diff --git a/packages/server/src/server/relay-transport.latency.e2e.test.ts b/packages/server/src/server/relay-transport.latency.e2e.test.ts
new file mode 100644
index 0000000000..84404da0c1
--- /dev/null
+++ b/packages/server/src/server/relay-transport.latency.e2e.test.ts
@@ -0,0 +1,61 @@
+import { execFile } from "node:child_process";
+import { mkdtemp, readFile, rm, writeFile } from "node:fs/promises";
+import os from "node:os";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import { promisify } from "node:util";
+import { expect, test } from "vitest";
+
+const run = promisify(execFile);
+
+// Opt in on Linux with passwordless sudo, unshare, ip, mount, and tc available:
+// RUN_RELAY_LATENCY_E2E=1 npx vitest run src/server/relay-transport.latency.e2e.test.ts
+// All networking and the hosts-file mount are private to the child namespaces.
+test.runIf(process.platform === "linux" && process.env.RUN_RELAY_LATENCY_E2E === "1")(
+  "connects relay control and data over a slow dual-stack route with an unreachable address",
+  async () => {
+    const directory = await mkdtemp(path.join(os.tmpdir(), "paseo-relay-latency-"));
+    try {
+      const hosts = path.join(directory, "hosts");
+      await writeFile(
+        hosts,
+        `${await readFile("/etc/hosts", "utf8")}\n127.0.0.1 relay-latency.test\n2001:db8:1::1 relay-latency.test\n`,
+      );
+      const fixture = fileURLToPath(
+        new URL("./test-utils/relay-latency-fixture.ts", import.meta.url),
+      );
+      for (const order of ["ipv4first", "ipv6first"]) {
+        const { stdout } = await run(
+          "sudo",
+          [
+            "-n",
+            "unshare",
+            "--mount",
+            "--net",
+            "sh",
+            "-ec",
+            `mount --make-rprivate /
+mount --bind "$1" /etc/hosts
+ip link set lo up
+ip link add relay-test type dummy
+ip addr add 192.0.2.2/32 dev relay-test
+ip -6 addr add 2001:db8::2/128 dev relay-test
+ip link set relay-test up
+tc qdisc add dev lo root netem delay 350ms
+exec "$2" --import tsx "$3" "$4"`,
+            "sh",
+            hosts,
+            process.execPath,
+            fixture,
+            order,
+          ],
+          { timeout: 20_000 },
+        );
+        expect(stdout).toContain("relay data round trip passed");
+      }
+    } finally {
+      await rm(directory, { recursive: true, force: true });
+    }
+  },
+  45_000,
+);
diff --git a/packages/server/src/server/relay-transport.ts b/packages/server/src/server/relay-transport.ts
index f7b73a5367..113cce605c 100644
--- a/packages/server/src/server/relay-transport.ts
+++ b/packages/server/src/server/relay-transport.ts
@@ -56,7 +56,13 @@ type ControlMessage =
 const CONTROL_PING_INTERVAL_MS = 10_000;
 const CONTROL_STALE_TIMEOUT_MS = 30_000;
 const CONTROL_READY_TIMEOUT_MS = 8_000;
-const RELAY_WEBSOCKET_OPTIONS = { handshakeTimeout: 10_000, perMessageDeflate: false } as const;
+const RELAY_WEBSOCKET_OPTIONS = {
+  handshakeTimeout: 10_000,
+  perMessageDeflate: false,
+  // Node cancels each non-final address when this expires, even if the next
+  // address is unreachable. Allow high-latency routes time to connect.
+  autoSelectFamilyAttemptTimeout: 5_000,
+} as const;
 
 function createDefaultRelayWebSocket(url: string): RelayWebSocketLike {
   return new WebSocket(url, RELAY_WEBSOCKET_OPTIONS);
diff --git a/packages/server/src/server/test-utils/relay-latency-fixture.ts b/packages/server/src/server/test-utils/relay-latency-fixture.ts
new file mode 100644
index 0000000000..5abcccf389
--- /dev/null
+++ b/packages/server/src/server/test-utils/relay-latency-fixture.ts
@@ -0,0 +1,75 @@
+import assert from "node:assert/strict";
+import dns from "node:dns";
+import { once } from "node:events";
+import net from "node:net";
+import { Writable } from "node:stream";
+import pino from "pino";
+import { WebSocketServer } from "ws";
+import { startRelayTransport } from "../relay-transport.js";
+
+// Match the reported runtime even on Node versions with a newer default.
+net.setDefaultAutoSelectFamilyAttemptTimeout(250);
+const order = process.argv[2];
+assert.ok(order === "ipv4first" || order === "ipv6first");
+dns.setDefaultResultOrder(order);
+
+const logs: string[] = [];
+const logger = pino(
+  { level: "debug" },
+  new Writable({
+    write(chunk, _encoding, callback) {
+      logs.push(chunk.toString());
+      callback();
+    },
+  }),
+);
+const relay = new WebSocketServer({ host: "127.0.0.1", port: 0 });
+await once(relay, "listening");
+const address = relay.address();
+assert.ok(address && typeof address !== "string");
+relay.on("connection", (socket, request) => {
+  const url = new URL(request.url!, "http://relay-latency.test");
+  if (url.searchParams.has("connectionId")) {
+    socket.on("message", (data) => socket.send(data));
+  } else {
+    socket.send(JSON.stringify({ type: "sync", connectionIds: ["latency-client"] }));
+  }
+});
+
+let complete!: () => void;
+let fail!: (error: Error) => void;
+const delivered = new Promise<void>((resolve, reject) => {
+  complete = resolve;
+  fail = reject;
+});
+const deadline = setTimeout(() => fail(new Error(logs.join(""))), 8_000);
+const transport = startRelayTransport({
+  logger,
+  relayEndpoint: `relay-latency.test:${address.port}`,
+  relayUseTls: false,
+  serverId: "latency-server",
+  attachSocket: async (socket) => {
+    socket.on("message", (data) => {
+      try {
+        assert.equal(String(data), "latency round trip");
+        complete();
+      } catch (error) {
+        fail(error as Error);
+      }
+    });
+    socket.send("latency round trip");
+  },
+});
+
+try {
+  await delivered;
+  assert.ok(logs.some((line) => JSON.parse(line).msg === "relay_control_connected"));
+  assert.ok(logs.some((line) => JSON.parse(line).msg === "relay_data_connected"));
+  assert.ok(!logs.some((line) => JSON.parse(line).msg === "relay_error"));
+  console.log("relay data round trip passed");
+} finally {
+  clearTimeout(deadline);
+  await transport.stop();
+  for (const socket of relay.clients) socket.terminate();
+  await new Promise<void>((resolve) => relay.close(() => resolve()));
+}