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RAW(7)                     Linux Programmer's Manual                    RAW(7)



NAME
       raw, SOCK_RAW - Linux IPv4 raw sockets

SYNOPSIS
       #include <sys/socket.h>
       #include <netinet/in.h>
       raw_socket = socket(AF_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
       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).

   Address Format
       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.

   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 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
       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.

VERSIONS
       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.

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 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.

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.

       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.

SEE ALSO
       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.

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-11-20                            RAW(7)