SYN floods (was: does history repeat itself?)

Thu Sep 12 18:41:09 UTC 1996

Curtis Villamizar writes:
> > I think its time for the larger providers to start filtering packets
> > coming from customers so that they only accept packets with the
> > customer's network number on it. 
> > 
> > Yes, its a load on routers. Yes, its nasty for the mobile IP weenies.
> > Unfortunately, the only known way to stop this.
[...]
> I'm not arguing against your point.  There are some feasibility issues
> right now that mean we can't just "turn this on".
> 
> Packet filtering on prefixes is only feasible as slower speeds with
> current routers and even then it can give smaller routers a tough
> time.

Fully agreed. This can't be done overnight. However, it has to be done
eventually. I'd say one to two years would be a reasonable
implementation timeframe, with a good fraction of the tail circuits
getting filtered in less than a year.

BTW, I would suggest that for a variety of applications, hardware
assisted filtering boxes that simply take in IP one end and put out
processed IP on the other end would be of use -- not just for this,
but also for helping in doing packet traces through high traffic
areas, for implementing firewalls, and for all sorts of other
things. Vendors, are you listening?

> There is also a problem with packet filtering due to assymetric
> routing.  You can legitimately end up with packets coming from
> addresses other than those that you route towards and should not black
> hole that traffic.

Yes, but this shouldn't be happening at the site of the customer tail
circuit, so thats not too bad a problem there.

> At the single homed connection a router option to reverse the sense of
> the forwarding table on a specific interface (look up the source in
> the forwarding table and only accept if the source is reachable
> through that next hop) seems to be a effective preventative that could
> be easily just "switched on".

A very good idea.

Perry