in.rdisc − network router discovery daemon


in.rdisc [ −sfa ]

in.rdisc −r [ −p preference ] [ −T interval ]


in.rdisc implements the ICMP router discover protocol. The first form of the command is used on hosts and the second form is used on routers. On a host in.rdisc is invoked at boot time to populate the network routing tables with default routes. On a router it is also invoked at boot time in order to start advertising the router to all the hosts.

Host (First Form)

On a host in.rdisc listens on the ALL_HOSTS ( multicast address for ROUTER_ADVERTISE messages from routers. The received messages are handled by first ignoring those listed router addresses with which the host does not share a network. Among the remaining addresses the ones with the highest preference are selected as default routers and a default route is entered in the kernel routing table for each one of them.

Optionally, in.rdisc can avoid waiting for routers to announce themselves by sending out a few ROUTER_SOLICITATION messages to the ALL_ROUTERS ( multicast address when it is started.

A timer is associated with each router address and the address will no longer be considered for inclusion in the the routing tables if the timer expires before a new advertise message is received from the router. The address will also be excluded from consideration if the host receives an advertise message with the preference being maximally negative.

Router (Second Form)

When in.rdisc is started on a router, it uses the SIOCGIFCONF ioctl(2) to find the interfaces configured into the system and it starts listening on the ALL_ROUTERS multicast address on all the interfaces that support multicast. It sends out advertise messages to the ALL_HOSTS multicast address advertising all its addresses. A few initial advertise messages are sent out during the first 30 seconds and after that it will transmit advertise messages approximately every 600 seconds.

When in.rdisc receives a solicitation message it sends an advertise message to the host that sent the solicitation message.

When in.rdisc is terminated by a signal it sends out an advertise message with the preference being maximally negative.



Accept all routers independently of the preference they have in their advertise messages. Normally in.rdisc only accepts (and enters in the kernel routing tables) the router or routers with the highest preference.


Run in.rdisc forever even if no routers are found. Normally in.rdisc gives up if it has not received any advertise message after after soliciting three times, in which case it exits with a non-zero exit code. If −f is not specified in the first form then −s must be specified.

−p preference

Set the preference transmitted in the solicitation messages. The default is zero.


Act as a router as opposed to a host.


Send three solicitation messages initially to quickly discover the routers when the system is booted. When −s is specified in.rdisc exits with a non-zero exit code if it can not find any routers. This can be overridden with the −f option.

−T interval

Set the interval between transmitting the advertise messages. The default time is 600 seconds.


in.routed(8C) ioctl(2), icmp(7), inet(7)

Deering, S.E.,ed ‘‘ICMP Router Discovery Messages,’’ RFC 1256, Network Information Center, SRI International, Menlo Park, Calif., September 1991.