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Docker, dig, consul

This is a description on how to use Consul in Docker so that it can serv other containers, as a DNS server.

start consul

To start a consul container locally on boot2docker:

$ BRIDGE_IP=$(docker run --rm debian:jessie ip ro | grep default | cut -d" " -f 3)
$ docker run -d \
    -h node1 \
    --name=consul \
    -p ${BRIDGE_IP}:53:53/udp \
    -p ${BRIDGE_IP}:8400:8400 \
    -p ${BRIDGE_IP}:8500:8500 \
    sequenceiq/consul:v0.4.1.ptr -server -bootstrap

To see that its started up:

$ docker exec -it consul consul members

Node   Address          Status  Type    Build     Protocol
node1  172.19.0.4:8301  alive   server  0.4.1ptr  2

To see the logs of the consul use: docker logs consul

Using consul as dns server in containers

Now you can start other containers which uses consul as DNS server

docker run -it --rm \
  --dns=$BRIDGE_IP \
  --dns=8.8.8.8 \
  --dns-search=node.consul \
  --dns-search=service.consul \
  sequenceiq/busybox

In that container you are able to get the consul A record via the DNS api

dig consul.service.consul

Getting the members via the HTTP api:

curl $(dig consul.service.consul +short):8500/v1/agent/members|jq .

If you want that all future containers use the dns and dns-search settings, you have to add the following options to docker daemon: --dns=<BRIDGE_IP> --dns=8.8.8.8 --dns-search=node.consul --dns-search=service.consul

For boot2docker you can use the EXTRA_ARGS env variable in /var/lib/boot2docker/profile. Instead of hand editing, here is the one-liner:

boot2docker ssh sudo "sed '$ a\EXTRA_ARGS=\"\$EXTRA_ARGS --dns=$BRIDGE_IP --dns=8.8.8.8 --dns-search=node.consul --dns-search=service.consul\"' /var/lib/boot2docker/profile"
boot2docker restart

Registrator

The next component to add to the secret sauce is registrator This will automatically register docker containers as services into consul. Registrator attaches to docker event stream,

docker run -d \
  --name=registrator \
  -v /var/run/docker.sock:/tmp/docker.sock \
  progrium/registrator consul://${BRIDGE_IP}:8500

Starting autoregistered services

Now if we start a postgres container, it will be automatically registered, because it exposes a port:

docker run -d \
  --name="postgresql" \
  -e "SERVICE_NAME=db" \
  -e "SERVICE_TAGS=master,backups" \
  -p 5432:5432 \
  postgres

To check that it's registered query the whole consul catalog:

docker run -it  sequenceiq/busybox bash -c "curl -s $BRIDGE_IP:8500/v1/catalog/services|jq ."

tl;dr

If you want to know the fine details read on ...

Understanding /etc/resolv.conf

nameserver 172.19.0.1
search node.consul service.consul

when not specified otherwise cli tools like: dig, host, nslookup will talk to nameserver. If you want to spare a couple of keystrokes, You can define the search domains, they will be appended to the short name

$ nslookup consul
Server:         172.19.0.1
Address:        172.19.0.1#53

Name:   consul.service.consul
Address: 172.19.0.18

dig doesn't honors search domanis by default, you have to force it with the +search option

$ dig +search consul +short
172.19.0.18

dig aliases

The firs alias to show query and answer sections only. The second alias produces the minimal output (cli-fu) with dns-search enabled.

alias digq='dig +search +nocmd +noall +answer +que'
alias digs='dig +search +short'