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Redirect Processing


The redirect code processes ICMP or ISO redirects learned by monitoring ICMP messages, or via the routing socket on systems that support it. It processes the redirect request and decides whether to accept the redirect. If the redirect is accepted, the route is installed in the gated routing table with the protocol redirect. Redirects are deleted from the routing table after 3 minutes.

If GateD determines that a redirect is not acceptable, it tries to figure out if the kernel forwarding table has been modified. On systems where ICMP messages are monitored this is accomplished by trying to second guess what the kernel would have done with the redirect. On systems with the routing socket, the kernel provides an indication of whether the redirect was accepted; GateD ignores redirects that were not processed.

If GateD has determined that the state of the kernel forwarding table has been changed, the necessary requests to the kernel are made to restore the correct state.

Note that on currently available systems it is not possible to disable the processing of ICMP redirects, even when the system is functioning as a router. To ignore the effects of redirects, GateD must process each one and actively restore any changes it made to the kernel's state. Because of the mechanism's involved there will be windows where the effects of redirects are present in the kernel.

By default, GateD ignores redirects when actively participating in an interior gateway protocol (RIP, HELLO, OSPF or IS-IS). It is not possible to enable redirects once they have been automatically disabled. Listening to RIP or HELLO in nobroadcast mode does not cause redirects to be ignored, nor does the use of EGP and BGP. Redirects must be manually configured off in these cases.

Note that in accordance with the latest IETF Router Requirements document, GateD insures that all ICMP net redirects are processed as host redirects. When an ICMP net redirect is accepted, GateD issues the requests to the kernel to make sure that the kernel forwarding table is updated to reflect a host redirect instead of a net redirect.

The redirect statement does not prevent the system from sending redirects, only from listening to them.

The Redirect Statement

redirect yes | no | on | off [ { preference preference ; interface interface_list [ noredirects ] | [ redirects ] ; trustedgateways gateway_list ; traceoptions trace_options ; } ] ;

Tracing options

There are no Redirect-specific tracing options. All non-error messages are traced under the normal class.

next up previous contents index
Next: Router Discovery Up: Configuration Guide Previous: ICMP

Laurent Joncheray
Wed Jun 12 15:35:22 EDT 1996