251 lines
9.3 KiB
Groff
251 lines
9.3 KiB
Groff
.\" Copyright (c) 1990, 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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.\" From: @(#)route.4 8.6 (Berkeley) 4/19/94
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.\" $Id$
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.\"
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.Dd October 8, 1996
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.Dt ROUTE 4
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.Os
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.Sh NAME
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.Nm route
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.Nd kernel packet forwarding database
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.Sh SYNOPSIS
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.Fd #include <sys/socket.h>
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.Fd #include <net/if.h>
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.Fd #include <net/route.h>
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.Ft int
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.Fn socket PF_ROUTE SOCK_RAW "int family"
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.Sh DESCRIPTION
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.Tn UNIX
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provides some packet routing facilities.
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The kernel maintains a routing information database, which
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is used in selecting the appropriate network interface when
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transmitting packets.
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.Pp
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A user process (or possibly multiple co-operating processes)
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maintains this database by sending messages over a special kind
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of socket.
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This supplants fixed size
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.Xr ioctl 2 Ns 's
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used in earlier releases.
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Routing table changes may only be carried out by the super user.
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.Pp
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The operating system may spontaneously emit routing messages in response
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to external events, such as receipt of a re-direct, or failure to
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locate a suitable route for a request.
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The message types are described in greater detail below.
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.Pp
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Routing database entries come in two flavors: for a specific
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host, or for all hosts on a generic subnetwork (as specified
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by a bit mask and value under the mask.
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The effect of wildcard or default route may be achieved by using
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a mask of all zeros, and there may be hierarchical routes.
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.Pp
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When the system is booted and addresses are assigned
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to the network interfaces, each protocol family
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installs a routing table entry for each interface when it is ready for traffic.
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Normally the protocol specifies the route
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through each interface as a
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.Dq direct
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connection to the destination host
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or network. If the route is direct, the transport layer of
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a protocol family usually requests the packet be sent to the
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same host specified in the packet. Otherwise, the interface
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is requested to address the packet to the gateway listed in the routing entry
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(i.e. the packet is forwarded).
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.Pp
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When routing a packet,
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the kernel will attempt to find
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the most specific route matching the destination.
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(If there are two different mask and value-under-the-mask pairs
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that match, the more specific is the one with more bits in the mask.
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A route to a host is regarded as being supplied with a mask of
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as many ones as there are bits in the destination).
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If no entry is found, the destination is declared to be unreachable,
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and a routing\-miss message is generated if there are any
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listers on the routing control socket described below.
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.Pp
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A wildcard routing entry is specified with a zero
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destination address value, and a mask of all zeroes.
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Wildcard routes will be used
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when the system fails to find other routes matching the
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destination. The combination of wildcard
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routes and routing redirects can provide an economical
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mechanism for routing traffic.
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.Pp
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One opens the channel for passing routing control messages
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by using the socket call shown in the synopsis above:
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.Pp
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The
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.Fa family
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parameter may be
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.Dv AF_UNSPEC
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which will provide
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routing information for all address families, or can be restricted
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to a specific address family by specifying which one is desired.
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There can be more than one routing socket open per system.
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.Pp
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Messages are formed by a header followed by a small
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number of sockadders (now variable length particularly
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in the
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.Tn ISO
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case), interpreted by position, and delimited
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by the new length entry in the sockaddr.
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An example of a message with four addresses might be an
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.Tn ISO
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redirect:
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Destination, Netmask, Gateway, and Author of the redirect.
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The interpretation of which address are present is given by a
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bit mask within the header, and the sequence is least significant
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to most significant bit within the vector.
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.Pp
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Any messages sent to the kernel are returned, and copies are sent
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to all interested listeners. The kernel will provide the process
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id. for the sender, and the sender may use an additional sequence
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field to distinguish between outstanding messages. However,
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message replies may be lost when kernel buffers are exhausted.
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.Pp
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The kernel may reject certain messages, and will indicate this
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by filling in the
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.Ar rtm_errno
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field.
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The routing code returns
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.Dv EEXIST
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if
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requested to duplicate an existing entry,
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.Dv ESRCH
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if
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requested to delete a non-existent entry,
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or
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.Dv ENOBUFS
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if insufficient resources were available
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to install a new route.
