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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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