java.net.UnknownHostException (Unknown host)

What it means

The player typed a name (like play.example.com) and their computer could not turn it into an IP address. Nothing was even sent to your server. It is purely a naming problem, which also means it can't be a port forward or firewall problem.

How to fix it

• Check the spelling, including the ending (.net versus .org). Copy and paste the address rather than retyping it. Stray spaces count.
• Check the name exists. From a Command Prompt, run nslookup your-name.example.com. "Non-existent domain" means the record isn't there: it was never created, or the domain lapsed.
• Give new records time. A just-created or just-changed record can take from a few minutes to a few hours to reach everyone, and a failed lookup can be remembered for a while. Restarting the game or flushing the DNS cache (ipconfig /flushdns) helps.
• Custom port? A plain name can only point at the default port automatically if you add an SRV record. Otherwise players type name:25566. The dynamic DNS guide covers both.
• The player's own network may be the problem. If other websites also fail to load for them, it is their internet connection, not your server.

More help

Coalstack's Auto Debugger scans a Java server's log for known errors like this one and links straight to the fix. Download Coalstack to try it.

• Connection timed out
• Connection refused
• Not white-listed
• All Minecraft server error codes
• Server guides