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What “private between hosts” means in Docker
A Docker bridge network is local to one Docker daemon. Containers attached to a bridge on host A cannot use that bridge to reach containers attached to a bridge on host B. For cross-host communication, Docker documents two broad choices: an overlay network, or ordinary operating-system routing that you manage yourself.
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An overlay is the normal Swarm-native design. Docker creates a virtual network that spans participating daemons and supplies container-to-container connectivity and name resolution across those nodes. The participating hosts must be members of the same Swarm even if the containers are ordinary standalone containers rather than Swarm services. The network must be created with --attachable so standalone containers can join it.
“Private” has several separate meanings that need to be secured independently:
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- Membership: only the intended containers should attach to the overlay.
- Host reachability: only participating Docker hosts should be able to exchange Swarm and overlay packets.
- Traffic confidentiality: overlay payloads should be encrypted in transit when the network path is not already trusted.
- Published services: a container port should not accidentally become reachable on every host interface.
A private overlay addresses the first and, with encryption enabled, the third item. Host firewalls and careful port publishing are still required for the other boundaries.
Plan the trust boundary before creating the network
Use private, reachable addresses for the nodes
Choose a private address on each Docker host that the other participating hosts can route to. The Swarm manager must advertise an address that workers can reach; advertising a public or otherwise ambiguous interface commonly produces a cluster that initializes successfully but cannot form a reliable overlay.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteKeep the hosts inside a private routed network, security-group boundary, or equivalent allowlist. Apply the same policy twice: once in the cloud or network firewall and again in the operating-system firewall. A rule that exists in only one layer is not a complete restriction.
Allow only Docker’s inter-host paths
For the hosts in this Swarm, permit the following traffic only between the participating node addresses:
| Port | Protocol | Purpose | Firewall scope |
|---|---|---|---|
2377 |
TCP | Swarm control plane | Allow from participating Docker hosts only |
4789 |
UDP | Overlay data path (VXLAN) | Allow from participating Docker hosts only |
7946 |
TCP | Node communication | Allow from participating Docker hosts only |
7946 |
UDP | Node communication and discovery | Allow from participating Docker hosts only |
Do not open these ports to the public Internet. They are node-to-node requirements, not client-facing application ports. If a host has multiple interfaces, make the allowlist specific to the private interface or private source addresses rather than permitting an entire address space.
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Decide whether encryption is required
Docker’s encrypted overlay option protects overlay traffic with IPsec at the VXLAN layer. It is appropriate when packets could cross an untrusted or only partially trusted network. Docker warns that encryption has a non-negligible performance penalty, so measure the workload you intend to run rather than assuming the impact is harmless. Encrypted overlays are unsupported for Windows containers.
Build an attachable encrypted overlay
The following sequence creates a two-or-more-host Swarm and an overlay that standalone containers can join. Run the first command on the host that will be the manager.
1. Initialize the manager on its private address
docker swarm init --advertise-addr <manager-private-ip>Docker prints a docker swarm join command. Save that command for the worker nodes. The address in --advertise-addr should be reachable from every node over the private network you selected.
2. Join each additional Docker host
On every worker, run the join command printed by the manager, substituting the manager’s private address if Docker’s output does not already contain it:
docker swarm join --token <join-token> <manager-private-ip>:2377Do not paste a worker token into an article, ticket, or shell history that other administrators do not need. If a token is exposed, rotate it from the manager and rejoin the affected node.
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3. Create the network from a manager
docker network create
--driver overlay
--attachable
--opt encrypted
private-netThe network is created once at the Swarm level. The --driver overlay flag makes it span hosts, --attachable permits standalone containers to connect, and --opt encrypted enables IPsec protection for overlay traffic. If you intentionally accept an unencrypted private path for performance reasons, omit the encryption option and document that decision; do not assume the network is encrypted by default.
4. Start containers on the overlay
docker run -d --name app --network private-net my-imageRun the same pattern on another Swarm node for a peer container. Containers on private-net can address one another by the Docker name or service/container DNS name provided on that overlay. Use stable names that describe the function, such as api or database, instead of relying on changing container IP addresses.
