Phil Howard phil at
Mon Dec 8 18:15:37 UTC 1997

Adrian Chadd writes...

> A couple of problems:
> * Filtering ALL ICMP is pretty silly, ICMP is there for more than just
>   pings, and some of it is important.

Such as MTU Discovery.  If your customers try to go down a path where the
MTU is smaller than what they are sending, and if their packets prohibit
fragmentation, ICMP is the only way to inform their stack to reduce the
size of the MTU.  Otherwise they may failures ranging from intermittent
to totality.  Intermittent ones would often be seen when just a small
amount of data sometimes gets through.  For example, an HTTP request could
be sent in a small packet, but posting a large form would not.

How many tech support people would know that the ability to get a page
and the inability to post a form could be caused by the customer's own
ISP blocking ICMP network unreachable?

So you need to leave "network unreachable" coming in.

Ref: "TCP/IP Illustrated, Volume 1, The Protocols", by W. Richard Stevens
See: page 340.

> * If people start doing this, someone with a smidgen of time on their
>   hands will write a ping flooder that uses random TCP or UDP packets
>   with spoofed from addresses.

Already been done.  I've had one of those for 4 years.

The ultimate solution to network security was published a few months ago
in the Dilbert series.

Phil Howard | suck5it2 at end7it92 at suck1it8 at
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