Broken PMTUD for . + TLD servers, was: Re: Smallest Transit MTU

Mark Andrews Mark_Andrews at
Mon Jan 10 11:42:28 UTC 2005

> I receive DNS responses > 500 bytes every day (reported by PIX firewall). So
> it is an issue, no matter wgat is recomended in RFC.

	And you most probable have EDNS clients (nameservers) inside
	your firewall making EDNS queries which return EDNS responses
	that are bigger than 512 bytes.  EDNS has been standards
	track for over 5 years now.  The majority of the nameservers
	in the world talk EDNS between themselves and have been for
	several years now.  Only a few queries caused the EDNS
	response to exceed 512 bytes.

	With the introduction of the AAAA records for A.GTLD-SERVERS.NET
	and B.GTLD-SERVERS.NET any EDNS referral from the root
	servers for COM/NET now exceeds 512 bytes (520 minimum).
	A plain DNS referral to COM/NET is 509 bytes so any referal
	for an name longer than is dropping glue records for
	the COM/NET servers.

	The correct thing to do is to fix your firewall to handle the
	EDNS responses.


	RFC 2671:   Extension Mechanisms for DNS (EDNS0)
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews at

More information about the NANOG mailing list