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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 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.