Bind container ports to the host

Estimated reading time: 3 minutes

The information in this section explains binding container ports within the Docker default bridge. This is a bridge network named bridge created automatically when you install Docker.

Note: The Docker networks feature allows you to create user-defined networks in addition to the default bridge network.

By default Docker containers can make connections to the outside world, but the outside world cannot connect to containers. Each outgoing connection will appear to originate from one of the host machine’s own IP addresses thanks to an iptables masquerading rule on the host machine that the Docker server creates when it starts:

$ sudo iptables -t nat -L -n

...
Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination
MASQUERADE  all  --  172.17.0.0/16       0.0.0.0/0
...

The Docker server creates a masquerade rule that let containers connect to IP addresses in the outside world.

If you want containers to accept incoming connections, you will need to provide special options when invoking docker run. There are two approaches.

First, you can supply -P or --publish-all=true|false to docker run which is a blanket operation that identifies every port with an EXPOSE line in the image’s Dockerfile or --expose <port> commandline flag and maps it to a host port somewhere within an ephemeral port range. The docker port command then needs to be used to inspect created mapping. The ephemeral port range is configured by /proc/sys/net/ipv4/ip_local_port_range kernel parameter, typically ranging from 32768 to 61000.

Mapping can be specified explicitly using -p SPEC or --publish=SPEC option. It allows you to particularize which port on docker server - which can be any port at all, not just one within the ephemeral port range – you want mapped to which port in the container.

Either way, you should be able to peek at what Docker has accomplished in your network stack by examining your NAT tables.

# What your NAT rules might look like when Docker
# is finished setting up a -P forward:

$ iptables -t nat -L -n

...
Chain DOCKER (2 references)
target     prot opt source               destination
DNAT       tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:49153 to:172.17.0.2:80

# What your NAT rules might look like when Docker
# is finished setting up a -p 80:80 forward:

Chain DOCKER (2 references)
target     prot opt source               destination
DNAT       tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:80 to:172.17.0.2:80

You can see that Docker has exposed these container ports on 0.0.0.0, the wildcard IP address that will match any possible incoming port on the host machine. If you want to be more restrictive and only allow container services to be contacted through a specific external interface on the host machine, you have two choices. When you invoke docker run you can use either -p IP:host_port:container_port or -p IP::port to specify the external interface for one particular binding.

Or if you always want Docker port forwards to bind to one specific IP address, you can edit your system-wide Docker server settings and add the option --ip=IP_ADDRESS. Remember to restart your Docker server after editing this setting.

Note: With hairpin NAT enabled (--userland-proxy=false), containers port exposure is achieved purely through iptables rules, and no attempt to bind the exposed port is ever made. This means that nothing prevents shadowing a previously listening service outside of Docker through exposing the same port for a container. In such conflicting situation, Docker created iptables rules will take precedence and route to the container.

The --userland-proxy parameter, true by default, provides a userland implementation for inter-container and outside-to-container communication. When disabled, Docker uses both an additional MASQUERADE iptable rule and the net.ipv4.route_localnet kernel parameter which allow the host machine to connect to a local container exposed port through the commonly used loopback address: this alternative is preferred for performance reasons.

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