Threat Level: green Handler on Duty: Didier Stevens

SANS ISC: IPv6 and SANS ISC InfoSec Forums

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IPv6 and

I spent some time last week to analyze the IPv6 traffic receives. To do so, I considered the last 90 days worth of logs. The full report can be found here.

A quick summary: IPv6 is still used by only 1.3% of hosts connecting to This is a considerable increase from about a year ago, which was about 0.5%. But the number of hits is still small. I am not able to proof this in every single case, but the overwhelming use of tunnels suggests that most if not all of these users would be able to reach via IPv4. The connection speed via IPv4 would probably be faster. For myself, the latency to via IPv6 is about double what it is via IPv4. Most of the overhead comes from the latency of my tunnel connection at home. The round-trip time from to our tunnel broker is only 12ms.

One of the important lessons from this analysis: A large number of hosts connecting to us appears to use automatically configured tunnels like 6to4 or Teredo. These tunnels are sometimes not managed, resulting in hosts unintentionally exposed to IPv6. Many firewalls are not configured to limit IPv6 or associated tunneling protocols, or don't even have the ability to do so. These hosts may be "naked" when it comes to IPv6.


  • We had IPv6 connections from about 13 thousand hosts.
  • about 2,500 of these used 6to4 (2002::/16 addresses) and 550 used Teredo.
  • only a very small fraction (815) of the IPs had PTR records configured for reverse DNS resolution.

 Full report: (PGP Signature)


Johannes B. Ullrich, Ph.D.
SANS Technology Institute

I will be teaching next: Defending Web Applications Security Essentials - SANS Cyber Defense Initiative 2021


4302 Posts
ISC Handler
Jan 12th 2010
I was one of the persons running IPv6. I installed Debian squeeze a few weeks ago and IPv6 is switched on [i found out]. It gave me problems reaching some websites, also the isc, website. I switched off IPv6 and the problems are over. If you have firefox with the ShowIp addon and you see green IP's beginning with 2002:: than you know IPv6 is switched on.
I have IPv6 PTRs configured on servers, but workstations have privacy extensions enabled and PTRs would defeat the purpose. Even with IPv4 making any decision based on PTRs is pretty obsolete.

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