINVAL
Invalid argument.
.TP
.B EMSGSIZE
Packet too big.
Either Path MTU Discovery is enabled (the
.B IP_MTU_DISCOVER
socket flag) or the packet size exceeds the maximum allowed IPv4
packet size of 64\ kB.
.TP
.B EOPNOTSUPP
Invalid flag has been passed to a socket call (like
.BR MSG_OOB ).
.TP
.B EPERM
The user doesn't have permission to open raw sockets.
Only processes with an effective user ID of 0 or the
.B CAP_NET_RAW
attribute may do that.
.TP
.B EPROTO
An ICMP error has arrived reporting a parameter problem.
.SH VERSIONS
.B IP_RECVERR
and
.B ICMP_FILTER
are new in Linux 2.2.
They are Linux extensions and should not be used in portable programs.
.PP
Linux 2.0 enabled some bug-to-bug compatibility with BSD in the
raw socket code when the
.B SO_BSDCOMPAT
socket option was set; since Linux 2.2,
this option no longer has that effect.
.SH NOTES
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
.B 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
.B IP_MTU_DISCOVER
socket option or the
.I /proc/sys/net/ipv4/ip_no_pmtu_disc
file, see
.BR 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.
.PP
A raw socket can be bound to a specific local address using the
.BR 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
.BR SO_BINDTODEVICE ;
see
.BR socket (7).
.PP
An
.B IPPROTO_RAW
socket is send only.
If you really want to receive all IP packets, use a
.BR packet (7)
socket with the
.B ETH_P_IP
protocol.
Note that packet sockets don't reassemble IP fragments,
unlike raw sockets.
.PP
If you want to receive all ICMP packets for a datagram socket,
it is often better to use
.B IP_RECVERR
on that particular socket; see
.BR ip (7).
.PP
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.
.PP
Linux never changes headers passed from the user (except for filling
in some zeroed fields as described for
.BR IP_HDRINCL ).
This differs from many other implementations of raw sockets.
.PP
Raw sockets are generally rather unportable and should be avoided in
programs intended to be portable.
.PP
Sending on raw sockets should take the IP protocol from
.IR sin_port ;
this ability was lost in Linux 2.2.
The workaround is to use
.BR IP_HDRINCL .
.SH BUGS
Transparent proxy extensions are not described.
.PP
When the
.B IP_HDRINCL
option is set, datagrams will not be fragmented and are limited to
the interface MTU.
.PP
Setting the IP protocol for sending in
.I sin_port
got lost in Linux 2.2.
The protocol that the socket was bound to or that
was specified in the initial
.BR socket (2)
call is always used.
.\" .SH AUTHORS
.\" This man page was written by Andi Kleen.
.SH SEE ALSO
.BR recvmsg (2),
.BR sendmsg (2),
.BR capabilities (7),
.BR ip (7),
.BR socket (7)
.PP
.B RFC\ 1191
for path MTU discovery.
.B RFC\ 791
and the
.I <linux/ip.h>
header file for the IP protocol.