Manpage of RAW
RAWSection: Linux Programmer's Manual (7)
NAMEraw - Linux IPv4 raw sockets
raw_socket = socket(AF_INET, SOCK_RAW, int protocol);
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.
The IPv4 layer generates an IP header when sending a packet unless the IP_HDRINCLsocket 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_RAWcapability in the user namespace that governs its network namespace.
All packets or errors matching the protocolnumber 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_RAWimplies enabled IP_HDRINCLand is able to send any IP protocol that is specified in the passed header. Receiving of all IP protocols via IPPROTO_RAWis 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_HDRINCLis 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_DONTROUTEis specified, the destination address should refer to a local interface, otherwise a routing table lookup is done anyway but gatewayed routes are ignored.
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).
Address formatFor sending and receiving datagrams (sendto(2), recvfrom(2), and similar), raw sockets use the standard sockaddr_inaddress structure defined in ip(7). The sin_portfield 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_portis set to zero.
Socket optionsRaw socket options can be set with setsockopt(2) and read with getsockopt(2) by passing the IPPROTO_RAWfamily flag.
- Enable a special filter for raw sockets bound to the IPPROTO_ICMPprotocol. 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_IPsocket 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_RECVERRflag is enabled. For connected sockets, only EMSGSIZEand EPROTOare 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_DISCOVERsocket flag) or the packet size exceeds the maximum allowed IPv4 packet size of 64 kB.
- 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_RAWattribute may do that.
- An ICMP error has arrived reporting a parameter problem.
VERSIONSIP_RECVERRand ICMP_FILTERare 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 EMSGSIZEwhen 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_DISCOVERsocket option or the /proc/sys/net/ipv4/ip_no_pmtu_discfile, 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_RAWsocket is send only. If you really want to receive all IP packets, use a packet(7) socket with the ETH_P_IPprotocol. 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_RECVERRon 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.
BUGSTransparent proxy extensions are not described.
When the IP_HDRINCLoption is set, datagrams will not be fragmented and are limited to the interface MTU.
Setting the IP protocol for sending in sin_portgot 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 ALSOrecvmsg(2), sendmsg(2), capabilities(7), ip(7), socket(7)
RFC 1191for path MTU discovery. RFC 791and the <linux/ip.h>header file for the IP protocol.
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