[dns-operations] DNS flag day 2020: Recommended EDNS buffer size discussion

Petr Špaček petr.spacek at nic.cz
Thu Sep 5 13:13:04 UTC 2019

On 03. 09. 19 19:26, Evan Hunt wrote:
> Paul Vixie <paul at redbarn.org> wrote:
>> and, there _is_ better information available, like the local routing table, or
>> the local interface table.
> Well, there's better information in existence, but it's not exactly
> "available" at the moment.  Anyway, not accessible to most DNS server
> implementations running on Linux or BSD.
> When that information does become straightforwardly accessible, then yes,
> we should all be able to use it. I'll try to compose some text for the
> "flag day 2020" site to make this clearer, if it isn't clear now.
> As far as I know, there's no plan to change EDNS or any other
> specification, or to permanently cap the size of UDP DNS messages. What
> we're talking about is an agreement between DNS server implementors to
> converge on a common *default* configuration setting, which we believe is
> likely to prevent fragmentation in *most* present-day network environments.
> EDNS packet size remains configurable. You will still be able to override
> the default in your own network if you wish to.  Default settings may be
> updated in the future if ground conditions change.  If kernels do end up
> making route table information available, we'll be able to use it.
> Despite the "flag day" branding, it isn't a breaking change.

To be precise:

It is breaking change only for non-compliant parties which send big DNS
answers *and at the same time* do not support TCP transport for DNS.

Petr Špaček  @  CZ.NIC

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