Debugging Hotel Wi-Fi Login on Arch Linux (and a Docker Gotcha)

Recently, while staying at a hotel, I tried connecting my Arch Linux laptop to the Wi-Fi. As usual, the network redirected to a captive portal login page. But this time, things didn’t work as expected.

First, I checked the captive portal detection URL with:

curl -I http://detectportal.firefox.com

That gave me a gateway URL like:

http://127.17.0.1/login.php

But when I tried to open it in a browser, I kept getting “Unable to connect”.

At first, I thought the hotel’s Wi-Fi was broken. Then I noticed something odd: the IP address started with 127.17…. That reminded me of Docker, since I had seen a similar subnet in docker info.

So I ran:

ip addr show

And sure enough, I saw this:

docker0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default
    link/ether 02:42:84:b7:e7:1e brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.1/16 brd 172.17.255.255 scope global docker0
       valid_lft forever preferred_lft forever
    inet6 fe80::42:84ff:feb7:e71e/64 scope link proto kernel_ll

Aha! Docker was hijacking the IP range 172.17.x.x, which happened to be the same network the hotel’s captive portal was using.

To fix it, I temporarily disabled the Docker bridge:

sudo ip link set docker0 down
sudo ip addr flush dev docker0

After that, I could finally access:

http://172.17.0.1/login.php

and log in to the Wi-Fi.