Section: Linux Programmer's Manual (7)
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.
Only processes with an effective user ID of 0 or the CAP_NET_RAW capability are allowed to open raw sockets.
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.
:<TABLE BORDER> <TR VALIGN="top| <TD ALIGN="center" COLSPAN="2|IP Header fields modified on sending by IP_HDRINCL
</TD> </TR> <TR VALIGN="top| <TD>IP Checksum</TD> <TD>Always filled in.
</TD> </TR> <TR VALIGN="top| <TD>Source Address</TD> <TD>Filled in when zero.
</TD> </TR> <TR VALIGN="top| <TD>Packet Id</TD> <TD>Filled in when zero.
</TD> </TR> <TR VALIGN="top| <TD>Total Length</TD> <TD>Always filled in.
</TD> </TR> </TABLE> :
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.
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).
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 should be always set to 0 (see BUGS). For incoming packets, sin_port is set to the protocol of the packet. See the <netinet/in.h> include file for valid IP protocols.
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.
IP_RECVERR and ICMP_FILTER are new in Linux 2.2. They are Linux extensions and should not be used in portable programs.
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.
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.
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.
RFC 1191 for path MTU discovery. RFC 791 and the <linux/ip.h> header file for the IP protocol.
This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.
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