Project Zomboid: "Server Has Port Closed" and Port Forwarding
The warning means the server could not confirm that the port it is listening on is reachable from outside your network. It does not always mean the server is broken, and plenty of people see it on a server their friends are joining perfectly well. What it does mean is that one of four things is off: the server is not actually listening yet, the port you forwarded is not the port the game uses, you forwarded the wrong protocol, or something in between is dropping the traffic.
The most common mistake by a distance is forwarding one port. Project Zomboid does not always use one.
Read from the shipped build. Checked September 17, 2026 against the Project Zomboid dedicated server (Steam buildid 24909836). The setting names and the descriptions quoted below are the game's own, taken from its server settings screen and its English translation file.
Which ports Project Zomboid actually uses
| Setting | Default | What the game itself says |
|---|---|---|
DefaultPort | 16261 | "Default starting port for player data. If UDP, this is this one of two ports used." The wording is the game's, typo included. |
UDPPort | 16262 | Sits on the Steam page of the server settings and is flagged Steam-only, so it applies to Steam-enabled servers rather than to every server. |
RCONPort | 27015 | "The port for the RCON (Remote Console)". Nothing to do with play traffic. Do not forward it unless you deliberately want remote console access from outside, and never without a strong RCON password. |
Read that first description again, because it is the whole answer for most people: the game calls the default port one of two. Forward 16261 alone and a client can look fine right up to the point where player data needs the other one. Forward the pair, 16261 and 16262, and the usual symptom goes away. If you changed DefaultPort, the second port moves with it, so forward the pair you actually configured rather than the pair from a forum post.
Check in this order
- Is the server actually listening? A port checker reports "closed" for a server that is still generating the map or has quietly exited. Read the console before you touch the router. If it died during startup, the launch termination guide covers the usual cause.
- Which port did you forward? Compare the rule against
DefaultPortin your own server settings. Two servers on one machine cannot share it. - Which protocol? Forward UDP. A TCP-only rule is one of the quietest ways to produce this exact warning while everything else looks correct.
- Both ports? See above. This is the step people skip.
- The firewall on the machine itself. Router forwarding does nothing if the host firewall drops the packet first. Test from a second machine on your own network to separate the two: if a LAN client joins and an outside one cannot, the fault is the router or the ISP, not the server.
- Carrier-grade NAT. If your public address begins 100.64 through 100.127, your ISP is sharing it and no amount of forwarding will work. That is the point where self-hosting stops being the cheaper option.
When the warning is harmless
Two cases where you can ignore it outright. If everyone who plays is on your own network, outside reachability is irrelevant and the warning is noise. And if your server is Steam-enabled and public, people can still find and join it through Steam: the game notes that "Steam-enabled servers are always visible in the Steam server browser", independently of the in-game browser setting.
Settings people mistake for this one
Publiconly controls whether the server shows in the in-game browser. It is not a connectivity setting and switching it on will not open anything.Opendecides whether people may join without a whitelist entry. The game's own description: "Clients may join without already having an account in the whitelist. If set to false, administrators must manually create username/password combos." A locked-downOpenproduces a join failure that looks nothing like a port problem, so check it before you rebuild firewall rules. See the whitelist guide.Passwordhas to be known by clients to join, and the game flags that it is "ignored when hosting a server via the Host button", which catches people moving from an in-game host to a dedicated one.
The full list of what you can set is in all server settings, and dedicated server setup covers getting there in the first place. If players can reach the server but cannot get in, the troubleshooting guide is the better starting point.
Skip the router entirely. A hosted Project Zomboid server arrives with its ports already open and reachable, which removes this whole class of problem.