feat: Enhance echo example to dynamically find free ports and improve address handling

- Added a function to find a free port on localhost.
- Updated the run function to use the new port finding logic when a non-positive port is provided.
- Modified address printing to handle multiple listen addresses correctly.
- Improved the get_available_interfaces function to ensure the IPv4 loopback address is included.

examples/echo/echo.py

@@ -1,4 +1,7 @@
 import argparse
+import random
+import secrets
+import socket
 
 import multiaddr
 import trio
@@ -12,23 +15,30 @@ from libp2p.crypto.secp256k1 import (
 from libp2p.custom_types import (
     TProtocol,
 )
-from libp2p.network.stream.net_stream import (
-    INetStream,
-)
 from libp2p.network.stream.exceptions import (
     StreamEOF,
 )
+from libp2p.network.stream.net_stream import (
+    INetStream,
+)
 from libp2p.peer.peerinfo import (
     info_from_p2p_addr,
 )
 from libp2p.utils.address_validation import (
-    get_optimal_binding_address,
+    get_available_interfaces,
 )
 
 PROTOCOL_ID = TProtocol("/echo/1.0.0")
 MAX_READ_LEN = 2**32 - 1
 
 
+def find_free_port():
+    """Find a free port on localhost."""
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
+        s.bind(("", 0))  # Bind to a free port provided by the OS
+        return s.getsockname()[1]
+
+
 async def _echo_stream_handler(stream: INetStream) -> None:
     try:
         peer_id = stream.muxed_conn.peer_id
@@ -47,19 +57,19 @@ async def _echo_stream_handler(stream: INetStream) -> None:
 
 async def run(port: int, destination: str, seed: int | None = None) -> None:
     # CHANGED: previously hardcoded 0.0.0.0
-    listen_addr = get_optimal_binding_address(port)
+    if port <= 0:
+        port = find_free_port()
+    listen_addr = get_available_interfaces(port)
 
     if seed:
-        import random
         random.seed(seed)
         secret_number = random.getrandbits(32 * 8)
         secret = secret_number.to_bytes(length=32, byteorder="big")
     else:
-        import secrets
         secret = secrets.token_bytes(32)
 
     host = new_host(key_pair=create_new_key_pair(secret))
-    async with host.run(listen_addrs=[listen_addr]), trio.open_nursery() as nursery:
+    async with host.run(listen_addrs=listen_addr), trio.open_nursery() as nursery:
         # Start the peer-store cleanup task
         nursery.start_soon(host.get_peerstore().start_cleanup_task, 60)
 
@@ -69,9 +79,10 @@ async def run(port: int, destination: str, seed: int | None = None) -> None:
             host.set_stream_handler(PROTOCOL_ID, _echo_stream_handler)
 
             # Print all listen addresses with peer ID (JS parity)
-            print("Listener ready, listening on:")
+            print("Listener ready, listening on:\n")
             peer_id = host.get_id().to_string()
-            print(f"{listen_addr}/p2p/{peer_id}")
+            for addr in listen_addr:
+                print(f"{addr}/p2p/{peer_id}")
 
             print(
                 "\nRun this from the same folder in another console:\n\n"

libp2p/utils/address_validation.py

@@ -67,6 +67,10 @@ def get_available_interfaces(port: int, protocol: str = "tcp") -> list[Multiaddr
         seen_v4.add(ip)
         addrs.append(Multiaddr(f"/ip4/{ip}/{protocol}/{port}"))
 
+    # Ensure IPv4 loopback is always included when IPv4 interfaces are discovered
+    if seen_v4 and "127.0.0.1" not in seen_v4:
+        addrs.append(Multiaddr(f"/ip4/127.0.0.1/{protocol}/{port}"))
+
     seen_v6: set[str] = set()
     for ip in _safe_get_network_addrs(6):
         seen_v6.add(ip)