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DESCRIPTION

In normal operation routed listens on the udp(4) socket for the route(8) service (see services(5)) for routing information packets. If the host is an internetwork router, it periodically supplies copies of its routing tables to any directly connected hosts and networks.

When routed is started, it uses the SIOCGIFCONF ioctl(2) 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. Routed 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.

When a request packet is received, 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.

Response packets received by routed are used to update the routing tables if one of the following conditions is satisfied:

1. No routing table entry exists for the destination network or host, and the metric indicates the destination is ``reachable'' (i.e. the hop count is not infinite).
2. 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.
3. 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.
4. 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.

When an update is applied, routed records the change in its internal tables and updates the kernel routing table. The change is reflected in the next response packet sent.

In addition to processing incoming packets, routed also 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 local internet.

Hosts acting as internetwork routers gratuitously 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 gratuitous responses. The reception of responses on each network is used to determine that the 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.

Options supported by routed:

-d

Enable additional debugging information to be logged, such as bad packets received.

-g

This flag is used on internetwork routers to offer a route 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.

-s

Supplying this option forces routed to supply routing information whether it is acting as an internetwork router or not. This is the default if multiple network interfaces are present, or if a point-to-point link is in use.

-q

This is the opposite of the -s option.

-t

If the -t option is specified, all packets sent or received are printed on the standard output. In addition, routed will not divorce itself from the controlling terminal so that interrupts from the keyboard will kill the process.

Any other argument supplied is interpreted as the name of file in which routed´s actions should be logged. This log contains information about any changes to the routing tables and, if not tracing all packets, a history of recent messages sent and received which are related to the changed route.

In addition to the facilities described above, routed supports the notion of ``distant'' passive and active gateways. When routed is started up, it reads the file /etc/gateways to find gateways which may not be located using only information from the SIOGIFCONF ioctl(2). 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 (i.e. they should have a routed process running on the machine). Routes through passive gateways are installed in the kernel's routing tables once upon startup. Such routes are not included in any routing information transmitted. 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. Gateways marked external 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 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.

The /etc/gateways is comprised of a series of lines, each in the following format:

The net or host keyword indicates if the route is to a network or specific host.

Name1 is the name of the destination network or host. This may be a symbolic name located in /etc/networks or /etc/hosts (or, if started after named(8), known to the name server), or an Internet address specified in ``dot'' notation; see inet(3).

Name2 is the name or address of the gateway to which messages should be forwarded.

Value is a metric indicating the hop count to the destination host or network.

One of the keywords passive, active or external indicates if the gateway should be treated as passive or active (as described above), or whether the gateway is external to the scope of the routed protocol.

Internetwork routers that are directly attached to the Arpanet or Milnet should use the Exterior Gateway Protocol (EGP) to gather routing information rather then using a static routing table of passive gateways. EGP is required in order to provide routes for local networks to the rest of the Internet system.