docker-nsupdate-ddns

This script pushes container/ip information from local Docker instance to a DNS server via the DNS update mechanism desribed in RFC 2136 (nsupdate). nsupdate is implemented by ISC Bind9.

Every REFRESH_INTERVAL seconds, it queries all the Docker containers on the local host, finds their IP and pushes it with the name to the DNS server.

Configuration

The script takes environment variables or a config file. A sample config file is provided in sample.config.env.

The file name of the config file can be passed as argument, but defaults to /config.env.

The names of the environment variables is the same as in the config file. Environment variables have precendence over the config file.

Config	Required	Default Value	Description
DOMAIN	Yes		Sets the domain in which the records are created. Needs to match the Bind zone.
NAMESERVER	Yes		Nameserver to push updates to.
TSIG_NAME	Yes		TSIG key name for secure updates.
TSIG_KEY	Yes		TSIG key value for secure updates.
DOCKER_SOCKET	No	/var/run/docker.sock	Sets the location of the Docker socket.
HOSTNAME_LABEL	No	nl.mtak.docker-nsupdate-ddns.hostname	Docker label to override the default record name with. Use with docker --label=nl.mtak.docker-nsupdate-ddns.hostname=nginx to get nginx.int.mtak.nl If the label value present on the container, use it as hostname otherwise the container name.
IGNORE_LABEL	No	nl.mtak.docker-nsupdate-ddns.ignore	Container label to exclude containers from DNS updates.
DNS_RECORD_TTL	No	60	Time to Live (TTL) for DNS records (seconds).
DEFAULT_NETWORK	No		Preferred network name to find IP for, in case there are multiple networks.
REFRESH_INTERVAL	No	60	Interval between checks for container changes (seconds).
ONE_SHOT	No	False	Run once and exit instead of continuously monitoring.

Environment variables

docker run -d \
   -v /var/run/docker.sock:/var/run/docker.sock \
   -e DOMAIN=int.mtak.nl \
   -e NAMESERVER=10.100.0.11 \
   -e TSIG_NAME=dck1 \
   -e TSIG_KEY=SyYXDCJ4kIs3qhvI= \
   -e DOCKER_SOCKET=/var/run/docker.sock \
   -e HOSTNAME_LABEL=nl.mtak.docker-nsupdate-ddns.hostname \
   -e IGNORE_LABEL=nl.mtak.docker-nsupdate-ddns.ignore \
   -e DNS_RECORD_TTL=60 \
   -e DEFAULT_NETWORK=10.100.0.192/26 \
   -e REFRESH_INTERVAL=60 \
   -e ONE_SHOT=true \
   merijntjetak/docker-nsupdate-ddns:latest

Config file

cat <<EOF > configfile
DOMAIN=int.mtak.nl
NAMESERVER=10.100.0.11
TSIG_NAME=dck1
TSIG_KEY=SyYXDCJ4kIs3qhvI=
DOCKER_SOCKET=/var/run/docker.sock
HOSTNAME_LABEL=nl.mtak.docker-nsupdate-ddns.hostname
IGNORE_LABEL=nl.mtak.docker-nsupdate-ddns.ignore
DNS_RECORD_TTL=60
DEFAULT_NETWORK=10.100.0.192/26
REFRESH_INTERVAL=60
ONE_SHOT=False
EOF

docker run -d \
  -v `pwd`/configfile:/configfile \
  -v /var/run/docker.sock:/var/run/docker.sock \
  merijntjetak/docker-nsupdate-ddns:latest /configfile

Bind9 integration

1. Generate a key

   tsig-keygen clientname > /etc/bind/keys/clientname.key

2. Include keys in your Bind9 named.config configuration file

   include "/etc/bind/keys/*";

3. Allow updates to your zone:

   zone "int.mtak.nl" {
       type master;
       file "/etc/bind/db/int.mtak.nl.zone";
       update-policy {
               grant clientname zonesub ANY;
       };
   };
   

Design

Requirements

• Eventual redundancy (Bind9 zone transfers to secondary)
• Support for multiple individual Docker servers
• IPv6 support
• Detect hostname in decreasing order of priority:
  • label
  • Container name
• Forward-only
• Clean up DNS (DNS is stateful but the script isn't, so there might be a mismatch)

Nice to have

• Add tests

Alternatives

CoreDNS didn't fit the requirements, because zone transfers out of a CoreDNS server do not include the records from the coredns-dockerdiscovery plugin.

K3s and k8s would probably do this, but incur significant complexity over a standalone Docker instance.