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.
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
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
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.
isn't set, then IP header options can be set on raw sockets
; 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
For sending and receiving datagrams (sendto(2)
similar), raw sockets use the standard sockaddr_in
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,
is set to zero.
Raw socket options can be set with setsockopt(2)
and read with
by passing the IPPROTO_RAW
- 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
In addition, all ip(7) IPPROTO_IP
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 IP_RECVERR
flag is enabled. For connected sockets,
are passed for compatibility. With
, 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.
- Invalid argument.
- 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.
- 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 CAP_NET_RAW
attribute may do that.
- An ICMP error has arrived reporting a parameter
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.
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
file, see ip(7)
When turned off, raw sockets will fragment outgoing packets that exceed the
interface MTU. However, disabling it is not recommended for performance and
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
; see socket(7)
socket is send only. If you really want to receive all IP
packets, use a packet(7)
socket with the ETH_P_IP
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
ability was lost in Linux 2.2. The workaround is to use IP_HDRINCL
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
call is always used.
for path MTU discovery. RFC 791
header file for the IP protocol.
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