raw(7)
NAME
raw, SOCK_RAW - Linux IPv4 raw sockets
SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>
raw_socket = socket(PF_INET, SOCK_RAW, int protocol);
DESCRIPTION
Raw sockets allow new IPv4 protocols to be implemented in
user space. A raw socket receives or sends the raw data
gram 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 speci
fied for the raw socket are passed to this socket. For a
list of the allowed protocols see RFC1700 assigned numbers
and getprotobyname(3).
A protocol of IPPROTO_RAW implies enabled IP_HDRINCL and
receives all IP protocols. Sending is not allowed.
+---------------------------------------------------+
|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
anyways 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 usu
ally 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).
ADDRESS FORMAT
Raw sockets use the standard sockaddr_in address structure
defined in ip(7). The 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.
SOCKET OPTIONS
Raw socket options can be set with setsockopt(2) and read
with getsockopt(2) by passing the SOL_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) SOL_IP socket options valid for
datagram sockets are supported.
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 IP_PMTU_DISCOVER section of
ip(7).
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 reassem
ble 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 par
ticular socket; see ip(7).
Raw sockets may tap all IP protocols in Linux, even proto
cols 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. Work around
is to use IP_HDRINCL.
ERROR HANDLING
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.
ERRORS
EMSGSIZE
Packet too big. Either Path MTU Discovery is
enabled (the IP_PMTU_DISCOVER socket flag) or the
packet size exceeds the maximum allowed IPv4 packet
size of 64KB.
EACCES User tried to send to a broadcast address without
having the broadcast flag set on the socket.
EPROTO An ICMP error has arrived reporting a parameter
problem.
EFAULT An invalid memory address was supplied.
EOPNOTSUPP
Invalid flag has been passed to a socket call (like
MSG_OOB).
EINVAL Invalid argument.
EPERM The user doesn't have permission to open raw sock
ets. Only processes with a effective user id of 0
or the CAP_NET_RAW attribute may do that.
VERSIONS
IP_RECVERR and ICMP_FILTER are new in Linux 2.2. They are
Linux extensions and should not be used in portable pro
grams.
Linux 2.0 enabled some bug-to-bug compatibility with BSD
in the raw socket code when the SO_BSDCOMPAT flag was set
- that has been removed in 2.2.
BUGS
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. This is
a limitation in Linux 2.2.
Setting the IP protocol for sending in sin_port got lost
in Linux 2.2. The protocol that socket was bound to or
that was specified in the initial socket(2) call is always
used.
AUTHORS
This man page was writen by Andi Kleen.
SEE ALSO
ip(7), socket(7), recvmsg(2), sendmsg(2).
RFC1191 for path MTU discovery.
RFC791 and the <linux/ip.h> include file for the IP proto
col.
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