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 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
       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
       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
       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
       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 5.11 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)