For a conventional Left 4 Dead 2 dedicated or listen server, use UDP 27015 for gameplay. Add TCP 27015 only if you need the SRCDS RCON console. Forward the selected port from your router to the server computer’s stable private address, allow it through the host firewall, and make the server configuration use that same port. If 27015 is occupied, Valve lists 27016–27019 and 27021–27032 as alternatives.
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Which ports Left 4 Dead 2 uses
The port number alone does not identify the job. The protocol determines how traffic is handled, so create rules for the protocol you actually need rather than forwarding every Steam-related range to the server.
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| Purpose | Protocol | Default local port | Forward it when |
|---|---|---|---|
| Dedicated/listen-server gameplay | UDP | 27015 | Players on other networks must connect directly to the server. |
| SRCDS RCON | TCP | 27015 | You will administer the server remotely with an RCON client; otherwise leave it unexposed. |
| Alternative gameplay port | UDP | 27016–27019 or 27021–27032 | 27015 is already occupied or blocked and you change the server setting to the selected value. |
Steam Support identifies UDP local port 27015 as the default gameplay port and TCP local port 27015 as the default SRCDS RCON port. Those are the dedicated-server rules relevant to a typical home or rented host; they are not a request to forward all ports used by the Steam client.
How port forwarding makes the server reachable
Your router has a public (WAN) address, while the game server normally has a private address such as 192.168.1.100. An Internet player reaches your public address and selected port; the router must translate that connection to the server’s private address and the port on which SRCDS is listening.
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The forwarding destination must remain stable. A DHCP reservation tied to the server’s network adapter is usually safer than relying on a changing lease. If the computer receives a new private address, a rule still pointing to the old address sends traffic nowhere even though the rule appears enabled.
Keep the number consistent across the launch command or configuration file, the router rule, and the operating-system firewall. Mapping a public port to a different internal port can work on some routers, but it adds an avoidable variable when diagnosing a game that cannot be found.
Port-forwarding procedure for a conventional server
- Reserve the server’s LAN address. In the router’s DHCP or LAN settings, reserve an address for the computer running L4D2. Record that address.
- Choose the listening port. Start with
27015in the SRCDS launch options or server configuration. If another service already owns it, select one of Valve’s alternate ports and use that value everywhere below. - Create the router rule. Open the router’s Port Forwarding, Port Range Forwarding, or similarly named page. Add the selected port, set the destination to the reserved LAN address, and select UDP for gameplay. Use the same external and internal port unless you have a specific reason not to.
- Add TCP only for RCON. If remote RCON is required, create a separate TCP rule for the RCON port. Restrict its source addresses to your administration network or VPN when the router supports that control; exposing an RCON password to the entire Internet increases the attack surface.
- Permit the traffic on the host. In Windows Defender Firewall, ufw, firewalld, or another host firewall, allow the selected UDP port for the game process. Add the TCP rule only when RCON is enabled. A router rule cannot override a host firewall that drops the packet after delivery.
- Start the server and verify its listener. Check the SRCDS console or log for the configured port and confirm that no other process failed to bind it. If the log shows a different port, correct the launch/configuration setting before changing the router again.
- Test from outside your home network. Have a friend connect from a different Internet connection, or test through a mobile hotspot. An apparent failure from a computer on the same LAN may be caused by the router’s lack of NAT loopback rather than a bad forwarding rule.
Players connecting from another network should use your public address and selected port when the game interface requires an explicit endpoint. Do not publish your private address to the Internet; it is only useful to clients on the same local network.
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If port 27015 is unavailable
Valve’s router guidance suggests trying 27016–27019 or 27021–27032. Pick one unused value rather than opening the entire range.
| Change | Required action |
|---|---|
| Server process | Set the SRCDS port launch option or equivalent configuration to the new value. |
| Router | Forward that UDP value to the server’s reserved LAN address; add TCP on the same value only if RCON uses it. |
| Host firewall | Allow the new protocol/port pair and remove or disable obsolete rules when they are no longer needed. |
| Players | Use the new public endpoint, including :port when the client or console requires it. |
Changing only the router rule is insufficient. If SRCDS still listens on 27015 while the router sends 27016, the packet reaches the computer but no process is waiting on that port.
Should TCP 27015 be open for RCON?
No, not for ordinary gameplay. RCON is a separate administrative channel. A server can accept player traffic over UDP while keeping TCP RCON disabled or reachable only from the LAN.
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- Use a long, unique RCON password and do not reuse an account password.
- Prefer a VPN, an administrator-only source-address rule, or LAN-only access instead of an unrestricted Internet rule.
- Remove the TCP forwarding rule when you are finished with remote administration.
- Review server logs for unexpected authentication attempts if RCON has been exposed publicly.
The gameplay connection does not become more reliable because TCP 27015 is open. Opening it changes the administrative exposure, not the UDP path used by players.
Same-router players: private addresses and NAT loopback
A client behind the same router may not be able to reach the server through the router’s public address. Some routers do not support hairpin (NAT loopback) connections, and others handle them differently from external traffic.
