Sets or get the IP options to be sent with every packet from this socket. The arguments are a pointer to a memory buffer containing the options and the option length. The setsockopt(2) call sets the IP options associated with a socket. The maximum option size for IPv4 is 40 bytes. See RFC 791 for the allowed options. When the initial connection request packet for a SOCK_STREAM socket contains IP options, the IP options will be set automatically to the options from the initial packet with routing headers reversed. Incoming packets are not allowed to change options after the connection is established. The processing of all incoming source routing options is disabled by default and can be enabled by using the accept_source_route sysctl. Other options like timestamps are still handled. For datagram sockets, IP options can be only set by the local user. Calling getsockopt(2) with IP_OPTIONS puts the current IP options used for sending into the supplied buffer.

Pass an IP_PKTINFO ancillary message that contains a pktinfo structure that supplies some information about the incoming packet. This only works for datagram oriented sockets. The argument is a flag that tells the socket whether the IP_PKTINFO message should be passed or not. The message itself can only be sent/retrieved as control message with a packet using recvmsg(2) or sendmsg(2).

ipi_ifindex is the unique index of the interface the packet was received on. ipi_spec_dst is the local address of the packet and ipi_addr is the destination address in the packet header. If IP_PKTINFO is passed to sendmsg(2) and ipi_spec_dst is not zero, then it is used as the local source address for the routing table lookup and for setting up IP source route options. When ipi_ifindex is not zero the primary local address of the interface specified by the index overwrites ipi_spec_dst for the routing table lookup.

If enabled the IP_TOS ancillary message is passed with incoming packets. It contains a byte which specifies the Type of Service/Precedence field of the packet header. Expects a boolean integer flag.

When this flag is set pass a IP_TTL control message with the time to live field of the received packet as a byte. Not supported for SOCK_STREAM sockets.

Pass all incoming IP options to the user in a IP_OPTIONS control message. The routing header and other options are already filled in for the local host. Not supported for SOCK_STREAM sockets.

Identical to IP_RECVOPTS but returns raw unprocessed options with timestamp and route record options not filled in for this hop.

Set or receive the Type-Of-Service (TOS) field that is sent with every IP packet originating from this socket. It is used to prioritize packets on the network. TOS is a byte. There are some standard TOS flags defined: IPTOS_LOWDELAY to minimize delays for interactive traffic, IPTOS_THROUGHPUT to optimize throughput, IPTOS_RELIABILITY to optimize for reliability, IPTOS_MINCOST should be used for "filler data" where slow transmission doesn't matter. At most one of these TOS values can be specified. Other bits are invalid and shall be cleared. Linux sends IPTOS_LOWDELAY datagrams first by default, but the exact behavior depends on the configured queueing discipline. Some high priority levels may require superuser privileges (the CAP_NET_ADMIN capability). The priority can also be set in a protocol independent way by the (SOL_SOCKET, SO_PRIORITY) socket option (see socket(7)).

Set or retrieve the current time to live field that is used in every packet sent from this socket.

If enabled the user supplies an IP header in front of the user data. Only valid for SOCK_RAW sockets. See raw(7) for more information. When this flag is enabled the values set by IP_OPTIONS, IP_TTL and IP_TOS are ignored.

Enable extended reliable error message passing. When enabled on a datagram socket all generated errors will be queued in a per-socket error queue. When the user receives an error from a socket operation the errors can be received by calling recvmsg(2) with the MSG_ERRQUEUE flag set. The sock_extended_err structure describing the error will be passed in an ancillary message with the type IP_RECVERR and the level IPPROTO_IP. This is useful for reliable error handling on unconnected sockets. The received data portion of the error queue contains the error packet.

The IP_RECVERR control message contains a sock_extended_err structure:

ee_errno contains the errno number of the queued error. ee_origin is the origin code of where the error originated. The other fields are protocol-specific. The macro SO_EE_OFFENDER returns a pointer to the address of the network object where the error originated from given a pointer to the ancillary message. If this address is not known, the sa_family member of the sockaddr contains AF_UNSPEC and the other fields of the sockaddr are undefined.

IP uses the sock_extended_err structure as follows: ee_origin is set to SO_EE_ORIGIN_ICMP for errors received as an ICMP packet, or SO_EE_ORIGIN_LOCAL for locally generated errors. Unknown values should be ignored. ee_type and ee_code are set from the type and code fields of the ICMP header. ee_info contains the discovered MTU for EMSGSIZE errors. The message also contains the sockaddr_in of the node caused the error, which can be accessed with the SO_EE_OFFENDER macro. The sin_family field of the SO_EE_OFFENDER address is AF_UNSPEC when the source was unknown. When the error originated from the network, all IP options (IP_OPTIONS, IP_TTL, etc.) enabled on the socket and contained in the error packet are passed as control messages. The payload of the packet causing the error is returned as normal payload. Note that TCP has no error queue; MSG_ERRQUEUE is illegal on SOCK_STREAM sockets. IP_RECVERR is valid for TCP, but all errors are returned by socket function return or SO_ERROR only.

