> as a gateway probe. The latter is basically a traceroute packet to
> sunsite.unc.edu or tsx-12.mit.edu, with a TTL of 2 (this usually elicts
> an ICMP redirect and/or a time exceeded, from which we can piece together
> the gateway's IP).
The gateway probe is tricky. You may start routing through one of the millions
of machines erroneously shipped by vendors with routing enabled or via the
wrong router. There is a router discovery protocol. Its less supported but a
lot safer. Listening for OSPF/RIP/RIP2 can also do the detection sometimes.
Its probably not that big an issue but if you install a network of machines
that accidentally all choose to route via the tiny isdn router on the edge
of the lan not via the main router bad things occur.
At the minimum you need to verify that the icmp time exceeded comes from a
host which you cannot directly reach. If you can directly reach the 2nd or
3rd hop that errors then you have found a secondary host misconfigured as
router, or a secondary router, and it just told you the correct router to
use. Sometimes you'll get a redirect, sometimes you dont.
If its an older 3com office connect firmware you actually start crashing the
3com box even ;)
Alan
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This archive was generated by hypermail 2b29 : Tue Feb 29 2000 - 21:00:17 EST