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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  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 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.

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_MTU_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 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. 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_MTU_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 sockets. 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 programs.

       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 written by Andi Kleen.

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.