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[SRX] How to allow EDNS queries to pass through the SRX firewall with DNS ALG enabled



Article ID: KB23569 KB Last Updated: 19 Mar 2013Version: 3.0
This article provides information how to allow the EDNS queries pass through the SRX firewall, with DNS ALG enabled.
  • The basic DNS protocol is not sufficient to support some required features. Moreover, DNS messages carried by UDP were restricted to 512 bytes; not considering the Internet Protocol (IP) and Transport Layer headers.

  • EDNS (Extension mechanisms for DNS) is a specification for expanding the size of several parameters of the Domain Name System (DNS) protocol, which had prior size restrictions.

  • In practice, difficulties can arise when using EDNS to traverse firewalls, as certain firewalls assume a maximum DNS message length of 512 bytes and block longer DNS packets.

  • EDNS (as per RFC 2671) queries are dropped by the SRX firewall, with DNS ALG enabled.


To allow EDNS queries to pass through the SRX firewall, with DNS ALG enabled, run the following command from the configuration mode:

user# set security alg dns maximum-message-length 8192

Note: The above setting is valid only from 10.1 or later, to 10.2. From 10.2 onwards, the limitation of 512 bytes will be removed; so the above command will no longer be required from 10.2 or later.

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