662 lines
17 KiB
Groff
662 lines
17 KiB
Groff
.\" Copyright (c) 1983, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)ip.4 8.2 (Berkeley) 11/30/93
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.\" $FreeBSD$
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.\"
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.Dd September 26, 2005
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.Dt IP 4
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.Os
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.Sh NAME
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.Nm ip
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.Nd Internet Protocol
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/socket.h
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.In netinet/in.h
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.Ft int
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.Fn socket AF_INET SOCK_RAW proto
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.Sh DESCRIPTION
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.Tn IP
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is the transport layer protocol used
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by the Internet protocol family.
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Options may be set at the
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.Tn IP
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level
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when using higher-level protocols that are based on
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.Tn IP
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(such as
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.Tn TCP
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and
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.Tn UDP ) .
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It may also be accessed
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through a
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.Dq raw socket
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when developing new protocols, or
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special-purpose applications.
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.Pp
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There are several
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.Tn IP-level
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.Xr setsockopt 2
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and
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.Xr getsockopt 2
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options.
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.Dv IP_OPTIONS
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may be used to provide
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.Tn IP
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options to be transmitted in the
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.Tn IP
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header of each outgoing packet
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or to examine the header options on incoming packets.
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.Tn IP
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options may be used with any socket type in the Internet family.
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The format of
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.Tn IP
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options to be sent is that specified by the
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.Tn IP
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protocol specification (RFC-791), with one exception:
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the list of addresses for Source Route options must include the first-hop
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gateway at the beginning of the list of gateways.
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The first-hop gateway address will be extracted from the option list
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and the size adjusted accordingly before use.
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To disable previously specified options,
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use a zero-length buffer:
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.Bd -literal
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setsockopt(s, IPPROTO_IP, IP_OPTIONS, NULL, 0);
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.Ed
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.Pp
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.Dv IP_TOS
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and
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.Dv IP_TTL
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may be used to set the type-of-service and time-to-live
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fields in the
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.Tn IP
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header for
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.Dv SOCK_STREAM , SOCK_DGRAM ,
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and certain types of
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.Dv SOCK_RAW
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sockets.
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For example,
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.Bd -literal
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int tos = IPTOS_LOWDELAY; /* see <netinet/ip.h> */
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setsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos));
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int ttl = 60; /* max = 255 */
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setsockopt(s, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl));
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.Ed
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.Pp
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.Dv IP_MINTTL
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may be used to set the minimum acceptable TTL a packet must have when
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received on a socket.
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All packets with a lower TTL are silently dropped.
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This option is only really useful when set to 255, preventing packets
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from outside the directly connected networks reaching local listeners
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on sockets.
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.Pp
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.Dv IP_DONTFRAG
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may be used to set the Don't Fragment flag on IP packets.
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Currently this option is respected only on
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.Xr udp 4
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and raw
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.Xr ip 4
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sockets, unless the
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.Dv IP_HDRINCL
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option has been set.
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On
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.Xr tcp 4
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sockets, the Don't Fragment flag is controlled by the Path
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MTU Discovery option.
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Sending a packet larger than the MTU size of the egress interface,
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determined by the destination address, returns an
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.Er EMSGSIZE
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error.
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.Pp
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If the
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.Dv IP_RECVDSTADDR
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option is enabled on a
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.Dv SOCK_DGRAM
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socket,
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the
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.Xr recvmsg 2
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call will return the destination
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.Tn IP
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address for a
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.Tn UDP
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datagram.
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The
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.Vt msg_control
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field in the
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.Vt msghdr
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structure points to a buffer
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that contains a
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.Vt cmsghdr
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structure followed by the
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.Tn IP
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address.
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The
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.Vt cmsghdr
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fields have the following values:
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.Bd -literal
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cmsg_len = sizeof(struct in_addr)
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cmsg_level = IPPROTO_IP
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cmsg_type = IP_RECVDSTADDR
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.Ed
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.Pp
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The source address to be used for outgoing
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.Tn UDP
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datagrams on a socket that is not bound to a specific
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.Tn IP
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address can be specified as ancillary data with a type code of
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.Dv IP_SENDSRCADDR .
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The msg_control field in the msghdr structure should point to a buffer
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that contains a
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.Vt cmsghdr
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structure followed by the
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.Tn IP
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address.
