|RAW(7)||Linux Programmer's Manual||RAW(7)|
NAMEraw - Linux IPv4 raw sockets
DESCRIPTIONRaw 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.
|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.
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).
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 formatRaw 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.
Socket optionsRaw socket options can be set with setsockopt(2) and read with getsockopt(2) by passing the IPPROTO_RAW family flag.
- 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 handlingErrors 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.
- 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 64KB.
- Invalid flag has been passed to a socket call (like MSG_OOB).
- 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 problem.
VERSIONSIP_RECVERR and ICMP_FILTER are new in Linux 2.2. They are Linux extensions and should not be used in portable programs.
NOTESBy 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.
BUGSTransparent proxy extensions are not described.
SEE ALSOrecvmsg(2), sendmsg(2), capabilities(7), ip(7), socket(7)
COLOPHONThis page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.