The Tool Desk
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What a private Docker network between hosts actually requires
A Docker bridge network is local to one Docker daemon. It can isolate containers on that host, but a container attached to a bridge on Host A cannot use that same bridge to reach a container on Host B. For multi-host communication, Docker documents two practical designs: an overlay network managed by Swarm, or a bridge network connected with routing that you manage at the operating-system or network layer.
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The usual Docker-native answer is an attachable overlay network. Swarm supplies node membership, control-plane coordination, service discovery and the virtual network fabric. The --attachable option lets ordinary, standalone containers join the overlay instead of requiring every workload to be declared as a Swarm service. The overlay can be encrypted in transit with --opt encrypted, which enables IPsec protection for the VXLAN data path.
“Private” has several separate meanings, and you should verify each one:
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- Membership privacy: only containers attached to the network should receive overlay connectivity.
- Host privacy: only the participating Docker hosts should be able to reach the Swarm and overlay ports.
- Transport privacy: overlay packets should be encrypted between hosts when the platform supports it.
- Application exposure: published ports should be bound only to the interface and addresses that really need them.
An encrypted overlay helps with transport privacy, but it does not authenticate your application protocol, replace container identity controls or make a deliberately published service private.
Recommended architecture: an attachable encrypted overlay
Put the Docker daemons on a private routed network, cloud security group or equivalent boundary. Initialize one Swarm manager with an address that other nodes can reach on that private network. Join the remaining hosts to that Swarm, then create the overlay from a manager. Finally, attach standalone containers or Swarm services to the network.
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docker swarm init --advertise-addr <manager-private-ip>Docker prints a worker-join command. Run that command on each worker host. If you need another manager, obtain a manager join token with docker swarm join-token manager and use the generated command on the additional host. Keep the join command private because its token grants membership to the Swarm.
After the nodes have joined, create the network on a manager:
docker network create
--driver overlay
--attachable
--opt encrypted
private-netThe network name is arbitrary; use a name that identifies its trust boundary and purpose. The command creates one logical network that Docker can materialize on participating nodes. A standalone container can then join it:
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docker run -d --name app --network private-net my-imageFor a Swarm service, specify the network when creating or updating the service:
docker service create --name app --network private-net my-imageContainers on the same overlay can resolve one another by container name or service name through Docker’s internal DNS. Use a stable service name for application configuration rather than hard-coding a task IP; task addresses can change when a container is rescheduled.
Firewall the exact host-to-host paths
Every participating Docker host must be able to reach the other participating hosts on the documented Swarm and overlay paths. Permit these ports only between the hosts that belong to this private Docker network:
| Port | Protocol | Purpose | Firewall scope |
|---|---|---|---|
| 2377 | TCP | Swarm control plane and node management | Allow only between trusted Swarm nodes; do not expose publicly |
| 4789 | UDP | Overlay VXLAN data path | Allow only between hosts carrying the overlay |
| 7946 | TCP | Node communication and membership traffic | Allow only between participating nodes |
| 7946 | UDP | Node communication and membership traffic | Allow only between participating nodes |
Apply the allowlist twice when you run in a cloud: once in the provider’s security group or network ACL and again in the host firewall. Permit the private addresses, security-group identity or another narrow identity for the Docker nodes, not an Internet-wide source such as 0.0.0.0/0. Keep the hosts on a private routed segment where possible. A private subnet alone is not a firewall policy; still deny unsolicited traffic from other networks.
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Keep published ports from escaping the private boundary
Docker’s warning is direct: “Publishing container ports is insecure by default.” A mapping such as -p 8080:80 asks Docker to listen on host addresses, not merely to make the service available to peers on the overlay. That listener may be reachable from every route to the host unless another firewall rule blocks it.
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For a host-only administrative endpoint, bind the published port to IPv4 loopback:
docker run -d
--name admin
--network private-net
-p 127.0.0.1:8080:80
admin-imageIf local clients use IPv6 loopback as well, publish an IPv6 loopback mapping explicitly:
docker run -d
--name admin
--network private-net
-p 127.0.0.1:8080:80
-p '[::1]:8080:80'
admin-imagePublish an application port only when an external client genuinely needs it. Bind it to the intended host interface or private address, and treat that published port as a firewall opening. Overlay membership does not override a host-level listener: a service can remain on the private network while also being exposed through -p.
