Do these 3 things before closing this tab:
1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsAn “unreachable” Arma Reforger server is almost always a networking or registration problem rather than a missing mod or game-content fault. Check the server’s UDP ports, registered address, host firewall, router forwarding and the difference between a local (LAN) and public (WAN) connection—in that order. The default setup uses UDP 2001 for game traffic and UDP 17777 for server-browser queries; RCON uses UDP 19999 only when remote administration is enabled.
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This guide separates a server that is running but hidden, a server that works on the same network but not from the internet, and a server that cannot complete the replication handshake. That distinction prevents you from changing settings that are unrelated to the actual failure.
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Arma Reforger clients must complete several network steps before you can join. The dedicated server first binds a UDP socket on the host, then registers an address and port with the backend. The browser may query the server separately, and the client finally establishes gameplay and replication traffic. A failure at any of those stages can look like the same message.
If the client log or in-game diagnostic shows Group 1 Replication reason=11 Server Unreachable, the replication connection could not reach or complete the server handshake. It does not identify a broken mission, mod or save file by itself. Treat it as a path, address, port or backend-registration symptom until a network test proves otherwise.
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Identify the symptom before changing anything
- Not listed in the server browser: investigate the query port, firewall rules and registration fields. A server can be running while its browser query is blocked.
- Listed but joining fails: verify the game port, the registered public address and router forwarding. Browser visibility does not guarantee that gameplay traffic can pass.
- Works for players on the same LAN but not over the internet: the server process is probably healthy; focus on WAN forwarding, the public address, CGNAT or an ISP policy.
- Fails for both LAN and WAN players: check that the process is running and listening, then inspect the host firewall and configuration.
- Many unrelated servers fail at once: consider a temporary backend outage after local settings have been ruled out.
Use the documented ports
Set the game and query rules to the documented defaults unless you have deliberately chosen different ports. Every device in the path must agree: the server configuration, the host firewall and the router’s port-forwarding rule.
| Function | Default port | Protocol | When it is needed |
|---|---|---|---|
| Public game traffic | 2001 | UDP | Required for players to connect and play. |
| Steam/A2S server-browser query | 17777 | UDP | Required for browser visibility and query responses. |
| RCON | 19999 | UDP | Optional; forward only if you use remote administration. |
The game and query ports are separate. Forwarding UDP 2001 alone can leave the server joinable by direct address while it remains absent or stale in the browser. Conversely, opening only UDP 17777 can make a listing appear without allowing gameplay. RCON is not part of ordinary joining; exposing it unnecessarily increases the attack surface.
Do not assume TCP is required
The documented defaults are UDP. Community instructions conflict about adding a TCP 2001 rule: some describe UDP exclusively, while others include TCP in a firewall example. The official server-port information does not establish TCP as a requirement. Start with UDP rules and verify the behavior of your particular build and host before adding any TCP exception.
Check the server configuration fields
Arma Reforger’s registration fields determine what the backend advertises, while the bind fields determine where the local process listens. A correct router rule cannot repair an address that the backend has been told to advertise incorrectly.
Understand each address and port
bindAddressis the local interface address. When omitted, its default is0.0.0.0, meaning the process can bind on available IPv4 interfaces rather than only one selected adapter.bindPortis the UDP socket port on the host. Its default is2001and the accepted range is 1–65535.publicPortis the UDP port registered with the backend. Its default is2001; it must match the externally forwarded game port.publicAddressis the public IPv4 address that clients use. If the field is missing or empty, the backend detects the address automatically. IPv6 is not supported for this server path.
The official configuration guidance says bindPort should generally be left out so it automatically uses the same value as publicPort. That avoids a subtle mismatch in which the process listens on one port while the backend registers another.
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Use current field names
Build 0.9.8.73 renamed two fields. Replace gameHostBindPort with bindPort, and replace gameHostRegisterBindAddress with publicAddress. A configuration copied from an older guide may therefore be silently ignored or leave the server registered with defaults.
A conservative configuration check
- Stop the dedicated server cleanly so it does not overwrite the file while you edit it.
- Locate the server configuration used by the launch command or hosting panel. Confirm that you are editing the active instance, not a template or an old profile.