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The cmsghdr fields should have the following values:
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.Bd -literal
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cmsg_len = sizeof(struct in_addr)
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cmsg_level = IPPROTO_IP
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cmsg_type = IP_SENDSRCADDR
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.Ed
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.Pp
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For convenience,
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.Dv IP_SENDSRCADDR
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is defined to have the same value as
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.Dv IP_RECVDSTADDR ,
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so the
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.Dv IP_RECVDSTADDR
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control message from
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.Xr recvmsg 2
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can be used directly as a control message for
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.Xr sendmsg 2 .
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.Pp
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If the
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.Dv IP_ONESBCAST
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option is enabled on a
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.Dv SOCK_DGRAM
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or a
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.Dv SOCK_RAW
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socket, the destination address of outgoing
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broadcast datagrams on that socket will be forced
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to the undirected broadcast address,
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.Dv INADDR_BROADCAST ,
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before transmission.
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This is in contrast to the default behavior of the
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system, which is to transmit undirected broadcasts
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via the first network interface with the
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.Dv IFF_BROADCAST flag set.
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.Pp
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This option allows applications to choose which
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interface is used to transmit an undirected broadcast
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datagram.
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For example, the following code would force an
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undirected broadcast to be transmitted via the interface
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configured with the broadcast address 192.168.2.255:
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.Bd -literal
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char msg[512];
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struct sockaddr_in sin;
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u_char onesbcast = 1; /* 0 = disable (default), 1 = enable */
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setsockopt(s, IPPROTO_IP, IP_ONESBCAST, &onesbcast, sizeof(onesbcast));
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sin.sin_addr.s_addr = inet_addr("192.168.2.255");
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sin.sin_port = htons(1234);
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sendto(s, msg, sizeof(msg), 0, &sin, sizeof(sin));
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.Ed
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.Pp
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It is the application's responsibility to set the
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.Dv IP_TTL option
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to an appropriate value in order to prevent broadcast storms.
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The application must have sufficient credentials to set the
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.Dv SO_BROADCAST
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socket level option, otherwise the
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.Dv IP_ONESBCAST option has no effect.
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.Pp
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If the
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.Dv IP_RECVTTL
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option is enabled on a
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.Dv SOCK_DGRAM
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socket, the
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.Xr recvmsg 2
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call will return the
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.Tn IP
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.Tn TTL
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(time to live) field for a
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.Tn UDP
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datagram.
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The msg_control field in the msghdr structure points to a buffer
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that contains a cmsghdr structure followed by the
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.Tn TTL .
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The cmsghdr fields have the following values:
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.Bd -literal
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cmsg_len = sizeof(u_char)
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cmsg_level = IPPROTO_IP
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cmsg_type = IP_RECVTTL
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.Ed
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.Pp
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If the
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.Dv IP_RECVIF
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option is enabled on a
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.Dv SOCK_DGRAM
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socket, the
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.Xr recvmsg 2
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call returns a
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.Vt "struct sockaddr_dl"
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corresponding to the interface on which the
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packet was received.
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The
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.Va msg_control
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field in the
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.Vt msghdr
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structure points to a buffer that contains a
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.Vt cmsghdr
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structure followed by the
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.Vt "struct sockaddr_dl" .
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The
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.Vt cmsghdr
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fields have the following values:
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.Bd -literal
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cmsg_len = sizeof(struct sockaddr_dl)
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cmsg_level = IPPROTO_IP
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cmsg_type = IP_RECVIF
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.Ed
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.Pp
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.Dv IP_PORTRANGE
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may be used to set the port range used for selecting a local port number
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on a socket with an unspecified (zero) port number.
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It has the following
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possible values:
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.Bl -tag -width IP_PORTRANGE_DEFAULT
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.It Dv IP_PORTRANGE_DEFAULT
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use the default range of values, normally
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.Dv IPPORT_HIFIRSTAUTO
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through
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.Dv IPPORT_HILASTAUTO .
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This is adjustable through the sysctl setting:
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.Va net.inet.ip.portrange.first
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and
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.Va net.inet.ip.portrange.last .
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.It Dv IP_PORTRANGE_HIGH
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use a high range of values, normally
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.Dv IPPORT_HIFIRSTAUTO
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and
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.Dv IPPORT_HILASTAUTO .