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In the current implementation, all routing process run locally,
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and the values for
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.Ar rtm_errno
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are available through the normal
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.Em errno
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mechanism, even if the routing reply message is lost.
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.Pp
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A process may avoid the expense of reading replies to
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its own messages by issuing a
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.Xr setsockopt 2
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call indicating that the
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.Dv SO_USELOOPBACK
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option
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at the
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.Dv SOL_SOCKET
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level is to be turned off.
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A process may ignore all messages from the routing socket
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by doing a
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.Xr shutdown 2
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system call for further input.
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.Pp
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If a route is in use when it is deleted,
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the routing entry will be marked down and removed from the routing table,
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but the resources associated with it will not
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be reclaimed until all references to it are released.
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User processes can obtain information about the routing
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entry to a specific destination by using a
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.Dv RTM_GET
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message,
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or by reading the
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.Pa /dev/kmem
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device, or by issuing a
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.Xr getkerninfo 2
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system call.
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.Pp
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Messages include:
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.Bd -literal
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#define RTM_ADD 0x1 /* Add Route */
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#define RTM_DELETE 0x2 /* Delete Route */
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#define RTM_CHANGE 0x3 /* Change Metrics, Flags, or Gateway */
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#define RTM_GET 0x4 /* Report Information */
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#define RTM_LOOSING 0x5 /* Kernel Suspects Partitioning */
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#define RTM_REDIRECT 0x6 /* Told to use different route */
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#define RTM_MISS 0x7 /* Lookup failed on this address */
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#define RTM_RESOLVE 0xb /* request to resolve dst to LL addr */
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.Ed
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.Pp
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A message header consists of:
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.Bd -literal
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struct rt_msghdr {
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u_short rmt_msglen; /* to skip over non-understood messages */
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u_char rtm_version; /* future binary compatibility */
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u_char rtm_type; /* message type */
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u_short rmt_index; /* index for associated ifp */
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pid_t rmt_pid; /* identify sender */
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int rtm_addrs; /* bitmask identifying sockaddrs in msg */
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int rtm_seq; /* for sender to identify action */
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int rtm_errno; /* why failed */
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int rtm_flags; /* flags, incl kern & message, e.g. DONE */
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int rtm_use; /* from rtentry */
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u_long rtm_inits; /* which values we are initializing */
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struct rt_metrics rtm_rmx; /* metrics themselves */
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};
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.Ed
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.Pp
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where
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.Dq Li "struct rt_metrics"
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and the flag bits are as defined in
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.Xr rtentry 9 .
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.Pp
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Specifiers for metric values in rmx_locks and rtm_inits are:
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.Bd -literal
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#define RTV_SSTHRESH 0x1 /* init or lock _ssthresh */
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#define RTV_RPIPE 0x2 /* init or lock _recvpipe */
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#define RTV_SPIPE 0x4 /* init or lock _sendpipe */
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#define RTV_HOPCOUNT 0x8 /* init or lock _hopcount */
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#define RTV_RTT 0x10 /* init or lock _rtt */
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#define RTV_RTTVAR 0x20 /* init or lock _rttvar */
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#define RTV_MTU 0x40 /* init or lock _mtu */
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.Ed
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.Pp
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Specifiers for which addresses are present in the messages are:
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.Bd -literal
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#define RTA_DST 0x1 /* destination sockaddr present */
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#define RTA_GATEWAY 0x2 /* gateway sockaddr present */
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#define RTA_NETMASK 0x4 /* netmask sockaddr present */
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#define RTA_GENMASK 0x8 /* cloning mask sockaddr present */
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#define RTA_IFP 0x10 /* interface name sockaddr present */
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#define RTA_IFA 0x20 /* interface addr sockaddr present */
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#define RTA_AUTHOR 0x40 /* sockaddr for author of redirect */
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.Ed
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.Sh SEE ALSO
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.Xr route 8 ,
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.Xr rtentry 9
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.Sh HISTORY
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A
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.Dv PF_ROUTE
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protocol family first appeared in
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.Bx 4.3 reno .
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