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NAME |SYNOPSIS |DESCRIPTION |ERRORS |VERSIONS |NOTES |BUGS |SEE ALSO |COLOPHON | |
raw(7) Miscellaneous Information Manualraw(7)raw - Linux IPv4 raw sockets
#include <sys/socket.h>#include <netinet/in.h>raw_socket = socket(AF_INET, SOCK_RAW, intprotocol);
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 theIP_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 theCAP_NET_RAWcapability in the user namespace that governs its network namespace. All packets or errors matching theprotocol 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/⟩ andgetprotobyname(3). A protocol ofIPPROTO_RAWimplies enabledIP_HDRINCLand is able to send any IP protocol that is specified in the passed header. Receiving of all IP protocols viaIPPROTO_RAWis not possible using raw sockets. ┌────────────────────────────────────────────────────┐ │ IP Header fields modified on sending byIP_HDRINCL│ ├───────────────────────┬────────────────────────────┤ │ IP Checksum │ Always filled in │ ├───────────────────────┼────────────────────────────┤ │ Source Address │ Filled in when zero │ ├───────────────────────┼────────────────────────────┤ │ Packet ID │ Filled in when zero │ ├───────────────────────┼────────────────────────────┤ │ Total Length │ Always filled in │ └───────────────────────┴────────────────────────────┘ IfIP_HDRINCLis specified and the IP header has a nonzero destination address, then the destination address of the socket is used to route the packet. WhenMSG_DONTROUTEis specified, the destination address should refer to a local interface, otherwise a routing table lookup is done anyway but gatewayed routes are ignored. IfIP_HDRINCLisn't set, then IP header options can be set on raw sockets withsetsockopt(2); seeip(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 standardsockaddr_in address structure defined inip(7). Thesin_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 withsetsockopt(2) and read withgetsockopt(2) by passing theIPPROTO_RAWfamily flag.ICMP_FILTER Enable a special filter for raw sockets bound to theIPPROTO_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, allip(7)IPPROTO_IPsocket 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 theIP_RECVERRflag is enabled. For connected sockets, onlyEMSGSIZEandEPROTOare passed for compatibility. WithIP_RECVERR, all network errors are saved in the error queue.
EACCESUser tried to send to a broadcast address without having the broadcast flag set on the socket.EFAULTAn invalid memory address was supplied.EINVALInvalid argument.EMSGSIZE Packet too big. Either Path MTU Discovery is enabled (theIP_MTU_DISCOVERsocket 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 (likeMSG_OOB).EPERMThe user doesn't have permission to open raw sockets. Only processes with an effective user ID of 0 or theCAP_NET_RAW attribute may do that.EPROTOAn ICMP error has arrived reporting a parameter problem.
IP_RECVERRandICMP_FILTERare 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 theSO_BSDCOMPATsocket option was set; since Linux 2.2, this option no longer has that effect.
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 returnEMSGSIZEwhen 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 theIP_MTU_DISCOVERsocket option or the/proc/sys/net/ipv4/ip_no_pmtu_disc file, seeip(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 thebind(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 usingSO_BINDTODEVICE; seesocket(7). AnIPPROTO_RAWsocket is send only. If you really want to receive all IP packets, use apacket(7) socket with theETH_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 useIP_RECVERRon that particular socket; seeip(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 forIP_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 fromsin_port; this ability was lost in Linux 2.2. The workaround is to useIP_HDRINCL.
Transparent proxy extensions are not described. When theIP_HDRINCLoption is set, datagrams will not be fragmented and are limited to the interface MTU. Setting the IP protocol for sending insin_port got lost in Linux 2.2. The protocol that the socket was bound to or that was specified in the initialsocket(2) call is always used.
recvmsg(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.
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17raw(7)Pages that refer to this page:icmp(7), ip(7), packet(7), udp(7)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |