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Basic Networking Guide for Podman

It seems once people master the basics of containers, networking is one of the first aspects they begin experimenting with. And in regards to networking, it takes very little experimentation before ending up on the deep end of the pool. The following guide shows the most common network setups for Podman rootfull and rootless containers. Each setup is supported with an example.

Differences between rootfull and rootless container networking

One of the guiding factors on networking for containers with Podman is going to be whether or not the container is run by a root user or not. This is because unprivileged users cannot create networking interfaces on the host. Therefore, with rootfull containers, the default networking mode is to use the Container Network Interface (CNI) plugins and specifically the bridge plugin. For rootless, the default network mode is slirp4netns. Because of the limited privileges, slirp4netns lacks some of the features of CNI networking; for example, slirp4netns cannot give containers a routable IP address.

Firewalls

The role of a firewall will not impact the setup and configuration of networking, but it will impact traffic on those networks. The most obvious is inbound network traffic to the container host, which is being passed onto containers usually with port mapping. Depending on the firewall implementation, we have observed firewall ports being opened automatically due to running a container with a port mapping (for example). If container traffic does not seem to work properly, check the firewall and allow traffic on ports the container is using. A common problem is that reloading the firewall deletes the cni iptables rules resulting in a loss of network connectivity for rootful containers. Podman v3 provides the podman network reload command to restore this without having to restart the container.

Basic Network Setups

Most containers and pods being run with Podman adhere to a couple of simple scenarios. By default, rootfull Podman will create a bridged network. This is the most straightforward and preferred network setup for Podman. Bridge networking creates an interface for the container on an internal bridge network, which is then connected to the internet via Network Address Translation(NAT). We also see users wanting to use macvlan for networking as well. The macvlan plugin forwards an entire network interface from the host into the container, allowing it access to the network the host is connected to. And finally, the default network configuration for rootless containers is slirp4netns. The slirp4netns network mode has limited capabilities but can be run on users without root privileges. It creates a tunnel from the host into the container to forward traffic.

Bridge

CNI defines a bridge network as where an internal network is created where both the container and host are attached. Then this network is capable of allowing the containers to communicate outside of the host.

bridge_network

Consider the above illustration. It depicts a laptop user running two containers: a web and db instance. These two containers are on the virtual network with the host. Additionally, by default, these containers can initiate communications outside the laptop (to the Internet for example). The containers on the virtual network typically have non-routable, also known as private IP addresses.

When dealing with communication that is being initiated outside the host, the outside client typically must address the laptop’s external network interface and given port number. Assuming the host allows incoming traffic, the host will know to forward the incoming traffic on that port to the specific container. To accomplish this, firewall rules are added to forward traffic when a container requests a specific port be forwarded.

Bridge networking is the default for Podman containers created as root. Podman provides a default bridge network, but you can create others using the podman network create command. Containers can be joined to a CNI network when they are created with the --network flag, or after they are created via the podman network connect and podman network disconnect commands.

As mentioned earlier, slirp4netns is the default network configuration for rootless users. But as of Podman version 3.0, rootless users can also use CNI networking. The user experience of rootless CNI is very akin to a rootfull CNI, except that there is no default network configuration provided. You simply need to create a network, and the one will be created as a bridge network.

$ podman network create

When rootless containers are run with a CNI networking configuration, CNI operations will be executed inside an extra network namespace. To join this namespace, use podman unshare --rootless-cni. Podman version 3.1 and earlier use a special “side-car” container called rootless-cni-infra. Do not remove this container while your rootless containers are running. If you remove this container (e.g. by accident), all attached containers lose network connectivity. In order to restore the network connectivity, all containers with networks must be restarted. This will automatically recreate the "side-car" container. When you are using version 3.2 or newer the “side-car” container can be safely removed. Therefore, it is no longer used. For rootfull containers, there is no extra namespace or “side-car” container as rootfull users have the permissions to create and modify network interfaces on the host.

Example

By default, rootfull containers use the CNI bridge plugin for its default configuration. In this case, no network name must be passed to Podman. However, you can create additional bridged networks with the podman create command. In that case, you will have to set the network name.

The following example shows how to set up a web server and expose it to the network outside the host as both rootfull and rootless. It will also show how an outside client can connect to the container.

(rootfull) $ sudo podman run -dt --name webserver -p 8080:80 quay.io/libpod/banner
00f3440c7576aae2d5b193c40513c29c7964e96bf797cf0cc352c2b68ccbe66a

As mentioned earlier, for rootless containers using CNI, a network must first be created.

$ podman network create
/home/baude/.config/cni/net.d/cni-podman1.conflist

Now run the container.

$ podman run -dt --name webserver --net cni-podman1 -p 8081:80 quay.io/libpod/banner
269fd0d6b2c8ed60f2ca41d7beceec2471d72fb9a33aa8ca45b81dc9a0abbb12

Note in the above run command, the container’s port 80 (where the Nginx server is running) was mapped to the host’s port 8080. Port 8080 was chosen to demonstrate how the host and container ports can be mapped for external access. The port could very well have been 80 as well (except for rootless users).

To connect from an outside client to the webserver, simply point an HTTP client to the host’s IP address at port 8080 for rootfull and port 8081 for rootless.

(outside_host): $ curl 192.168.99.109:8080
   ___           __
  / _ \___  ___/ /_ _  ___ ____
 / ___/ _ \/ _  /  ' \/ _ `/ _ \
/_/   \___/\_,_/_/_/_/\_,_/_//_/

(outside_host): $ curl 192.168.99.109:8081
   ___           __
  / _ \___  ___/ /_ _  ___ ____
 / ___/ _ \/ _  /  ' \/ _ `/ _ \
/_/   \___/\_,_/_/_/_/\_,_/_//_/

Macvlan

With macvlan, the container is given access to a physical network interface on the host. This interface can configure multiple subinterfaces. And each subinterface is capable of having its own MAC and IP address. In the case of Podman containers, the container will present itself as if it is on the same network as the host.

macvlan_network

In the illustration, outside clients will be able to access the web container by its IP address directly. Usually the network information, including IP address, is leased from a DHCP server like most other network clients on the network. If the laptop is running a firewall, such as firewalld, then accommodations will need to be made for proper access.

