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[ScreenOS] Can 'EDNS0' packets pass through the firewall?



Article ID: KB15540 KB Last Updated: 20 Jul 2014Version: 4.0

This article explains that there are no known issues with passing Extension mechanisms for DNS (EDNS) traffic through a firewall that is running ScreenOS.


EDNS is a special extension of DNS that sends packets with a payload size greater than 512 bytes. For more information, refer to RFC 2671.



With standard DNS, the payload is generally less than 512 bytes; but recently, more servers are employing EDNS0 (as defined in RFC 2671), to allow a requester to advertise UDP packets that are larger than 512 bytes. This has been tested in ScreenOS 5.3.0 and later; there are no known issues with passing EDNS0 traffic through a firewall that is running ScreenOS.

If using Deep Inspection (DI), the permitted DNS size must be increased to allow EDNS packets.  Please refer to KB23925 for instructions.

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