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The quickest way to run a vanilla Left 4 Dead 2 dedicated server is Docker’s host network mode:
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docker run --name l4d2 --network host left4devops/l4d2On Linux, this lets the container use the host’s network directly. The image documentation prefers it because the server can see players’ real IP addresses, which is important for bans and RCON protection. If you use Docker Desktop on Windows or macOS, enable its host-networking support first. Bridge mode also works, but you must publish the selected game port on both TCP and UDP with identical host and container values.
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Prepare the Docker host before launching the server. You need:
- Docker Engine, or Docker Desktop on Windows or macOS.
- A host that can remain online whenever friends need to join.
- Enough disk space for the dedicated-server files, updates and any add-ons you install.
- Administrative access to allow the game port through the host firewall and, for players outside your home network, to forward that port from your router to the Docker host.
- A stable public address, or a Steam Group route, if players will connect over the internet.
The official dedicated-server package is Steam application ID 222860. Docker images normally install or update this package for you, but knowing the ID is useful when you need to install it manually with SteamCMD or validate a damaged installation.
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Choose Docker networking before you start
Host networking: the simplest Linux setup
With host networking, the container shares the host’s network namespace. There is no -p option because Docker does not translate or publish a separate container port. The server listens directly on the host’s selected port.
The Left4DevOps image documentation prefers this mode because the game server can identify connecting player IP addresses correctly. That allows server-side bans to target the right address and helps protect RCON access. Docker Desktop users must enable host-networking support; without that feature, use bridge networking instead.
Bridge networking: portable, explicit port publishing
Bridge mode gives the container its own network interface. You publish the game port from the host with two mappings: one for TCP and one for UDP.
docker run --name l4d2-bridged
-p 27015:27015/tcp
-p 27015:27015/udp
left4devops/l4d2The left-hand and right-hand port numbers must match. The project documentation warns:
“When publishing your ports with bridge networking, ensure the same port is used for both client and host. Otherwise matchmaking will advertise the wrong port and new players will not be able to connect.”
Bridge mode is often the practical choice on Docker Desktop and on hosts where you need Docker’s normal port-management behavior. It requires more careful configuration because the port advertised to matchmaking must be the port players can actually reach.
Host and bridge at a glance
| Decision | Host network | Bridge network |
|---|---|---|
| Container command | --network host |
-p HOST:CONTAINER/tcp and -p HOST:CONTAINER/udp |
| IP visibility | Preferred by the image documentation for correct player IP detection | Uses Docker’s network translation |
| Port configuration | No Docker port publishing; the server uses the host network | TCP and UDP mappings are both required |
| Windows and macOS | Requires Docker Desktop host-networking support | Works with standard Docker Desktop port publishing |
| Main failure mode | Host-networking support is unavailable or disabled | TCP/UDP mappings or the advertised PORT value do not match |
Start a vanilla server
Linux or a Docker Desktop host with host networking enabled
- Pull and start the image in one operation:
docker run --name l4d2 --network host left4devops/l4d2 - Watch the startup output in another terminal:
docker logs -f l4d2 - Wait for the image to finish installing or updating the dedicated-server files and for the Source server process to begin listening. Keep the log window available when diagnosing connection or add-on problems.
Bridge mode on port 27015
Use the following command when host networking is unavailable:
docker run --name l4d2-bridged
-p 27015:27015/tcp
-p 27015:27015/udp
left4devops/l4d2Allow TCP and UDP 27015 through the host firewall. If players connect from outside your LAN, forward both protocols on your router to the Docker host. Give players the public address and port, not the container’s private Docker address.
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Choosing a different port
The image uses the PORT environment variable for the game and RCON port. If you do not set it explicitly, the image normally selects the next free port after 27015. Setting it explicitly removes ambiguity, especially when several servers share one host.
docker run --name l4d2-bridged
-e PORT=27016
-p 27016:27016/tcp
-p 27016:27016/udp
left4devops/l4d2Every occurrence must agree: the PORT value, the host-side mapping, the container-side mapping and the port you give to players. A mismatch can make matchmaking advertise a port that is not reachable.