5. Keep attachment intentional
Because the network is attachable, any principal that can control a Docker daemon in this Swarm may be able to attach a container. Treat Docker Engine access as a privileged capability. Limit membership in the Swarm, protect manager credentials, and avoid giving untrusted automation unrestricted access to the Docker socket.
Separate private overlay traffic from published ports
Docker states that “Publishing container ports is insecure by default.” A mapping such as -p 8080:80 publishes the container’s port on host addresses; it does not merely make the port available to other containers on the private overlay. A published port is therefore a host-level exposure and must be reviewed like a firewall opening.
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docker run -d
--name admin
--network private-net
-p 127.0.0.1:8080:80
admin-imageThis mapping accepts connections through the host’s IPv4 loopback interface, so remote machines cannot reach it through the host’s ordinary network addresses. If IPv6 loopback access is required, add an explicit IPv6 mapping:
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docker run -d
--name admin
--network private-net
-p 127.0.0.1:8080:80
-p '[::1]:8080:80'
admin-imageDo not add a wildcard binding merely to make a service easier to test. If an external client genuinely needs the application, publish its port on the intended host interface and constrain that interface with the host and upstream firewall. Overlay membership does not make a published port private.
Validate the design before putting it into service
Check Swarm membership and advertised addresses
- On a manager, run
docker node lsand confirm that every expected host is present and ready. - Confirm that each node advertises a reachable private address, not an interface that the other nodes cannot route to.
- From the firewall or security-group view, verify that the only node-to-node allowances are the documented port groups:
2377/tcp,4789/udp, and7946/tcpplus7946/udp.
Inspect the overlay options on every node
docker network inspect private-netInspect the result on each host that will run a container. Verify that the network uses the overlay driver, is attachable, and has the encrypted option enabled when encryption is part of your policy. A local network with a similar name is not equivalent; the driver and options must match the intended Swarm network.
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From a container on the overlay, resolve the peer by its Docker name or service name and connect to the application’s container port. Test the path from both directions if the application is bidirectional. Keep this test separate from a host-published port test: the first confirms overlay connectivity, while the second confirms a deliberate host exposure.
Test from an untrusted network
From a network that is not in the node allowlist, verify that no Swarm administrative port and no host-only administrative endpoint is reachable. From an allowed private node, verify that only the intended application ports respond. This two-sided test catches the common mistake of validating only that the service works, without checking that it is inaccessible from places that should not see it.
Benchmark before production
Run a representative workload over an encrypted overlay and compare it with the same workload over an unencrypted overlay if your policy permits that test. Measure the latency and throughput that matter to your application. Docker documents a performance penalty for encryption; the size of that penalty depends on the workload and host environment, so a generic number would be misleading.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Overlay versus an OS-routed bridge network
Docker documents an overlay as the multi-host network driver and a bridge as host-local unless you build and maintain routing outside Docker. The following comparison helps choose deliberately rather than treating both designs as interchangeable.
| Decision point | Overlay network | OS-routed bridge network |
|---|---|---|
| Cross-host scope | Built for containers on different Swarm nodes. | Bridge remains local to a daemon; cross-host traffic requires your own routing design. |
| Encryption in transit | Optional IPsec protection with --opt encrypted; Docker documents a non-negligible performance cost. |
Not supplied by the bridge itself; encryption must come from another layer. |
| Control-plane requirements | Requires a Swarm and node-to-node access on 2377/tcp, 4789/udp, and 7946/tcp/udp. |
No overlay control plane, but you must operate routing, address allocation, and filtering. |
| Container DNS and discovery | Docker overlay naming can resolve peer containers or services on the network. | Docker does not turn separate local bridges into one shared discovery domain; provide and secure a separate mechanism. |
| Operational complexity | Swarm membership and firewall rules are required, but Docker manages the virtual network. | Fewer Swarm dependencies, but routing, encryption, discovery, and failure handling become your responsibility. |
| Portability across hosts | Containers can use the same logical network name on participating nodes. | Depends on your routed address plan and the network configuration of every host. |
| Performance considerations | Overlay encapsulation adds overhead; encryption adds further overhead. | May avoid overlay encapsulation, but performance and security depend on the routing implementation. |
Choose the routed-bridge approach only when you have a reason to manage those additional components yourself. A bridge alone is not a private multi-host network.
