refactor: split gateway server helpers and tests

src/gateway/net.ts

@@ -0,0 +1,25 @@
+import { pickPrimaryTailnetIPv4 } from "../infra/tailnet.js";
+
+export function isLoopbackAddress(ip: string | undefined): boolean {
+  if (!ip) return false;
+  if (ip === "127.0.0.1") return true;
+  if (ip.startsWith("127.")) return true;
+  if (ip === "::1") return true;
+  if (ip.startsWith("::ffff:127.")) return true;
+  return false;
+}
+
+export function resolveGatewayBindHost(
+  bind: import("../config/config.js").BridgeBindMode | undefined,
+): string | null {
+  const mode = bind ?? "loopback";
+  if (mode === "loopback") return "127.0.0.1";
+  if (mode === "lan") return "0.0.0.0";
+  if (mode === "tailnet") return pickPrimaryTailnetIPv4() ?? null;
+  if (mode === "auto") return pickPrimaryTailnetIPv4() ?? "0.0.0.0";
+  return "127.0.0.1";
+}
+
+export function isLoopbackHost(host: string): boolean {
+  return isLoopbackAddress(host);
+}