Custom DynDNS for FritzBox

💡 Idea

At home, I have a FritzBox router, a home server, and Home Assistant running on a Raspberry Pi. Since I don’t have a static IP address, I need a way to access the server and Pi from outside through DNS. That requires a dynamic DNS record that updates whenever the router’s IP address changes. My router has a built-in DynDNS function, but it doesn’t work with Cloudflare. Since it only sends a GET request to a specific URL, I can use a serverless function to handle the request and update the DNS record. The code for this is quite simple, and can be seen here fritz-dns

I adapted the idea from the old PHP code I previously hosted on my server. Since I didn’t want to pay for another server, I decided to use Vercel serverless functions. Original idea

I only had to migrate it to JavaScript and use the Cloudflare library to update the DNS record. A random token ensures that only I can send the request, then the function updates the DNS record with the new IP address.

That’s it. This week was kinda short, but that’s because I didn’t do that much, since it took like 2 hours to find out what the problem was 😂

🛠️ Code

import { NextRequest, NextResponse } from "next/server";
import Cloudflare from "cloudflare";

const client = new Cloudflare({
  apiEmail: process.env["CLOUDFLARE_EMAIL"],
  apiKey: process.env["CLOUDFLARE_API_KEY"],
});

function isValidIPv4(ip: string) {
  return /^(\d{1,3}\.){3}\d{1,3}$/.test(ip);
}
function isValidIPv6(ip: string) {
  return /^[a-fA-F0-9:]+$/.test(ip);
}

export async function GET(req: NextRequest) {
  const params = Object.fromEntries(req.nextUrl.searchParams.entries());
  const { cf_key, domain, ipv4, ipv6, log, proxy } = params;

  if (cf_key !== process.env["CLOUDFLARE_TOKEN"]) {
    return NextResponse.json(
      { error: "Invalid Cloudflare token" },
      { status: 403 },
    );
  }

  // Logging helper (console only)
  function wlog(level: string, msg: string) {
    if (log === "true") {
      // Optionally, write to a file or external log here
      console.log(`${new Date().toISOString()} - ${level} - ${msg}`);
    }
  }

  wlog("INFO", "===== Starting Script =====");

  if (!cf_key || !domain) {
    wlog("ERROR", "Parameter(s) missing or invalid");
    wlog("INFO", "Script aborted");
    return NextResponse.json(
      { error: "Parameter(s) missing or invalid" },
      { status: 400 },
    );
  }

  let validIPv4 = ipv4 && isValidIPv4(ipv4) ? ipv4 : null;
  let validIPv6 = ipv6 && isValidIPv6(ipv6) ? ipv6 : null;

  if (!validIPv4) {
    wlog("ERROR", "Neither IPv4 nor IPv6 available.");
    wlog("INFO", "Script aborted");
    return NextResponse.json(
      { error: "Neither IPv4 nor IPv6 available." },
      { status: 400 },
    );
  }

  const proxied = proxy === "true";
  wlog("INFO", `Record will${proxied ? "" : " not"} be proxied by Cloudflare`);

  // Automatically fetches more pages as needed.
  for await (const zone of client.zones.list()) {
    if (domain.includes(zone.name)) {
      const list = await client.dns.records.list({ zone_id: zone.id });
      const record = list.result.find((r: any) => r.name === domain);
      if (!record) {
        wlog("ERROR", `Record ${domain} not found`);
        wlog("INFO", "Script aborted");
        return NextResponse.json(
          { error: `Record ${domain} not found` },
          { status: 404 },
        );
      }
      await client.dns.records.update(record.id, {
        zone_id: zone.id,
        type: "A",
        name: domain,
        content: validIPv4,
        ttl: 2 * 60,
      });
      break;
    }
  }

  wlog("INFO", "===== Script completed =====");
  return NextResponse.json(
    { message: "IP updated successfully" },
    { status: 200 },
  );
}