L2 Linker


Has anyone implemented EDNS on their network? How does the firewall treat it? Is it just as DNS? Does it block it because the packets are too big?

Does anyone know if there is a plan to make it its own discreet application?

Thanks for the info...


L5 Sessionator


Hello bgranholm,

Today, we do not support the EDNS RFC 2671. Usually, if DNS servers enabled with EDNS tries to talk to a non-EDNS server, the non-EDNS servers will simply ignore the OPT request and will not negotiate a larger packet size.

However, I do see a feature request (FR) submitted to our development team to potentially add it to our upcoming releases. You can also request your account's SE to vote for it.

FR ID : 2315

Hope that information helps!

Thanks and regards,

Kunal Adak

L2 Linker


So how does the Firewall classify this traffic now? unknown-udp or unknown-tcp?

L5 Sessionator


Hello bgranholm,

The DNS decoder does not enforce a length check for EDNS, and so the traffic should still be identified as DNS.

Thanks and regards,

Kunal Adak

L6 Presenter


To summarize my findings regarding EDNS:

If you are using BIND then add one of these values to your BIND config:

max-udp-size 1460;

edns-udp-size 1460;


max-udp-size 1280;

edns-udp-size 1280;

whatever floats your boat.

According to http://www.cisco.com/en/US/docs/security/asa/asa83/asdm63/configuration_guide/ddns.html one could say that:

- Old standard: max 512 bytes for UDP (DNS).

- New standard: max 4096 bytes for UDP (DNS).

- Microsoft standard: max 1280 bytes for UDP (DNS).

where 1280 seems to be connected to that 1280 is the smallest allowed MTU for IPv6 networks.




If the IPv6 stack does not support IPV6_USE_MIN_MTU, then steps

should be taken to prevent PMTUD occuring.  These include, but are

not limited to, setting the MTU of the interface the packets are

being sent over to the minimum IPv6 MTU (1280 bytes), or restricing

DNS/UDP packets to no more than 1280 bytes including IPv6 headers.





Note that a 512-octet UDP payload requires a 576-octet IP

reassembly buffer.  Choosing 1280 on an Ethernet connected

requestor would be reasonable.  The consequence of choosing too

large a value may be an ICMP message from an intermediate

gateway, or even a silent drop of the response message.




1280 bytes of DNS data is chosen as the new default to provide a

generous allowance for IP headers and still be within the highly

prevalent approximately Ethernet size or larger MTU and buffering

generally available today.

An IPv6 server should enable fragmentation on UDP replies.  While

fragmentation will not be frequent if the above guidelines are

followed, it may occur on occasion. In principle, IPv6 headers and

options could be huge, resulting in a very large UDP packet even

though the DNS payload is limited, but this should not occur in



Way to test if you are affected of any EDNS bugs in your infrastructure:

>dig +short rs.dns-oarc.net txt

and then check the logs of your firewalls etc.

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