dns packet size

[Mark Andrews]

In message <20081009151332.GA23819 at nic.fr>, Stephane Bortzmeyer writes:
> On Thu, Oct 09, 2008 at 09:49:57AM -0400,
>  jeff donovan <donovan at beth.k12.pa.us> wrote 
>  a message of 29 lines which said:
> 
> > i have been noticing my firewall denying udp packets that exceed
> > 512.  
> 
> Very bad idea. It will prevent any DNS feature younger than twelve
> years (DNSSEC, IDN, IPv6).
> 
> > What is the correct packet size ,
> 
> 64k, the maximum size of an UDP packet.

	A better answer is the maximun of whatever EDNS UDP sizes
	your nameservers and other DNS clients advertise.  Named
	will advertise a size up to 4096 bytes, which is also the
	default.

	See RFC 2671 and edns-udp-size in named.conf.

Network Working Group                                            P. Vixie
Request for Comments: 2671                                            ISC
Category: Standards Track                                     August 1999

                  Extension Mechanisms for DNS (EDNS0)

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