Use of Wifi vs. Mobile Data - Printable Version

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Use of Wifi vs. Mobile Data - kqlnut - 01-03-2021

I just found out that I accidentally used up my mobile data by updating my Manjaro Phosh Pinephone while connected to a wifi network. Looking at the routing tables it seems the wifi connection (via wlan0) would be preferred when available since it has lower metric values than the mobile data connection (via wwan0). This was not the case though. Looking around I came across this thread which has some contradictory info on whether the mobile data connection might be used when IPv6 is not available on the wifi. This led me to disable IPv6 on my Pinephone and everything worked as expected afterwards (no more use of mobile data when connected to wifi). So it seems that it actually is a problem when IPv6 is not available. My phone gets an IPv6 address from wifi network I'm connected to, but IPv6 is disabled on the router, so connection attempts must fail.
I would like to file a bug report for this or at least put a warning about this on the wiki since it might easily become a mysterious problem for Pinephone users. But I can't login to the wiki with my forum credentials and I have no idea where to file a bug report for this. Does anyone know where would be the right place for that or if this is already a known problem that maybe even is already being worked on?

RE: Use of Wifi vs. Mobile Data - kqlnut - 01-07-2021

Quick follow-up: I disabled IPv6 functionality via sysctl as described in the Arch Wiki. While that does disable IPv6 after running 'systemctl restart systemd-sysctl', it doesn't survive modem reboots/toggling of airplane mode/deep sleep (the systemd service has to be restarted manually every time). Seems to be a bug from what I've found. The easiest way to go about this I have found is just do disable IPv6 for the mobile data APN in Network Manager (e. g. via 'nm-connection-editor' or through nmcli). This leaves IPv6 functionality for all other connections like wifi, but prevents accidentally using your mobile data when IPv6 is not available on the wifi.