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Docker HostDNS

Update BIND nameserver zone with Docker hosts via DNS Updates.

Usage

Docker HostDNS can be run by docker-hostdns wrapper script or directly with python -m docker_hostdns.

usage: docker-entrypoint [-h] [--zone ZONE] [--dns-server DNS_SERVER]
                         [--dns-key-secret DNS_KEY_SECRET]
                         [--dns-key-name DNS_KEY_NAME]
                         [--dns-key-alg {...}]
                         [--name NAME] [--network NETWORK] [--verbose]
                         [--syslog [SYSLOG]] [--clear-on-exit]

Update BIND nameserver zone with Docker hosts via DNS Updates.

optional arguments:
  -h, --help            show this help message and exit
  --zone ZONE           dns zone to update, defaults to "docker"
  --dns-server DNS_SERVER
                        address of DNS server which will be updated, defaults
                        to 127.0.0.1
  --dns-key-secret DNS_KEY_SECRET
                        DNS Server key secret for use when updating zone, use
                        '-' to read from stdin
  --dns-key-name DNS_KEY_NAME
                        DNS Server key name for use when updating zone
  --dns-key-alg {...}
                        DNS Server key algorithm for use when updating zone
  --name NAME           name to differentiate between multiple instances
                        inside same dns zone, defaults to current hostname
  --network NETWORK     network to fetch container names from, defaults to
                        docker default bridge, can be used multiple times
  --verbose, -v         give more output - option is additive, and can be used
                        up to 3 times
  --syslog [SYSLOG]     enable logging to syslog, defaults to "/dev/log", you
                        can provide path to unix socket or uri:
                        <tcp|udp|unix>://<path_or_host>[:<port>]
  --clear-on-exit       clear zone on exit

The --daemonize options is only available when you have installed python-daemon3 package.

Example named.conf zone configuration with key auth:

include "/etc/bind/docker.key";

zone "docker" in {
    type master;
    file "/var/bind/dyn/docker.zone";
    allow-update {
      key "docker-key";
    };
};

docker.key can be generated by:

rndc-confgen -a -c docker.key -k docker-key

And then:

echo 'my base64 key secret' | docker-hostdns --dns-key-name docker-key --dns-key-secret -

Host names

Host name is created by using container name and slugifying & trimming it. So /example2::docker will result with example2-docker. In case of name duplication a "-<number>" will be appended, resulting with eg. example2-docker-1

Following dns records are created for each container, given example hostname and docker zone:

  • IPv4: example.docker
  • IPv4: *.example.docker
  • IPv6: example.docker
  • IPv6: *.example.docker
  • TXT: _container_<name>.docker with container name as value and instance name as <name>

TXT record is used for keeping track of added hosts so when app is stopped or resumed it keeps its state.

Custom host names

You can set custom host name by using container label pl.glorpen.hostname, its content will be used as container name.

Docker Image

Docker image is available at glorpen/hostdns. For help try docker run --rm -it glorpen/hostdns:latest --help.

Remember to mount /run/docker.sock inside container.

Build image from GitHub

  • git clone <repo>
  • cd docker-hostdns/
  • docker build -t "<image name>" .

Docker environment variables

  • DNS_SERVER: address of DNS server which will be updated, defaults to 127.0.0.1
  • DNS_ZONE: DNS zone to update, defaults to docker
  • DNS_KEY_NAME: DNS Server key name for use when updating zone
  • DNS_KEY_SECRET: DNS Server key secret for use when updating zone
  • DNS_KEY_SECRET_FILE: path of file with secret as its content
  • DNS_KEY_ALGORITHM: DNS Server key algorithm for use when updating zone
  • NAME: name to differentiate between multiple instances inside same dns zone, defaults to current hostname
  • NETWORK: network to fetch container names from, defaults to docker default bridge, accepts multiple networks as comma delimited list (e.g. network1,network2,network3,..)
  • VERBOSITY: give more output, accepts 0 to 3, defaults to 0 (equivalent to -v, -vv, -vvv arguments on the command line)
  • SYSLOG: enable logging to syslog, if set true or yes defaults to "/dev/log", or you can provide path to unix socket or uri: <tcp|udp|unix>://<path_or_host>[:<port>]
  • CLEAR_ON_EXIT: clear zone on exit, defaults to false (accepts true or yes)

Securing DNS secret key

To secure secret key (the dns-key-secret option) you can:

  • passing its contents to env var DNS_KEY_SECRET
  • setting env var DNS_KEY_SECRET_FILE to path of file with secret as its content

Option --dns-key-secret - will be then automatically prepended and secret key piped to docker-hostdns process.

Working with docker-compose

When using docker-compose for development you can create custom docker network and use it as domain names source.

To do this, create docker network with docker network create example-dns and then run Docker HostDNS with --network example-dns argument.

Next, with example docker-compose.yml:

version: '2.2'
services:
  app:
    image: example
    labels:
      pl.glorpen.hostname: example
    networks:
      default: ~
      dns: ~

networks:
  dns:
    external: true
    name: example-dns

you can start container that would be accessible by host as example.docker domain.