Wifi Tethering - Phones IP is random. How predict?

Grantelbart

I want to do communication between a tethering device and and a downstream device.
Each time I restart tethering ( / or the phone, its always both at the same time), the gateway IP from perspective of the downstream is a different one inside of the 192.168.0.0/16 space each time.
The software I use needs at least one side to listen and one side to connect.

But with random IPs neither downstream nor phone is stable.
Is there any way to get this automatically?
Is there a locally available DNS-Name for the gateway maybe?
Without modifying the application and thereby maintaining a fork I can only input IPs or DNS Names.

A very weird solution would be to use some kind of DynDNS scheme for that. Which is propably easier with the downstream device as initiator. But then I need to involve external hosts for DNS.
Downstream has name -> Phone doesn't know it in its dns cache. -> Roundtrip to outside dns and back.
Phone has name -> Unknown if it exposes the tethering gateway IP under that name and if I can even manipulate the phone resolver on unmodified GOS. -> So likely also roundtrip through external DNS.

What is a good way to solve this?

Grantelbart

Use an overlay network.

The thing I want to do this with is an overlay network albeit manually configured.
Its the official Wireguard client. In VPN - Block not-VPN connections mode. So the only way to talk locally is if the phone and downstream are Wireguard-peers as well.

Using externally managed overlay networks like Tailscale and similar is not an option.
Modifying the Wireguard app is possible but tedious.

trilogy6202

Grantelbart On the downstream device you should be able to disable DHCP and choose a static IP in the WiFi settings of the hotspot network provided by the upstream device.

trilogy6202

Grantelbart Alternatively, you could try to see if the multicast DNS (mDNS) is working on the hotspot network. If it does, you should be able to reach the upstream device on YourDeviceName.local where YourDeviceName is the host name of your phone.

Grantelbart

trilogy6202

While that works in theory, it does not work in practice.

Because even the network part of the address range is random.
Its always a 192.168.x.y/24.
So it could be any of 255 /24 nets and the phones device is not always the 1 as y but also random.

For example this time its ...199.0/24 and the phone as gateway is the 52 and the laptop gets 63 from dhcp.
Last hotspot restart it was ..31.0/24.

Grantelbart

trilogy6202
Nope, MDNS doesn't work because of VPN stuff at that stage. It likely would work inside the tunnel.

gh4mut

I'm struggle with the same thing: hotspot using different subnets :(
I was able to configure mDNS on my linux laptops and this works between them, but if i try for example to ping the hostname.local of a laptop from Termux app on GOS, It can't do it.