Re: (PM) Avalanche! Help! (fwd)

Chris Adams (cadams@ro.com)
26 Jan 1998 05:16:09 GMT

According to MegaZone <megazone@livingston.com>:
>Say you have a route on the gateway saying 1.1.1.1 is on PM2. Well, that
>IP is in the pool but the user isn't on. Someone sends a packet for 1.1.1.1
>and the gateway sends it to PM2. PM2 doesn't have a route for 1.1.1.1 as
>it is not active, so it ends up sending it to the default route - back to
>the gateway. Which then sends it back - and it bounces until TTL is up.
>
>There is a request open to have a PM squelch packets that come in for a
>pool IP address when that address isn't in use. But is hasn't been a
>high priority as it hasn't been a major issue for most users. And the
>ofilter on the ether0 as describe can do it today. People just want it
>to be automatic.

The reason I have requested this is that I think the current behavoir of
ComOS is technically wrong (note I just said technically - it doesn't
break anything, it just isn't really right). When using OSPF, the PM
advertises the dialup pool IP addresses. However, when traffic is
directed to an address that is not currently in use, the PM sends it on
to the default route, which is usually a router on the ethernet, which
is getting the OSPF routing info, so it sends it back to the PM, etc.

I suppose someone could ping through your address pool to find addresses
not in use and spoof packets to those addresses, causing them to bounce
around your network and maybe clog it up some. Also, if your default
route is back out a T1 (if this is a remote POP), those packets bouncing
back and forth could chew up some bandwidth.

A filter on the port with the default route can be used to capture
these, but then the packets just die. Shouldn't the PM instead return a
host unreachable (or maybe network unreachable) packet? If ComOS had a
null interface (bit bucket) and had a default route to null with a high
metric for advertised addresses, this wouldn't be a problem.

-- 
Chris Adams - cadams@ro.com
System Administrator - Renaissance Internet Services
I don't speak for anybody but myself - that's enough trouble.
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