For raw sockets, IP_RECVERR enables passing of all received ICMP errors to the application, otherwise errors are only reported on connected sockets

It sets or retrieves an integer boolean flag. IP_RECVERR defaults to off.

Sets or receives the Path MTU Discovery setting for a socket. When enabled, Linux will perform Path MTU Discovery as defined in RFC 1191 on this socket. The don't fragment flag is set on all outgoing datagrams. The system-wide default is controlled by the ip_no_pmtu_disc sysctl for SOCK_STREAM sockets, and disabled on all others. For non-SOCK_STREAM sockets it is the user's responsibility to packetize the data in MTU sized chunks and to do the retransmits if necessary. The kernel will reject packets that are bigger than the known path MTU if this flag is set (with EMSGSIZE ).

Retrieve the current known path MTU of the current socket. Only valid when the socket has been connected. Returns an integer. Only valid as a getsockopt(2).

Pass all to-be forwarded packets with the IP Router Alert option set to this socket. Only valid for raw sockets. This is useful, for instance, for user space RSVP daemons. The tapped packets are not forwarded by the kernel, it is the users responsibility to send them out again. Socket binding is ignored, such packets are only filtered by protocol. Expects an integer flag.

Set or reads the time-to-live value of outgoing multicast packets for this socket. It is very important for multicast packets to set the smallest TTL possible. The default is 1 which means that multicast packets don't leave the local network unless the user program explicitly requests it. Argument is an integer.

Sets or reads a boolean integer argument whether sent multicast packets should be looped back to the local sockets.

Join a multicast group. Argument is an ip_mreqn structure.

imr_multiaddr contains the address of the multicast group the application wants to join or leave. It must be a valid multicast address. imr_address is the address of the local interface with which the system should join the multicast group; if it is equal to INADDR_ANY an appropriate interface is chosen by the system. imr_ifindex is the interface index of the interface that should join/leave the imr_multiaddr group, or 0 to indicate any interface.

For compatibility, the old ip_mreq structure is still supported. It differs from ip_mreqn only by not including the imr_ifindex field. Only valid as a setsockopt(2).

Leave a multicast group. Argument is an ip_mreqn or ip_mreq structure similar to IP_ADD_MEMBERSHIP.

Set the local device for a multicast socket. Argument is an ip_mreqn or ip_mreq structure similar to IP_ADD_MEMBERSHIP.

When an invalid socket option is passed, ENOPROTOOPT is returned.

[New with kernel 2.2.13; in earlier kernel versions this feature was controlled at compile time by the CONFIG_IP_ALWAYS_DEFRAG option; this option is not present in 2.4.x and later]

When this boolean frag is enabled (not equal 0) incoming fragments (parts of IP packets that arose when some host between origin and destination decided that the packets were too large and cut them into pieces) will be reassembled (defragmented) before being processed, even if they are about to be forwarded.

Only enable if running either a firewall that is the sole link to your network or a transparent proxy; never ever use it for a normal router or host. Otherwise fragmented communication can be disturbed if the fragments travel over different links. Defragmentation also has a large memory and CPU time cost.

This is automagically turned on when masquerading or transparent proxying are configured.

Not documented.

Set the default time-to-live value of outgoing packets. This can be changed per socket with the IP_TTL option.

Enable dynamic socket address and masquerading entry rewriting on interface address change. This is useful for dialup interface with changing IP addresses. 0 means no rewriting, 1 turns it on and 2 enables verbose mode.

Enable IP forwarding with a boolean flag. IP forwarding can be also set on a per interface basis.

Contains two integers that define the default local port range allocated to sockets. Allocation starts with the first number and ends with the second number. Note that these should not conflict with the ports used by masquerading (although the case is handled). Also arbitrary choices may cause problems with some firewall packet filters that make assumptions about the local ports in use. First number should be at least >1024, better >4096 to avoid clashes with well known ports and to minimize firewall problems.

If enabled, don't do Path MTU Discovery for TCP sockets by default. Path MTU discovery may fail if misconfigured firewalls (that drop all ICMP packets) or misconfigured interfaces (e.g., a point-to-point link where the both ends don't agree on the MTU) are on the path. It is better to fix the broken routers on the path than to turn off Path MTU Discovery globally, because not doing it incurs a high cost to the network.

If set, allows processes to bind(2) to non-local IP addresses, which can be quite useful, but may break some applications.

Time in seconds to keep an IPv6 fragment in memory.

Regeneration interval (in seconds) of the hash secret (or lifetime for the hash secret) for IPv6 fragments.

If the amount of queued IP fragments reaches ipfrag_high_thresh, the queue is pruned down to ipfrag_low_thresh. Contains an integer with the number of bytes.

See arp(7).