.\" $OpenBSD: ip.4,v 1.29 2011/02/03 16:08:24 otto Exp $ .\" $NetBSD: ip.4,v 1.3 1994/11/30 16:22:19 jtc Exp $ .\" .\" Copyright (c) 1983, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)ip.4 8.2 (Berkeley) 11/30/93 .\" .Dd $Mdocdate: February 3 2011 $ .Dt IP 4 .Os .Sh NAME .Nm ip .Nd Internet Protocol .Sh SYNOPSIS .Fd #include .Fd #include .Ft int .Fn socket AF_INET SOCK_RAW proto .Sh DESCRIPTION .Tn IP is the network layer protocol used by the Internet protocol family. Options may be set at the .Tn IP level when using higher-level protocols that are based on .Tn IP (such as .Tn TCP and .Tn UDP ) . It may also be accessed through a .Dq raw socket when developing new protocols, or special-purpose applications. .Pp There are several .Tn IP-level .Xr setsockopt 2 Ns / Ns Xr getsockopt 2 options. .Dv IP_OPTIONS may be used to provide .Tn IP options to be transmitted in the .Tn IP header of each outgoing packet or to examine the header options on incoming packets. .Tn IP options may be used with any socket type in the Internet family. The format of .Tn IP options to be sent is that specified by the .Tn IP protocol specification (RFC 791), 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. To disable previously specified options, use a zero-length buffer: .Bd -literal -offset indent setsockopt(s, IPPROTO_IP, IP_OPTIONS, NULL, 0); .Ed .Pp .Dv IP_TOS and .Dv IP_TTL may be used to set the type-of-service and time-to-live fields in the .Tn IP header for .Dv SOCK_STREAM and .Dv SOCK_DGRAM sockets. For example, .Bd -literal -offset indent int tos = IPTOS_LOWDELAY; /* see */ setsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos)); int ttl = 60; /* max = 255 */ setsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl)); .Ed .Pp If the .Dv IP_RECVDSTADDR option is enabled on a .Dv SOCK_DGRAM socket, the .Xr recvmsg 2 call will return the destination .Tn IP address for a .Tn UDP datagram. The .Va msg_control field in the .Vt msghdr structure points to a buffer that contains a .Vt cmsghdr structure followed by the .Tn IP address. The .Vt cmsghdr fields have the following values: .Bd -literal -offset indent cmsg_len = CMSG_LEN(sizeof(struct in_addr)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVDSTADDR .Ed .Pp If the .Dv IP_RECVDSTPORT option is enabled on a .Dv SOCK_DGRAM socket, the .Xr recvmsg 2 call will return the destination port for a .Tn UDP datagram. The .Va msg_control field in the .Vt msghdr structure points to a buffer that contains a .Vt cmsghdr structure followed by the port in 16-bit network byte order. The .Vt cmsghdr fields have the following values: .Bd -literal -offset indent cmsg_len = CMSG_LEN(sizeof(u_int16_t)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVDSTPORT .Ed .Pp If the .Dv IP_RECVTTL option is enabled on a .Dv SOCK_DGRAM or .Dv SOCK_RAW socket, the .Xr recvmsg 2 call will return the .Tn TTL of the received datagram. The .Va msg_control field in the .Vt msghdr structure points to a buffer that contains a .Vt cmsghdr structure followed by the .Tn TTL value. The .Vt cmsghdr fields have the following values: .Bd -literal -offset indent cmsg_len = CMSG_LEN(sizeof(u_int8_t)) cmsg_level = IPPROTO_IP cmsg_type = IP_RECVTTL .Ed .Pp The .Dv IP_MINTTL option may be used on .Dv SOCK_STREAM sockets to discard packets with a TTL lower than the option value. This can be used to implement the .Em Generalized TTL Security Mechanism (GTSM) according to RFC 3682. To discard all packets with a TTL lower than 255: .Bd -literal -offset indent int minttl = 255; setsockopt(s, IPPROTO_IP, IP_MINTTL, &minttl, sizeof(minttl)); .Ed .Pp The .Dv IP_PORTRANGE option causes the default allocation policy for when the kernel is asked to choose a free port number. Three choices are available: .Pp .Bl -tag -width IP_PORTRANGE_DEFAULT -compact -offset indent .It Dv IP_PORTRANGE_DEFAULT The regular range of non-reserved ports. .It Dv IP_PORTRANGE_HIGH A high range, for fun. .It Dv IP_PORTRANGE_LOW Reserved ports; between 600 and 1023. .El .Ss "Multicast Options" .Tn IP multicasting is supported only on .Dv AF_INET sockets of type .Dv SOCK_DGRAM and .Dv SOCK_RAW , and only on networks where the interface driver supports multicasting. .Pp The .Dv IP_MULTICAST_TTL option changes the time-to-live (TTL) for outgoing multicast datagrams in order to control the scope of the multicasts: .Bd -literal -offset indent u_char ttl; /* range: 0 to 255, default = 1 */ setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl)); .Ed .Pp 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 network, 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. .Pp For hosts with multiple interfaces, each multicast transmission is sent from the primary network interface. The .Dv IP_MULTICAST_IF option overrides the default for subsequent transmissions from a given socket: .Bd -literal -offset indent struct in_addr addr; setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr)); .Ed .Pp where .Va addr is the local .Tn IP address