What causes the “address already in use” socket exception in Unity?

Two processes (or two components in one process) tried to bind the same port — a previous server instance is still running, the socket lingers in TIME_WAIT after a crash, or the port is claimed by other software.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Fix “SocketException: Address already in use” in a Unity Game Server

Quick answer: Kill the stale process holding the port, enable address reuse where appropriate, bind to port 0 for ephemeral cases, and make your server shut its listener down cleanly on exit and domain reload.

Your host or dedicated server throws this on start because the port it wants is taken — often by a zombie copy of itself. Here is how to find the holder and make restarts reliable.

How to fix it

1. Find what holds the port

Use netstat -ano (Windows) or lsof -i :7777 (macOS/Linux) to see the owning process — it is frequently the previous run of your own server that never released it.

2. Close listeners deterministically

Stop the transport in OnApplicationQuit and on editor domain reload; a listener owned by a leaked thread survives Play Mode exits and blocks the next run.

3. Enable reuse for quick restarts

Set SO_REUSEADDR (or your transport's equivalent) so a socket in TIME_WAIT after a crash does not block rebinding during development and rolling restarts.

4. Make the port configurable

Read the port from config or command line and surface a clear startup error naming the port, so ops can move an instance instead of debugging a stack trace.

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.

The bug you can't reproduce isn't gone — it's just invisible until you capture it from the player's device.