offscreen.js

async function detectLanIPv4() {
  const ips = new Set();
  const peerConnection = new RTCPeerConnection({ iceServers: [] });
  peerConnection.createDataChannel("x");

  peerConnection.onicecandidate = (e) => {
    const candidate = e?.candidate?.candidate;
    if (!candidate) {
      return;
    }

    const m = candidate.match(/\b(\d{1,3}(?:\.\d{1,3}){3})\b/);
    if (m?.[1]) ips.add(m[1]);
  };

  await peerConnection.setLocalDescription(await peerConnection.createOffer());
  await new Promise((r) => setTimeout(r, 800));
  peerConnection.close();

  return [...ips].filter((ip) => isPrivate(ip) && !ip.startsWith("127."))[0] || "";
}

function isPrivate(ip) {
  const [a, b] = ip.split(".").map((n) => parseInt(n, 10));
  if ([a, b].some(Number.isNaN)) {
    return false;
  }

  return (
    a === 10 || (a === 172 && b >= 16 && b <= 31) || (a === 192 && b === 168)
  );
}

chrome.runtime.onMessage.addListener((msg, _sender, sendResponse) => {
  if (msg?.type === "GET_LAN_IP") {
    detectLanIPv4()
      .then((ip) => sendResponse({ ip }))
      .catch(() => sendResponse({ ip: "" }));

    return true;
  }
});