Docker and iptables

Estimated reading time: 4 minutes

On Linux, Docker manipulates iptables rules to provide network isolation. While this is an implementation detail and you should not modify the rules Docker inserts into your iptables policies, it does have some implications on what you need to do if you want to have your own policies in addition to those managed by Docker.

If you’re running Docker on a host that is exposed to the Internet, you will probably want to have iptables policies in place that prevent unauthorized access to containers or other services running on your host. This page describes how to achieve that, and what caveats you need to be aware of.

Add iptables policies before Docker’s rules

Docker installs two custom iptables chains named DOCKER-USER and DOCKER, and it ensures that incoming packets are always checked by these two chains first.

All of Docker’s iptables rules are added to the DOCKER chain. Do not manipulate this chain manually. If you need to add rules which load before Docker’s rules, add them to the DOCKER-USER chain. These rules are applied before any rules Docker creates automatically.

Rules added to the FORWARD chain -- either manually, or by another iptables-based firewall -- are evaluated after these chains. This means that if you expose a port through Docker, this port gets exposed no matter what rules your firewall has configured. If you want those rules to apply even when a port gets exposed through Docker, you must add these rules to the DOCKER-USER chain.

Restrict connections to the Docker host

By default, all external source IPs are allowed to connect to the Docker host. To allow only a specific IP or network to access the containers, insert a negated rule at the top of the DOCKER-USER filter chain. For example, the following rule restricts external access from all IP addresses except 192.168.1.1:

$ iptables -I DOCKER-USER -i ext_if ! -s 192.168.1.1 -j DROP

Please note that you will need to change ext_if to correspond with your host’s actual external interface. You could instead allow connections from a source subnet. The following rule only allows access from the subnet 192.168.1.0/24:

$ iptables -I DOCKER-USER -i ext_if ! -s 192.168.1.0/24 -j DROP

Finally, you can specify a range of IP addresses to accept using --src-range (Remember to also add -m iprange when using --src-range or --dst-range):

$ iptables -I DOCKER-USER -m iprange -i ext_if ! --src-range 192.168.1.1-192.168.1.3 -j DROP

You can combine -s or --src-range with -d or --dst-range to control both the source and destination. For instance, if the Docker daemon listens on both 192.168.1.99 and 10.1.2.3, you can make rules specific to 10.1.2.3 and leave 192.168.1.99 open.

iptables is complicated and more complicated rules are out of scope for this topic. See the Netfilter.org HOWTO for a lot more information.

Docker on a router

Docker also sets the policy for the FORWARD chain to DROP. If your Docker host also acts as a router, this will result in that router not forwarding any traffic anymore. If you want your system to continue functioning as a router, you can add explicit ACCEPT rules to the DOCKER-USER chain to allow it:

$ iptables -I DOCKER-USER -i src_if -o dst_if -j ACCEPT

Prevent Docker from manipulating iptables

It is possible to set the iptables key to false in the Docker engine’s configuration file at /etc/docker/daemon.json, but this option is not appropriate for most users. It is not possible to completely prevent Docker from creating iptables rules, and creating them after-the-fact is extremely involved and beyond the scope of these instructions. Setting iptables to false will more than likely break container networking for the Docker engine.

For system integrators who wish to build the Docker runtime into other applications, explore the moby project.

Setting the default bind address for containers

By default, the Docker daemon will expose ports on the 0.0.0.0 address, i.e. any address on the host. If you want to change that behavior to only expose ports on an internal IP address, you can use the --ip option to specify a different IP address. However, setting --ip only changes the default, it does not restrict services to that IP.

Integration with Firewalld

If you are running Docker version 20.10.0 or higher with firewalld on your system with --iptables enabled, Docker automatically creates a firewalld zone called docker and inserts all the network interfaces it creates (for example, docker0) into the docker zone to allow seamless networking.

Consider running the following firewalld command to remove the docker interface from the zone.

# Please substitute the appropriate zone and docker interface
$ firewall-cmd --zone=trusted --remove-interface=docker0 --permanent
$ firewall-cmd --reload

Restarting dockerd daemon inserts the interface into the docker zone.