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NAME | SYNOPSIS | DESCRIPTION | ERRORS | VERSIONS | NOTES | BUGS | SEE ALSO | COLOPHON |
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raw(7) Miscellaneous Information Manual raw(7)
raw - Linux IPv4 raw sockets
#include <sys/socket.h>
#include <netinet/in.h>
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 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.
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.
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.
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.
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.
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.
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-05-02 raw(7)
Pages that refer to this page: icmp(7), ip(7), packet(7), udp(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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