in.routed 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.
At boot time, in.routed is invoked with either
the -s or -q option. Options are described
below. The -s option is appropriate for a router. The in.routed -s command is invoked under the following
- A machine has multiple interfaces
- The interfaces are not configured by DHCP
- The defaultrouter(4)
and /etc/notrouter files are not present
Otherwise, the in.routed -q command
In normal operation, in.routed listens on udp(7P) socket
520 (decimal) 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 in.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
the host will forward packets between networks. in.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, in.routed formulates a reply based on the information maintained
in its internal tables. The response packet 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.
request packets received by in.routed are used to update the routing tables if one of the following
conditions is satisfied:
- 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).
- 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.
- 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.
- 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, in.routed records the
change in its internal tables and generates a response
packet to all directly connected hosts and networks. in.routed waits a short period of time (no more than 30 seconds) before
modifying the kernel's routing tables to allow possible unstable situations
In addition to processing incoming packets, in.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 internet.
Hosts acting as internetwork routers gratuitously supply their routing
tables every 30 seconds to all directly connected hosts and networks.
In addition to the facilities described above, in.routed supports the notion of "distant" passive and active gateways. When in.routed is started up, it reads the file gateways
to find gateways which may not be identified using the SIOCGIFCONFioctl. 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 in.routed process running on the machine). Routes through passive
gateways are installed in the kernel's routing tables once upon startup.
They may change, depending upon routing information they receive from other
gateways. Information regarding their existence is 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
The gateways is comprised of a series of lines,
each in the following format:
< net | host > filename1 gateway filename2 metric value < passive | active >
The net or host keyword indicates
if the route is to a network or specific host.
filename1 is the name of the destination
network or host. This may be a symbolic name located in networks or hosts, or an Internet address specified
in "dot" notation; see inet(3SOCKET).
filename2 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.
The keyword passive or active
indicates if the gateway should be treated as passive or active (as described