syncthing-android: Android API level 30 breaks interface address discovery

API level 30(Android 11) will be mandatory starting November 2021 for apps on the PlayStore but seems to break golangs interface address discovery: https://github.com/golang/go/issues/40569

This seems to be an intended privacy related change which directly affects how golangs stdlib retrieves addresses for an interface:

NetworkInterface.getHardwareAddress() returns null for every interface.

Apps cannot use the bind() function on NETLINK_ROUTE sockets.

The ip command does not return information about interfaces.

Apps cannot send RTM_GETLINK messages.

Source: https://developer.android.com/training/articles/user-data-ids#mac-11-plus

About this issue

Original URL
State: closed
Created 3 years ago
Reactions: 5
Comments: 18 (14 by maintainers)

Most upvoted comments

Alternative workaround, which I prefer over rooting:

Give one (or more) of your devices a reserved DHCP address, e.g., 192.168.0.231.
On your other devices, update the addresses field for that device to use QUIC specifying that reserved DHCP address, e.g., quic://192.168.0.231 instead of default.

This turns discovery off completely.

StephenCleary on May 3, 2022

https://github.com/golang/go/pull/61089 might solve our remaining problems on Android but so far local discovery should work with next syncthing-android release.

bt90 on Sep 12, 2023

Did some research this morning, and it seems to me that the best way forward is to adopt the tailscale workaround, which uses the higher-level APIs recommended by the Android docs. Is it just lack of time preventing this from being addressed?

The workarounds are fine for home networks, but I’m currently traveling with two Android devices and using ST for syncing travel plans/documents The hotel wifis tend to disallow UPnP, so even global discovery doesn’t work. Of course I can still do the workarounds, but I have to redo them each time we return to the hotel which is tedious. Not complaining; just describing a use case. 😉

If this is still open after my vacation, I could contribute a PR.

StephenCleary on Jun 9, 2022

For anyone struggling: running syncthing as root on Android is a workaround, but please remember that it’s very inadvisable.

Maybe the IP address discovery issue could be partially worked around if Syncthing detected that there’s an ip binary/symlink available (provided by e.g. Busybox (I tested that binary in a terminal and it returned everything needed, without root))? Of course, this still requires a rooted phone, but doesn’t require Syncthing to run as root on its own. Note that this is still just a workaround, not a solution for most users.

Atrate on Apr 20, 2022

We could have an endpoint to set extra external addresses for announcement though, in addition to one populating the discovery cache. I could even see other potential uses for it (peeps running it in k8s behind a load balancer etc).

But both local and global discovery take the source of the packets as the address if none is given, that should work for most cases on mobile as well? We just lose out on announcing our LAN addresses…

calmh on Feb 15, 2022

We already have a wrapper which is able to access this APIs. It’s probably easier to leverage our REST API and allow external sources to feed syncthing with discovered IPs.

bt90 on Feb 15, 2022