Factorio Server Health Check Worker

README.md

Factorio Server Health Check Worker

A Cloudflare Worker that checks the health of a Factorio server listed in the public multiplayer lobby.

It wraps the Factorio Matchmaking API with automatic SRV record resolution.
Useful for monitoring and dashboards — returns a 200 OK if the server is online, and 404 Not Found if it isn’t.

---

How It Works

1. SRV Lookup
   Uses DNS-over-HTTPS (https://1.1.1.1/dns-query) to resolve: _factorio._udp.example.com
2. Resolve Target
   Resolves the SRV target hostname to an IPv4 (or IPv6 if needed).
3. Query Factorio Matchmaking API
   Calls https://wiki.factorio.com/Matchmaking_API (https://multiplayer.factorio.com/get-game-details/$IP:$PORT).
4. Return Health

• 200 OK → server is online (response includes JSON with server details)
• 404 Not Found → server is offline or not visible in the public lobby

Disclaimer

This Worker was quickly generated using ChatGPT. It was built as a quick way to perform out-of-band monitoring of a self-hosted Factorio server on a dynamic IP using the public Matchmaking API.

factorio-dns-srv-health-check.js

export default {
  async fetch(request, env, ctx) {
    try {
      const srvName = "_factorio._udp.example.com"; // CHANGE ME: Requires a DNS SRV Record, See https://wiki.factorio.com/Multiplayer#DNS_SRV_Records.
      const dohBase = "https://1.1.1.1/dns-query";

      // --- helpers ---
      const doh = async (name, type) => {
        const url = `${dohBase}?name=${encodeURIComponent(name)}&type=${encodeURIComponent(type)}`;
        const res = await fetch(url, {
          headers: { "accept": "application/dns-json" },
          // Keep DoH results hot for a short time on the edge
          cf: { cacheTtl: 20, cacheEverything: true }
        });
        if (!res.ok) throw new Error(`DoH ${type} query failed (${res.status})`);
        return res.json();
      };

      const pickSrv = (answers) => {
        // SRV RDATA: "priority weight port target"
        // Prefer lowest priority; if tie, highest weight.
        const parsed = answers
          .filter(a => a.type === 33 && typeof a.data === "string")
          .map(a => {
            const [priStr, wtStr, portStr, ...targetParts] = a.data.trim().split(/\s+/);
            const target = targetParts.join(" ").replace(/\.+$/, ""); // strip trailing dot
            return {
              priority: parseInt(priStr, 10),
              weight: parseInt(wtStr, 10),
              port: parseInt(portStr, 10),
              target
            };
          });

        if (parsed.length === 0) throw new Error("No SRV answers found");
        parsed.sort((a, b) => a.priority - b.priority || b.weight - a.weight);
        return parsed[0];
      };

      const firstA = (answers) => {
        const a = answers.find(a => a.type === 1 && a.data);
        return a?.data ?? null;
      };

      const firstAAAA = (answers) => {
        const aaaa = answers.find(a => a.type === 28 && aaaa?.data); // will fail; fix below
        return aaaa?.data ?? null;
      };

      // --- 1) SRV lookup ---
      const srvResp = await doh(srvName, "SRV");
      if (!srvResp.Answer || !Array.isArray(srvResp.Answer)) {
        throw new Error("SRV lookup returned no Answer");
      }
      const srv = pickSrv(srvResp.Answer);

      // --- 2) Resolve target to IP (A first, then AAAA) ---
      const aResp = await doh(srv.target, "A");
      let ip = (aResp.Answer && Array.isArray(aResp.Answer)) ? firstA(aResp.Answer) : null;

      if (!ip) {
        const aaaaResp = await doh(srv.target, "AAAA");
        const answers = (aaaaResp.Answer && Array.isArray(aaaaResp.Answer)) ? aaaaResp.Answer : [];
        const aaaaRec = answers.find(x => x.type === 28 && x.data);
        ip = aaaaRec?.data ?? null;
      }
      if (!ip) throw new Error(`Could not resolve IP for ${srv.target}`);

      // Wrap IPv6 literal for host:port
      const hostForUrl = ip.includes(":") ? `[${ip}]` : ip;

      // --- 3) Query Factorio get-game-details ---
      const detailsUrl = `https://multiplayer.factorio.com/get-game-details/${hostForUrl}:${srv.port}`;
      const detailsRes = await fetch(detailsUrl, {
        // Factorio endpoint is public; set a small cache
        cf: { cacheTtl: 10, cacheEverything: true }
      });

      // Pass through status & body; add friendly CORS
      const body = await detailsRes.text();
      const headers = new Headers(detailsRes.headers);
      headers.set("access-control-allow-origin", "*");
      headers.set("access-control-allow-methods", "GET, OPTIONS");
      headers.set("access-control-allow-headers", "Content-Type");
      headers.set("cache-control", "public, max-age=10");

      return new Response(body, { status: detailsRes.status, headers });
    } catch (err) {
      return new Response(
        JSON.stringify({ error: String(err?.message ?? err) }, null, 2),
        {
          status: 502,
          headers: {
            "content-type": "application/json",
            "access-control-allow-origin": "*"
          }
        }
      );
    }
  }
};