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.

   In order to create a raw socket, a process must  have  the  CAP_NET_RAW
   capability in the user namespace that governs its network namespace.

   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   the  IANA  list  of  assigned  protocol  numbers  at
   http://www.iana.org/assignments/protocol-numbers/                 and

   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.

   Starting with 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
   handlers (e.g., kernel protocol modules).

   Address format
   For  sending  and  receiving  datagrams  (sendto(2),  recvfrom(2),  and
   similar),  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 later,
   and should be always set  to  0  (see  BUGS).   For  incoming  packets,
   sin_port is set to zero.

   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 passed to the  user  only  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>  header
   file for the IP protocol.


   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at


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