TsuNAME vulnerability could enable DDoS attacks on major DNS servers

DDoS Attack on a screen

Security researchers have discovered a severe flaw in domain name system (DNS) resolvers that hackers could exploit to launch reflection-based denial-of-service attacks against authoritative DNS servers.

Researchers from SIDN Labs and InternetNZ discovered the vulnerability, which they dubbed TsuNAME.

Authoritative DNS servers translate web domains, such as www.google.com, into IP addresses, such as To understand how the vulnerability works, you must know the difference between an authoritative and recursive DNS server.

Currently, most servers on the web are recursive, meaning they forward DNS queries from users to authoritative DNS servers that act as a phone book and return DNS responses for specific domain names. Under normal circumstances, millions of recursive DNS servers send billions of DNS queries to authoritative DNS servers every day.

Large companies and organizations, like content delivery networks, tech giants, ISPs, domain registrars, and government agencies, typically run authoritative DNS server.

The flaw affects DNS resolvers and can be exploited to attack authoritative servers. Resolvers vulnerable to the flaw will send nonstop queries to authoritative servers that have cyclic dependent records. Among the DNS resolvers known to be unaffected are Unbound, BIND, and KnotDNS.

“While one resolver is unlikely to overwhelm an authoritative server, the aggregated effect from many looping, vulnerable recursive resolvers may as well do,” said researchers in an advisory.

If a DDoS attack brings down such a DNS server using the bug, this could result in nationwide internet outages.

“What makes TsuNAME particularly dangerous is that it can be exploited to carry out DDoS attacks against critical DNS infrastructure like large TLDs or ccTLDs, potentially affecting country-specific services,” said researchers in a technical report.

Researchers added that they had observed 50% traffic increases due to TsuNAME in production in .nz traffic, which was due to a configuration error and not a real attack. The issue increased traffic from 800 million to 1.2 billion daily queries.

The researchers noted that after private disclosure of the bug, they were contacted by an anonymous European ccTLD that had experienced 10-fold traffic growth when two domains were misconfigured with cyclic dependencies.

Old DNS resolvers are particularly vulnerable, with Google's public DNS resolver turning out to be a source of repeated queries. Google fixed the problem by adding code to resolvers to detect cyclic dependency and ended query loops.

Researchers also found similar problems with Cisco's OpenDNS, as it would loop in the presence of continuous incoming queries. According to researchers, Cisco has fixed this issue.

Administrators can check for interdependencies using the open-source software CycleHunter.

Michael Barragry, operations lead and security consultant at edgescan, told ITPro this is a fairly serious, exploitable condition that’s remained undetected for quite some time.

"It’s unclear how prevalent this vulnerability really is in the wild, although initial testing performed by the researchers indicate that it may not be widespread," he said. "If an attacker successfully affected the performance of a top-level-domain name server, this could potentially have a large downstream impact.”

Rene Millman

Rene Millman is a freelance writer and broadcaster who covers cybersecurity, AI, IoT, and the cloud. He also works as a contributing analyst at GigaOm and has previously worked as an analyst for Gartner covering the infrastructure market. He has made numerous television appearances to give his views and expertise on technology trends and companies that affect and shape our lives. You can follow Rene Millman on Twitter.