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.
Only processes with an effective user id of 0 or the CAP_NET_RAW
capability are allowed to open raw sockets.
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
RFC1700 assigned 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
Always filled in.
Filled in when zero.
Filled in when zero.
Always filled in.
If IP_HDRINCL is specified and the IP header has a non-zero 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 anyways
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.
In Linux 2.2 all IP header fields and options can be set using IP socket
options. This means raw sockets are usually only needed 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
Raw sockets use the standard sockaddr_in address structure defined in
ip(7). The The sin_port field could be used to specify the IP
protocol number, but it is ignored for sending in Linux 2.2 and should be
always set to 0 (see BUGS) For incoming packets sin_port is set to the
protocol of the packet. See the <netinet/in.h> include file for
valid IP protocols.
Raw sockets fragment a packet when its total length exceeds the interface MTU
(but see BUGS). A more network friendly and faster alternative is to implement
path MTU discovery as described in the IP_MTU_DISCOVER section of
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
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. Work around is to use IP_HDRINCL.
Errors originating from the network are only passed to the user 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.