Packet filtering and firewalls (2024)

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

If you're running Docker on a host that is exposed to the Internet, you willprobably want to have iptables policies in place that prevent unauthorizedaccess to containers or other services running on your host. This pagedescribes how to achieve that, and what caveats you need to be aware of.

Docker installs two custom iptables chains named DOCKER-USER and DOCKER,and it ensures that incoming packets are always checked by these two chainsfirst. These chains are part of the FORWARD chain.

All of Docker's iptables rules are added to the DOCKER chain. Do notmanipulate this chain manually. If you need to add rules which load beforeDocker's rules, add them to the DOCKER-USER chain. These rules are appliedbefore any rules Docker creates automatically.

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

Match the original IP and ports for requests

When packets arrive to the DOCKER-USER chain, they have already passed througha Destination Network Address Translation (DNAT) filter. That means that theiptables flags you use can only match internal IP addresses and ports ofcontainers.

If you want to match traffic based on the original IP and port in the networkrequest, you must use theconntrack iptables extension.For example:

$ sudo iptables -I DOCKER-USER -p tcp -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT$ sudo iptables -I DOCKER-USER -p tcp -m conntrack --ctorigsrc 1.2.3.4 --ctorigdstport 80 -j ACCEPT

Important
Using the conntrack extension may result in degraded performance.

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 anegated rule at the top of the DOCKER-USER filter chain. For example, thefollowing rule restricts external access from all IP addresses except 192.168.1.1:

Docker on a router

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

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

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 themoby project.

Setting the default bind address for containers

By default, the Docker daemon binds published container ports to the 0.0.0.0address. When you publish a container's ports as follows:

docker run -p 8080:80 nginx

This publishes port 8080 to all network interfaces on the host, potentiallymaking them available to the outside world. Unless you've disabled IPv6 at thekernel level, the port gets published on both IPv4 and IPv6.

You can change the default binding address for published container ports so thatthey're only accessible to the Docker host by default. To do that, you canconfigure the daemon to use the loopback address (127.0.0.1) instead.To do so, configure the "ip" key in the daemon.json configuration file:

{ "ip": "127.0.0.1"}

This changes the default binding address to 127.0.0.1 for published containerports on the default bridge network.Restart the daemon for this change to take effect.Alternatively, you can use the dockerd --ip flag when starting the daemon.

Note
Changing the default bind address doesn't have any effect on Swarm services.Swarm services are always exposed on the 0.0.0.0 network interface.

To configure this setting for user-defined bridge networks, usethe com.docker.network.bridge.host_binding_ipv4driver option when you create the network.

$ docker network create mybridge \ -o "com.docker.network.bridge.host_binding_ipv4=127.0.0.1"

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

Docker and ufw

Uncomplicated Firewall(ufw) is a frontend that ships with Debian and Ubuntu,and it lets you manage firewall rules. Docker and ufw use iptables in waysthat make them incompatible with each other.

When you publish a container's ports using Docker, traffic to and from thatcontainer gets diverted before it goes through the ufw firewall settings.Docker routes container traffic in the nat table, which means that packetsare diverted before it reaches the INPUT and OUTPUT chains that ufw uses.Packets are routed before the firewall rules can be applied,effectively ignoring your firewall configuration.