# L4D2 Dedicated Server Install Guide

A short guide for installing a Left 4 Dead 2 dedicated server with SourceMod and MetaMod:Source on a Debian or Ubuntu VPS.

**Assumptions:**
- Fresh Debian 12+ or Ubuntu 22.04+ VPS
- ~10 GB free disk, 2 GB+ RAM
- Non-root user with sudo (commands below assume your user, not root)
- Open ports: 27015/udp (game), 27020/udp (SourceTV)

Time estimate: 30–60 minutes including game file download.

---

## 1. Prep the system

L4D2 dedicated server is a 32-bit (i386) binary. Multi-arch is required.

```bash
sudo dpkg --add-architecture i386
sudo apt update
sudo apt upgrade -y
sudo apt install -y curl wget file tar bzip2 gzip unzip bsdmainutils \
    python3 util-linux ca-certificates binutils bc jq tmux netcat-openbsd \
    lib32gcc-s1 lib32stdc++6 libcurl4-openssl-dev:i386 libsdl2-2.0-0:i386
```

(`libsdl2-2.0-0:i386` is what L4D2 needs at runtime; missing it = silent crash on first launch.)

---

## 2. Create a dedicated user

Never run game servers as root.

```bash
sudo adduser --disabled-password --gecos "" l4d2server
sudo passwd l4d2server   # set a password if you want SSH access
sudo su - l4d2server
```

The rest of the guide runs as `l4d2server`.

---

## 3. Install LinuxGSM

LinuxGSM is the standard wrapper for Source-engine dedicated servers — handles updates, start/stop, monitoring, console capture.

```bash
wget -O linuxgsm.sh https://linuxgsm.sh
chmod +x linuxgsm.sh
bash linuxgsm.sh l4d2server
```

This downloads the LinuxGSM script tailored for L4D2 and saves it as `l4d2server` in your home directory.

---

## 4. Install the game files

```bash
./l4d2server install
```

The installer will:
- Check dependencies
- Prompt for hostname, rcon password, admin SteamID (you can leave blanks and edit later)
- Run SteamCMD to download ~10 GB of L4D2 server files (takes 5–20 min depending on connection)

When it finishes you should see `Install complete` and the prompt comes back.

---

## 5. Get a Steam Game Server Login Token (GSLT)

Required to show up as a public server (without one, your server is invisible to the in-game browser).

1. Open https://steamcommunity.com/dev/managegameservers in your browser
2. Sign in with a Steam account
3. Create a GSLT for **App ID 222860** (L4D2)
4. Give it a memo like "my-l4d2-vps"
5. Copy the token string

On the VPS:

```bash
nano ~/lgsm/config-lgsm/l4d2server/l4d2server.cfg
```

Add (or uncomment and set):

```
gslt="YOUR_TOKEN_HERE"
```

Save (Ctrl+O, Enter, Ctrl+X).

---

## 6. Configure server basics

Edit the gameplay config:

```bash
nano ~/serverfiles/left4dead2/cfg/server.cfg
```

Minimum useful contents:

```
hostname "My L4D2 Server"
sv_search_key "anything"
rcon_password "USE_OPENSSL_RAND_HEX_24"
sv_lan 0
mp_gamemode "coop"
sv_allow_lobby_connect_only 0
sv_steamgroup_exclusive 0
sv_region 1
```

Generate a strong RCON password with `openssl rand -hex 24` (run in another terminal). Don't paste passwords anywhere afterward.

LinuxGSM-specific config (start parameters, hostname, etc.) lives in `~/lgsm/config-lgsm/l4d2server/l4d2server.cfg`. The two configs are loaded at different times — `l4d2server.cfg` (LinuxGSM) is for startup parameters, `server.cfg` (game engine) is exec'd on each map load.

---

## 7. Patch executable stack flags (required on modern Linux)

Modern Linux kernels refuse to load shared libraries flagged with executable stacks. L4D2's bundled `.so` files have this flag set, causing the server to fail silently on startup.

Save this Python script as `~/clear_execstack.py`:

```python
#!/usr/bin/env python3
import os, struct, sys

def clear_execstack(path):
    with open(path, 'r+b') as f:
        if f.read(4) != b'\x7fELF':
            return False
        f.seek(0x10)
        e_type = struct.unpack('<H', f.read(2))[0]
        if e_type not in (2, 3):  # EXEC or DYN
            return False
        f.seek(0x20)
        e_phoff = struct.unpack('<I', f.read(4))[0]
        f.seek(0x2A)
        e_phentsize = struct.unpack('<H', f.read(2))[0]
        e_phnum = struct.unpack('<H', f.read(2))[0]
        for i in range(e_phnum):
            f.seek(e_phoff + i * e_phentsize)
            p_type = struct.unpack('<I', f.read(4))[0]
            if p_type == 0x6474e551:  # PT_GNU_STACK
                f.seek(e_phoff + i * e_phentsize + 0x18)
                flags = struct.unpack('<I', f.read(4))[0]
                if flags & 1:  # PF_X set
                    f.seek(e_phoff + i * e_phentsize + 0x18)
                    f.write(struct.pack('<I', flags & ~1))
                    return True
        return False

if __name__ == '__main__':
    target = sys.argv[1] if len(sys.argv) > 1 else '/home/l4d2server/serverfiles'
    fixed = 0
    for root, _, files in os.walk(target):
        for fname in files:
            if fname.endswith('.so') or fname.endswith('.so.1'):
                path = os.path.join(root, fname)
                try:
                    if clear_execstack(path):
                        print(f"Patched: {path}")
                        fixed += 1
                except Exception as e:
                    print(f"Skip {path}: {e}")
    print(f"\n{fixed} files patched.")
```

Run it:

```bash
chmod +x ~/clear_execstack.py
python3 ~/clear_execstack.py
```

You'll need to re-run this after every L4D2 game update (Valve ships fresh `.so` files unpatched).

