fix(dns): refuse a --proxy host name instead of NODATA'ing every live name

src/dns.mjs

@@ -2231,6 +2231,17 @@ export async function dnsCommand(args = [], out = console.log, deps = {}) {
     if (proxyIndex >= 0) {
       const given = rest[proxyIndex + 1];
       const host = given && !given.startsWith("-") ? given : null;
+      // A host name passes the reachability probe (connect resolves it) but a
+      // DNS answer can only carry an address — isIP would leave both families
+      // null, so the mode would announce success and then NODATA every live
+      // name. That is the very outage the gate below exists to refuse, so it is
+      // refused here for the same reason rather than warned about.
+      if (host && !isIP(host)) {
+        out(`! --proxy needs an IP address, not a host name like "${host}"`);
+        out("  a name here answers every live Moshpit name with nothing, which reads");
+        out("  as a total outage — pass the proxy's address (127.0.0.1 or ::1) instead.");
+        return 1;
+      }
       const candidates = host ? [host] : ["127.0.0.1", "::1"];
       const reachable = [];
       for (const candidate of candidates) {

test/dns-proxy-mode.test.mjs

@@ -189,3 +189,28 @@ test("an explicit --proxy host is the only one probed", async () => {
     await held.release();
   }
 });
+
+test("--proxy with a host name refuses instead of NODATA'ing every live name", async () => {
+  // A name like `localhost` passes the reachability probe (connect resolves it)
+  // but cannot go in an A/AAAA answer, so the mode would announce success and
+  // then answer every live name with nothing — the outage the gate exists for.
+  // Hold the resolver port so that even without the fix `start` cannot bind and
+  // sit on the loop — the assertion is about the refusal, not the bind.
+  const held = await holdUdp();
+  try {
+    const lines = [];
+    let probed = false;
+    const code = await dnsCommand(["start", "--proxy", "localhost", "--port", String(held.port)], (l) => lines.push(l), {
+      tlds: async () => ["eggs"],
+      proxyReachableImpl: async () => { probed = true; return true; },
+    });
+
+    assert.equal(code, 1);
+    assert.equal(probed, false, "a host name is rejected before anything is probed");
+    const text = lines.join("\n");
+    assert.match(text, /needs an IP address/);
+    assert.doesNotMatch(text, /proxying every live name/, "it must not claim to have started proxying");
+  } finally {
+    await held.release();
+  }
+});