WiFi at Deutsche Bahn + Ubuntu + Docker == trouble

Recently Deutsche Bahn started wifi for everyone travelling with a ICE. So far I had trips where it just worked great, on other trips I could not even connect – not on mobile phone nor on my laptop.

Today it was different

We have great signal strength, wifi on my phone works like a charm. Connecting to the wifi works nicely on the laptop as well. But I cannot connect to the login page for accepting t&c.

What happened – the analysis

I have a Thinkpad X1 Yoga laptop running Ubuntu 16.04. Among a gazillion of other packages docker is installed – mostly for dealing with lots of neo4j databases (of course ;-). The wifi (SSID: WIFIOnICE) itself is not authenticated but upon accessing the first webpage you get redirected to URL http://www.wifionice.de. Here I got a “cannot connect” error message in the browser. DNS lookups however worked fine – on couple of other WIFI issues DNS is a common culprit. Using dig www.wifionice.de I’ve learned that this hostname resolves to IP address 172.18.10.10. Next to check are the routing tables:

stefan@x1 ➜  sudo route -n
Kernel-IP-Routentabelle
Ziel Router Genmask Flags Metric Ref Use Iface
0.0.0.0 172.16.0.1 0.0.0.0 UG 600 0 0 wlp4s0
169.254.0.0 0.0.0.0 255.255.0.0 U 1000 0 0 br-034d1e2af367
172.16.0.0 0.0.0.0 255.255.0.0 U 600 0 0 wlp4s0
172.17.0.0 0.0.0.0 255.255.0.0 U 0 0 0 docker0
172.18.0.0 0.0.0.0 255.255.0.0 U 0 0 0 br-034d1e2af367

Interesting, packages to 172.18.0.0/16 are routed to a weird interface called br-034d1e2af367 and are therefore not set via the wifi device. This finding justifies a loud WTF! This bridge interface is established by docker. Since I’m just a docker user without deep understanding of its internals I cannot really explain its exact purpose. But I don’t have to 😉

the workaround

It’s good enough to just disable the bridge network interface while doing the wifi authentication:

ihtsudo ifconfig br-034d1e2af367 down
open http://www.wifionice.de in your webbrowser and press the "go online" button
sudo ifconfig br-034d1e2af367 up

After that operation I could use the internet on a ICE train without hassle.

a more elegant solution

Don’t have this one yet. I seems you can tweak the bridge’s IP number using --bip <CIDR> upon docker startup. But I couldn’t find out the details for now. Happy to read your helpful comments here.

finally {}

This post is mainly intended as a self-reminder for future train trips. If it’s helpful to others as well I’m more than happy. As a reference I’ve posted this in German language to a question on a forum of Deutsch Bahn as well.