A failing Swarm service does not, by itself, explain why other healthy containers lost overlay connectivity. Diagnose the service and its tasks separately from network attachment and the inter-node data path; without task history, network inspection, node and firewall details, and logs, the cause of this incident cannot be identified.
Why can healthy containers no longer communicate over a Docker Swarm overlay network?
Swarm services describe desired state, including which networks a service should use; tasks are the running or attempted instances that implement that state. A task may be restarting, rejected, or pending while a container elsewhere reports healthy. Neither service status nor a container health check establishes that peers can communicate across an overlay.
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Start by separating the control state (what Swarm wants and where tasks are placed) from actual network reachability. Treat task failure, missing network membership, and a broken cross-node data path as distinct diagnostic branches, not as proof of a single cause.
How do I check whether a Swarm service task is attached to an overlay network?
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On a manager, run
docker service ls, thendocker service ps <service-name>. Record desired and current states, node placement, task errors, and task history. Docker documentsdocker service psas a way to list tasks and their networks. Identify whether the affected containers are tasks of the failing service or belong to other services.#1 Best Overall
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Inspect the service definition and its network list, then run
docker network inspect <network-name>for the named overlay. Compare the failing service’s tasks with the peer services and the containers or tasks shown on that network. Check whether every participant is attached to the same intended overlay. -
Compare the task timeline with the first reported connectivity failure. Swarm managers try to reconcile actual tasks with desired service state, so a current service summary alone may not show when tasks were replaced or where they ran.
Network membership is configurable: Docker supports adding an overlay to a new or existing service and removing one during an update. That makes an unintended removal or mismatched attachment worth checking, but neither is established as the explanation here. Do not change the service until inspection indicates what is wrong; Docker documents docker service update --network-add <network> <service> and --network-rm for configuration changes.
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Does the failure happen only between nodes?
Test communication between affected tasks or containers on the same host, then across different hosts. A failure limited to cross-node traffic points toward the inter-node path; same-host failures mean you should not assume that opening an inter-node port will address the problem.
For multi-node Swarms, Docker documents these inter-node requirements:
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TCP and UDP 7946: used for network discovery.
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UDP 4789: used for the overlay data path by default. If the cluster is configured with an alternate data-path port, verify that port instead.
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Discovery and overlay data transport are different functions, so a block affecting one is not interchangeable with a block affecting the other. Check routing and firewall reachability between Swarm nodes under the cluster’s security policy; these requirements are not a reason to expose ports broadly to the public internet.
Did the connectivity failure coincide with an update or task replacement?
Compare the first failure time with service updates, node availability changes, and task replacements. If symptoms began during a deployment, inspect the update status and policy as well as task history. Docker documents a default --update-monitor period of 30 seconds: a task failure in that period counts toward the service update failure threshold, while a failure after it does not. This timing helps interpret update outcomes, but it does not prove that an update caused overlay connectivity to fail.
Could the number of containers on a host matter?
Check scale only if the deployment approaches Docker’s documented condition. Docker Docs’ “Overlay network driver” warns that Linux kernel limitations can make overlay networks unstable and inter-container communication may break when 1,000 containers are co-located on one host. This is a specific co-location condition, not a universal container limit or an explanation to apply to a smaller deployment without matching evidence.
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What evidence is needed to identify this incident’s cause?
The available incident description does not include the details needed to distinguish these branches or name a root cause. Gather the following for the affected time window:
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Docker Engine and kernel versions, cluster size and topology, and the affected node list.
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Task placement and replacement history, update status and policy, and the first known failure time.
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Results of same-node and cross-node connectivity checks, plus node routing and firewall configuration for the required inter-node ports.
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Relevant Docker daemon and application logs, including health-check results and errors from affected tasks.
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With those artifacts, an operator can determine whether the evidence points to task state, network membership, cross-node transport, deployment timing, or the documented high-density condition. Without them, attributing the loss of reachability to the failing service would go beyond what is known.
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