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

Paul Hoffman phoffman at proper.com
Sat Oct 8 20:54:46 UTC 2016

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.

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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