For a Docker Swarm, allow node-to-node traffic only on TCP 2377, TCP and UDP 7946, and UDP 4789 by default. Restrict those ports to the network that contains your managers and workers. Then control application ports separately, because Swarm’s routing mesh can make a published port reachable on every node, including nodes that are not running that service.
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Keep Docker’s generated firewall chains intact. Put administrator rules in DOCKER-USER for container and published-port policy, while host-level Swarm control traffic is filtered in the host’s input path. Docker’s documented nftables backend is not supported when the daemon runs in Swarm mode, so a Swarm host should continue using the iptables path (including an iptables-nft compatibility command only when Docker’s support statement for your release permits it).
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The ports Docker Swarm needs
Every participating node must be able to reach the other nodes on the control, membership, and overlay data paths. The minimum rules are:
| Traffic | Protocol and port | Who should be allowed | Purpose |
|---|---|---|---|
| Swarm manager control plane | TCP 2377 | Only trusted Swarm nodes | Manager join and control traffic |
| Node discovery and membership | TCP and UDP 7946 | All participating managers and workers | Node discovery and membership gossip |
| Overlay/VXLAN data path | UDP 4789 by default | All participating managers and workers | Overlay networks, including ingress routing-mesh traffic |
| Encrypted overlay data | IP protocol 50 (ESP) | Participating nodes when encrypted overlays are used | Encrypted overlay packets |
| Published services | Application-specific TCP or UDP ports | Only intended clients or load balancers, on every Swarm node | Client access to services published with Swarm |
Port 4789 is configurable. If the cluster was initialized with a different data-path port, allow that UDP port instead of 4789 on every node. Port 7946 is not configurable. These rules are for the private or otherwise trusted network carrying Swarm traffic, not for arbitrary Internet sources.
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Why the trust boundary matters
VXLAN on UDP 4789 does not authenticate its packets. An Internet-facing firewall must never expose that port to untrusted networks. A perimeter firewall should allow it only across the private link, VPN, or dedicated segment used by the nodes. The same source restriction should be applied to TCP 2377 and both protocols on 7946.
Do not confuse “the host has a public address” with “the Swarm network is public.” A node can have a public interface for client traffic and a separate private interface for cluster traffic. Bind the inter-node rules to the private interface or its CIDR whenever your firewall supports interface and source restrictions.
Inventory the cluster before changing rules
Firewall changes are safer when you first write down the traffic you intend to permit. Record:
- Every manager and worker address, including the subnet used for node-to-node communication.
- Which interface carries Swarm control and overlay traffic.
- Whether the cluster uses encrypted overlay networks.
- The data-path UDP port selected at initialization. A cluster created with
docker swarm init --data-path-port 14789, for example, needs UDP 14789 rather than UDP 4789. - Every port published by
docker service createordocker service update, with its protocol and the client networks that should reach it. - Any external load balancer, reverse proxy, monitoring system, or backup host that must connect to a published service.
Keep manager and worker addresses in a small, explicit source set. A broad “allow from the whole data center” rule is easier to create but increases the consequence of a compromised host.
Understand Docker’s iptables chains
Docker creates and maintains several chains, including DOCKER-USER, DOCKER-FORWARD, DOCKER, DOCKER-BRIDGE, DOCKER-INTERNAL, DOCKER-CT, and DOCKER-INGRESS. Docker’s generated rules handle forwarding, address translation, bridges, and Swarm ingress.
DOCKER-USER is the administrator insertion point for forwarded container traffic. Docker processes it before DOCKER-FORWARD and DOCKER-INGRESS. An accept or drop there can prevent later chains from being evaluated, which makes it the right place for source allowlists and published-port restrictions.
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Do not flush or delete Docker’s chains as a way to “start clean.” Disabling Docker’s iptables management without replacing all required forwarding and translation rules can break bridge networking, overlay networks, ingress, and service publication. Also inspect the host’s FORWARD policy: Docker may enable IP forwarding and set that policy to DROP, so the resulting rule path must be intentional.
Build the host-level Swarm rules with iptables
Node control and overlay packets arrive at the host itself, so place their source-restricted permits in the host input path. The following example uses 10.20.0.0/24 as the private Swarm-node network; substitute the actual node CIDR and, where appropriate, add an interface match.
- Allow manager control traffic:
iptables -I INPUT -s 10.20.0.0/24 -p tcp --dport 2377 -j ACCEPT - Allow membership and discovery on both protocols:
iptables -I INPUT -s 10.20.0.0/24 -p tcp --dport 7946 -j ACCEPTiptables -I INPUT -s 10.20.0.0/24 -p udp --dport 7946 -j ACCEPT - Allow the overlay data path:
iptables -I INPUT -s 10.20.0.0/24 -p udp --dport 4789 -j ACCEPT
Use the configured custom UDP port if the cluster was initialized with one. - If encrypted overlays are enabled, allow IP protocol 50 between the participating nodes. Keep this rule limited to the node network and the interface that carries overlay traffic.
