man > raw(7)

raw(7)                            Miscellaneous Information 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  op‐
       tion  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 ]8;;http://www.iana.org/assignments/protocol-numbers/\http://www.iana.org/assignments/protocol-numbers/]8;;\ 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 pos‐
       sible 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 des‐
       tination 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  pro‐
       tocol 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 effec‐
              tive 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 mod‐
       ule 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  proto‐
       col.

Linux man-pages 6.7                          2023-10-31                                       raw(7)