Firewall and driver caveats
Do not rewrite Docker’s bridge rules casually
On Linux, Docker creates firewall rules for bridge-network isolation, published ports, NAT, and masquerading. Docker advises administrators not to modify the rules it creates for bridge networks. If a host firewall tool needs to coexist with Docker, place your policy in the supported administration layer and test changes against both container connectivity and published-port behavior.
Host, macvlan, and ipvlan do not receive the same Docker rules
The host, macvlan, and ipvlan drivers do not receive Docker-created firewall rules in the way bridge networks do. Designs using those drivers therefore require explicit operating-system or upstream filtering. Do not infer that a driver has the same isolation properties as an overlay simply because both can be attached to a container.
Remember the Windows limitation
Docker’s encrypted overlay mode is unsupported for Windows containers. If Windows containers are part of the deployment, verify the platform-specific networking and encryption capabilities before choosing this design; do not enable the option and assume that the same protection applies.
Troubleshoot by symptom
Workers cannot join or remain unavailable
- Confirm that the worker can route to the manager’s advertised private address.
- Check that
2377/tcpis allowed from the worker to the manager and that the join token is current. - Check the node-communication paths on
7946/tcpand7946/udpin both cloud and host firewalls. - Make sure the manager advertised the private interface rather than an unreachable public, loopback, or secondary address.
The overlay exists but containers on different hosts cannot connect
- Verify that both daemons are members of the same Swarm and that both containers are attached to the same overlay network.
- Check
4789/udpin both directions; the overlay data path cannot work when that path is blocked. - Inspect
docker network inspect private-neton each node and confirm that the network is the intended attachable overlay. - Test by container name as well as by application port so that DNS and application-listener problems are not confused with routing problems.
Standalone containers cannot attach
Recreate the network with --attachable if it was created without that option. A Swarm overlay that is not attachable is intended for Swarm-managed workloads and will reject the standalone-container workflow shown above.
Encryption causes failures or unacceptable latency
- Confirm that the workload is not running in a Windows container, where encrypted overlays are unsupported.
- Compare encrypted and unencrypted performance in a controlled test and inspect host CPU and network capacity.
- Keep encryption enabled when its confidentiality benefit is required; optimize the hosts or network rather than silently exposing sensitive traffic.
An administrative endpoint is reachable remotely
Inspect the container’s port mapping. Replace an unqualified mapping such as -p 8080:80 with an explicit loopback binding such as -p 127.0.0.1:8080:80, add the IPv6 loopback mapping when needed, and remove any upstream firewall rule that is no longer necessary. A service that is private on the overlay can still be public through a wildcard host binding.
Production checklist
- Every participating daemon is in one Swarm and advertises a reachable private address.
- The overlay was created with
--driver overlayand--attachable. --opt encryptedis enabled when traffic confidentiality is required and the deployment does not use unsupported Windows containers.2377/tcp,4789/udp, and7946/tcp/udpare allowed only between the participating hosts.- No Swarm or overlay port is exposed to the public Internet.
- Administrative container ports are bound to loopback or another explicitly restricted interface.
- Application ports are published only when an external client needs them, with host and upstream firewall rules matching that intent.
- Network inspection confirms the expected driver, attachability, and encryption option.
- Cross-host DNS and application connectivity work from each required node.
- Untrusted-network tests show that administrative ports and unintended services are unreachable.
- Encrypted and unencrypted performance has been measured where the trade-off affects capacity planning.
Bottom line
For containers that must communicate across Docker hosts, create an attachable overlay inside a Swarm, advertise and firewall the hosts over private addresses, and allow only Docker’s documented control, data, and node-communication paths. Enable IPsec overlay encryption when its confidentiality benefit justifies the documented performance cost, and handle published ports separately: bind host-only services to loopback and expose application ports only on deliberately filtered interfaces.
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