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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 4.13 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/.