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Install or update the server with SteamCMD
If you want a separate installation step, or you are building your own image, SteamCMD installs the dedicated-server package by application ID.
docker run -it -v "$(pwd)/steamapps:/steamapps"
--name steamcmd left4devops/steamcmd
+login gaben +app_update 222860 +quitThis command mounts a local steamapps directory at /steamapps, logs in as shown, downloads App ID 222860 and exits. Keeping the mount means the downloaded files remain on the host when the temporary SteamCMD container is removed. Treat Steam credentials as sensitive and do not place them in a public script or image layer.
Some implementations run validation during an update:
steamcmd +app_update 222860 validateValidation checks the installed files and redownloads files that do not match Steam’s copy. The alternative image documentation also exposes a VALIDATE environment variable: when it is set to a non-empty value, the image runs SteamCMD validation before startup.
Automatic updates in the production image
The production Left4DevOps image can update the server automatically at startup. Allow that first run to complete, then inspect the logs with docker logs -f l4d2. If you need a deterministic maintenance window, stop the container, run a separate SteamCMD update or validation against the persistent files, and start the server again.
Persist files so container recreation is safe
A container is disposable. If the server directory exists only inside the container, removing and recreating that container downloads the files again and can discard changes. Use a Docker volume or bind mount for the server data, and mount custom content separately.
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SteamCMD data mount
The SteamCMD example above persists its download cache and application files with:
-v "$(pwd)/steamapps:/steamapps"Keep that host directory backed up if it contains configuration, downloaded maps or other files you cannot easily reproduce.
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An alternative implementation documents this bind mount:
-v /srv/l4d2:/l4d2With that layout, the host’s /srv/l4d2 directory holds the server files exposed in the container at /l4d2. Use the path required by the image you selected; container paths are not interchangeable between images.
Mount add-ons
For the Left4DevOps image, custom content can be mounted into /addons/:
docker run --name l4d2
--network host
-v ./addons:/addons/
left4devops/l4d2Place the add-on files in the host-side ./addons directory before starting the container. If you recreate the container, keep the same mount so the add-ons are available immediately.
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Set configuration values on the first docker run, or recreate the container with revised values. The documented variables are:
| Variable | Purpose | Important detail |
|---|---|---|
HOSTNAME |
Name shown in the server browser | Choose a distinctive name so friends can identify the server |
MOTD |
Message-of-the-day setting | Use with the image’s documented MOTD behavior |
HOST_CONTENT |
Controls host content for the MOTD | Use the value format expected by the image |
MOTD_CONTENT |
Text or content for the message of the day | Set it alongside the MOTD-related options when you want custom text |
REGION |
Numeric region hint used for server selection | Use one of the documented codes below |
STEAM_GROUP |
Associates the server with a Steam Group | Players can select the group’s server route |
STEAM_GROUP_EXCLUSIVE |
Restricts ordinary lobby selection to group members | Set to true; non-members can still use mm_dedicated_force_servers |
PORT |
Game and RCON port | Match bridge mappings exactly when using bridge networking |
Region codes
| Code | Region |
|---|---|
| 0 | East Coast USA |
| 1 | West Coast USA |
| 2 | South America |
| 3 | Europe |
| 4 | Asia |
| 5 | Australia |
| 6 | Middle East |
| 7 | Africa |
| 255 | World; documented default |
Example configured launch
docker run --name l4d2-bridged
-e HOSTNAME="Weekend Co-op"
-e REGION=3
-e STEAM_GROUP="your-group"
-e STEAM_GROUP_EXCLUSIVE=true
-e PORT=27015
-p 27015:27015/tcp
-p 27015:27015/udp
-v ./addons:/addons/
left4devops/l4d2Replace the example group value with the group identifier expected by the image. Keep the port, mappings and advertised address consistent with your firewall and router configuration.
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How friends join the Docker server
The image documentation lists several supported connection paths. Use the one that fits your setup:
Best Available Dedicated in a lobby
In the Left 4 Dead 2 lobby, select Best Available Dedicated. Matchmaking chooses an available dedicated server according to its region and group settings.
Force a specific server from the developer console
- Enable and open the game’s developer console.
- Enter:
mm_dedicated_force_servers IP:PORT - Replace
IP:PORTwith the reachable address and the exact value configured inPORT.
This method is also available to non-members when STEAM_GROUP_EXCLUSIVE=true, according to the image documentation.
