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ip(7) Miscellaneous Information Manual ip(7) NAME ip - Internet Protocol SYNOPSIS #include <sys/socket.h> #include <netinet/in.h> s = socket(AF_INET, SOCK_RAW, proto); DESCRIPTION The Internet Protocol (IP) is the transport layer protocol used by the Internet Protocol family. Options may be set at the IP level when us- ing higher-level protocols that are based on IP (such as the Transmis- sion Control Protocol (TCP) and the User Datagram Package (UDP)). It may also be accessed through a raw socket when developing new proto- cols, or special purpose applications. IP_OPTIONS is used to provide IP options to be transmitted in the IP header of each outgoing packet. Options are set with the setsockopt() function and examined with the getsockopt() function. The format of IP options to be sent is that specified by the IP specification, with one exception: the list of addresses for Source Route options must include the first-hop gateway at the beginning of the list of gateways. The first-hop gateway address will be extracted from the option list and the size adjusted accordingly before use. IP options may be used with any socket type in the Internet family. Other options supported by the getsocket() and setsocket() functions can be found in the <netinet/in.h> header file. Raw IP sockets are connectionless, and are normally used with the sendto() and recvfrom() calls, though the connect() call may also be used to fix the destination for future packets, in which case the read() or recv() and write() or send() functions may be used. If proto is 0 (zero), the default protocol IPPROTO_RAW is used for out- going packets, and only incoming packets destined for that protocol are received. If proto is nonzero, that protocol number will be used on outgoing packets and to filter incoming packets. Outgoing packets automatically have an IP header prepended to them (based on the destination address and the protocol number the socket is created with), unless the IP_HDRINCL option is set. IP_HDRINCL speci- fies whether the IP header is provided by the sent packet. Incoming packets are received with IP header and options intact. Multicasting IP multicasting is supported only on AF_INET sockets of type SOCK_DGRAM and SOCK_RAW, and only on networks where the interface driver supports multicasting. The IP_MULTICAST_TTL option changes the time-to-live (TTL) for outgoing multicast datagrams in order to control the scope of the multicasts; for example: u_char ttl; /* range: 0 to 255, default = 1 */ set- sockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl)); Datagrams with a TTL of 1 are not forwarded beyond the local network. Multicast datagrams with a TTL of 0 will not be transmitted on any net- work, but may be delivered locally if the sending host belongs to the destination group and if multicast loopback has not been disabled on the sending socket (see below). Multicast datagrams with TTL greater than 1 may be forwarded to other networks if a multicast router is attached to the local network. For hosts with multiple interfaces, each multicast transmission is sent from the primary network interface. The IP_MULTICAST_IF option over- rides the default for subsequent transmissions from a given socket: struct in_addr addr; setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr)); The addr parameter specifies the local IP address of the desired interface or INADDR_ANY to specify the default interface. An interface's local IP address and multicast capability can be obtained through the SIOCGIFCONF and SIOCGIFLAGS ioctls. Normal applications should not need to use this option. If a multicast datagram is sent to a group to which the sending host itself belongs (on the outgoing interface), a copy of the datagram is, by default, looped back by the IP layer for local delivery. The IP_MULTICAST_LOOP option gives the sender explicit control over whether or not subsequent datagrams are looped back, for example: u_char loop; /* 0 = disable, 1 = enable (default) */ setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop)); This option improves perfor- mance for applications that may have no more than one instance on a single host (such as a router demon), by eliminating the overhead of receiving their own transmissions. It should generally not be used by applications for which there may be more than one instance on a single host (such as a conferencing program) or for which the sender does not belong to the destination group (such as a time querying program). A multicast datagram sent with an initial TTL greater than 1 may be de- livered to the sending host on a different interface from that on which it was sent, if the host belongs to the destination group on that other interface. The loopback control option has no effect on such delivery. A host must become a member of a multicast group before it can receive datagrams sent to the group. To join a multicast group, use the IP_ADD_MEMBERSHIP option, for example: struct ip_mreq mreq; setsock- opt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq)); The mreq parameter is the following structure: struct ip_mreq { struct in_addr imr_multiaddr; /* multicast group to join */ struct in_addr imr_interface; /* interface to join on */ } The imr_interface should be INADDR_ANY to choose the default multicast in- terface, or the IP address of a particular multicast-capable interface if the host is multihomed. Membership is associated with a single in- terface; programs running on multihomed hosts may need to join the same group on more than one interface. Up to IP_MAX_MEMBERSHIPS (currently 20) memberships may be added on a single socket. To drop a membership, use the following: struct ip_mreq mreq; setsock- opt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq)); The mreq parameter contains the same values as used to add the membership. Mem- berships are dropped when the socket is closed or the process exits. The IP_RECVDSTADDR option enables a SOCK_DGRAM socket to receive the destination IP address for a UDP datagram. The IP_RECVOPTS option en- ables a SOCK_DGRAM socket to receive the Internet Protocol options. ERRORS If a socket operation fails, errno may be set to one of the following values: The socket is already connected. This error occurs when trying to establish connection on a socket or when trying to send a datagram with the destination address specified. The destination address of a datagram was not specified, and the socket has not been connected. The system ran out of memory for an internal data structure. An attempt was made to create a socket with a network address for which no network interface exists. The following errors specific to IP may occur when setting or getting IP options: An unknown socket option name was given. The IP option field was improperly formed; an option field was shorter than the mini- mum value or longer than the option buffer provided. RELATED INFORMATION Functions: getsockopt(2), send(2), recv(2) Network Information: netintro(7), icmp(7), inet(7) delim off ip(7)