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.  Receiv‐
       ing 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 destina‐
       tion 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 han‐
       dlers (e.g., kernel protocol modules).

   Address format
       For sending and receiving datagrams (sendto(2), recvfrom(2), and sim‐
       ilar), 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 pack‐
       ets, 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 con‐
       nected sockets, only EMSGSIZE and EPROTO are passed for compatibil‐
       ity.  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 64KB.

       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 4.08 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                            2016-10-08                           RAW(7)