test: print resolved IP

ip.test.ts

@@ -0,0 +1,11 @@
+import { ipv4, ipv6 } from "./ip.ts";
+import { assertEquals } from "https://deno.land/std@0.208.0/assert/mod.ts";
+
+Deno.test("ip.ts", async (t) => {
+  await t.step(`ipv4`, async () =>
+    assertEquals(await ipv4("localhost"), ["127.0.0.1"])
+  );
+  await t.step(`ipv6`, async () =>
+    assertEquals(await ipv6("localhost"), ["::1"])
+  );
+});

ip.ts

@@ -0,0 +1,26 @@
+const resolveHostname =
+  (recordType: "A" | "AAAA") =>
+  async (hostname: string): Promise<string[]> => {
+    const aRecords = await Deno.resolveDns(hostname, recordType);
+    if (aRecords.length > 0) return aRecords;
+    const cname = await Deno.resolveDns(hostname, "CNAME");
+    const cnameIps = (
+      await Promise.all(
+        cname.map((cname) => {
+          try {
+            return resolveHostname(recordType)(cname);
+          } catch {
+            return null;
+          }
+        })
+      )
+    )
+      .flat()
+      .filter((ip): ip is string => ip !== null);
+
+    if (cnameIps.length > 0) return cnameIps;
+    throw new Error(`Could not determine ${recordType} for ${hostname}!`);
+  };
+
+export const ipv4 = resolveHostname("A");
+export const ipv6 = resolveHostname("AAAA");

test.ts

@@ -1,5 +1,6 @@
 import udp from "node:dgram";
 import { assertEquals } from "https://deno.land/std@0.208.0/assert/mod.ts";
+import { ipv4, ipv6 } from "./ip.ts";
 
 const hostname = Deno.env.get("HOSTNAME") ?? "localhost";
 const proto = Deno.env.get("PROTO") ?? "udp4";
@@ -10,7 +11,9 @@ const port = 2444;
 
 Deno.test("UDP echo server", async (t) => {
   await t.step(
-    `the UDP server at ${hostname}:${port} (${proto}) should echo a given string`,
+    `the UDP server at ${hostname}:${port} (${(
+      await (proto === "udp6" ? ipv6 : ipv4)(hostname)
+    ).join(", ")}) should echo a given string`,
     async () => {
       const msg = `Hello World (${Date.now()})!`;