Use a Steam Group server
If the server is associated with STEAM_GROUP, select the Steam Group server route in the game. The image documentation also states that LAN servers are shown automatically as Steam Group servers.
Browse or connect directly through Steam
- Open the game’s server browser with
openserverbrowser. - Use Steam’s Game Servers view to find the address.
- Open the developer console and enter:
connect IP:PORT
For direct connections, players must be able to reach the public IP and selected port. A private Docker address or an address that works only inside your LAN will not work for friends on the internet.
Keep the server running after crashes and reboots
Watchdog behavior
The image starts the server through srcds_run. Its watchdog restarts the server process after a crash. Follow the container output with:
docker logs -f l4d2If the process repeatedly crashes, leave the logs attached while reproducing the problem; they usually reveal whether the failure occurs during SteamCMD updates, add-on loading or Source startup.
Restart after a host reboot
Add Docker’s restart policy when you create the container:
docker run --name l4d2
--restart unless-stopped
--network host
left4devops/l4d2unless-stopped starts the container again after a Docker daemon or host reboot, while respecting a deliberate manual stop.
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Safe update cycle
- Tell players the server will restart and stop accepting new games.
- Stop the container with
docker stop l4d2. - Update or validate App ID 222860 using the persistent SteamCMD files.
- Start the existing container with
docker start l4d2. - Check
docker logs -f l4d2and make a test connection before inviting players back.
Troubleshooting checklist
Players cannot connect in bridge mode
- Confirm both mappings exist: one
/tcpand one/udp. - Confirm the host and container ports are identical in each mapping.
- Confirm
PORT, if set, is the same number. - Allow that port through the host firewall and forward both protocols on the router for internet players.
- Use the public IP and port externally; use the LAN address only for players on the same network.
- Read the startup and matchmaking information with
docker logs -f l4d2.
Matchmaking advertises the wrong port
This is usually a configuration mismatch. Recreate the container so PORT and both -p mappings use one number. Do not map host port 27016 to container port 27015 while leaving PORT=27015; the documentation specifically warns that mismatches advertise an unreachable port.
The server downloads everything again
The server directory was not persisted, or the recreated container uses a different mount. Reattach the same Docker volume or bind mount. For the documented alternative layout, verify that /srv/l4d2 is mounted to /l4d2. For SteamCMD, keep the steamapps bind mount.
Add-ons do not appear
Verify that the host directory contains the add-on files and that it is mounted at the image’s expected path. The Left4DevOps example uses -v ./addons:/addons/. Then restart the container and inspect the logs for load errors.
Host networking fails on Windows or macOS
Docker Desktop requires host-networking support to be enabled for the host-mode command. If that feature is unavailable in your installation, use bridge mode with matching TCP and UDP mappings instead.
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Run docker logs l4d2 without -f to inspect the complete output. Check SteamCMD authentication and App ID 222860 installation first, then validate the files. If the image supports VALIDATE, set it to a non-empty value for a validation pass before startup.
Docker implementation choices
The Left4DevOps image is the most direct documented route because it provides a one-command vanilla launch, automatic update capability, configurable hostname, region, Steam Group and port settings, add-on mounting, log access through Docker and the srcds_run watchdog. An alternative implementation is useful when you specifically need its documented /srv/l4d2:/l4d2 persistence layout or optional validation behavior.
| Capability | Left4DevOps route | Alternative implementation documented in the setup examples |
|---|---|---|
| Vanilla launch | docker run --name l4d2 --network host left4devops/l4d2 |
Uses its own image command and layout |
| Steam package | Can update automatically | Documents steamcmd +app_update 222860 validate |
| Persistence | Use a volume or the image’s documented data path | Example mount: /srv/l4d2:/l4d2 |
| Add-ons | Mount host content at /addons/ |
Use the paths documented by that image |
| Validation trigger | Not stated as a separate variable in the supplied image description | Non-empty VALIDATE runs validation before startup |
| Logging and recovery | Docker logs plus srcds_run watchdog |
Use the image’s documented process behavior and Docker logs |
For most administrators, the decision is networking and persistence rather than image branding: choose host networking when the platform supports it and accurate player IP handling matters; choose bridge mode when you need explicit Docker port publishing; in both cases, persist the server files and keep the advertised port consistent.
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