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       raw, SOCK_RAW - Linux IPv4 raw sockets


       #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
       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 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 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.

              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.


       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
              maximum 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
              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.


       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.


       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

       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.


       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> include file for the IP protocol.


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