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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(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  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 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  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 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  han-
       dlers (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  proto-
       col  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 getsock-
       opt(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  con-
       nected  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 max-
              imum 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 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.

VERSIONS
       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.

NOTES
       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_DISCOVER
       socket  option  or  the  ip_no_pmtu_disc sysctl, 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 perfor-
       mance 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 3.05 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                             1998-10-02                            RAW(7)

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