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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 con-
       tain 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 inter-
       face, 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 set-
       sockopt(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  appli-
       cation  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  addi-
       tion, 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 proto-
       col 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 information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2008-11-20                            RAW(7)

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