Section (7) raw
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_zsingle_quotesz_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.
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_zsingle_quotesz_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_zsingle_quotesz_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_zsingle_quotesz_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.
SEE ALSO
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.
COLOPHON
This page is part of release 4.16 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
t This man page is Copyright (C) 1999 Andi Kleen <akmuc.de>. %%%LICENSE_START(VERBATIM_ONE_PARA) Permission is granted to distribute possibly modified copies of this page provided the header is included verbatim, and in case of nontrivial modification author and date of the modification is added to the header. %%%LICENSE_END $Id: raw.7,v 1.6 1999/06/05 10:32:08 freitag Exp $ |
Section (8) raw
Name
raw — bind a Linux raw character device
Synopsis
raw
/dev/raw/raw<N>
<major> <minor>
raw
/dev/raw/raw<N>
/dev/<blockdev>
raw -q
/dev/raw/raw<N>
raw -qa
DESCRIPTION
raw is used to bind a Linux raw character device to a block device. Any block device may be used: at the time of binding, the device driver does not even have to be accessible (it may be loaded on demand as a kernel module later).
raw is used
in two modes: it either sets raw device bindings, or it
queries existing bindings. When setting a raw device,
/dev/raw/raw<N>
is the device name of an existing raw device node in the
filesystem. The block device to which it is to be bound can
be specified either in terms of its major
and minor
device numbers, or as a
path name /dev/<blockdev>
to an
existing block device file.
The bindings already in existence can be queried with the
−q
option, which is used
either with a raw device filename to query that one device,
or with the −a
option to
query all bound raw devices.
Unbinding can be done by specifying major and minor 0.
Once bound to a block device, a raw device can be opened, read and written, just like the block device it is bound to. However, the raw device does not behave exactly like the block device. In particular, access to the raw device bypasses the kernel_zsingle_quotesz_s block buffer cache entirely: all I/O is done directly to and from the address space of the process performing the I/O. If the underlying block device driver can support DMA, then no data copying at all is required to complete the I/O.
Because raw I/O involves direct hardware access to a process_zsingle_quotesz_s memory, a few extra restrictions must be observed. All I/Os must be correctly aligned in memory and on disk: they must start at a sector offset on disk, they must be an exact number of sectors long, and the data buffer in virtual memory must also be aligned to a multiple of the sector size. The sector size is 512 bytes for most devices.
OPTIONS
−q
,−−query
-
Set query mode. raw will query an existing binding instead of setting a new one.
−a
,−−all
-
With
−q
, specify that all bound raw devices should be queried. −h
,−−help
-
Display help text and exit.
−V
,−−version
-
Display version information and exit.
BUGS
The Linux dd(1) command should be
used without the bs=
option, or the blocksize
needs to be a multiple of the sector size of the device (512
bytes usually), otherwise it will fail with Invalid
Argument messages (EINVAL).
Raw I/O devices do not maintain cache coherency with the Linux block device buffer cache. If you use raw I/O to overwrite data already in the buffer cache, the buffer cache will no longer correspond to the contents of the actual storage device underneath. This is deliberate, but is regarded either a bug or a feature depending on who you ask!
NOTES
Rather than using raw devices applications should prefer open(2) devices, such as /dev/sda1, with the O_DIRECT flag.