#include <sys/socket.h>
       #include <netinet/in.h>
       raw_socket = socket(AF_INET, SOCK_RAW, int protocol);

       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  capa-
       bility 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 proto-
       cols 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 sock-
       ets 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 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 han-
       opt(2) by passing the IPPROTO_RAW family flag.

              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  connected or the IP_RECVERR flag is enabled.  For con-
       nected sockets, only EMSGSIZE and EPROTO are passed for  compatibility.
       With IP_RECVERR, all network errors are saved in the error queue.

       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.

              Packet too big.  Either  Path  MTU  Discovery  is  enabled  (the
              IP_MTU_DISCOVER socket flag) or the packet size exceeds the max-
              imum allowed IPv4 packet size of 64KB.

              Invalid flag has been passed to a socket call (like MSG_OOB).

       EPERM  The user doesn't have permission to open raw sockets.  Only pro-
              cesses  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.

       IP_RECVERR and ICMP_FILTER are new in Linux 2.2.  They are Linux exten-
       sions 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.

       By default, raw sockets do path MTU (Maximum Transmission Unit) discov-
       ery.  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_DIS-
       COVER socket option or the /proc/sys/net/ipv4/ip_no_pmtu_disc file, see

       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.

       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.

       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 protocol.

       This  page  is  part of release 3.54 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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