Example

The following example demonstrates how to set up a web container on a macvlan and how to access that container from outside the host. First, create the macvlan network. You need to know the network interface on the host that connects to the routable network. In the example case, it is eth0.

$ sudo podman network create -d macvlan -o parent=eth0 webnetwork
/etc/cni/net.d/webnetwork.conflist

The next step is to ensure that the DHCP CNI plugin is running. This plugin facilitates the DHCP lease from the network.

$ sudo /usr/libexec/cni/dhcp daemon

Now run the container and be certain to attach it to the network we created earlier.

$ sudo podman run -dt --name webserver --network webnetwork quay.io/libpod/banner
03d82083c434d7e937fc0b87c25401f46ab5050007df403bf988e25e52c5cc40
[baude@localhost ~]$ sudo podman exec webserver ip address show eth0
2: eth0@if3: <BROADCAST,MULTICAST,UP,LOWER_UP,M-DOWN> mtu 1500 qdisc noqueue state
UP
link/ether 0a:3c:e2:eb:87:0f brd ff:ff:ff:ff:ff:ff
inet 192.168.99.186/24 brd 192.168.99.255 scope global eth0
valid_lft forever preferred_lft forever
inet6 fe80::83c:e2ff:feeb:870f/64 scope link
valid_lft forever preferred_lft forever

Because the container has a routable IP address (on this network) and is not being managed by firewalld, no change to the firewall is needed.

(outside_host): $ curl http://192.168.99.186
   ___           __
  / _ \___  ___/ /_ _  ___ ____
 / ___/ _ \/ _  /  ' \/ _ `/ _ \
/_/   \___/\_,_/_/_/_/\_,_/_//_/

Slirp4netns

Slirp4netns is the default network setup for rootless containers and pods. It was invented because unprivileged users are not allowed to make network interfaces on the host. Slirp4netns creates a TAP device in the container’s network namespace and connects to the usermode TCP/IP stack. Consider the following illustration.

slirp_network

The unprivileged user on this laptop has created two containers: a DB container and a web container. Both of these containers have the ability to access content on networks outside the laptop. And outside clients can access the containers if the container is bound to a host port and the laptop firewall allows it. Remember, unprivileged users must use ports 1024 through 65535 as lower ports require root privileges. (CAP_NET_BIND_SERVICE) Note: this can be adjusted using the sysctl net.ipv4.ip_unprivileged_port_start

One of the drawbacks of slirp4netns is that the containers are completely isolated from each other. Unlike the bridge approach, there is no virtual network. For containers to communicate with each other, they can use the port mappings with the host system, or they can be put into a Pod where they share the same network namespace. See Communicating between containers and pods for more information.

Example

The following example will show how two rootless containers can communicate with each other where one is a web server. Then it will show how a client on the host’s network can communicate with the rootless web server.

First, run the rootless web server and map port 80 from the container to a non-privileged port like 8080.

$ podman run -dt --name webserver -p 8080:80 quay.io/libpod/banner
17ea33ccd7f55ff45766b3ec596b990a5f2ba66eb9159cb89748a85dc3cebfe0

Because rootless containers cannot communicate with each other directly with TCP/IP via IP addresses, the host and the port mapping are used. To do so, the IP address of the host (interface) must be known.

$ ip address show eth0
3: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group
default qlen 1000
link/ether 3c:e1:a1:c1:7a:3f brd ff:ff:ff:ff:ff:ff
altname eth0
inet 192.168.99.109/24 brd 192.168.99.255 scope global dynamic noprefixroute eth0
valid_lft 78808sec preferred_lft 78808sec
inet6 fe80::5632:6f10:9e76:c33/64 scope link noprefixroute
valid_lft forever preferred_lft forever

From another rootless container, use the host’s IP address and port to communicate between the two rootless containers successfully.

$ podman run -it quay.io/libpod/banner curl http://192.168.99.109:8080
   ___           __
  / _ \___  ___/ /_ _  ___ ____
 / ___/ _ \/ _  /  ' \/ _ `/ _ \
/_/   \___/\_,_/_/_/_/\_,_/_//_/

From a client outside the host, the IP address and port can also be used:

(outside_host): $ curl http://192.168.99.109:8080
   ___           __
  / _ \___  ___/ /_ _  ___ ____
 / ___/ _ \/ _  /  ' \/ _ `/ _ \
/_/   \___/\_,_/_/_/_/\_,_/_//_/

Communicating between containers and pods

Most users of containers have a decent understanding of how containers communicate with each other and the rest of the world. Usually each container has its own IP address and networking information. They communicate amongst each other using regular TCP/IP means like IP addresses or, in many cases, using DNS names often based on the container name. But pods are a collection of one or more containers, and with that, some uniqueness is inherited.

By definition, all containers in a Podman pod share the same network namespace. This fact means that they will have the same IP address, MAC addresses, and port mappings. You can conveniently communicate between containers in a pod by using localhost.

slirp_network

The above illustration describes a Pod on a bridged network. As depicted, the Pod has two containers “inside” it: a DB and a Web container. Because they share the same network namespace, the DB and Web container can communicate with each other using localhost (127.0.0.1). Furthermore, they are also both addressable by the IP address (and DNS name if applicable) assigned to the Pod itself.

For more information on container to container networking, see Configuring container networking with Podman.