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RAW
Section: Linux Programmer's Manual (7)Updated: 2017-09-15
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NAME
raw - Linux IPv4 raw socketsSYNOPSIS
#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 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.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'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'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.
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 4.15 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/.
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Time: 04:45:57 GMT, September 16, 2022 Content-type: text/html; charset=UTF-8
RAW
Section: System Administration (8)Updated: August 1999
Index Return to Main Contents
NAME
raw - bind a Linux raw character deviceSYNOPSIS
raw /dev/raw/raw<N> <major> <minor>raw /dev/raw/raw<N> /dev/<blockdev>
raw -q /dev/raw/raw<N>
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'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'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.AUTHOR
Stephen Tweedie (sct@redhat.com)AVAILABILITY
The raw command is part of the util-linux package and is available from https://www.kernel.org/pub/linux/utils/util-linux/.
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This document was created by man2html, using the manual pages.
Time: 04:45:59 GMT, September 16, 2022
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