blog: Strictly Filtering Docker Containers

Author: erig0

blog/_posts/2024-03-22-strictly-filtering-docker-containers.md

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+---
+layout: post
+title: "Strict Filtering of Docker Containers"
+section: Blog
+date: 2024-04-03T16:00:00
+author: Eric Garver
+---
+
+## Introduction
+
+Docker supports publishing ports for a container. This allows external
+access to the container. When firewalld is running these published ports
+are honored and a hole is opened in firewalld. For most users, e.g.
+workstations, this a good thing as docker works transparently.
+
+For some users, this is not expected. They want firewalld to be strict.
+They want to only allow traffic explicitly via firewalld. Fortunately
+this can be achieve with some configuration.
+
+## Docker Configuration
+
+To have full control of docker containers via firewalld one must first
+disable iptables in docker.
+
+This can be done by adding `iptables: false` to the daemon
+configuration.
+
+```
+# cat /etc/docker/daemon.json
+{
+    "iptables": false
+}
+```
+
+Then the host must be rebooted. Restarting Docker is not enough to clean
+up all the pre-existing iptables rules.
+
+```
+# reboot
+```
+
+Now the containers won't have any iptables firewall rules automatically
+created.
+
+## Verify Docker Does Not Install iptables rules
+
+This is optional and just for illustration purposes.
+
+After the reboot starting a docker container will cause containers to
+not have internet access. This means that docker is not setting up
+iptables rules.
+
+```
+# docker run -it --rm debian:stable
+
+# apt update
+Ign:1 http://deb.debian.org/debian stable InRelease
+Ign:2 http://deb.debian.org/debian stable-updates InRelease
+Ign:3 http://deb.debian.org/debian-security stable-security InRelease
+0% [Connecting to deb.debian.org]
+```
+
+In the next step this will be fixed by doing the networking natively in
+firewalld. Restarting the container is not necessary.
+
+## Firewalld Configuration
+
+Now setup firewalld to natively perform all networking for docker with
+the following configuration.
+
+```
+firewall-cmd --permanent --zone docker --add-source 172.17.0.1/16
+
+firewall-cmd --permanent --new-policy dockerToWorld
+firewall-cmd --permanent --policy dockerToWorld --add-ingress-zone docker
+firewall-cmd --permanent --policy dockerToWorld --add-egress-zone ANY
+firewall-cmd --permanent --policy dockerToWorld --set-target ACCEPT
+firewall-cmd --permanent --policy dockerToWorld --add-masquerade
+
+firewall-cmd --reload
+```
+
+This creates a policy, `dockerToWorld`, to give the container internet
+access. Note that the `--add-source` above assumes the default address
+range used by docker.
+
+### Verify Firewalld Rules
+
+After the firewalld rules are created verify the container has internet
+access.
+
+```
+# apt update
+Get:1 http://deb.debian.org/debian stable InRelease [151 kB]
+Get:2 http://deb.debian.org/debian stable-updates InRelease [55.4 kB]
+Get:3 http://deb.debian.org/debian-security stable-security InRelease [48.0 kB]
+Get:4 http://deb.debian.org/debian stable/main amd64 Packages [8786 kB]
+Get:5 http://deb.debian.org/debian stable-updates/main amd64 Packages [12.7 kB]
+Get:6 http://deb.debian.org/debian-security stable-security/main amd64 Packages [150 kB]
+Fetched 9203 kB in 1s (7189 kB/s)
+Reading package lists... Done
+Building dependency tree... Done
+Reading state information... Done
+8 packages can be upgraded. Run 'apt list --upgradable' to see them.
+```
+
+### Adding Firewalld Native Forward Ports
+
+Since docker is no longer using iptables ports published with
+`--publish` will no longer work. It's simply ignored. Ports must be
+exposed with firewalld.
+
+The first step is to create another policy, `dockerFwdPort`, to allow
+external access to the container.
+
+```
+firewall-cmd --permanent --new-policy dockerFwdPort
+firewall-cmd --permanent --policy dockerFwdPort --add-ingress-zone ANY
+firewall-cmd --permanent --policy dockerFwdPort --add-egress-zone HOST
+```
+
+Note: Older firewalld versions (before v2.0.z) require using
+`egress-zone=ANY`.
+
+To add port forwarding (equivalent of docker --publish) to a specific
+container use `--add-forward-port` in the `dockerFwdPort` policy.  This
+example forwards port `8080` to to `80`. Note that the containers IP
+address must be known.
+
+```
+firewall-cmd --permanent --policy dockerFwdPort --add-forward-port port=8080:proto=tcp:toport=80:toaddr=172.17.0.2
+```
+
+Lastly reload the firewall.
+
+```
+firewall-cmd --reload
+```
+
+## Results
+
+This small amount of configuration allows firewalld to strictly filter
+docker container network traffic by doing all the networking natively in
+firewalld.
+
+## A Note About Podman
+
+A similar configuration can be done with Podman. Podman 4.x can use
+environment variable `NETAVARK_FW=none` to disable the network plugin.
+Podman 5.x will have a `containers.conf` for it.
+
+The firewalld configuration is the same except that the `podman` zone is
+used instead of the `docker` zone.
+
+## Bugs Referencing This Topic
+
+This topic has been discussed and referenced in numerous reports.
+
+- [How to manage docker exposed port by firewall-cmd?](https://github.com/firewalld/firewalld/issues/869)
+- [Conflict with CNI DNAT rule and top-level accept of DNAT'd packets](https://github.com/firewalld/firewalld/issues/556)
+- [podman-quadlet loads NAT rules that bypass firewall restrictions](https://issues.redhat.com/browse/RHEL-26522)