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



NAME
       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 nonzero 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  setsock-
       opt(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 needed only 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 proto-
       cols.

   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 passed to the user only when the socket is con-
       nected 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 sock-
       ets.

       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> header file for the IP pro-
       tocol.

COLOPHON
       This page is part of release 3.53 of the Linux man-pages project.  A  description  of  the
       project,     and    information    about    reporting    bugs,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.



Linux                                       2012-05-10                                     RAW(7)

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