docker-bind

A Docker image of the ISC Bind/Bind9/Named DNS service that has been set up so it is easy to configure when running inside a container.

There are both Debian and Alpine images available, and they install the server version available for their respective package manager which means they differ slightly on the minor version. Useful configuration files and folders from the Debian image are copied to the Alpine one for a consistent experience across both.

There is also an Ansible role using this image, if that is of interest.

Acknowledgments and Thanks

This repository was originally a fork of ventz/docker-bind, but what was supposed to be just a small pull request turned into a complete rewrite. While very little of the original code remains it would be dishonest to not keep the commit history since without it I would not have found inspiration to make my own version.

Usage

The amount of options available for Bind is absolutely enormous, and what options to use will be very different depending on how you intend to run your instance so I will not even try to list suggestions here. But to just get you started we will set up a super simple forwarding server, so it becomes easier to understand what configuration files are needed and where they are expected to be found.

Available Environment Variables

ℹ️ It is possible to change these environment variables, but you will most likely break things.

• BIND_LOG: Input argument for configuring stderr or file logging (default: -f)
• BIND_USER: The username the service will run as (default: alpine=named, debian=bind)

Configuration Files

When the container starts it will launch Bind which will read the main config file /etc/bind/named.conf, however, that one has only the following content:

include "/etc/bind/local-config/named.conf.logging";
include "/etc/bind/local-config/named.conf.options";
include "/etc/bind/local-config/named.conf.local";

What this means is that these three files are expected to be present when the container starts, and some basic (but fully functioning) examples are available inside the example-configs/ folder.

⚠️ Logging in Bind is a little bit weird, so if anything in the config is wrong it will not output any logs unless you set BIND_LOG=-g. Use this for debugging and then switch back to default.

Create rndc.key

At the top of the named.conf.local file we include an "rndc key" which needs to be created manually by you as it should be unique and secret. You can have the file written to the example-configs/ folder by running the following command:

docker run -it --rm \
    -v $(pwd)/example-configs:/etc/bind/local-config \
    --entrypoint=/bin/sh \
    jonasal/bind:9 \
    -c 'rndc-confgen -a -A hmac-sha256 -b 256 -u "${BIND_USER}" -c /etc/bind/local-config/rndc.key'

Volumes

There are two locations that are important for this image, and the first one is /etc/bind/local-config/ since that is the place where it expects to find your custom configuration files. It should be very difficult to miss as the image wont start without this folder properly populated.

The other important location is the working directory of the server that is defined at the top of the named.conf.options file. This is the location where your primary zone files are expected to be found and to where slave zone files will be written, so for persistence I recommend to host mount this folder and make sure Bind is allowed to write to it:

mkdir zones && sudo chown root:101 zones && sudo chmod 775 zones

While you could change it to anything you want the examples below expects it to remain as it is now.

Custom Entrypoint Scripts

There is a third location that might be of interest and that is the folder /entrypoint.d/ since the main entrypoint.sh will look inside this folder for any files ending with .sh and try to execute them in alphabetical order. This allows you to run custom commands before the Bind service is started.

Input Arguments

Any extra input arguments provided as the command, when starting the image, will be appended directly to the Bind service. Please take a look at the last line in entrypoint.sh to see how it works.

Run

After you have read through all the steps above we can finally start the image:

docker run -it --rm \
    -p 54:53 -p 54:53/udp -p 953:953 \
    -v $(pwd)/example-configs:/etc/bind/local-config \
    -v $(pwd)/zones:/var/cache/bind \
    jonasal/bind:9 \
    -4

Important to note here is that we forward port 54 on the host to the "correct" port 53 inside the container. I do this because some Linux distributions comes with the systemd-resolved service running which already use port 53. This is a problem if you want to run this image as a real DNS server, so you will have to disable it if it causes you trouble.

Furthermore, at the very end of the command we include -4, and this tells Bind to not enable any IPv6 functionality. The reason for this is that by default no IPv6 traffic is handled by Docker and unnecessary error messages will be printed unless the flag is provided. Read more about this in the Docker Network Mode section.

Verify

In order to verify that Bind works, after running the command above, is to just make a quick query to your machine on port 54 and see if anything is printed in the container logs.

dig @127.0.0.1 -p 54 google.se

Docker Network Mode

As was previously mentioned Docker does not have IPv6 enabled by default, so it is recommended to start Bind with the -4 flag to tell it to run in just IPv4 mode. But if you do want to run it for both IP versions I would suggest you first read this to get a better understanding of the quirks that currently exist, and I would actually suggest you just run this container on the host network to make your life easier.

Also, don't forget to change the listen-on-v6 directive in the options config file.

docker run -it --rm \
    --network host \
    -v $(pwd)/example-configs:/etc/bind/local-config \
    -v $(pwd)/zones:/var/cache/bind \
    jonasal/bind:9

You could probably do some fiddling with macvlan to achieve the same stuff, but I would not bother.

Further reading

As was mentioned in the beginning there exists a plethora of ways on how to configure Bind, so you will need to do some of your own research in order to function just as you want. However, here is a collection of links from where you can start your journey:

• https://wiki.debian.org/Bind9
• https://help.ubuntu.com/community/BIND9ServerHowto
• https://www.zytrax.com/books/dns/ch7/
• https://www.digitalocean.com/community/tutorials/how-to-configure-bind-as-a-private-network-dns-server-on-ubuntu-18-04
• https://kb.isc.org/docs/aa-01526
• https://www.zytrax.com/books/dns/ch7/logging.html