[dns-operations] Does anyone pay attention to the EDNS0 UDP payload size in responses?

Mark Andrews marka at isc.org
Sat Oct 8 21:35:00 UTC 2016

In message <D3E7347A-1B0F-459A-B99C-AC12785B3D65 at proper.com>, "Paul Hoffman" writes:
> Greetings. As I was doing a bit of unrelated research, I found that 
> Google DNS always returns 512 for the UDP payload size in EDNS0 
> responses, even when the response itself is bigger. For instance, "dig 
> @ dns-oarc.org dnskey +dnssec" sends back a message of 1597 
> bytes, but
>     ; EDNS: version: 0, flags: do; udp: 512
> Reading RFC 6891, I see nothing that indicates that the MTU in a 
> response is actually relevant to anything. Section 6.2.4 talks about the 
> size probably being constant over short periods of time, but not about 
> it actually being relevant.

It is a capability.

e.g.  When you are sending UPDATE messages you can use the advertised
size to send larger UDP UPDATE messages.

> Is there client software out there that looks at the MTU in the response 
> and does something with that value if it seems "small"? If so, what?
> --Paul Hoffman
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Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org

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