[Bugfix] Fix MessageQueue connect_ip for cross-node data parallelism (#35429)

Signed-off-by: Lu Fang <fanglu@fb.com>
Co-authored-by: Lu Fang <30275821+houseroad@users.noreply.github.com>

tests/distributed/test_mq_connect_ip.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""
+Test that MessageQueue uses the local node's IP for binding,
+not a remote master_addr. This validates the fix for cross-node
+data-parallel where each DP group leader must bind to its own IP.
+
+The bug: multiproc_executor used `parallel_config.master_addr` as
+`connect_ip` for every DP group's MessageQueue. For DP groups whose
+leader is NOT on the master node, binding to master_addr fails with
+"Cannot assign requested address".
+
+The fix: use `get_ip()` (local node IP) instead of `master_addr`.
+"""
+
+import pytest
+import zmq
+
+from vllm.distributed.device_communicators.shm_broadcast import MessageQueue
+from vllm.utils.network_utils import get_ip
+
+
+def test_mq_bind_with_local_ip():
+    """MessageQueue with remote readers should successfully bind
+    when connect_ip is the local node's IP."""
+    # n_reader=2, n_local_reader=1 means 1 remote reader,
+    # which triggers the remote ZMQ socket bind.
+    mq = MessageQueue(
+        n_reader=2,
+        n_local_reader=1,
+        connect_ip=get_ip(),
+    )
+    handle = mq.export_handle()
+    assert handle.remote_subscribe_addr is not None
+    # The bound address should contain our local IP
+    local_ip = get_ip()
+    assert (
+        local_ip in handle.remote_subscribe_addr
+        or f"[{local_ip}]" in handle.remote_subscribe_addr
+    )
+    del mq
+
+
+def test_mq_bind_with_non_local_ip_fails():
+    """MessageQueue should fail to bind when connect_ip is a
+    non-local IP address (simulating the bug where master_addr
+    from a different node was used)."""
+    # Use a non-local IP that we definitely can't bind to.
+    # 198.51.100.1 is from TEST-NET-2 (RFC 5737), never locally assigned.
+    non_local_ip = "198.51.100.1"
+    with pytest.raises(zmq.error.ZMQError, match="Cannot assign requested address"):
+        MessageQueue(
+            n_reader=2,
+            n_local_reader=1,
+            connect_ip=non_local_ip,
+        )
+
+
+def test_mq_bind_defaults_to_local_ip():
+    """When connect_ip is None, MessageQueue should auto-detect
+    the local IP and bind successfully."""
+    mq = MessageQueue(
+        n_reader=2,
+        n_local_reader=1,
+        connect_ip=None,  # should fallback to get_ip()
+    )
+    handle = mq.export_handle()
+    assert handle.remote_subscribe_addr is not None
+    del mq
+
+
+if __name__ == "__main__":
+    test_mq_bind_with_local_ip()
+    print("PASSED: test_mq_bind_with_local_ip")
+    test_mq_bind_with_non_local_ip_fails()
+    print("PASSED: test_mq_bind_with_non_local_ip_fails")
+    test_mq_bind_defaults_to_local_ip()
+    print("PASSED: test_mq_bind_defaults_to_local_ip")
+    print("\nAll tests passed!")

vllm/v1/executor/multiproc_executor.py

@@ -44,6 +44,7 @@ from vllm.logger import init_logger
 from vllm.tracing import instrument, maybe_init_worker_tracer
 from vllm.utils.network_utils import (
     get_distributed_init_method,
+    get_ip,
     get_loopback_ip,
     get_open_port,
 )
@@ -128,11 +129,23 @@ class MultiprocExecutor(Executor):
             # For leader node within each dp rank,
             # each dp will have its own leader multiproc executor.
             max_chunk_bytes = envs.VLLM_MQ_MAX_CHUNK_BYTES_MB * 1024 * 1024
+            mq_connect_ip = get_ip()
+            logger.info(
+                "DP group leader: node_rank=%d, node_rank_within_dp=%d, "
+                "master_addr=%s, mq_connect_ip=%s (local), "
+                "world_size=%d, local_world_size=%d",
+                self.parallel_config.node_rank,
+                self.parallel_config.node_rank_within_dp,
+                self.parallel_config.master_addr,
+                mq_connect_ip,
+                self.world_size,
+                self.local_world_size,
+            )
             self.rpc_broadcast_mq = MessageQueue(
                 self.world_size,
                 self.local_world_size,
                 max_chunk_bytes=max_chunk_bytes,
-                connect_ip=self.parallel_config.master_addr,
+                connect_ip=mq_connect_ip,
             )
             scheduler_output_handle = self.rpc_broadcast_mq.export_handle()
         # Create workers