ICMP Redirect Query?

Question

If I have a router with a single interface on 192.168.1.254 configured as default gateway for LAN clients on 192.168.1.x and it has 2 routes i.e:

192.168.2.0 route through 192.168.1.1
0.0.0.0 route through 192.168.1.2

and ICMP Redirect is enabled, can anyone confirm under what circumstances traffic from LAN clients on 192.168.1.x attempts to go via the default gateway on 192.168.1.254?

My understanding is that if a client tries to go to

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the default gateway does an ICMP Redirect and tells the client to use

192.168.1.2 directly, and the client will store/cache the route.

If the client then tries

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would it attempt to go via the default gateway and again be redirected, or would the client know from the first redirect that all traffic to 0.0.0.0 should go directly to

192.168.1.2?

TIA

stephen · Accepted Answer

this sounds like homework, so treat it as an "understanding" Q....

1st point is the default g/w isnt doing anything useful, so your design is poor.

ICMP will happen assuming: redirects are enabled (nb - they get turned off by 1st hop protocols such as VRRP / HSRP). the default gateway uses redirect (a firewall probably wont ).

if the redirect is sent, the client can cache it, but doesnt have to (and the cache time could be short compared to the packet rate, so it has no effect).

in real life redirects are per destination address, so getting 1 redirect doesnt imply anything about a different destination.

Thrill5 · Answer

Some clients ignore ICMP redirects because it is a security vulnerability. You network design should avoid this type of configuration and put the router that is the default gateway behind  the router the clients are talking to.

ICMP Redirect Query?

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