Red Hat OpenShift Development I: Introduction to Containers with Podman

Podman comes with a network called podman. By default, containers are attached to this network and can use it to communicate with one another.

However, you might need to create a new Podman network to better suit the increased communication needs of most applications. For example, the containers running an application API and database can use a separate Podman network to isolate their communication from other containers. Similarly, that same API container can use yet another network to isolate communication with a third container that hosts the application UI.

Figure 2.8: Example of isolated communication by using Podman networks

In the preceding example diagram, the UI and API containers are attached to the ui-network Podman network. The API and database containers are attached to the api-network Podman network.

Podman network management is done via the podman network subcommand. This subcommand includes the following operations:

For example, the following command creates a new Podman network called example-net:

[user@host ~]$ podman network create example-net

To connect a new container to this Podman network, use the --net option. The following example command creates a new container called my-container, which is connected to the example-net network.

[user@host ~]$ podman run -d --name my-container \
--net example-net container-image:latest

When you create new containers, you can connect them to multiple networks by specifying network names in a comma-separated list. For example, the following command creates a new container called double-connector that connects to both the postgres-net and redis-net networks.

[user@host ~]$ podman run -d --name double-connector \
--net postgres-net,redis-net \
container-image:latest

Alternatively, if the my-container container is already running, then run the following command to connect it to the example-net network:

[user@host ~]$ podman network connect example-net my-container

When you use the default Podman network, the domain name system (DNS) for other containers in that network is disabled. To enable DNS resolution between containers, create a Podman network and connect your containers to that network.

When using a network with DNS enabled, a container's hostname or alias is the name assigned to the container. For example, if a container is started with the following command, then the other containers on the test-net network can make requests to the first container by using the basic-container hostname. The basic-container hostname resolves to the current IP address of the basic-container container.

[user@host ~]$ podman run --net test-net --name basic-container example-image

You can connect containers to one or more Podman networks. After a container connects to a network, the container can communicate with other containers on that network. However, even though the containers are reachable to one another, other components might prevent connections. For example, firewall rules might block a connection coming from another container. By default, a container is available within any network that the container connects to.

For example, consider a running container called nginx-host that uses the example-net network. The container exposes an HTTP server on port 8080. Within another container that uses the example-net network, the following curl command resolves to the root of the HTTP server.

[user@host ~]$ curl http://nginx-host:8080