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 }>();
 

web/src/views/settings/AboutSettings.tsx

@@ -16,7 +16,8 @@ interface Nodes {
 
 function mailNodeLabel(m: Nodes["mailServer"]): string {
   if (!m.address) return t("{host} does not resolve", { host: m.host });
-  return m.name ? `${m.name} (${m.address})` : m.address;
+  // The node's hostname; the bare address only when it has no PTR record.
+  return m.name ?? m.address;
 }
 
 export function AboutSettings() {