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The fastest diagnosis is to join the host over its local LAN address. If that works, the server is running and the remaining fault is between the router, ISP and public backend registration. If LAN access fails, concentrate on the process, configuration and host firewall before changing router settings.
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What “Server Unreachable” means in Arma Reforger
Arma Reforger has several separate stages between starting a dedicated server and joining it. The server process must start, bind a UDP socket, register an address and port with the backend, pass through the host firewall and router, and finally receive traffic from the client. A failure at any stage can produce a server that is visible but cannot be joined, absent from the browser, or accompanied by the message Group 1 Replication, reason=11, Server Unreachable.
That distinction matters because changing mods or reinstalling the game does not repair a blocked UDP port, an incorrect public address or carrier-grade NAT. Treat the message as a connectivity symptom and work from the inside out: process, configuration, host network, router, ISP and backend.
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Ports Arma Reforger uses by default
| Purpose | Default port | Protocol | When to expose it |
|---|---|---|---|
| Public game traffic | 2001 | UDP | Required for players to reach the game server. |
| Steam/A2S server-browser queries | 17777 | UDP | Required for browser visibility and query responses. |
| RCON remote administration | 19999 | UDP | Optional; forward only when you actually use remote administration. |
The values above are the documented defaults. A host may choose different ports, but the same chosen value must be reflected in the server configuration, firewall rules and router forwarding. Do not forward RCON simply because it appears in a template; leaving an unused administration port exposed increases attack surface.
The documentation identifies UDP as the default transport. Some community instructions suggest adding a TCP rule for port 2001, while other guidance says the game uses UDP exclusively. The official configuration reference does not resolve that disagreement, so start with UDP and do not add TCP forwarding unless a specific version or tool you use documents a TCP requirement.
Fix the server in priority order
1. Confirm that the server is running and listening
On the machine hosting the server, confirm that the Arma Reforger process remains running after startup and that it does not exit with a configuration or binding error. A crash loop, an occupied port or a service bound only to an unavailable interface cannot be repaired by a router rule.
Check the server log for the selected game port and for a successful registration message. If another application already uses the selected port, stop that application or assign a different port and then apply the same number everywhere. The default bind port is 2001 and the documented valid range is 1–65535.
2. Verify the address and port fields
Four fields determine how the host listens and what the backend advertises:
- bindAddress is the local address on which the socket listens. When omitted, it defaults to
0.0.0.0, meaning all local IPv4 interfaces. - bindPort is the UDP port used by the listening socket. It defaults to 2001. The official guidance says this entry should generally be omitted so it automatically uses the same value as publicPort.
- publicPort is the UDP port registered with the backend. It defaults to 2001 and must be the port players can reach from outside.
- publicAddress is the public IP address clients use. If the field is missing or empty, the backend detects the address automatically.
Use an explicit public address only when automatic detection registers the wrong address. Enter the public IPv4 address assigned to the router, not the host’s private LAN address such as an address used only inside your home network. IPv6 is not supported for this server connection, so an IPv6 address cannot substitute for a reachable public IPv4 address.
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A minimal illustration, preserving your existing configuration format, looks like this:
{
"bindAddress": "0.0.0.0",
"publicAddress": "",
"publicPort": 2001
}Leaving bindPort out is normally preferable. If you do set it, make it identical to publicPort unless you have a deliberate, documented reason to separate the listening and registered ports.
3. Replace obsolete field names
Build 0.9.8.73 renamed two configuration entries. gameHostBindPort became bindPort, and gameHostRegisterBindAddress became publicAddress. A guide or old configuration using the former names may be ignored by a current server, leaving it on defaults or registering an unusable address. Remove the obsolete keys and use the current names, then restart the server so the new values are read.
4. Allow the ports through the host firewall
Both the operating-system firewall and the router must permit the traffic. On Windows, open Windows Defender Firewall with Advanced Security, choose Inbound Rules, select New Rule, choose Port, select UDP, and enter the game port. Create a second UDP rule for 17777 if you need server-browser visibility. Add UDP 19999 only for an RCON service you intentionally operate. Restrict the rule to the server executable or the necessary profiles when your environment permits it.
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After changing rules, restart the server and inspect the firewall’s active rules rather than assuming a previous exception still applies. Community reports describe firewall utilities silently putting Arma Reforger or BattlEye back on a blocked list after an update. If the server worked until a game or security-product update, look for a newly disabled or duplicated rule.
On Linux or another host operating system, make the equivalent inbound UDP allowances in the active firewall manager. Do not enable every protocol or open an unrestricted range as a shortcut; permit only the ports the server actually uses.
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5. Forward the same UDP ports on the router
Sign in to the router that owns the internet connection and find its Port Forwarding, NAT or Virtual Server page. Create rules that send UDP 2001 and, when browser listing is required, UDP 17777 to the host’s fixed private IPv4 address. If you changed the game port, substitute that value consistently. Forward UDP 19999 only when RCON is enabled and protected.
Reserve the host’s LAN address with the router’s DHCP reservation feature or configure a static address outside the DHCP pool. A forwarding rule aimed at yesterday’s address will appear correct but send packets to the wrong device after a reboot. Avoid forwarding the same external port to multiple machines; a router cannot decide which internal server should receive the packet.
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Some routers ask for separate internal and external ports. Use the same number in both fields unless your configuration intentionally translates them, and then ensure publicPort reflects the externally reachable port registered with the backend.
6. Test from the local network first
Use a second device on the same LAN and connect to the host’s private IPv4 address. If the client offers a direct address entry, enter that address and the game port. This test bypasses the internet-facing side of the router.
