Technical data

Gateway Routing Daemon (GATED) Configuration Reference
A.14 Redirect Processing
A.14 Redirect Processing
The redirect code is passed 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, a 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 and 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.
You cannot 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 mechanisms involved there will be windows where the effects of redirects
are present in the kernel.
By default, GATED removes redirects when actively participating in an interior
gateway protocol (RIP or OSPF). It is not possible to enable redirects once they
have been automatically disabled. Listening to RIP 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 is formatted as follows:
redirect yes | no | on | off
[{
preference preference ;
interface interface_list
[ noredirects ] | [redirects ] ;
trustedgateways gateway_list ;
traceoptions trace_options ;
}];
In the redirect statement:
• preference sets the preference for a route learned from a redirect. The
default is 30.
Gateway Routing Daemon (GATED) Configuration Reference A–19