RAW(7)                    (2020-08-13)                     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 protocols, see the IANA list of assigned proto-
          col numbers at 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.
               tab(:) allbox; c s l l.  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 infor-
          mation.

          Starting with Linux 2.2, all IP header fields and options
          can be set using IP socket options.  This means raw sockets

     Page 1                        Linux             (printed 5/22/22)

     RAW(7)                    (2020-08-13)                     RAW(7)

          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.

          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 connected 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 hav-
               ing 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 maximum allowed IPv4 packet size of 64 kB.

     Page 2                        Linux             (printed 5/22/22)

     RAW(7)                    (2020-08-13)                     RAW(7)

          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 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 prob-
               lem.

     VERSIONS
          IP_RECVERR and ICMP_FILTER are new in Linux 2.2.  They are
          Linux extensions and should not be used in portable pro-
          grams.

          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) 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 dis-
          covery 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 perfor-
          mance 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 reassem-
          ble 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 partic-
          ular socket; see ip(7).

          Raw sockets may tap all IP protocols in Linux, even proto-
          cols like ICMP or TCP which have a protocol module in the

     Page 3                        Linux             (printed 5/22/22)

     RAW(7)                    (2020-08-13)                     RAW(7)

          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 wor-
          karound 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 5.10 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/.

     Page 4                        Linux             (printed 5/22/22)