systemd-resolved keeps swapping DNS servers and breaking name resolution

[Jordi Miralles]

Hi,

I think the name is self explanatory, let me paste some logs of what does my syslog look like at any given time:

https://paste.ubuntu.com/25089356/

Jul 14 16:49:23 Tuxedo systemd-resolved[1120]: Using system hostname 'Tuxedo'.
Jul 14 16:49:52 Tuxedo systemd-resolved[1120]: Switching to fallback DNS server 8.8.8.8.
Jul 14 16:50:17 Tuxedo systemd-resolved[1120]: Switching to fallback DNS server 8.8.4.4.
Jul 14 16:50:17 Tuxedo systemd-resolved[1120]: Switching to fallback DNS server 2001:4860:4860::8888.
Jul 14 16:50:17 Tuxedo systemd-resolved[1120]: Switching to fallback DNS server 2001:4860:4860::8844.
Jul 14 16:50:18 Tuxedo systemd-resolved[1120]: Switching to fallback DNS server 8.8.8.8.
Jul 14 16:50:18 Tuxedo systemd-resolved[1120]: Switching to fallback DNS server 8.8.4.4.
Jul 14 16:50:18 Tuxedo systemd-resolved[1120]: Using degraded feature set (UDP+EDNS0+DO) for DNS server 8.8.4.4.
Jul 14 16:50:18 Tuxedo systemd-resolved[1120]: Switching to fallback DNS server 2001:4860:4860::8888.
Jul 14 16:50:19 Tuxedo systemd-resolved[1120]: Switching to fallback DNS server 2001:4860:4860::8844.

(...)

Jul 14 16:50:43 Tuxedo systemd-resolved[1120]: Using degraded feature set (UDP+EDNS0+DO) for DNS server 78.46.223.24.
Jul 14 16:50:43 Tuxedo systemd-resolved[1120]: Switching to DNS server 162.242.211.137 for interface tun0.
Jul 14 16:50:44 Tuxedo systemd-resolved[1120]: Switching to DNS server 78.46.223.24 for interface tun0.
Jul 14 16:50:45 Tuxedo systemd-resolved[1120]: Switching to DNS server 162.242.211.137 for interface tun0.
Jul 14 16:50:48 Tuxedo systemd-resolved[1120]: Switching to DNS server 78.46.223.24 for interface tun0.
Jul 14 16:50:48 Tuxedo systemd-resolved[1120]: Switching to DNS server 162.242.211.137 for interface tun0.
Jul 14 16:50:49 Tuxedo systemd-resolved[1120]: Switching to DNS server 78.46.223.24 for interface tun0.
Jul 14 16:50:50 Tuxedo systemd-resolved[1120]: Switching to DNS server 162.242.211.137 for interface tun0.
Jul 14 16:50:50 Tuxedo systemd-resolved[1120]: Switching to DNS server 78.46.223.24 for interface tun0.

This happens both over the VPN and without (only than without the VPN I don't get the log messages, just a huge packet loss):

tux@Tuxedo:~$ ping google.es
^T192PING google.es (216.58.201.131) 56(84) bytes of data.
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=1 ttl=55 time=26.9 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=5 ttl=55 time=19.5 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=9 ttl=55 time=24.4 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=10 ttl=55 time=22.3 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=14 ttl=55 time=20.5 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=20 ttl=55 time=20.7 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=22 ttl=55 time=26.4 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=24 ttl=55 time=29.2 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=25 ttl=55 time=20.1 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=26 ttl=55 time=22.7 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=27 ttl=55 time=21.0 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=28 ttl=55 time=30.4 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=29 ttl=55 time=23.2 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=30 ttl=55 time=29.1 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=37 ttl=55 time=30.1 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=40 ttl=55 time=24.8 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=43 ttl=55 time=23.3 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=45 ttl=55 time=22.9 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=46 ttl=55 time=21.9 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=53 ttl=55 time=23.7 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=61 ttl=55 time=21.1 ms
64 bytes from mad06s25-in-f3.1e100.net (216.58.201.131): icmp_seq=62 ttl=55 time=19.3 ms
^C
--- google.es ping statistics ---
62 packets transmitted, 22 received, 64% packet loss, time 61869ms
rtt min/avg/max/mdev = 19.394/23.851/30.417/3.404 ms

Let me know if I can provide any other details.

[Jordi Miralles (jmiralles)]

Hi,

The release is Ubuntu 17.10

[Jordi Miralles (jmiralles)]

Found out it's related to some occasional packet loss. Why this happens, I'm not sure, but appears to be more related to the wireless kernel mod than systemd in this case.