Chapter 12 of 16 · Visual guide

Networking: user-defined bridge and Docker DNS

Create your own network and Docker gives you service discovery: containers find each other by name.

key idea / Docker itselfcontainer / good pathhost / watch outproblem / blockedneutral

The embedded DNS server

Every container on a user-defined network is pointed at 127.0.0.11.

Why names beat IPs

Aliases and round-robin

What resolves, and what does not

Try it yourself

1. Step 1 · Create a network and two named containers

   $ docker network create app
   $ docker run -d --name api --rm --network app alpine:3.20 sleep 300
   $ docker run -d --name web --rm --network app alpine:3.20 sleep 300

2. Step 2 · Read the DNS server

   On a user-defined network the container’s resolver is 127.0.0.11.

   $ docker exec web cat /etc/resolv.conf
   nameserver 127.0.0.11

3. Step 3 · Ping the name

   $ docker exec web ping -c 1 api
   PING api (172.18.0.2): 56 data bytes
   64 bytes from 172.18.0.2: seq=0 ttl=64

4. Step 4 · Recreate api and ping the name again

   The IP can change. The name still resolves.

   $ docker stop api
   $ docker run -d --name api --rm --network app alpine:3.20 sleep 300
   $ docker exec web ping -c 1 api
   PING api (172.18.0.3): 56 data bytes
   $ docker stop api web
   $ docker network rm app

Recap

• A user-defined bridge comes with DNS at 127.0.0.11; the default bridge does not.
• Containers resolve each other by container name or network alias.
• Unknown names are forwarded to the host resolver, so the internet still works.
• Resolution is scoped to shared networks.

Quick check