feat: initial etrelay UDP relay with master heartbeat

2026-09-26 15:51:43 +02:00
commit 7f715979e1
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+# etrelay
+
+UDP relay + master heartbeat for a Wolfenstein: Enemy Territory server reachable only through a tunnel: runs on the front server (public IP) and makes the server show up in the in-game browser.
+
+```
+players / masters ──> front :27960 (etrelay) ══wg══> <wg-peer>:27960 (nginx stream) ──> ET <et-server>:27960
+```
+
+- **Proxy**: one upstream socket per client address, packets forwarded as-is both ways, session dropped after 2 min of silence.
+- **Heartbeat**: every 5 min, `getinfo` to the ET server; if it answers, `heartbeat EnemyTerritory-1` to the masters **from the public socket**. When it stops answering, one `heartbeat ETFlatline-1`.
+
+Masters check (`getChallenge`, `getInfo`) the address the heartbeat came from: the public `:27960`. Those packets go through the relay like any client, the ET server answers them, and the master lists the front public address.
+
+How it works in detail, with diagrams: [ENGINE.md](ENGINE.md).
+
+## Usage
+
+```bash
+etrelay <UPSTREAM> [--listen 0.0.0.0:27960] [--master host:port]... [--debug]
+```
+
+Default masters: `etmaster.idsoftware.com:27950` (id Software, the one ET 2.60b clients query) and `etmaster.net:27950` (ET: Legacy community). Passing `--master` replaces both.
+
+`--debug` logs sessions (open/close, packet counts), connectionless packets by command name only (`getinfo`, `getstatus`, `connect`...: no arguments, `connect` carries the userinfo), probes and heartbeats. In-game packets are only counted.
+
+```
+probe <wg-peer>:27960: up
+ET server <wg-peer>:27960 is up
+heartbeat EnemyTerritory-1 -> etmaster.net:27950 (198.51.100.10:27950)
+session + 198.51.100.10:27950 (2 active)
+198.51.100.10:27950 -> ET  getChallenge (27 B)
+198.51.100.10:27950 <- ET  challengeResponse (32 B)
+198.51.100.10:27950 -> ET  getInfo (22 B)
+198.51.100.10:27950 <- ET  infoResponse (303 B)
+session - <player>:27960: 5321 packets to ET, 4876 from ET
+```
+
+As a service: add `--debug` to `ExecStart`, then `systemctl daemon-reload && systemctl restart etrelay`.
+
+## Build
+
+```bash
+make build     # dynamic (glibc)
+make static    # static (musl), runs on any x86_64 Linux
+make release   # both
+```
+
+Artifacts land in `dist/`, named `<name>-v<version>-<target triple>` with a `.sha256` checksum:
+
+```
+dist/etrelay-v0.1.0-x86_64-unknown-linux-gnu    # dynamic
+dist/etrelay-v0.1.0-x86_64-unknown-linux-musl   # static
+```
+
+## Run as a service
+
+1. Binary and unit on the front server ([etrelay.service](etrelay.service) is a template: replace `@UPSTREAM@` with the ET server `ip:port` as seen from the front, `@ARGS@` with extra flags or nothing):
+   ```bash
+   install -m 755 dist/etrelay-v<version>-x86_64-unknown-linux-musl /usr/local/bin/etrelay
+   sed -e 's#@UPSTREAM@#<ip:port>#' -e 's#@ARGS@##' etrelay.service > /etc/systemd/system/etrelay.service
+   systemctl daemon-reload && systemctl enable --now etrelay
+   ```
+2. Nothing else on the front may capture UDP 27960 (DNAT rule, nginx `stream`...), or players bypass the relay: `iptables -t nat -S | grep 27960` must print nothing.
+3. ET server `server.cfg`: stop its own heartbeats, they leave through the home connection with the wrong IP:
+   ```
+   set sv_master1 ""
+   set sv_master2 ""
+   set sv_master3 ""
+   set sv_master4 ""
+   set sv_master5 ""
+   ```
+
+## Check
+
+```bash
+journalctl -u etrelay -n 50                      # "ET server ... is up"
+tcpdump -ni any udp port 27950                   # heartbeat out, getChallenge/getInfo in, answers out
+```
+
+The server then shows up on the master (in-game server browser).