Ubuntu Manpage: raw - Linux IPv4 raw sockets

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.