For a local player, use the server’s private address in private-ip:port form when the game’s connection interface permits it—for example, 192.168.1.100:27015. Replace both values with the server’s actual LAN address and configured port. This local test does not prove that Internet forwarding works; perform a second test from an external network.
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Docker deployments
Container networking adds another translation layer. With Docker bridge networking, publish the game port for both protocols so the host can receive UDP gameplay and any TCP traffic required by the deployment:
-p 27015:27015/udp -p 27015:27015/tcpUse the actual port selected for the server in place of 27015. The published host port, the container’s listening port, the router rule, and the host firewall must agree. The Left4DevOps L4D2 Docker documentation recommends host networking when accurate identification of connecting players matters; bridge mode can obscure client-address information because of its additional NAT layer. Follow the project’s networking guidance for the image and runtime you use, and do not assume that publishing a port automatically creates the router’s WAN forwarding rule.
When Steam Networking can remove the forwarding requirement
Valve documents Steam Networking paths that use Steam Datagram Relay and can let users create joinable self-hosted servers without traditional port forwarding. This is an alternative reachability method, not a change to the UDP/TCP roles of a direct SRCDS listener.
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Whether it applies depends on the server configuration, game branch, and connection path. If a supported Steam Networking setup is in use, follow that configuration’s join instructions and test with players on an external network. If the server is using a conventional direct public endpoint, the router and firewall procedure above still applies.
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Steam client ranges you should not forward automatically
Steam’s general support material also lists broader service ranges, including UDP 27000–27250 for game traffic and UDP 3478, 4379, and 4380 for peer-to-peer or voice contexts. Those values describe Steam client and Steamworks requirements across many games. They are not a blanket instruction to send every port in those ranges to a home Left 4 Dead 2 server.
Forward only the selected gameplay port, plus the RCON port if you have deliberately enabled remote administration. Narrow rules are easier to audit and reduce unintended exposure.
Troubleshooting an unreachable server
| Symptom or check | Likely cause | Fix |
|---|---|---|
| The server log shows a different port than the router rule. | Launch options or configuration were not changed together with the forwarding rule. | Choose one port and make the server, router, firewall, and player endpoint use it consistently. |
| External players time out, but the process is running. | UDP is missing at the router or host firewall, or the rule targets an old LAN address. | Confirm the current DHCP reservation, UDP protocol, destination address, and host-firewall allowance. |
| RCON fails while gameplay works. | TCP is not allowed, RCON is disabled, or the RCON port/password is wrong. | Check the RCON configuration and add a restricted TCP rule only if remote administration is necessary. |
| A same-LAN player cannot join via the public address. | The router lacks NAT loopback or handles hairpin traffic differently. | Connect to the private address and port from inside the LAN; test the public endpoint externally. |
| Bridge-mode Docker clients appear with incorrect addresses or cannot connect. | The container port was published for only one protocol, or bridge NAT interferes with address visibility. | Publish UDP (and TCP where needed), match container and host ports, or evaluate host networking. |
| No forwarding rule works from the Internet. | The selected Steam Networking path may use relays, or the ISP may place the router behind another NAT. | Verify whether the server is intended to use Steam Networking; otherwise check for an upstream modem/router and confirm that the router’s WAN address is publicly reachable. |
- Confirm SRCDS is listening on the intended interface and port, not just that its process exists.
- Check the router’s event or connection log for an arriving packet during an external test.
- Temporarily test with a narrowly scoped firewall rule rather than disabling the entire host firewall.
- After selecting an alternate port, search every configuration layer for the old value.
Choosing a hosting and reachability method
| Method | Reachability | Protocol exposure | Deployment considerations | Best fit |
|---|---|---|---|---|
| Direct bare-metal or listen server | Router forwards a public port to the host. | UDP gameplay; optional TCP RCON. | Few translation layers; requires a stable LAN address and firewall rules. | Administrators who control the router and want a conventional endpoint. |
| Docker bridge networking | Router forwards to the Docker host, which publishes the container port. | Publish UDP for gameplay and TCP where the container requires it. | Host-to-container NAT must be configured; client-address fidelity can be affected. | Containerized deployments where bridge isolation is desired. |
| Docker host networking | Router forwards directly to the host’s listening socket. | Same UDP/TCP choices as the server process. | Less port translation and generally better client identification, with less network isolation. | Deployments following the project’s recommendation for accurate player identification. |
| Steam Networking relay path | Steam Datagram Relay can provide joinability without traditional inbound forwarding. | Managed by the supported Steam Networking configuration. | Availability depends on the game/server path; direct-port assumptions may not apply. | Hosts whose configuration supports Valve’s relay-based self-hosting. |
For most conventional home servers, the smallest working rule set is one UDP gameplay forward to a reserved LAN address, a matching host-firewall allowance, and an external test. Add TCP only for a deliberate RCON requirement, and use a relay-based Steam Networking path when the server configuration supports it.
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