man-pages/man7/raw.7

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.TH RAW  7 1998-10-02 "Linux Man Page" "Linux Programmer's Manual" 
.SH NAME
raw, SOCK_RAW \- Linux IPv4 raw sockets
.SH SYNOPSIS
.B #include <sys/socket.h>
.br
.B #include <netinet/in.h>
.br
.BI "raw_socket = socket(PF_INET, SOCK_RAW, int " protocol ); 

.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.

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.

Only processes with an effective user ID of 0 or the 
.B CAP_NET_RAW 
capability are allowed to open raw sockets.

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 RFC\ 1700 assigned numbers and
.BR getprotobyname (3).

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.

.TS
tab(:) allbox;
c s
l l.
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.
.TE
.PP

If 
.B 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 
.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.

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.

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).  

.SH "ADDRESS FORMAT"

Raw sockets use the standard 
.B sockaddr_in 
address structure defined in 
.BR ip (7).
The 
The 
.B 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 
.B sin_port 
is set to the protocol of the packet. 
See the
.B <netinet/in.h>
include file for valid IP protocols. 

.SH "SOCKET OPTIONS"
Raw socket options can be set with
.BR setsockopt (2)
and read with
.BR getsockopt (2)
by passing the 
.I SOL_RAW 
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 SOL_IP 
socket options valid for datagram sockets are supported.

.SH NOTES
Raw sockets fragment a packet when its total length exceeds the interface MTU
(but see BUGS).
A more network friendly and faster alternative is to implement path MTU 
discovery as described in the
.B IP_MTU_DISCOVER 
section of
.BR ip (7).

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
.B SO_BINDTODEVICE;
see 
.BR socket (7).

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.

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).

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
.BR 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 
.B sin_port;
this ability was lost in Linux 2.2. Work around is to use
.BR IP_HDRINCL .

.SH "ERROR HANDLING"
Errors originating from the network are only passed to the user 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
.B IP_RECVERR
all network errors are saved in the error queue. 
.SH ERRORS
.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 64KB.  
.TP
.B EACCES
User tried to send to a broadcast address without having the broadcast flag
set on the socket.
.TP
.B EPROTO
An ICMP error has arrived reporting a parameter problem.
.TP
.B EFAULT
An invalid memory address was supplied.
.TP
.B EOPNOTSUPP
Invalid flag has been passed to a socket call (like 
.BR MSG_OOB ).
.TP
.B EINVAL
Invalid argument.
.TP
.B EPERM
The user doesn't have permission to open raw sockets. Only processes
with a effective user ID of 0 or the 
.B CAP_NET_RAW 
attribute may do that.

.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.

Linux 2.0 enabled some bug-to-bug compatibility with BSD in the raw socket code
when the SO_BSDCOMPAT flag was set \(em that has been removed in 2.2.

.SH BUGS
Transparent proxy extensions are not described.

When the 
.B IP_HDRINCL
option is set datagrams will not be fragmented and are limited to the interface
MTU.  This is a limitation in Linux 2.2.

Setting the IP protocol for sending in
.B sin_port
got lost in Linux 2.2. The protocol that 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)

.B RFC\ 1191 
for path MTU discovery.

.B RFC\ 791 
and the
.B <linux/ip.h>
include file for the IP protocol.