routed.8c

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)routed.8c	8.2	10/25/90
.TH routed 8c
.SH Name
routed \- network routing daemon
.SH Syntax
.B /etc/routed
[
.I options
] [
.I logfile
]
.SH Description
.NX "routed program"
.NX "routing table" "managing"
.NXS "Xerox NS Routing Information Protocol" "routed program"
The
.PN routed
program
is invoked at boot time to manage the network routing tables.
The routing daemon uses a variant of the Xerox NS Routing
Information Protocol in maintaining up-to-date kernel routing
table entries.
.PP
In normal operation the 
.PN routed
program listens on a
.MS udp 4p 
socket for packets of routing information.  If the host is an
internetwork router, it periodically supplies copies
of its routing tables to any directly connected hosts
and networks.
.PP
When
.PN routed
is started, it uses the SIOCGIFCONF
.I ioctl
to find those
directly connected interfaces configured into the
system and marked up (the software loopback interface
is ignored).  If multiple interfaces
are present, it is assumed that the host will forward packets
between networks.
The
.PN routed
command then transmits a request
packet on each interface using a broadcast packet, if
the interface supports it, and enters a loop, listening
for request and response
packets from other hosts.
.PP
When a request packet is received, 
.PN routed
formulates a reply based on the information maintained in its
internal tables.  The response
packet generated contains a list of known routes, each marked
with a hop count metric. A count of 16 or greater is  
considered infinite.  The metric associated with each
route returned provides a metric "relative to the sender".
.PP
The response packets received by
.PN routed
are used to update the routing tables if one of the following
conditions is satisfied:
.IP \(bu 4
No routing table entry exists for the destination network
or host, and the metric indicates the destination is reachable.
That is, the hop count is not infinite.
.IP \(bu
The source host of the packet is the same as the router in the
existing routing table entry.  That is, updated information is
being received from the very internetwork router through which
packets for the destination are being routed.
.IP \(bu
The existing entry in the routing table has not been updated for
some time (defined to be 90 seconds) and the route is at least
as cost effective as the current route.
.IP \(bu
The new route describes a shorter route to the destination than
the one currently stored in the routing tables. The metric of
the new route is compared against the one stored in the table
to decide this.
.PP
When an update is applied, the
.PN routed
command records the change in its internal tables and generates a
response packet to all directly connected hosts and networks.
The
.PN routed
command waits a short period
of time (no more than 30 seconds) before modifying the kernel's
routing tables to allow possible unstable situations to settle.
.PP
In addition to processing incoming packets,
the
.PN routed
command periodically checks the routing table entries.
If an entry has not been updated for 3 minutes, the entry's metric
is set to infinity and marked for deletion.  Deletions are delayed
an additional 60 seconds to insure the invalidation is propagated
throughout the internet.
.PP
Hosts acting as internetwork routers supply their
routing tables every 30 seconds to all directly connected hosts
and networks.  The response is sent to the broadcast address on nets 
capable of that function, to the destination address on point-to-point 
links, and to the router's own address on other networks.  The normal
routing tables are bypassed when sending responses.
The reception of responses on each network is used to determine
if that network and interface are functioning correctly.  If no
response is received on an interface, another route may be chosen
to route around the interface, or the route may be dropped if no
alternative is available.
.PP
The
.PN routed
program supports the notion of distant passive and active gateways.  
.NXR "passive gateway"
.NXR "active gateway"
When 
.PN routed
is started up, it reads the file 
.PN /etc/gateways
to find gateways which may not be identified using
the SIOGIFCONF
.IR ioctl .
Gateways specified in this manner should be marked passive
if they are not expected to exchange routing information,
while gateways marked active
should be willing to exchange routing information (that is,
they should have a
.PN routed
process running on the machine).
Passive gateways are maintained indefinitely in 
routing tables.  Note, however, that passive gateways
are known only to the local host that lists them in
its 
.PN /etc/gateways 
file.  Information about
passive gateways is not included in any routing information
that is transmitted.
.PP
Active gateways are treated equally to network
interfaces.  Routing information is distributed
to the gateway and if no routing information is
received for a period of time, the associated
route is deleted.
.PP
External gateways are also passive, but are not placed in the kernel
routing table nor are they included in routing updates.
The function of external entries is to inform 
.PN routed
that another routing process will install such a route, and that 
alternate routes to that destination should not be installed.
Such entries are only required when both routers may learn of routes to 
the same destination.
.PP
The 
.PN /etc/gateways
is a series of lines, each in the following format:
.NXR "gateways file" "entry format"
.PP
< \fBnet\fP | \fBhost\fP > \fIname1\fP \fBgateway\fP \fIname2\fP 
\fBmetric\fP \fIvalue\fP < \fBpassive\fP | \fBactive\fP | \fBexternal\fP >
.PP
The 
.B net
or
.B host
keyword indicates if the route is to a network or specific host.
.PP
The
.I name1
is the name of the destination network or host.  This may be a
symbolic name located in 
.PN /etc/networks
or 
.PN /etc/hosts ,
or an Internet address specified in dot notation.  For further
information, see 
.MS inet 3n .
.PP
The
.I name2
is the name or address of the gateway to which messages should
be forwarded.
.PP
The
.I value
is a metric indicating the hop count to the destination host
or network.
.PP
The keywords
.BR passive ,
.BR active ,
or
.B external
indicate if the gateway should be treated as 
passive or active
(as previously described), or whether the gateway is external 
to the scope of the
.PN routed
protocol.
.PP
Any other argument supplied is interpreted as the name
of a file in which the actions of 
.PN routed 
should be logged.  This log contains information
about any changes to the routing tables and a history of
recent messages sent and received which are related to
the changed route.
.SH Options
.IP \fB\-d\fR 5
Enables additional debugging information to be logged, such as bad 
packets received.
.IP \fB\-g\fR
Offers a route, on internetwork routers, to the default
destination.  This is typically used on a gateway to the Internet, 
or on a gateway that uses another routing protocol whose routes 
are not reported to other local routers.
.IP \fB\-s\fR
Forces 
.PN routed
to supply routing information whether it is acting as an internetwork
router or not.
.NXR "routed program" "options"
.IP \fB\-q\fR
Opposite of the
.PN \-s
option.
.IP \fB\-t\fR
Prints all packets, sent or received, on the standard output.  In addition,
.PN routed
continues to receive input from the controlling terminal,
so that interrupts from the keyboard will kill the process.
.SH Restrictions
.NXR "routed program" "restricted"
The kernel's routing tables may not correspond to those of
.PN routed
for short periods of time while processes utilizing existing
routes exit; the only remedy for this is to place the routing
process in the kernel.
.PP
The
.PN routed
command
should listen to intelligent interfaces, such as an IMP, and
to error protocols, such as ICMP, to gather more information.
However, it does not always detect unidirectional failures in 
network interfaces, such as when the output side fails.
.SH Files
.TP 15
.PN /etc/gateways
For distant gateways
.SH See Also
udp(4p), htable(8)