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[ScreenOS] Functionality of the DNS ALG



Article ID: KB24599 KB Last Updated: 29 Jul 2020Version: 2.0

This article provides information about the functionality of the DNS ALG on ScreenOS 6.0 or later.


The DNS ALG is different than most of the other protocol ALG’s. There are no secondary connections to be managed; rather, DNS ALG closes the firewall session, as soon as a reply is returned from the DNS server. DNS does not have any session tear down procedure and the sessions will remain, until the protocol time out expires.

The time out needs to be long enough to support the iterative query process, which could be involved. With the ALG in place, completed DNS queries are removed from the session table in a timely manner; without the need to adjust the protocol time out to a level, which could cause issues for slower valid queries.

Modification History:
2020-7-29-: Content reviewed for accuracy. No changes needed
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