- Look for
publicPort. Set it to the UDP port forwarded for game traffic; use2001if you have not intentionally changed the default. - Remove an unnecessary
bindPortentry, or set it to the same number aspublicPort. Leaving it out follows the documented automatic behavior. - Leave
publicAddressempty unless automatic detection registers the wrong address. If you must specify it, use the server’s current public IPv4 address, not a private address such as192.168.x.x,10.x.x.xor172.16.x.x–172.31.x.x. - Leave
bindAddressout unless you have a specific multi-interface requirement. The default0.0.0.0is normally the safest starting point. - Save the file, restart the server and watch its startup output for the bound port and registration result.
Do not paste a complete configuration from an unrelated version into a working installation. Preserve unrelated settings and change only the address and port entries you are testing.
Allow the ports through the host firewall
The operating-system firewall and any endpoint-security firewall must allow inbound UDP traffic to the game and query ports. A router rule cannot help if the packet reaches the host and is discarded there.
Windows host checks
- Open the firewall’s advanced inbound-rules view.
- Create or verify an inbound rule for UDP 2001 on the profiles used by the server (Private, Public or both, according to your network policy).
- Create or verify a separate inbound rule for UDP 17777 if you need server-browser visibility.
- Add UDP 19999 only when RCON is enabled and restrict its remote addresses when the firewall supports that control.
- Ensure the rule applies to the actual dedicated-server executable and profile. A rule for the game client does not necessarily cover the server process.
- Restart the server and confirm that the rule remains enabled. Some security products silently restore an application to a blocked list after an update; check their event log if a rule appears correct but traffic is still dropped.
Do not run “fix” scripts copied from forum posts. They can disable the firewall, add broad permanent exceptions or change unrelated networking settings. Make the smallest rule that permits the documented UDP traffic.
Linux or managed-host checks
Confirm the host firewall, cloud security group and any provider-level network policy all allow inbound UDP 2001 and UDP 17777. If the provider gives you a separate public firewall, an operating-system rule alone is insufficient. Keep RCON closed at every layer unless you actively use it.
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Forward the ports on the router
For a server behind a home router, create a port-forward (sometimes called a virtual-server or NAT rule) from the router’s public interface to the server’s fixed LAN IPv4 address.
- Give the server a DHCP reservation or a static LAN address so the destination cannot change after a reboot.
- Create a UDP rule for external port 2001 to the server’s LAN address and internal port 2001, unless you intentionally use another public port.
- Create a UDP rule for external port 17777 to the server’s LAN address and internal port 17777 for browser queries.
- Create UDP 19999 only for RCON, and limit the allowed source addresses if your router supports it.
- Save or apply the rules, then restart the router only if its interface requires that step.
- Check the router’s WAN status page. The address shown there must be a real public IPv4 address for inbound forwarding to work.
Use UDP in the protocol selector. If the interface offers “Both” and cannot create a UDP-only rule, verify the resulting rule rather than assuming TCP is needed. Extra TCP exposure is not a substitute for a missing UDP rule.
Match external and internal ports deliberately
Using the same external and internal port makes diagnosis easiest: public 2001 to LAN 2001 and public 17777 to LAN 17777. If you choose a different external port, set that number consistently as publicPort, in the router rule and in any firewall rule. A server listening on 2001 while the backend advertises 2101 will remain unreachable even though both ports are individually open.
Test from the LAN before testing the internet
Have a second device on the same network join using the server’s private LAN IPv4 address. This test bypasses the router’s public-side translation and answers a key question: can the host and game process communicate locally?
- LAN join succeeds: the process and local firewall are likely functional. Investigate WAN forwarding, the registered public address and ISP routing.
- LAN join fails: check that the server is running, listening on the expected port, bound to the correct interface and allowed by the host firewall.
- LAN join works but the public address fails from inside the house: the router may not support NAT hairpinning (also called loopback). Many consumer routers cannot send an internal client back through the router’s own public address. Test from a genuinely external network instead of treating this result as proof that the server is down.
For the LAN test, use the private address assigned to the server, not the address shown by an internet “what is my IP” page. Do not advertise the private address to internet players; it is only a diagnostic route inside your network.
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Rule out carrier-grade NAT
Port forwarding requires the router to receive unsolicited traffic on a public IPv4 address. With carrier-grade NAT (CGNAT), the ISP places another translation device in front of your router. Your router can show a WAN address and still have no way to receive inbound connections from the internet.
Compare the two WAN addresses
- Record the IPv4 address displayed on the router’s WAN or internet-status page.
- From a device on that connection, check the public IPv4 address reported by a trusted web service.
- If the values differ, or the router WAN address is in a shared/private range such as
100.64.0.0/10, ask the ISP whether CGNAT is enabled.
