[dns-operations] DNSviz and G-root: EDNS issue?

Casey Deccio casey at deccio.net
Fri Oct 15 12:54:56 UTC 2021


> On Oct 13, 2021, at 1:07 AM, Stephane Bortzmeyer <bortzmeyer at nic.fr> wrote:
> 
> On Tue, Oct 12, 2021 at 01:01:08PM -0400,
> Matthew Pounsett <matt at conundrum.com> wrote 
> a message of 11 lines which said:
> 
>>> This might be a known intermittent IPv6 routing issue with DNSviz, do
>>> you see this problem for v4 and/or v6 ?
>> 
>> That would show up as a non-answer over IPv6, rather than an apparent
>> PMTU/EDNS problem.
> 
> DNSviz (and similar tools) may wrongly diagnose a PMTU problem if
> there are random losses.

That's true.

> 1) Try with bufsize=4096. No answer, because of a random packet loss.
> 2) Retry with bufsize=1024. Answer received, therefore it must be a
>   PMTU problem.

Actually, it give it several tries before reducing PMTU.  Here is the full order of diagnostic queries when a timeout occurs:

# 1 - no change
# 2 - change timeout to 2 seconds
# 3 - no change
# 4 - reduce udp max payload to 512; change timeout to 1 second
# 5 - change timeout to 2 seconds
# 6 - remove EDNS option (if any); change timeout to 1 second
# 7 - remove EDNS option (if any)
# 8 - remove EDNS option (if any)
# 9 - remove EDNS option (if any)
# 10 - clear DO flag;
# 11 - disable EDNS
# 12 - return (give up)

(See https://github.com/dnsviz/dnsviz/blob/master/dnsviz/query.py#L1766)

Casey


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