systemd-resolved-docker

Provides systemd-resolved and docker DNS integration.

1. A DNS server is configured to listen on the docker interface's IP address. This is used to expose the systemd-resolved DNS service (127.0.0.53) to docker containers by proxying DNS requests, which doesn't work by default due to the differing network namespaces.

2. Allows containers to be referenced by hostname by adding a DNS servers to a dummy interface using the systemd-resolved D-Bus API.

Features

Container domain addresses

Based on the container's properties multiple domain names may be generated. For this the default_domain (DEFAULT_DOMAIN) and allowed domains (ALLOWED_DOMAINS) options are used. The list of allowed domains specifies which domains may be handled. An entry starting with . (example: .docker) allows all matching subdomains, otherwise an exact match is required. If a generated domain address doesn't match the list of allowed domains, then the default_domain is appended.

1. <container_id>.<default_domain>

   All containers will be reachable by their container_id:

   docker run --rm -it alpine                                        #  d6d51528ac46.docker
   docker ps
   CONTAINER ID        IMAGE                     COMMAND                  CREATED             STATUS              PORTS                    NAMES
   d6d51528ac46        alpine                    "/bin/sh"                8 seconds ago       Up 6 seconds                                 relaxed_cartwright
   

2. <container_hostname>.<default_domain>, <container_hostname>.<container_domain>.<default_domain>, <container_hostname>.<container_domain>

   If an explicit --hostname is provided then that may also be used:

   docker run --rm -it --hostname test      alpine                   # test.docker
   

   Glob matching is supported in the --hostname, with which wildcard domains are supported:

   docker run --hostname '*.test' --rm -it alpine                    # anything.test
   

   If an explicit --domainname is provided then that may also be used:

   docker run --rm -it --hostname test --domainname mydomain alpine  # test.mydomain.docker
   

   When the domain name is in the list of allowed domains (ALLOWED_DOMAINS=.docker,.local), then the default_domain will not be appended:

   docker run --rm -it --hostname test --domainname local    alpine  # test.local
   

3. <container_name>.<container_network>.<default_domain>, <container_name>.<container_network>

   If a non-default network is used (not bridge or host) then a name will be generated based on the network's name:

   docker run --rm -it           --network testnet alpine            # zealous_jones.testnet.docker
   docker run --rm -it --name db --network testnet alpine            # db.testnet.docker
   

   When the network's name is in the list of allowed domains (ALLOWED_DOMAINS=.docker,.somenet), then the default_domain will not be appended:

   docker run --rm -it           --network somenet alpine            # zealous_jones.somenet
   docker run --rm -it --name db --network somenet alpine            # db.somenet.docker
   

4. <service>.<project>.<default_domain>, <service>.<project>, <container_number>.<service>.<project>.<default_domain>, <container_number>.<service>.<project>

   If docker-compose is used then names will be generated based on the service and project names. If a service has multiple containers then the reply will contain all instances:

   host   webserver.someproject.docker                        #   webserver.someproject.docker has address 172.16.238.3
                                                              #   webserver.someproject.docker has address 172.16.238.4
   host 1.webserver.someproject.docker                        # 1.webserver.someproject.docker has address 172.16.238.3
   

   When the project's name is in the list of allowed domains (ALLOWED_DOMAINS=.docker,.someproject), then the default_domain will not be appended.

   If a <service> name is unique, then it is available also as <service>.<default_domain>.

   host   webserver.docker                                    #   webserver.docker has address 172.16.238.3
   

If configured correctly then resolvectl status should show the configured link-specific DNS server:

$ resolvectl status
...
Link 7 (srd-dummy)
Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
     Protocols: -DefaultRoute +LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
   DNS Servers: 127.0.0.153
    DNS Domain: ~docker
... 

A dummy interface (srd-dummy by default) is created to add the custom DNS server to systemd-resolved. This is required because the lifecycle of the docker0 depends on there being running containers on the default network, even if there are running containers on other networks.

127.0.0.53 / systemd-resolved within containers

If docker is configured to use the provided DNS server then the container domain names may also be resolved within containers:

$ docker run --dns 1.1.1.1 --rm -it alpine
/ # apk add bind
/ # host test.docker
Host test.docker not found: 3(NXDOMAIN)

$ docker run --dns 172.17.0.1 --rm -it alpine
/ # apk add bind
/ # host test.docker
/ # host test.docker
test.docker has address 172.17.0.3
Host test.docker not found: 3(NXDOMAIN)
Host test.docker not found: 3(NXDOMAIN)

If there are link-local, VPN or other DNS servers configured then those will also work within containers.

Configuration

systemd-resolved-docker may be configured using environment variables. When installed using the RPM /etc/sysconfig/systemd-resolved-docker may also be modified to update the environment variables.

Install

Fedora / COPR

For Fedora and RPM based systems COPR contains pre-built packages.

1. Enabled the COPR repository

   dnf copr enable flaktack/systemd-resolved-docker
   

2. Install the package

   dnf install systemd-resolved-docker
   

3. Start and optionally enable the service

   systemctl start  systemd-resolved-docker
   systemctl enable systemd-resolved-docker
   

4. Docker should be updated to use the DNS server provided by systemd-docker-resolved. This may be done globally by editing the docker daemon's configuration (daemon.json) or per-container using the --dns flag.

   "dns": [
     "172.17.0.1" // docker0 interface's IP address
   ]
   

5. NetworkManager may reset the docker interface's configuration for systemd-resolved. If that happens than the interface needs to be unmanaged. This may be done by creating a /etc/NetworkManager/conf.d/99-docker.conf:

   [main]
   plugins=keyfile
   
   [keyfile]
   unmanaged-devices=interface-name:docker0
   

Build

setup.py may be used to create a python package.

tito may be used to create RPMs.

Links

Portions are based on docker-auto-dnsmasq and dnslib.