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This is adjustable through the sysctl setting:
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.Va net.inet.ip.portrange.hifirst
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and
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.Va net.inet.ip.portrange.hilast .
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.It Dv IP_PORTRANGE_LOW
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use a low range of ports, which are normally restricted to
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privileged processes on
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.Ux
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systems.
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The range is normally from
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.Dv IPPORT_RESERVED
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\- 1 down to
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.Li IPPORT_RESERVEDSTART
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in descending order.
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This is adjustable through the sysctl setting:
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.Va net.inet.ip.portrange.lowfirst
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and
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.Va net.inet.ip.portrange.lowlast .
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.El
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.Pp
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The range of privileged ports which only may be opened by
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root-owned processes may be modified by the
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.Va net.inet.ip.portrange.reservedlow
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and
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.Va net.inet.ip.portrange.reservedhigh
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sysctl settings.
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The values default to the traditional range,
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0 through
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.Dv IPPORT_RESERVED
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\- 1
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(0 through 1023), respectively.
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Note that these settings do not affect and are not accounted for in the
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use or calculation of the other
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.Va net.inet.ip.portrange
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values above.
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Changing these values departs from
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.Ux
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tradition and has security
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consequences that the administrator should carefully evaluate before
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modifying these settings.
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.Pp
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Ports are allocated at random within the specified port range in order
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to increase the difficulty of random spoofing attacks.
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In scenarios such as benchmarking, this behavior may be undesirable.
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In these cases,
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.Va net.inet.ip.portrange.randomized
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can be used to toggle randomization off.
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If more than
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.Va net.inet.ip.portrange.randomcps
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ports have been allocated in the last second, then return to sequential
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port allocation.
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Return to random allocation only once the current port allocation rate
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drops below
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.Va net.inet.ip.portrange.randomcps
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for at least
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.Va net.inet.ip.portrange.randomtime
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seconds.
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The default values for
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.Va net.inet.ip.portrange.randomcps
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and
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.Va net.inet.ip.portrange.randomtime
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are 10 port allocations per second and 45 seconds correspondingly.
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.Ss "Multicast Options"
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.Pp
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.Tn IP
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multicasting is supported only on
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.Dv AF_INET
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sockets of type
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.Dv SOCK_DGRAM
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and
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.Dv SOCK_RAW ,
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and only on networks where the interface
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driver supports multicasting.
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.Pp
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The
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.Dv IP_MULTICAST_TTL
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option changes the time-to-live (TTL)
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for outgoing multicast datagrams
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in order to control the scope of the multicasts:
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.Bd -literal
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u_char ttl; /* range: 0 to 255, default = 1 */
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setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));
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.Ed
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.Pp
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Datagrams with a TTL of 1 are not forwarded beyond the local network.
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Multicast datagrams with a TTL of 0 will not be transmitted on any network,
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but may be delivered locally if the sending host belongs to the destination
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group and if multicast loopback has not been disabled on the sending socket
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(see below).
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Multicast datagrams with TTL greater than 1 may be forwarded
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to other networks if a multicast router is attached to the local network.
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.Pp
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For hosts with multiple interfaces, each multicast transmission is
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sent from the primary network interface.
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The
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.Dv IP_MULTICAST_IF
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option overrides the default for
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subsequent transmissions from a given socket:
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.Bd -literal
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struct in_addr addr;
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setsockopt(s, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));
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.Ed
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.Pp
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where "addr" is the local
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.Tn IP
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address of the desired interface or
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.Dv INADDR_ANY
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to specify the default interface.
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An interface's local IP address and multicast capability can
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be obtained via the
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.Dv SIOCGIFCONF
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and
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.Dv SIOCGIFFLAGS
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ioctls.
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Normal applications should not need to use this option.
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.Pp
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If a multicast datagram is sent to a group to which the sending host itself
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belongs (on the outgoing interface), a copy of the datagram is, by default,
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looped back by the IP layer for local delivery.
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The
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.Dv IP_MULTICAST_LOOP
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option gives the sender explicit control
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over whether or not subsequent datagrams are looped back:
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.Bd -literal
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u_char loop; /* 0 = disable, 1 = enable (default) */
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setsockopt(s, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));
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.Ed
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.Pp
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This option
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improves performance for applications that may have no more than one
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instance on a single host (such as a router daemon), by eliminating
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the overhead of receiving their own transmissions.
