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IBM AIX Packet Filter module
From: brummie () SECURE I1 NET (Brumbles)
Date: Mon, 25 Oct 1999 14:45:19 -0500

I have tried unsuccessfully to get any response from IBM on the following,
apparently unless you have a support contract you cant report bugs..
(well.. you can.. "Program Services", but thats a link to /dev/null

AixLevel AIX4.3.2
Packet Filtering Module, in particular the command genfilt does not allow
the addition of filters with port numbers greater than 32767

genfilt -v 4 -a D -s -m -d -M \
-c udp -o any -O eq  -P 123 -l n -w I -i all

Works fine... but...

genfilt -v 4 -a D -s -m -d -M -c udp \
-o any -O eq  -P 32768 -l n -w I -i all

Fails with:
Bad destination port/ICMP type "32768".

All is well if you use port 32767.

Simply put, the -P (port) parameter will not accept an argument greater
than 32767.

Obviously there are a lot of things above 32768 that you might want to filter,
e.g. rstatd. and other RPC programs, and also if I wanted to ensure that
my users arent opening up any services that sit on high ports, they can
circumvent any protection I layer on top by starting their service above

As the AIX4.3.2 packet filtering module is based upon the commercial IBM
firewall, I would be very interested to see if this weakness also exists
in that product.

I believe this opens up a security problem for anyone using the AIX
filtering that wants to continue using RPC on an internal interface, but
wishes to present only certain ports to an external side.


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