dockerdns

coredns-dockerdns

Docker discovery plugin for coredns

Name

dockerdns - add/remove A/AAAA DNS records for docker containers.

Syntax

docker [ZONES...] {
    endpoint DOCKER_ENDPOINT
    by_domain
    by_hostname
    by_label
    by_compose_domain
    enabled_by_default
    ttl TTL
    networks NETWORKS...
    no_reverse
    fallthrough [ZONES...]
}

• ZONES: zones to apply for plugin (i.e.: loc, docker.local)
• DOCKER_ENDPOINT: the path to the docker socket. If unspecified, defaults to unix:///var/run/docker.sock. It can also be TCP socket, such as tcp://127.0.0.1:999.
• by_domain: expose container in dns by container name. Default is false
• by_hostname: expose container in dns by hostname. Default is false
• by_label: expose container in dns by label. Default is true, so it is of no use. This directive is always true
• by_compose_domain: expose container in dns by compose_domain. Default is false
• enabled_by_default: default is false
• TTL: change the DNS TTL (in seconds) of the records generated (forward and reverse). The default is 3600 seconds (1 hour).
• networks: filter list of networks for dns resolver to apply
• no_reverse: disable the automatic generation of the in-addr.arpa or ip6.arpa entries for the hosts.
• fallthrough: If zone matches and no record can be generated, pass request to the next plugin. If [ZONES...] is omitted, then fallthrough happens for all zones for which the plugin is authoritative. If specific zones are listed (for example in-addr.arpa and ip6.arpa), then only queries for those zones will be subject to fallthrough.

COREDNS docker container may have env variables:

• COREDNS_DOCKER_ENDPOINT
• COREDNS_DOCKER_NETWORKS
• COREDNS_DOCKER_AUTOENABLE
• COREDNS_DOCKER_TTL This variables are equivalent to config variables. All env variables overwrite config values

Apply next host resolve rules:

• if by_domain == true:
  container_name.zone
• if by_hostname == true:
  hostname.zone
• if by_label == true:
  label value (must have the same zone as plugin)
• if by_compose_domain == true:
  service.project.zone

Dockerdns plugin works with hosts, forward and other plugins as well. See configs below

# works correct (add except directive to forward)
# enable specific container with `enable` label  
. {
    reload 10s
    hosts {
        172.28.0.4  whoami.gat
        172.28.0.4  whoami.nit
        fallthrough
    }
    docker docker.loc {
        by_domain
        by_hostname
        by_compose_domain
    }
    forward . 1.1.1.1 8.8.8.8  {
        except docker.loc
    }
    errors
}

# works correct
# enable specific container with `enable` label  
loc:15353 groc:15353 {
    reload 10s
    docker {
        by_domain
        by_hostname
        by_compose_domain
    }
    errors
}

# works correct too
# all containers will be resolved with zone `moc.`
# by domain and hostname
# in that case you may expose too many containers, be cautious
moc:15353 {
    reload 10s
    docker {
        by_domain
        by_hostname
        enabled_by_default
    } 
    errors
}

How To Build

GO111MODULE=on go get -u github.com/coredns/coredns
GO111MODULE=on go get github.com/karantin2020/coredns-dockerdns
cd ~/go/src/github.com/coredns/coredns
echo "docker:github.com/karantin2020/coredns-dockerdns" >> plugin.cfg
cat plugin.cfg | uniq > plugin.cfg.tmp
mv plugin.cfg.tmp plugin.cfg
make all
~/go/src/github.com/coredns/coredns/coredns --version

Alternatively, you can use the following manual steps:

1. Checkout coredns: go get github.com/coredns/coredns.
2. cd $GOPATH/src/github.com/coredns/coredns
3. echo "docker:github.com/karantin2020/coredns-dockerdns" >> plugin.cfg
4. go generate
5. make

Alternatively, run insider docker container

docker build -t coredns-dockerdns .
docker run --rm -v ${PWD}/Corefile:/etc/Corefile -v /var/run/docker.sock:/var/run/docker.sock -p 15353:15353/udp coredns-dockerdns -conf /etc/Corefile

Run tests

go test -v

Example

Corefile:

loc:15353 groc:15353 {
    reload 10s
    docker {
        by_domain
        by_hostname
        by_compose_domain
    }
    errors
}

moc:15353 {
    reload 10s
    docker a.moc b.moc {
        by_domain
        by_hostname
        by_compose_domain
    }
    errors
}

Start CoreDNS:

$ ./coredns

groc.:15353
loc.:15353
moc.:15353         
CoreDNS-1.11.0

Start a docker container:

$ docker run -d --name my-alpine --hostname alpine alpine sleep 1000
78c2a06ef2a9b63df857b7985468f7310bba0d9ea4d0d2629343aff4fd171861

Use CoreDNS as your resolver to resolve the my-alpine.loc or alpine.moc, alpine.groc, alpine.a.moc, alpine.b.moc:

$ dig @localhost -p 15353 my-alpine.loc

; <<>> DiG 9.10.3-P4-Ubuntu <<>> @localhost -p 15353 my-alpine.docker.loc
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 61786
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;my-alpine.docker.loc.            IN      A

;; ANSWER SECTION:
my-alpine.docker.loc.     3600    IN      A       172.17.0.2

;; Query time: 0 msec
;; SERVER: 127.0.0.1#15353(127.0.0.1)
;; WHEN: Thu Apr 26 22:39:55 EDT 2018
;; MSG SIZE  rcvd: 63

Stop the docker container will remove the corresponded DNS entries:

$ docker stop my-alpine
78c2a

$ dig @localhost -p 15353 my-alpine.loc

;; QUESTION SECTION:
;my-alpine.docker.loc.            IN      A

Container will be resolved by label as nginx.loc

docker run --label=coredns.dockerdns.host=nginx.loc nginx

See receipt how install for local development