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It should generally not
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be used by applications for which there may be more than one instance on a
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single host (such as a conferencing program) or for which the sender does
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not belong to the destination group (such as a time querying program).
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.Pp
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A multicast datagram sent with an initial TTL greater than 1 may be delivered
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to the sending host on a different interface from that on which it was sent,
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if the host belongs to the destination group on that other interface.
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The loopback control option has no effect on such delivery.
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.Pp
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A host must become a member of a multicast group before it can receive
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datagrams sent to the group.
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To join a multicast group, use the
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.Dv IP_ADD_MEMBERSHIP
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option:
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.Bd -literal
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struct ip_mreq mreq;
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setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq));
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.Ed
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.Pp
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where
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.Fa mreq
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is the following structure:
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.Bd -literal
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struct ip_mreq {
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struct in_addr imr_multiaddr; /* IP multicast address of group */
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struct in_addr imr_interface; /* local IP address of interface */
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}
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.Ed
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.Pp
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.Va imr_interface
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should be set to
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.Dv INADDR_ANY
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to choose the default multicast interface,
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or the
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.Tn IP
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address of a particular multicast-capable interface if
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the host is multihomed.
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Since
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.Fx 4.4 ,
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if the
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.Va imr_interface
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member is within the network range
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.Li 0.0.0.0/8 ,
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it is treated as an interface index in the system interface MIB,
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as per the RIP Version 2 MIB Extension (RFC-1724).
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.Pp
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Membership is associated with a single interface;
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programs running on multihomed hosts may need to
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join the same group on more than one interface.
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Up to
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.Dv IP_MAX_MEMBERSHIPS
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(currently 20) memberships may be added on a
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single socket.
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.Pp
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To drop a membership, use:
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.Bd -literal
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struct ip_mreq mreq;
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setsockopt(s, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));
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.Ed
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.Pp
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where
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.Fa mreq
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contains the same values as used to add the membership.
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Memberships are dropped when the socket is closed or the process exits.
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.\"-----------------------
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.Ss "Raw IP Sockets"
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.Pp
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Raw
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.Tn IP
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sockets are connectionless,
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and are normally used with the
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.Xr sendto 2
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and
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.Xr recvfrom 2
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calls, though the
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.Xr connect 2
|
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call may also be used to fix the destination for future
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packets (in which case the
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.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
|
|
#include <netinet/in_systm.h>
|
|
#include <netinet/ip.h>
|
|
|
|
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
|
|
ip->ip_v = IPVERSION;
|
|
ip->ip_hl = hlen >> 2;
|
|
ip->ip_id = 0; /* 0 means kernel set appropriate value */
|
|
ip->ip_off = offset;
|
|
.Ed
|
|
.Pp
|
|
The
|
|
.Va ip_len
|
|
and
|
|
.Va ip_off
|
|
fields
|
|
.Em must
|
|
be provided in host byte order .
|
|
All other fields must be provided in network byte order.
|
|
See
|
|
.Xr byteorder 3
|
|
for more information on network byte order.
|
|
If the
|
|
.Va ip_id
|
|
field is set to 0 then the kernel will choose an
|
|
appropriate value.
|
|
If the header source address is set to
|
|
.Dv INADDR_ANY ,
|
|
the kernel will choose an appropriate address.
|
|
.Sh ERRORS
|
|
A socket operation may fail with one of the following errors returned:
|
|
.Bl -tag -width Er
|
|
.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 has not 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 Er
|
|
.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
|
|
.Pp
|
|
The following errors may occur when attempting to send
|
|
.Tn IP
|
|
datagrams via a
|
|
.Dq raw socket
|
|
with the
|
|
.Dv IP_HDRINCL
|
|
option set:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EINVAL
|
|
The user-supplied
|
|
.Va ip_len
|
|
field was not equal to the length of the datagram written to the socket.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr getsockopt 2 ,
|
|
.Xr recv 2 ,
|
|
.Xr send 2 ,
|
|
.Xr byteorder 3 ,
|
|
.Xr icmp 4 ,
|
|
.Xr inet 4 ,
|
|
.Xr intro 4
|
|
.Sh HISTORY
|
|
The
|
|
.Nm
|
|
protocol appeared in
|
|
.Bx 4.2 .
|