raw

RAW(7)                     Linux Programmer's Manual                    RAW(7)

NAME
       raw - Linux IPv4 raw sockets

SYNOPSIS
       #include <sys/socket.h>
       #include <netinet/in.h>
       raw_socket = socket(AF_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.

       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 proto-
       cols,   see   the   IANA   list   of   assigned   protocol  numbers  at
       <http://www.iana.org/assignments/protocol-numbers/>   and   getprotoby-
       name(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 sock-
       ets 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 han-
       dlers (e.g., kernel protocol modules).

   Address format
       For sending and receiving datagrams (sendto(2), recvfrom(2), and  simi-
       lar),  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 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 passed to the  user  only  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 64 kB.

       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_DIS-
       COVER 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 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>  header
       file for the IP protocol.

COLOPHON
       This  page  is  part of release 4.15 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                            RAW(7)
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