If CGNAT is confirmed, ordinary port forwarding will not make the server publicly reachable. The practical options are an ISP-provided public IPv4 address, a business/static service where available, or hosting the server on a network with a directly reachable address. Do not keep changing local firewall rules expecting them to overcome an upstream NAT boundary.
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After restarting, allow time for the server to register. Check the server’s own startup messages for the selected bind port, public port and registration status. If the server registers an address that belongs to an old ISP connection, a VPN adapter or another machine, set publicAddress to the current public IPv4 address or clear it and let automatic detection run again.
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Keep the query path separate from the join path during diagnosis. UDP 17777 controls Steam/A2S-style browser queries; UDP 2001 carries the game connection. A listing that appears after opening 17777 is evidence that queries can reach the host, not that the game handshake on 2001 is working.
Online port-check websites are not definitive for UDP. Many cannot create a valid Arma Reforger or A2S query, and a failed generic check does not prove that your rule is closed. Prefer a real browser refresh or an external player’s join attempt, while observing the router and host firewall logs for an incoming packet.
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Check for address and version mismatches
Configuration examples written before build 0.9.8.73 may use the retired names gameHostBindPort and gameHostRegisterBindAddress. Replace them with bindPort and publicAddress; do not keep both versions and assume the server will choose correctly.
Also check for these common mismatches:
- The router forwards UDP 2001, but
publicPortadvertises another number. - The server binds to a single obsolete adapter address instead of the default
0.0.0.0. publicAddresscontains a LAN address, a stale public address or an IPv6 address. IPv6 is not supported for this setup.- The firewall rule covers the client executable but not the dedicated-server executable.
- A second server on the same host already occupies the chosen UDP port.
Consider a backend incident only after local checks
The backend is involved in server registration and matchmaking, so a service-side fault can produce unreachable errors even when the host, router and configuration are correct. Reports from players have described backend errors in otherwise valid setups, but no uptime or incident status is established here.
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A complete troubleshooting sequence
Use this order so each test narrows the fault domain instead of introducing several changes at once.
- Stop and identify the instance. Confirm the correct server process, profile and configuration file.
- Record the intended ports. Start with UDP 2001 for gameplay and UDP 17777 for queries; reserve UDP 19999 for RCON only.
- Correct field names. Use
bindPortandpublicAddresson builds that include the 0.9.8.73 rename. - Simplify the address settings. Omit
bindPortunless needed, leavepublicAddressempty for automatic detection and leavebindAddressat its default unless you have a reason to pin an interface. - Restart and read startup output. Look for the actual bind and registration values rather than relying on the file alone.
- Test by LAN IP. A local join separates process and host-firewall problems from internet routing.
- Check host firewall rules. Permit inbound UDP on the exact game and query ports for the dedicated-server process.
- Check router forwarding. Send the same UDP ports to a fixed LAN address and confirm the router has a real public IPv4 WAN address.
- Test from outside. Use a player or connection on another network; do not rely on a hairpin test from inside the same house.
- Investigate CGNAT or an upstream firewall. Compare router WAN and externally observed addresses and ask the ISP when they differ.
- Check for a wider backend problem. Do this only after both local and external paths have been tested.
What not to change
- Do not reinstall the game merely because the server is unreachable; the symptom concerns connectivity and registration.
- Do not open every port on the router. The documented game and query paths are sufficient for ordinary operation.
- Do not expose RCON to the whole internet when you do not use it.
- Do not switch to IPv6 as a workaround; IPv6 is not supported for this server path.
- Do not add TCP rules as a first response to a UDP failure. Verify the UDP path and the version-specific behavior first.
- Do not trust a browser listing alone as proof that players can join; query traffic and gameplay traffic use different ports.
When to escalate to the host or ISP
Contact the hosting provider or ISP with a concise packet of facts rather than a generic “server unreachable” report. Include the Reforger build, the server’s LAN address, the configured publicPort, whether UDP 2001 and 17777 are forwarded, the router WAN address, whether a LAN join works, and the exact time of an external failed attempt. Ask specifically whether inbound UDP is filtered, whether CGNAT is active and whether the assigned public address is shared.
If you administer the machine yourself, preserve the server log from the failed attempt and the router’s firewall/NAT event. Those records can show whether a packet reached the router, was forwarded, arrived at the host and received a response. That is more useful than repeatedly changing unrelated game settings.
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