- Review the resulting order with
iptables-saveand confirm that an earlier drop rule is not reached first.
These permits should not be opened to 0.0.0.0/0. If managers and workers use more than one private subnet, add one narrowly scoped rule per subnet instead of replacing the source restriction with “any.”
Enforce encrypted overlay traffic
When encrypted overlay networking is in use, Docker recommends accepting only encrypted packets on the data-path port. Its documented example drops unencrypted inbound VXLAN packets with:
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iptables -I INPUT -m udp --dport 4789 -m policy --dir in --pol none -j DROP
Adapt the port, rule position, and interface to your host. Insert the allow rule for the trusted node network before this drop when needed, and test a real encrypted overlay afterward. A custom data-path port must replace 4789 in both the allow and drop logic.
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Put published-service policy in DOCKER-USER
Swarm’s routing mesh listens on a published port on every node. A request sent to a worker that has no local task can still be forwarded to a task on another node. Consequently, a firewall rule on only the node currently running a container is insufficient.
Create a separate allowlist for each published application port and apply it to every node. For example, if TCP 8443 is intended only for clients in 192.0.2.0/24, a forwarded-traffic policy can be organized as follows:
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- Permit return traffic before restrictive rules:
iptables -I DOCKER-USER -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT - Permit the approved client network to the published port:
iptables -I DOCKER-USER -s 192.0.2.0/24 -p tcp --dport 8443 -j ACCEPT - Drop other attempts to that published port:
iptables -I DOCKER-USER -p tcp --dport 8443 -j DROP
Place the rules in an order that preserves the established-connection exception and the approved source permit. If several client networks are valid, add each source-specific accept before the final drop. Repeat the pattern for UDP services, using the service’s actual published port. Do not add a blanket drop in DOCKER-USER without first accounting for other container traffic that the host must continue to forward.
Docker may perform address translation before a packet reaches the user chain. Verify which destination and port are visible on your Docker Engine release, and use counters or logs to confirm that the intended rule is matching.
Using UFW without losing Docker’s rules
UFW is a front end, not a replacement for Docker’s generated chains. Use it to express host input policy for the Swarm ports, with the node CIDR as the source. For the example private network, the rule forms are:
ufw allow from 10.20.0.0/24 to any port 2377 proto tcpufw allow from 10.20.0.0/24 to any port 7946 proto tcpufw allow from 10.20.0.0/24 to any port 7946 proto udpufw allow from 10.20.0.0/24 to any port 4789 proto udp
Use the custom data-path port when applicable. Add the encrypted-overlay requirement separately if the deployment uses protocol 50; UFW’s ordinary TCP/UDP syntax does not express every IP-protocol policy, so retain an iptables rule for that case.
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Published Docker ports require special care. Confirm that UFW’s generated rules and Docker’s chains produce the intended result, then put source restrictions in DOCKER-USER when UFW’s high-level rules do not constrain forwarded container traffic. Never assume that a successful ufw status display proves the routing mesh is restricted.
firewalld and other distribution front ends
firewalld, nftables command sets, and persistence packages differ by distribution. The policy remains the same: permit the three inter-node port groups only from the node network, permit ESP when encrypted overlays require it, and maintain a separate allowlist for every published application port.
When a front end reloads, inspect the actual iptables rules afterward. Confirm that Docker’s chains still exist, that DOCKER-USER is reached before Docker’s forwarding chains, and that the host’s FORWARD policy has not changed in a way that blocks overlay or ingress traffic. Use the distribution’s documented mechanism to save and restore rules across reboot; there is no single persistence command that applies to every Linux distribution.
Why nftables native mode is a Swarm problem
Docker documents its nftables backend as experimental. Overlay-network rules have not been migrated to that backend, so Docker states that nftables cannot be enabled while the daemon is running in Swarm mode. A system may provide an iptables-nft compatibility command, but that does not by itself mean Docker is using a supported native nftables backend for Swarm.
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Runtime iptables or UFW changes disappear unless the distribution saves them. After adding rules:
- Save the active rules with the mechanism supplied by the distribution or firewall package.
- Reload the firewall in a maintenance window, preferably with console access or an out-of-band channel in case remote access is blocked.
