Docker breaks Deutsche Bahn Wi-Fi on my laptop

I usually travel by train, so I often use Deutsche Bahn’s on-board Wi-Fi. On one laptop, however, the Wi-Fi captive portal simply would not open, even though the laptop was connected to the network. I usually worked around this by using my smartphone, where everything worked fine. Still, I wanted to understand why Deutsche Bahn Wi-Fi failed on that specific computer.

During one train ride, I did a deeper analysis. The Wi-Fi connection was up, but pinging the Deutsche Bahn router’s IP address did not work. DNS resolution, however, did work. That was a bit weird. After trying many things and checking the IP addresses with ip addr, I found the issue: the on-board Wi-Fi used the network 172.18.0.0 (interface wlp3s0 in the screenshot below), and I also had a Docker network on the same range (interface br-0ae16a6fdb3f) from a container project I was working on.

OK, so this was a conflicting network configuration. Having two different interfaces configured in the same subnet usually does not make sense and can lead to exactly this behavior. Once I knew the cause, the solution was simple: after removing the Docker network with docker network prune, the captive portal opened and internet access worked again.

One thing I still cannot fix is the cellular dead zones along the railroad tracks in Germany.