Section: Linux Programmer's Manual (7)
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raw - Linux IPv4 raw sockets
raw_socket = socket(AF_INET, SOCK_RAW, int protocol);
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
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
capability are allowed to open raw sockets.
All packets or errors matching the
for the raw socket are passed to this socket.
For a list of the allowed protocols see RFC 1700 assigned numbers and
A protocol of
and is able to send any IP protocol that is specified in the passed
Receiving of all IP protocols via
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.|
is specified and the IP header has a nonzero destination address, then
the destination address of the socket is used to route the packet.
is specified, the destination address should refer to a local interface,
otherwise a routing table lookup is done anyway but gatewayed routes
isn't set, then IP header options can be set on raw sockets with
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 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).
Raw sockets use the standard
address structure defined in
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,
is set to the protocol of the packet.
include file for valid IP protocols.
Raw socket options can be set with
and read with
by passing the
Enable a special filter for raw sockets bound to the
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
socket options valid for datagram sockets are supported.
Errors originating from the network are passed to the user only when the
socket is connected or the
flag is enabled.
For connected sockets, only
are passed for compatibility.
all network errors are saved in the error queue.
User tried to send to a broadcast address without having the
broadcast flag set on the socket.
An invalid memory address was supplied.
Packet too big.
Either Path MTU Discovery is enabled (the
socket flag) or the packet size exceeds the maximum allowed IPv4
packet size of 64KB.
Invalid flag has been passed to a socket call (like
The user doesn't have permission to open raw sockets.
Only processes with an effective user ID of 0 or the
attribute may do that.
An ICMP error has arrived reporting a parameter problem.
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
socket option was set --- since Linux 2.2,
this option no longer has that effect.
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
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
socket option or the
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
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
socket is send only.
If you really want to receive all IP packets, use a
socket with the
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
on that particular socket; see
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
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
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
this ability was lost in Linux 2.2.
The workaround is to use
Transparent proxy extensions are not described.
option is set, datagrams will not be fragmented and are limited to
the interface MTU.
Setting the IP protocol for sending in
got lost in Linux 2.2.
The protocol that the socket was bound to or that
was specified in the initial
call is always used.
for path MTU discovery.
header file for the IP protocol.
This page is part of release 3.74 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
- Address format
- Socket options
- Error handling
- SEE ALSO