of the desired interface or .Dv INADDR_ANY to specify the default interface. An interface's local IP address and multicast capability can be obtained via the .Dv SIOCGIFCONF and .Dv SIOCGIFFLAGS .Xr ioctl 2 Ns 's . Normal applications should not need to use this option. .Pp 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 .Dv IP_MULTICAST_LOOP option gives the sender explicit control over whether or not subsequent datagrams are looped back: .Bd -literal -offset indent u_char loop; /* 0 = disable, 1 = enable (default) */ setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop)); .Ed .Pp This option improves performance for applications that may have no more than one instance on a single host (such as a router daemon), 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). .Pp A multicast datagram sent with an initial TTL greater than 1 may be delivered 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. .Pp 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 .Dv IP_ADD_MEMBERSHIP option: .Bd -literal -offset indent struct ip_mreq mreq; setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq)); .Ed .Pp where .Fa mreq is the following structure: .Bd -literal -offset indent struct ip_mreq { struct in_addr imr_multiaddr; /* multicast group to join */ struct in_addr imr_interface; /* interface to join on */ } .Ed .Pp .Va imr_interface should be .Dv INADDR_ANY to choose the default multicast interface, or the .Tn IP address of a particular multicast-capable interface if the host is multihomed. Membership is associated with a single interface; programs running on multihomed hosts may need to join the same group on more than one interface. Up to .Dv IP_MAX_MEMBERSHIPS (currently 4095) memberships may be added on a single socket. .Pp To drop a membership, use: .Bd -literal -offset indent struct ip_mreq mreq; setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq)); .Ed .Pp where .Fa mreq contains the same values as used to add the membership. Memberships are dropped when the socket is closed or the process exits. .\"----------------------- .Ss "Raw IP Sockets" Raw .Tn IP sockets are connectionless, and are normally used with the .Xr sendto 2 and .Xr recvfrom 2 calls, though the .Xr connect 2 call may also be used to fix the destination for future packets (in which case the .Xr read 2 or .Xr recv 2 and .Xr write 2 or .Xr send 2 system calls may be used). .Pp If .Fa proto is 0, the default protocol .Dv IPPROTO_RAW is used for outgoing packets, and only incoming packets destined for that protocol are received. If .Fa proto is non-zero, that protocol number will be used on outgoing packets and to filter incoming packets. .Pp Outgoing packets automatically have an .Tn IP header prepended to them (based on the destination address and the protocol number the socket is created with), unless the .Dv IP_HDRINCL option has been set. Incoming packets are received with .Tn IP header and options intact. .Pp .Dv IP_HDRINCL indicates the complete IP header is included with the data and may be used only with the .Dv SOCK_RAW type. .Bd -literal -offset indent #include int hincl = 1; /* 1 = on, 0 = off */ setsockopt(s, IPPROTO_IP, IP_HDRINCL, &hincl, sizeof(hincl)); .Ed .Pp Unlike previous .Bx releases, the program must set all the fields of the IP header, including the following: .Bd -literal -offset indent ip->ip_v = IPVERSION; ip->ip_hl = hlen >> 2; ip->ip_id = 0; /* 0 means kernel set appropriate value */ ip->ip_off = htons(offset); ip->ip_len = htons(len); .Ed .Pp Additionally note that starting with .Ox 2.1 , the .Va ip_off and .Va ip_len fields are in network byte order. If the header source address is set to .Dv INADDR_ANY , the kernel will choose an appropriate address. .Sh DIAGNOSTICS A socket operation may fail with one of the following errors returned: .Bl -tag -width [EADDRNOTAVAIL] .It Bq Er EISCONN when trying to establish a connection on a socket which already has one, or when trying to send a datagram with the destination address specified and the socket is already connected; .It Bq Er ENOTCONN when trying to send a datagram, but no destination address is specified, and the socket hasn't been connected; .It Bq Er ENOBUFS when the system runs out of memory for an internal data structure; .It Bq Er EADDRNOTAVAIL when an attempt is made to create a socket with a network address for which no network interface exists. .It Bq Er EACCES when an attempt is made to create a raw IP socket by a non-privileged process. .El .Pp The following errors specific to .Tn IP may occur when setting or getting .Tn IP options: .Bl -tag -width EADDRNOTAVAILxx .It Bq Er EINVAL An unknown socket option name was given. .It Bq Er EINVAL The IP option field was improperly formed; an option field was shorter than the minimum value or longer than the option buffer provided. .El .Sh SEE ALSO .Xr getsockopt 2 , .Xr ioctl 2 , .Xr recv 2 , .Xr send 2 , .Xr icmp 4 , .Xr inet 4 , .Xr netintro 4 .Sh HISTORY The .Nm protocol appeared in .Bx 4.2 .