If another container needs the endpoint, prefer its overlay DNS name and container port instead of publishing the port at all. For example, an API container can call http://admin:80 on private-net while the host exposes nothing.
Verify that the network is really private and encrypted
Use a repeatable check before placing production traffic on the network:
- Check Swarm membership. Run
docker node lson a manager. Every intended host should appear with the expected role and an available status. A node advertising an unreachable address will cause overlay failures even when the containers themselves are healthy. - Check the host firewall. From each participating node, verify that TCP 2377 and TCP/UDP 7946 work only to the other approved nodes, and that UDP 4789 is allowed between overlay hosts. From an unrelated network, these ports should be filtered.
- Inspect the network. Run
docker network inspect private-neton a manager and examine the driver, attachable setting and options. Confirm that the network reports the encrypted option you requested and that the expected containers or services are attached. - Test name resolution. From one attached container, resolve the peer by its service or container DNS name. If the name does not resolve, verify that both workloads use the same overlay and that the peer is running.
- Test the application path. Connect to the peer’s container port from an attached container. Do not test only a published host port; that would verify the host listener rather than overlay connectivity.
- Test from an untrusted network. Confirm that administrative ports and any service that should remain private are not reachable from outside the approved host or client boundary.
- Measure before production. Compare representative traffic on encrypted and unencrypted overlays in your environment. Docker documents a non-negligible performance penalty for overlay encryption, so CPU use, latency and throughput should be checked with your actual workload.
Encryption is a property of the overlay data path, not a guarantee that every packet generated by a container is protected. Traffic leaving the overlay through a published port, host networking or another interface follows that design’s controls.
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Overlay versus OS-routed bridge networking
An overlay is not the only multi-host design. You can use ordinary bridge networks and provide routing between host subnets yourself. That can fit environments with an existing routed fabric, but it moves service discovery, address planning and filtering outside Docker’s overlay control plane.
| Decision axis | Attachable overlay | OS-routed bridge networks |
|---|---|---|
| Encryption in transit | Optional IPsec protection with --opt encrypted; Docker warns of a performance cost. |
Not supplied by the bridge itself; use your network’s VPN, IPsec or other transport security. |
| Operational complexity | Requires Swarm membership and the documented node ports, but Docker manages the virtual network. | Requires host routes, subnet planning, return paths and filtering that you manage separately. |
| Firewall requirements | TCP 2377, UDP 4789 and TCP/UDP 7946 between participating nodes, plus any application ports you publish. | No overlay control or VXLAN requirement, but routed container subnets and application ports need explicit policy. |
| Portability across hosts | Containers can join the same logical network on participating Swarm nodes. | Portability depends on consistent routes, addresses and network configuration on every host. |
| DNS and service discovery | Docker’s overlay and Swarm mechanisms provide service-name discovery. | You must provide an equivalent discovery method or configure addresses yourself. |
| Performance overhead | VXLAN encapsulation adds overhead; encryption adds more and should be benchmarked. | A routed path can avoid overlay encapsulation, but its performance and security depend on your network. |
Choose the routed design when your organization already operates the required routing and security controls and wants Docker to remain host-local. Choose an overlay when you want Docker to coordinate membership, cross-host discovery and connectivity with a standard Swarm workflow.
Driver-specific firewall caveats
On Linux, Docker creates firewall rules for bridge-network isolation, port publishing, NAT and masquerading. Docker advises leaving those Docker-created bridge rules intact. Removing or rewriting them can break isolation and published-port behavior.
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That automation does not apply uniformly to every network driver:
- Bridge: host-local by default, with Docker-managed rules for common isolation and NAT behavior.
- Overlay: spans Swarm nodes and needs the inter-host ports listed above. Encryption is configured as an overlay option.
- Host: the container shares the host’s network namespace, so host firewall and service configuration become the primary boundary.
- macvlan and ipvlan: Docker does not create the same firewall rules it creates for bridge networks. Apply explicit operating-system, upstream-switch or network-firewall filtering.
