raw(7) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       raw - Linux IPv4 raw sockets

SYNOPSIS         top

       #include <sys/socket.h>
       #include <netinet/in.h>
       raw_socket = socket(AF_INET, SOCK_RAW, int protocol);

DESCRIPTION         top

       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
       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.

       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.

       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         top

       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 maximum 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 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.

VERSIONS         top

       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.

NOTES         top

       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 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         top

       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         top

       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         top

       This page is part of release 5.11 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                          2021-03-22                         RAW(7)

Pages that refer to this page: icmp(7)ip(7)packet(7)udp(7)