fix: prefer ipv4 when resolving ssh host names (#1365)

plugins/ssh-terminal/src/backend/helpers.ts

@@ -154,6 +154,17 @@ export function isRetriableDnsError(error: unknown): boolean {
   );
 }
 
+type LookupResult = { address: string; family: number };
+
+// Prefer IPv4: ssh2 gets one address and has no fallback, and a name that
+// also has an AAAA record often points at a v6 address sshd is not bound to.
+export function pickSshAddress(result: LookupResult | LookupResult[]): string {
+  const list = Array.isArray(result) ? result : [result];
+  if (list.length === 0)
+    throw Object.assign(new Error("No address found"), { code: "ENOTFOUND" });
+  return (list.find((entry) => entry.family === 4) ?? list[0]).address;
+}
+
 /** Resolves a hostname up front, retrying transient container DNS failures. */
 export async function resolveHostForSshConnect(
   host: string,
@@ -168,10 +179,10 @@ export async function resolveHostForSshConnect(
 
   for (let attempt = 0; ; attempt += 1) {
     try {
-      const result = await lookup(normalized);
+      const address = pickSshAddress(await lookup(normalized, { all: true }));
       return {
-        host: result.address,
-        resolvedAddress: result.address,
+        host: address,
+        resolvedAddress: address,
         attempts: attempt + 1,
       };
     } catch (error) {

plugins/ssh-terminal/tests/backend/ssh-dns.test.ts

@@ -0,0 +1,22 @@
+import { describe, expect, it, vi } from "vitest";
+import {
+  pickSshAddress,
+  resolveHostForSshConnect,
+} from "../../src/backend/helpers.js";
+
+describe("resolveHostForSshConnect", () => {
+  it("prefers IPv4 over IPv6", async () => {
+    const lookup = vi.fn().mockResolvedValue([
+      { address: "fe80::1", family: 6 },
+      { address: "192.168.1.20", family: 4 },
+    ]);
+    const result = await resolveHostForSshConnect("nas", lookup);
+    expect(result.host).toBe("192.168.1.20");
+  });
+
+  it("keeps an IPv6-only answer", () => {
+    expect(pickSshAddress({ address: "2001:db8::5", family: 6 })).toBe(
+      "2001:db8::5",
+    );
+  });
+});

src/backend/hosts/ssh-dns.ts

@@ -25,6 +25,17 @@ export function shouldResolveBeforeSshConnect(host: string): boolean {
   return net.isIP(normalized) === 0;
 }
 
+type LookupResult = { address: string; family: number };
+
+// Prefer IPv4: ssh2 gets one address and has no fallback, and a name that
+// also has an AAAA record often points at a v6 address sshd is not bound to.
+export function pickSshAddress(result: LookupResult | LookupResult[]): string {
+  const list = Array.isArray(result) ? result : [result];
+  if (list.length === 0)
+    throw Object.assign(new Error("No address found"), { code: "ENOTFOUND" });
+  return (list.find((entry) => entry.family === 4) ?? list[0]).address;
+}
+
 export async function resolveHostForSshConnect(
   host: string,
   lookup: Lookup = dns.lookup,
@@ -38,10 +49,10 @@ export async function resolveHostForSshConnect(
 
   for (let attempt = 0; ; attempt += 1) {
     try {
-      const result = await lookup(normalized);
+      const address = pickSshAddress(await lookup(normalized, { all: true }));
       return {
-        host: result.address,
-        resolvedAddress: result.address,
+        host: address,
+        resolvedAddress: address,
         attempts: attempt + 1,
       };
     } catch (error) {

src/backend/tests/hosts/ssh-dns.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, it, vi } from "vitest";
 import {
   isRetriableDnsError,
+  pickSshAddress,
   resolveHostForSshConnect,
   resolveSshConnectConfigHost,
   shouldResolveBeforeSshConnect,
@@ -76,4 +77,25 @@ describe("SSH DNS resolution", () => {
       resolvedHost: "10.0.0.6",
     });
   });
+
+  it("prefers an IPv4 address when a name has both", async () => {
+    const lookup = vi.fn().mockResolvedValue([
+      { address: "::1", family: 6 },
+      { address: "10.0.0.7", family: 4 },
+    ]);
+
+    await expect(resolveHostForSshConnect("box", lookup)).resolves.toEqual({
+      host: "10.0.0.7",
+      resolvedAddress: "10.0.0.7",
+      attempts: 1,
+    });
+    expect(lookup).toHaveBeenCalledWith("box", { all: true });
+  });
+
+  it("falls back to IPv6 when that is all there is", () => {
+    expect(pickSshAddress([{ address: "2001:db8::2", family: 6 }])).toBe(
+      "2001:db8::2",
+    );
+    expect(() => pickSshAddress([])).toThrow();
+  });
 });