11 August 2020
UTC timezone
Doors open at 12:45 UTC. Session starts at 13:00 UTC

Defragmenting DNS - Determining the optimal maximum UDP response size for DNS

11 Aug 2020, 14:00
Standard Presentation Online Workshop


Axel Koolhaas (University of Amsterdam) Tjeerd Slokker (University of Amsterdam)


DNS uses the connectionless User Datagram Protocol (UDP) by default, which causes problems with Path MTU Discovery. This is because DNS servers are stateless, and do not remember queries they have already answered. The Path MTU (PMTU) should be used as maximum size to stop fragmentation from happening. Extension Mechanisms for DNS (EDNS(0)) expands DNS with the UDP Message Size field, which communicates the response size capability of the resolver. This allows resolvers to specify the EDNS(0) they support.
This presentation reports on a research, with as aim to provide data for a considered optimal maximum EDNS(0) UDP message size, by measuring the PMTU to which resolvers and stub resolvers on the Internet are subject. We did this by creating an environment to serve different sized DNS responses and querying this environment across the Internet. This aligns with the goals DNS Flag Day 2020. Our ambition is to suggest defaults for the maximum EDNS(0) message size for DNS.

Talk Duration 20 minutes
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Primary authors

Axel Koolhaas (University of Amsterdam) Tjeerd Slokker (University of Amsterdam) Willem Toorop (NLnet Labs)

Presentation Materials