routed - network routing daemon
/etc/routed [ options ] [ logfile ]
The program is invoked at boot time to manage the network routing tables. The routing daemon uses a variant of the Xerox NS Routing Infor-
mation Protocol in maintaining up-to-date kernel routing table entries.
In normal operation the program listens on a socket for packets of routing information. If the host is an internetwork router, it periodi-
cally supplies copies of its routing tables to any directly connected hosts and networks.
When is started, it uses the SIOCGIFCONF 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 net-
works. The 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.
When a request packet is received, formulates a reply based on the information maintained in its internal tables. The response packet gen-
erated 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".
The response packets received by are used to update the routing tables if one of the following conditions is satisfied:
o 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.
o 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.
o 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.
o 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, the command records the change in its internal tables and generates a response packet to all directly connected
hosts and networks. The 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.
In addition to processing incoming packets, the 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.
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.
The program supports the notion of distant passive and active gateways. When is started up, it reads the file to find gateways which may
not be identified using the SIOGIFCONF 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
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 file. Information about passive gateways is not included in any routing information
that is 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.
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 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 is a series of lines, each in the following format:
< net | host > name1 gateway name2 metric value < passive | active | external >
The net or host keyword indicates if the route is to a network or specific host.
The name1 is the name of the destination network or host. This may be a symbolic name located in or or an Internet address specified in
dot notation. For further information, see
The name2 is the name or address of the gateway to which messages should be forwarded.
The value is a metric indicating the hop count to the destination host or network.
The keywords passive, active, or 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 protocol.
Any other argument supplied is interpreted as the name of a file in which the actions of 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.
-d Enables additional debugging information to be logged, such as bad packets received.
-g 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.
-s Forces to supply routing information whether it is acting as an internetwork router or not.
-q Opposite of the option.
-t Prints all packets, sent or received, on the standard output. In addition, continues to receive input from the controlling terminal,
so that interrupts from the keyboard will kill the process.
The kernel's routing tables may not correspond to those of 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.
The 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.
For distant gateways