About: find the mail node's name with resolvePtr

dns.reverse came back empty inside the image while the resolver answered
the PTR, so About showed only the address. Ask for the PTR record of the
in-addr.arpa / ip6.arpa name directly.

server/src/nodes.test.ts

@@ -1,6 +1,6 @@
 import { test, beforeEach } from "node:test";
 import assert from "node:assert/strict";
-import { clearNodeCache, mailNode } from "./nodes.js";
+import { clearNodeCache, mailNode, ptrName } from "./nodes.js";
 
 /**
  * About names the mail node by the PTR of the address the server's name
@@ -38,3 +38,12 @@ test("the answer is cached for a minute, then looked up again", async () => {
   await mailNode("https://mail.example.com", count, rev, 61_000);
   assert.equal(calls, 2);
 });
+
+test("the PTR name is built for IPv4 and IPv6 alike", () => {
+  assert.equal(ptrName("192.0.2.52"), "52.2.0.192.in-addr.arpa");
+  assert.equal(
+    ptrName("2001:db8::25"),
+    "5.2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa",
+  );
+  assert.equal(ptrName("::1"), "1" + ".0".repeat(31) + ".ip6.arpa");
+});

server/src/nodes.ts

@@ -12,7 +12,8 @@
  * node has one: mail/mx2/mx3). The server offers its node name only to
  * administrators, so asking it would leave everyone else with nothing.
  */
-import { lookup as dnsLookup, reverse as dnsReverse } from "node:dns/promises";
+import { lookup as dnsLookup, resolvePtr } from "node:dns/promises";
+import { isIPv4 } from "node:net";
 import { hostname } from "node:os";
 import { config } from "./config.js";
 
@@ -33,6 +34,22 @@ export interface Nodes {
 type Lookup = (host: string) => Promise<{ address: string }>;
 type Reverse = (address: string) => Promise<string[]>;
 
+/**
+ * The reverse-lookup name for an address: 1.2.0.192.in-addr.arpa, or the
+ * nibble form under ip6.arpa. Asked for directly, because `dns.reverse` came
+ * back empty in the image while the resolver answered the PTR (2026-09-26).
+ */
+export function ptrName(address: string): string {
+  if (isIPv4(address)) return `${address.split(".").reverse().join(".")}.in-addr.arpa`;
+  const [head = "", tail = ""] = address.split("::");
+  const groups = (part: string) => (part ? part.split(":") : []);
+  const h = groups(head), t = groups(tail);
+  const full = [...h, ...Array(8 - h.length - t.length).fill("0"), ...t];
+  return `${full.map((g) => g.padStart(4, "0")).join("").split("").reverse().join(".")}.ip6.arpa`;
+}
+
+const dnsReverse: Reverse = (address) => resolvePtr(ptrName(address));
+
 const CACHE_MS = 60_000;
 const cache = new Map<string, { node: MailNode; at: number }>();