docker-mailserver/docs/content/config/advanced/podman.md

title
Advanced | Podman

Introduction

Podman is a daemonless container engine for developing, managing, and running OCI Containers on your Linux System.

!!! warning "About Support for Podman"

Please note that Podman **is not** officially supported as DMS is built and verified on top of the _Docker Engine_. This content is entirely community supported. If you find errors, please open an issue and provide a PR.

!!! warning "About this Guide"

This guide was tested with Fedora 34 using `systemd` and `firewalld`. Moreover, it requires Podman version >= 3.2. You may be able to substitute `dnf` - Fedora's package manager - with others such as `apt`.

!!! warning "About Security"

Running podman in rootless mode requires additional modifications in order to keep your mailserver secure.
Make sure to read the related documentation.

Installation in Rootfull Mode

While using Podman, you can just manage docker-mailserver as what you did with Docker. Your best friend setup.sh includes the minimum code in order to support Podman since it's 100% compatible with the Docker CLI.

The installation is basically the same. Podman v3.2 introduced a RESTful API that is 100% compatible with the Docker API, so you can use Docker Compose with Podman easily. Install Podman and Docker Compose with your package manager first.

sudo dnf install podman docker-compose

Then enable podman.socket using systemctl.

systemctl enable --now podman.socket

This will create a unix socket locate under /run/podman/podman.sock, which is the entrypoint of Podman's API. Now, configure docker-mailserver and start it.

export DOCKER_HOST="unix:///run/podman/podman.sock"
docker compose up -d mailserver
docker compose ps

You should see that docker-mailserver is running now.

Self-start in Rootfull Mode

Podman is daemonless, that means if you want docker-mailserver self-start while boot up the system, you have to generate a systemd file with Podman CLI.

podman generate systemd mailserver > /etc/systemd/system/mailserver.service
systemctl daemon-reload
systemctl enable --now mailserver.service

Installation in Rootless Mode

Running rootless containers is one of Podman's major features. But due to some restrictions, deploying docker-mailserver in rootless mode is not as easy compared to rootfull mode.

• a rootless container is running in a user namespace so you cannot bind ports lower than 1024
• a rootless container's systemd file can only be placed in folder under ~/.config
• a rootless container can result in an open relay, make sure to read the security section.

Also notice that Podman's rootless mode is not about running as a non-root user inside the container, but about the mapping of (normal, non-root) host users to root inside the container.

!!! warning

In order to make rootless DMS work we must modify some settings in the Linux system, it requires some basic linux server knowledge so don't follow this guide if you not sure what this guide is talking about. Podman rootfull mode and Docker are still good and security enough for normal daily usage.

First, enable podman.socket in systemd's userspace with a non-root user.

systemctl enable --now --user podman.socket

The socket file should be located at /var/run/user/$(id -u)/podman/podman.sock. Then, modify compose.yaml to make sure all ports are bindings are on non-privileged ports.

services:
  mailserver:
    ports:
      - "10025:25"   # SMTP  (explicit TLS => STARTTLS)
      - "10143:143"  # IMAP4 (explicit TLS => STARTTLS)
      - "10465:465"  # ESMTP (implicit TLS)
      - "10587:587"  # ESMTP (explicit TLS => STARTTLS)
      - "10993:993"  # IMAP4 (implicit TLS)

Then, setup your mailserver.env file follow the documentation and use Docker Compose to start the container.

export DOCKER_HOST="unix:///var/run/user/$(id -u)/podman/podman.sock"
docker compose up -d mailserver
docker compose ps

Security in Rootless Mode

In rootless mode, podman resolves all incoming IPs as localhost, which results in an open gateway in the default configuration. There are two workarounds to fix this problem, both of which have their own drawbacks.

Enforce authentication from localhost

The PERMIT_DOCKER variable in the mailserver.env file allows to specify trusted networks that do not need to authenticate. If the variable is left empty, only requests from localhost and the container IP are allowed, but in the case of rootless podman any IP will be resolved as localhost. Setting PERMIT_DOCKER=none enforces authentication also from localhost, which prevents sending unauthenticated emails.

Use the slip4netns network driver

The second workaround is slightly more complicated because the compose.yaml has to be modified. As shown in the fail2ban section the slirp4netns network driver has to be enabled. This network driver enables podman to correctly resolve IP addresses but it is not compatible with user defined networks which might be a problem depending on your setup.

Rootless Podman requires adding the value slirp4netns:port_handler=slirp4netns to the --network CLI option, or network_mode setting in your compose.yaml.

You must also add the ENV NETWORK_INTERFACE=tap0, because Podman uses a hard-coded interface name for slirp4netns.

!!! example

```yaml
services:
  mailserver:
    network_mode: "slirp4netns:port_handler=slirp4netns"
    environment:
      - NETWORK_INTERFACE=tap0
      ...
```

!!! note

`podman-compose` is not compatible with this configuration.

Self-start in Rootless Mode

Generate a systemd file with the Podman CLI.

podman generate systemd mailserver > ~/.config/systemd/user/mailserver.service
systemctl --user daemon-reload
systemctl enable --user --now mailserver.service

Systemd's user space service is only started when a specific user logs in and stops when you log out. In order to make it to start with the system, we need to enable linger with loginctl

loginctl enable-linger <username>

Remember to run this command as root user.

Port Forwarding

When it comes to forwarding ports using firewalld, see https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/securing_networks/using-and-configuring-firewalld_securing-networks#port-forwarding_using-and-configuring-firewalld for more information.

firewall-cmd --permanent --add-forward-port=port=<25|143|465|587|993>:proto=<tcp>:toport=<10025|10143|10465|10587|10993>
...

# After you set all ports up.
firewall-cmd --reload

Notice that this will only open the access to the external client. If you want to access privileges port in your server, do this:

firewall-cmd --permanent --direct --add-rule <ipv4|ipv6> nat OUTPUT 0 -p <tcp|udp> -o lo --dport <25|143|465|587|993> -j REDIRECT --to-ports <10025|10143|10465|10587|10993>
...
# After you set all ports up.
firewall-cmd --reload

Just map all the privilege port with non-privilege port you set in compose.yaml before as root user.