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

Evan Hunt each at isc.org
Tue Sep 3 17:26:27 UTC 2019

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.

Evan Hunt -- each at isc.org
Internet Systems Consortium, Inc.

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