[dns-operations] PMTUD of .org servers

Florian Weimer fw at deneb.enyo.de
Thu Jun 18 18:33:48 UTC 2009

* Michael Graff:

> Florian Weimer wrote:
>> Is it just me, or do some of the .org servers (notably
>> ignore ICMP "fragmentation
>> needed, but DF bit set" message, even though they set the DF bit in
>> the IP header of the response packet?
> Is PMTU even defined on UDP?

Yes, it is.  RFC 1191 is somewhat protocol agnostic.

> IMHO, UDP should not have DF set in the common case, and never for
> DNS.  I thought PMTU was more of a TCP thing anyway?

EDNS0 requires PMTUD for DNS, see section 4.5.2 of RFC 2671.  It is
debatable if this requirement is desirable.

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