Tobias Fiebig, Head of the Internet Infrastructures Research Unit at TU Wien Informatics, has co-authored a new Internet Engineering Task Force (IETF) document on operating the Domain Name System (DNS) in today’s mixed IPv4/IPv6 Internet. Written together with Momoka Yamamoto of the WIDE project, RFC 10001, “Operational Guidelines for DNS Transport in IPv4/IPv6 Environments,” updates guidance first published more than two decades ago.

Published by the IETF in August, RFC 10001 reflects years of rigorous discussion, empirical measurement, peer review, and consensus-building within the Internet standards community. The document replaces RFC 3901, the IETF’s DNS transport guidance from 2004, and recommends that both authoritative DNS servers and recursive resolvers support IPv4 and IPv6.

Why DNS Must Work Across Both IPv4 and IPv6

The Domain Name System (DNS) translates names such as www.example.org into the numerical IP addresses used by computers. It is a fundamental component of Internet infrastructure: before a user can visit a website, send an email, or connect to many online services, the DNS typically helps identify where that service can be reached.

When RFC 3901 was published, IPv4, the Internet’s initial addressing system, was still dominant. IPv4 uses 32-bit addresses, which allows for about 4.3 billion unique addresses. Enough for the early Internet, but not for today’s billions of devices. IPv6, introduced in 1998, solves this by using 128-bit addresses, providing a virtually unlimited address space. But adoption has been slow. The 2004 guidance prioritized reliable DNS operation over IPv4 while treating IPv6 support as optional.

For years, the Internet has operated as a patchwork of:

• IPv4-only networks (still common in many regions),
• Dual-stack networks (supporting both IPv4 and IPv6), and
• IPv6-only networks (growing in mobile and newer infrastructure).

This fragmentation creates a problem. If a DNS server only supports one protocol, parts of the Internet can become unreachable. The issue is not only technical. IPv4 addresses have become scarce, and access to them is not equally distributed around the world. As Fiebig notes: “The issue of running out of IPv4 addresses is, of course, also a socio-economic problem. Not having access to IPv4 addresses for your services, that’s harming communities.” RFC 10001 updates the IETF’s earlier guidance to reflect this reality: the DNS must work seamlessly across both IPv4 and IPv6.

Does IPv6 Really Work?

For more than a decade, discussions about revising RFC 3901 stalled over a fundamental question: Could the DNS reliably operate over IPv6 under real-world conditions? To answer this, the researchers developed an independent measurement infrastructure, testing DNS resolution over both IPv4 and IPv6 under varying network conditions, including maximum packet sizes and packet loss. Fiebig compares the challenge to sending a package without knowing the size limits: “It’s a little bit like you have to send something with post.at, and they don’t tell you what the maximum packet size is you can fit in there without paying extra.”

The measurements showed that DNS resolution over IPv6 performs just as reliably as resolution over IPv4. In his inaugural lecture at TU Wien, Fiebig summarised the result: “What our measurements ultimately showed was that, in practice, there’s not really a difference whether you use IPv4 or IPv6 for your DNS resolution. Deploying IPv6 for the DNS is not a form of religious zealotry. It just works.” After years of debate, RFC 10001 now provides operators with clear guidance for ensuring the DNS remains accessible across both IPv4 and IPv6.

Congratulations to Tobias and Momoka on this achievement!

Watch Tobias Fiebig’s Inaugural Lecture to learn more about RFC 10001

About Tobias Fiebig

Tobias Fiebig studied cognitive science at the University of Osnabrück, focusing on cognitive psychology, philosophy of mind, artificial intelligence, computational linguistics, and computer science. He then completed a master’s degree in System and Network Engineering at the University of Amsterdam. For his doctoral studies, he moved to TU Berlin, where he earned his PhD in 2017 with a dissertation on the topic ‘An Empirical Evaluation of Misconfiguration in Internet Services’. During his dissertation period, Tobias Fiebig gained international research experience at a research visit to the University of California, Santa Barbara. Following his doctorate, he was offered an assistant professorship at TU Delft. At the same time, he took on responsibilities in the European research project “CyberSecurity4Europe.” In 2022, he moved from TU Delft to the Max Planck Institute for Computer Science, where he was appointed Senior Researcher in 2023. In 2024, he was also appointed as Associate Fellow at Saarland University. Since 2026, Tobias Fiebig is full professor and head of the Internet Infrastructures Research Unit at TU Wien Informatics.

About the Internet Infrastructures Research Unit

The Internet Infrastructures Research Unit investigates the world’s largest digital infrastructure, the Internet. Its research spans the development and analysis of communication protocols, empirical network measurements, and the design, implementation, and operation of networks and systems. It examines how individuals, organizations, and society shape digital infrastructures and how these infrastructures, in turn, shape society.

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