---

## 8. First start

```bash
./l4d2server start
./l4d2server details
```

`details` shows hostname, IPs, ports, current status. The server should be running on `27015/udp`.

To watch the live console:

```bash
./l4d2server console
```

(Detach without killing the server: `Ctrl+B` then `D`.)

If it crashes immediately, check:

```bash
tail -50 ~/log/console/l4d2server-console.log
```

---

## 9. Test the connection

In L4D2 client, open the in-game console (~ key, must be enabled in Keyboard/Mouse options) and:

```
connect YOUR_VPS_IP:27015
```

You should load into the default map (c5m1_waterfront unless you've changed it).

---

## 10. Install MetaMod:Source

MetaMod is the loader that lets SourceMod hook into the engine.

```bash
cd /tmp
wget https://mms.alliedmods.net/mmsdrop/1.12/mmsource-latest-linux.tar.gz
tar xzf mmsource-latest-linux.tar.gz -C ~/serverfiles/left4dead2/
```

Verify:

```bash
ls ~/serverfiles/left4dead2/addons/
# Should show: metamod  metamod.vdf
```

---

## 11. Install SourceMod

```bash
cd /tmp
wget https://sm.alliedmods.net/smdrop/1.12/sourcemod-latest-linux.tar.gz
tar xzf sourcemod-latest-linux.tar.gz -C ~/serverfiles/left4dead2/
```

---

## 12. Make yourself an admin

Find your SteamID in Steam-style format (STEAM_1:1:xxxxx). Easiest way: https://steamid.io

Edit:

```bash
nano ~/serverfiles/left4dead2/addons/sourcemod/configs/admins_simple.ini
```

Add at the end:

```
"STEAM_1:1:YOUR_NUMBER"  "z"
```

`z` = root flag (all admin powers).

---

## 13. Restart and verify SourceMod loaded

```bash
./l4d2server restart
sleep 15
./l4d2server send "meta version"
sleep 2
./l4d2server send "sm version"
sleep 2
tail -30 ~/log/console/l4d2server-console.log | grep -iE "metamod|sourcemod"
```

You should see Metamod and SourceMod version banners with no errors.

---

## 14. Useful day-to-day commands

```bash
./l4d2server start            # start the server
./l4d2server stop             # stop the server
./l4d2server restart          # restart
./l4d2server details          # show status / IPs / ports
./l4d2server console          # attach to live console (Ctrl+B D to detach)
./l4d2server send "command"   # send a console command
./l4d2server update           # update game files via SteamCMD (re-run clear_execstack.py after)
./l4d2server update-lgsm      # update LinuxGSM scripts
./l4d2server backup           # back up server files (config + plugins, not game files)
```

---

## 15. Basic hardening

```bash
# Firewall
sudo apt install -y ufw
sudo ufw default deny incoming
sudo ufw default allow outgoing
sudo ufw allow 22/tcp comment 'SSH'
sudo ufw allow 27015/udp comment 'L4D2 game'
sudo ufw allow 27020/udp comment 'SourceTV'
sudo ufw enable

# Auto-apply security updates
sudo apt install -y unattended-upgrades
sudo dpkg-reconfigure -plow unattended-upgrades

# SSH brute-force protection
sudo apt install -y fail2ban
sudo systemctl enable --now fail2ban
```

Don't expose 27015/tcp (RCON) unless you specifically use external RCON tools — most admins don't, and it's a brute-force target.

---

## 16. Common gotchas

**Server won't start, no obvious error:**
- Run `clear_execstack.py` — missing this is the #1 silent failure on fresh Debian/Ubuntu
- Check `libsdl2-2.0-0:i386` is installed

**LinuxGSM uses `l4d2server.cfg`, not `server.cfg`:**
- `~/lgsm/config-lgsm/l4d2server/l4d2server.cfg` — LinuxGSM startup parameters (GSLT, hostname, ports)
- `~/serverfiles/left4dead2/cfg/server.cfg` — engine-level gameplay cvars, exec'd on map load

**Server not visible in browser:**
- GSLT not set, set wrong, or banned by Valve (request a new one)
- `sv_lan 1` accidentally set (use `0` for public)
- Firewall blocking 27015/udp

**Steam "Invalid platform" on SteamCMD:**
- Sometimes needed for L4D2 updates: prefix with `+@sSteamCmdForcePlatformType windows` before login (rare workaround)

**Can't open in-game console:**
- Steam → Settings → In-Game → enable Developer Console
- Default key is `~`, can be rebound in keyboard options

---

## Where to go from here

Now you have a base server. Optional next steps:

- Install plugins (see https://forums.alliedmods.net for L4D2 plugins)
- Set up persistent player stats with a web UI (separate guide)
- Add custom maps, mutations, or gamemodes
- Set up automated backups and a daily cron restart

The server will run indefinitely with LinuxGSM monitoring it (`l4d2server monitor` can be cronned every few minutes to auto-restart on crash).