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From: John.Airey at (
Subject: Potential denial of service bug in Cisco Pix Firewall IOS resolve
 d in 6.3(3)105

Brief Description

Users of Cisco Pix Firewalls may discover that their pool of NAT'ted IP
addresses is running out, and that a reboot, reload or "clear xlate" on the
firewall clears the problem.


The problem is caused by the Firewall being swamped by incoming ICMP packets
on the global pool IP addresses. If these are not intercepted by a router
beforehand, the incoming echo requests (that are emanating from
Nachi/Welchia worm infected machines) are preventing the release of the
address translation. ie, the Pix is detecting the blocked traffic as
indication that the translation is still in use.

Permanent fix

I have been informed by Cisco that this is resolved in 6.3(3)105. The
relevant case ID is CSCec 47609 detailed at and
referenced at

Hopefully Cisco will mention this fix in their next major release, as
6.3(3)105 is an engineering release.

John Airey, BSc (Jt Hons), CNA, RHCE
Internet systems support officer, ITCSD, Royal National Institute of the
Bakewell Road, Peterborough PE2 6XU,
Tel.: +44 (0) 1733 375299 Fax: +44 (0) 1733 370848 

The translation of the Bible into English has shaped the languages and
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