Do not select macvlan, ipvlan or host networking merely to avoid configuring an overlay. They change the exposure model and may remove Docker’s normal filtering assumptions. Document the interface, subnet, gateway and upstream policy for any design that bypasses bridge rules.
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Containers on different hosts cannot resolve each other
Confirm that both containers are attached to the same overlay name and that the network was created with --attachable if they are standalone containers. A local bridge with the same name on two hosts is still two unrelated networks. For Swarm services, inspect the service’s network attachment and verify that tasks are running on nodes that can reach one another.
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The overlay exists, but traffic times out
Check all four inter-host paths, not only the control port. TCP 2377 handles Swarm control, UDP 4789 carries VXLAN data, and TCP plus UDP 7946 support node communication. A security group that allows 2377 but drops UDP 4789 can produce a healthy-looking Swarm with unusable cross-host traffic. Check host firewalls, cloud rules, route tables and any network appliance between the private addresses.
A node joined with the wrong address
Leave and rejoin the node using an address reachable from every participating host. On a manager, inspect node details and the advertised address. Hosts with public and private interfaces can accidentally advertise the public route or an interface that peers cannot reach.
Name resolution works, but the application refuses connections
Use the peer’s container port, not a host-published port, for overlay-to-overlay calls. Confirm that the application listens on the container interface rather than only on 127.0.0.1 inside its own namespace. Also check the application’s bind address, internal access-control list and protocol configuration.
An administrator can reach a service from another machine
Inspect the container’s published mappings with docker ps and docker port. Replace an unqualified mapping with an explicit private or loopback bind, then remove any cloud or host firewall rule that permits unnecessary sources. Remember that a published port is independent of overlay membership.
Encrypted traffic is too slow
Docker documents a non-negligible performance penalty for encrypted overlays. Benchmark the encrypted and unencrypted versions with representative payloads, connection counts and node placement. If policy requires encryption, size CPU capacity for the cost or reduce unnecessary cross-host traffic by co-locating tightly coupled containers. Do not disable encryption solely because a small synthetic test is slow; measure the security requirement and workload together.
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Windows containers fail to use an encrypted overlay
Docker documents encrypted overlays as unsupported for Windows containers. Do not assume that enabling --opt encrypted provides the same result on a Windows container workload. Use a supported transport-security design for that platform, such as protection at the private network or host layer, and validate it against the exact Windows and Docker versions in use.
Operational hardening checklist
- Use a dedicated private address or security-group identity for Swarm advertisements and node-to-node traffic.
- Allow 2377/TCP, 4789/UDP and 7946/TCP plus 7946/UDP only between participating Docker hosts.
- Keep the Swarm manager endpoint off the public Internet and protect join tokens as credentials.
- Create overlays as attachable only when standalone containers need to join; otherwise limit membership to declared services.
- Enable
--opt encryptedfor supported workloads that require confidentiality between hosts, then benchmark its cost. - Use service and container DNS names instead of task IP addresses.
- Do not publish a port unless an external client needs it. Bind host-only endpoints to
127.0.0.1and, when required,[::1]. - Review every published mapping as a firewall opening, including mappings added by deployment tooling.
- Keep Docker-managed bridge firewall rules intact and write explicit policy for host, macvlan and ipvlan designs.
- Repeat the reachability test from both an approved private node and an untrusted network after every firewall or topology change.
Practical decision sequence
- If the containers are on one host only, use a user-defined bridge and avoid publishing ports that peers do not need.
- If containers must communicate across hosts and you want Docker-managed discovery, create an attachable overlay inside a Swarm.
- If traffic must be confidential between hosts and the workload is supported, add
--opt encryptedand benchmark before rollout. - If your organization already provides routed container subnets and its own security controls, evaluate an OS-routed bridge instead of adding overlay encapsulation.
- Whichever design you choose, narrow the host firewall, bind published ports deliberately and validate from trusted and untrusted vantage points.
The result is a layered boundary: Swarm limits which Docker hosts participate, the overlay limits which containers share a virtual network, encryption protects supported overlay traffic in transit, and host or cloud firewalls prevent unrelated systems from reaching the control and data paths. Published ports remain a separate exposure decision and must be configured accordingly.
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