Filtering Source Addresses on gw-internet

Jon Lewis jlewis at
Tue Aug 12 20:16:38 UTC 1997

On Tue, 12 Aug 1997, C. Jon Larsen wrote:

> Thats what I thought at first. But if the permit comes first, then packets
> with valid source addresses (a.b.c.d) get out because they pass that rule.
> So a packet built like:
> Source-> a.b.c.d  Dest->
> will get out and be passed to the ISP, wasting bandwidth. Thats why I deny
> them first, and then do the permit later on in the list. 

Ah...I wasn't reading your rules closely enough.  As a trick to reduce
access-list length on my internet gatway, I nailed up static routes like:

ip route   null0 250
ip route   null0 250
ip route       null0 250
ip route     null0 250
ip route     null0 250

The first two are our netblocks.  The rest just stop packets for the
"private IP" nets from leaving our network.  I also have access-list rules
that prevent them from entering our network.  I'm assuming here that route
cache hits are faster than access-list testing.

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