- LAN connection succeeds: the process, bind settings and local firewall are probably functional. Investigate forwarding, public registration, NAT and the ISP.
- LAN connection fails: recheck that the server is listening, that the client and host are on the same network, and that the host firewall allows the selected UDP port.
- Browser listing fails but direct LAN connection works: focus on UDP 17777 and backend registration; browser queries are separate from gameplay traffic.
Do not use a public IP from inside the same LAN as your only test. Many consumer routers lack NAT hairpinning, the feature that loops an internal client back through the router’s public address. Such a failure does not prove that external players are blocked. Test the public path from a genuinely external network, such as a phone hotspot or a friend’s connection.
7. Check the public address and internet-side NAT
Compare the address shown as the router’s internet or WAN address with the public IPv4 address that your ISP assigns. If the router has one address but outside services see another, the router may be behind an additional modem, another router or carrier-grade NAT (CGNAT).
CGNAT is important because port forwarding can work only on equipment that receives the public address. When the ISP translates many customers behind a shared address, unsolicited inbound traffic never reaches your router, so no local forwarding rule can make the server publicly reachable. Ask the ISP whether the connection uses CGNAT and whether a public IPv4 address or an inbound-capable service is available.
If there are two routers in series, either place the inner router in the outer router’s designated exposed-host or bridge arrangement, or forward the selected UDP ports through both devices. Make the change deliberately and document which device owns the public address; indiscriminate double forwarding often creates more confusion than it solves.
8. Recheck backend registration
The backend uses publicAddress and publicPort to tell clients where to connect. An empty publicAddress normally allows automatic detection, but an incorrect explicit value can register a private, stale or otherwise unreachable address. Clear the field and restart if automatic detection is appropriate, or enter the current public IPv4 address when detection is wrong.
After a restart, allow time for the server browser and backend registration to refresh. A server can be running locally while its listing is temporarily absent. If LAN access works, the ports and address are correct, and external clients still receive an unreachable error, the fault may be on the official backend rather than on your machine. Check Bohemia Interactive’s service announcements and community channels for an incident before repeatedly rebuilding a working configuration.
Read the symptom before changing settings
| Observed symptom | Most likely area | Next check |
|---|---|---|
| Server process stops or never binds | Local service, port conflict or configuration | Read startup logs; verify current field names and available port. |
| LAN clients cannot join | Bind address, host firewall or wrong local port | Connect to the private IPv4 address and inspect UDP rules. |
| LAN works, internet clients cannot join | Router forwarding, public address, CGNAT or upstream firewall | Test from an external network and compare WAN and public addresses. |
| Direct connection works, browser listing does not | UDP 17777 or backend registration | Open the query port and verify publicAddress and publicPort. |
| Only a public-IP test from inside the LAN fails | Missing NAT hairpinning | Use the LAN address internally and an outside network for WAN testing. |
| RCON is unavailable while the game works | Optional administration port | Check UDP 19999 only if RCON is enabled; do not alter game-port rules. |
Common fixes that make the problem worse
Forwarding TCP because a guide says “port 2001”
A port number without a protocol is incomplete. The documented defaults are UDP. A TCP-only rule does not permit UDP game traffic, while adding broad TCP exposure can create an unnecessary service surface. Keep the UDP rule and verify the requirement for your exact build before adding anything else.
Using the host’s private address as publicAddress
Addresses that work only inside the LAN cannot be reached by internet clients. Leave publicAddress empty for automatic detection or enter the router’s public IPv4 address, never the host’s private address.
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Changing bindPort and publicPort independently
If the socket listens on one port while the backend advertises another, the listing can appear healthy but connections go nowhere. Omit bindPort in ordinary setups so it follows publicPort, or keep the two values intentionally aligned.
Relying on an online port checker as proof
UDP checkers cannot always reproduce the game’s handshake, and a failed web-based probe is not conclusive evidence that a port is closed. Combine external gameplay testing with server logs and router counters. A successful player connection is stronger evidence than a generic checker result.
Running forum PowerShell “fix” scripts
Do not execute a script merely because it promises to repair firewall or NAT settings. Such scripts can add unsafe rules, change unrelated services or contain malicious commands. Make individual, reversible changes through the firewall and router interfaces, and remove rules you no longer need.
A clean restart-and-test procedure
- Stop the dedicated server and record the port and address values currently configured.
- Replace obsolete field names with
bindPortandpublicAddress; remove an unnecessarybindPortentry so it followspublicPort. - Confirm UDP 2001 for gameplay and UDP 17777 for browser queries, using your custom values if you deliberately changed them.
- Reserve the host’s private IPv4 address in the router.
- Allow the selected UDP ports through the host firewall and forward the same ports to that reserved address.
- Start the server and read the log for a successful bind and registration.
- Join from a LAN client using the private address.
- Join from an external network using the public listing or direct public address.
- If LAN succeeds but external access fails, investigate CGNAT, double NAT, stale publicAddress data or a backend incident instead of reinstalling the game.
What to collect before asking for help
Support requests are resolved faster when they include the server build, operating system, configured publicPort, whether UDP 17777 is open, the result of the LAN test, the result of an external test, and whether the router’s WAN address matches the public IPv4 address. Include the relevant startup and registration log lines, but redact public credentials, RCON passwords and unrelated personal network information.
Describe the exact symptom: not listed, listed but fails to join, LAN failure, or only public-IP failure from inside the home. Those cases point to different layers, and separating them prevents repeated changes to a configuration that is already correct.
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