- Restarting Docker must not erase your
DOCKER-USERpolicy. Verify the chain and its rule order after a Docker restart. - Reboot a test node, restore the saved rules, and confirm that Swarm membership, overlay traffic, and published services recover.
Do not rely on a rule count alone. A persistence system can restore rules into a different table or chain order than the one you tested.
Verification plan
Test from three perspectives: an allowed client, a deliberately denied client, and each Swarm node.
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- Inspect membership with
docker node ls. A node that repeatedly changes between Ready and Down often indicates blocked control or membership traffic. - Create or use a test overlay service and verify that tasks on different nodes can communicate.
- From an approved client network, connect to each node’s published application port, not only the node currently running the task.
- From a denied network, confirm that the same published port is rejected or times out according to your intended policy.
- Inspect counters with
iptables -L -v -nand the complete configuration withiptables-save. Counters should increase on the expected allow or drop rule. - If overlays are encrypted, verify that encrypted traffic works and that an unencrypted packet cannot use the data-path port.
Run these checks after firewall reloads, Docker upgrades, node replacement, and changes to service publication. A test that succeeds only from the manager is not enough because routing mesh behavior is node-wide.
Troubleshooting common failures
| Symptom | Likely firewall cause | What to inspect |
|---|---|---|
| Worker cannot join the Swarm | TCP 2377 is blocked or limited to the wrong source network | Manager input rules, advertised address, and the source CIDR |
| Nodes join but show intermittent membership failures | TCP or UDP 7946 is blocked in one direction | Both protocols on every node and intermediate firewalls |
| Overlay service traffic fails across hosts | The UDP data-path port is blocked, or a custom port was overlooked | The initialization setting, node routes, and UDP counters |
| Encrypted overlay does not establish | ESP protocol 50 is blocked, or the data-path policy drops required packets | Protocol-50 permits and the order of the policy match/drop rules |
| A published service works on one node but not another | The routing-mesh port is allowed only on the node holding the task | Published-port rules on every node and DOCKER-USER counters |
| All containers lose connectivity after a firewall change | Docker chains were flushed, disabled, or bypassed by a broad forward drop | DOCKER-USER, DOCKER-FORWARD, DOCKER-INGRESS, and the FORWARD policy |
| Rules vanish after reboot or Docker restart | The distribution’s persistence mechanism was not configured, or chain order changed | Saved rules, service startup order, and post-restart iptables-save output |
Choosing a firewall approach
There is no universally best front end. Compare the implementation you plan to deploy against these operational requirements:
| Decision axis | Questions to answer |
|---|---|
| Docker and Swarm support | Is the backend supported by the Docker Engine release, especially while the daemon runs in Swarm mode? |
| Network scoping | Can rules be limited to the node CIDR and the interface carrying Swarm traffic? |
| Docker chain integrity | Will Docker’s generated chains remain intact and will administrator policy still enter through DOCKER-USER? |
| Routing-mesh control | Can you restrict each published port on every node, rather than only on nodes running tasks? |
| Persistence | Are rules restored after reboot, firewall reload, and Docker restart? |
| Observability | Can operators inspect counters, logs, and the effective order of rules? |
A simple, auditable iptables policy is preferable to a sophisticated backend that Docker does not support for Swarm overlays. Whichever front end you select, document the node CIDRs, data-path port, encrypted-overlay requirement, published ports, and rollback procedure beside the cluster configuration.
Operational hardening checklist
- Allow TCP 2377 only between trusted managers and workers.
- Allow TCP and UDP 7946 between every participating node.
- Allow UDP 4789, or the configured replacement port, only across the node network.
- Allow IP protocol 50 when encrypted overlays are deployed.
- Never expose VXLAN UDP 4789 to an untrusted perimeter.
- Keep Docker’s generated chains and place forwarded container policy in
DOCKER-USER. - List every published service port and its approved client networks.
- Apply routing-mesh restrictions on every node, including nodes without a local task.
- Check the
FORWARDpolicy after enabling IP forwarding. - Persist rules using the host distribution’s supported mechanism.
- Test allowed, denied, cross-node, encrypted-overlay, reboot, and Docker-restart paths.
Safe change and rollback procedure
For a live cluster, add narrowly scoped allow rules first, verify connectivity, and only then add restrictive drops. Keep an existing administrative session open while testing and have console access available. If a change blocks membership or overlay traffic, remove the newest restrictive rule from the affected chain, restore the last known-good saved ruleset, and retest one node before propagating the correction.
When changing the data-path port, treat it as a cluster-wide change: update the firewall on all participating nodes and ensure every intermediate firewall permits the same UDP port. A node left with the old port can appear healthy for control traffic while overlay applications fail, so include cross-node service tests in the change window.
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