Hi Mike, On 2020-05-16 10:57, Mike Przybylski wrote: > Hi, Aurelien, > > Thank you for looking into this. I really appreciate it. > > > Which nameserver do you use? > > Google DNS ( 184.108.40.206 and 220.127.116.11 ) > > > As the answer is probably large, it might > > be interesting to check if it supports TCP connections as a fallback. > > Alternative you might want to enable edns0 if it's not already done. > > I will definitely look into that. > > > Could you try with other nameservers, there are many public DNS servers > > available to test. > > I’m sorry that I didn’t think of that. Everything works fine with OpenDNS ( > 18.104.22.168 and 22.214.171.124 ). > > > Finally would it be possible to get a tcpdump trace of the issue? That > > would likely help to understand the issue. > > Please see the attached pcap file.
Thanks a lot for the pcap file. I got a look at it, and there is indeed something fishy: - 10.0.2.15 asks 126.96.36.199/UDP for A download.docker.com (query 0xd009) - 10.0.2.15 asks 188.8.131.52/UDP for AAAA download.docker.com (query 0x710a) - 184.108.40.206 answers query 0xd009 - 220.127.116.11 answers query 0x710a but marks it as truncated as it it too big. Up to there all looks normal. As expected the AAAA query 0x710a is retried using TCP: - 10.0.2.15 asks 18.104.22.168/TCP for AAAA download.docker.com (query 0x710a) - 22.214.171.124 answers to query *0xd009* with the *A* records. This is totally unexpected. The glibc resolvers therefore retries with the second name server: - 10.0.2.15 asks 126.96.36.199/TCP for AAAA download.docker.com (query 0x710a) - 188.8.131.52 answers to query *0xd009* with the *A* records, the same way as 184.108.40.206. This is again totally unexpected. As both TCP queries failed, glibc concludes there is a server error. I have no idea what could explain that, it seems there is something between the Google DNS servers and you host mangling the answers. I noticed that the IP of your host is 10.0.2.15. Could it be a QEMU or Virtualbox VM running with the user mode network stack? Regards, Aurelien -- Aurelien Jarno GPG: 4096R/1DDD8C9B aurel...@aurel32.net http://www.aurel32.net