[dns-operations] PMTUD of .org servers

Mark Andrews marka at isc.org
Fri Jun 19 01:08:27 UTC 2009

In message <873a9x4aue.fsf at mid.deneb.enyo.de>, Florian Weimer writes:
> * Rick Jones:
> > However, as this can sometimes call for successive resends, IIRC many
> > (most?) stacks may disable setting DF on IPv4 datagrams carrying UDP
> > datagrams.
> Linux sets the DF bit on outgoing UDP packets by default.

	Linux and Solaris set DF by default.  Linux boxes allow it
	to be modified on a per-socket basis.  On Solaris boxes it
	is a global flag.  You can't disable DF on UDP without also
	disabling DF on TCP.

	Other boxes that implement path mtu discovery appear to only
	do it on TCP connections by default.

	Named turns off the setting of DF for UDP on all platforms
	where we are aware of a per socket control.

	Setting DF on UDP/DNS responses is detrimental to DNS.

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org

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