How to Fix Clients Unable to Connect After NetworkManager StartHost

A host that works in a self-test but rejects every remote client is almost always bound to loopback. The transport's addresses default to localhost. Here is how to set them for real connections.

How to fix it

1. Listen on all interfaces

On the host, set the Unity Transport's listen address to 0.0.0.0 via SetConnectionData("0.0.0.0", port, "0.0.0.0") so it binds every interface, not just loopback.

2. Point clients at the real IP

On clients, set the connect address to the host's LAN or public IP (not 127.0.0.1) before StartClient(). The default connection data targets localhost, which only works on the same machine.

3. Open the port and verify reachability

Forward or open the UDP port on the host's firewall and router, and test reachability from outside. A correctly configured transport still fails if the port is closed upstream.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every Unity error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.