Yesterday I updated my systems to Debian 10.7 and noticed problems with apt update against my apt-cacher-ng server on Debian 10.6/7. It goes like 0% ["waiting for headers"] then repeats the waiting message and finally reports an Err for the file and says

W: Failed to fetch http://security.debian.org/debian[...]  Connection failed [IP: 172.XX.YY.ZZ 3142]

If you repeatedly call apt update, it's completely random which file fails and the problem exists on Debian, Ubuntu and OpenWrt clients. There seem to be no issues with the server or network, ssh and the acgn webserver respond properly. When updating, tcpdump says that a TCP connection is established, nothing happens for 30 seconds, then the connection is terminated. The error log stays empty and outgoing files are in the log file. I really don't know what to do here. Does anybody have any ideas?

Update: I used today's openssl security update (which effectively affects every Debian machine on my network) to disable apt-cacher-ng on all of them, which seems seriously lame.

Is there any other place to ask this question?

  • I checked on my system (zgrep and zless) and the last apt-cacher-ng update seems to have been on 2020-06-12, almost 6 month ago. I'm pretty sure that there have been security updates after that time, which would have led me to notice the problem. It seems that apt-cacher-ng just "went crazy" without any reason or rhyme. I really didn't touch the host at all, except for installing updates whenever a security updates was annouced via [email protected].
    – OttoEisen
    Dec 8, 2020 at 18:26
  • Somebody directed me to snapshot.debian.org to debug the problem but then deleted the comment, that's the reason for the above comment...very helpful...
    – OttoEisen
    Dec 8, 2020 at 18:29
  • That was me but then I thoguht that indeed I didn't know what was upgraded or not between clients systems and acng system, so I didn't know if the comment was helpful , and reproducing the problem would be difficult
    – A.B
    Dec 9, 2020 at 10:32

1 Answer 1


Finally figured it out: IPv6 was to blame. When I started tcpdumping the outgoing connections, I noticed a lot of SYNs being send, but without receiving a reply. For now I have disabled IPv6 on the apt-cacher-ng and everything is just fine. Still trying to figure out exactly where (me-tunnelbroker-server) the problem is located but for now, at least it works.

In case somebody else has these issues, here are further symptoms I've noticed: Ubuntu clients worked, but much more slowly. HTTPS repositories generated an error Reading from proxy failed - select (115: Operation now in progress)

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