I have a docker-compose.yml file that I use to run a web application.
version: '3'
services:
web:
entrypoint: gunicorn myapp.wsgi:application --bind 0.0.0.0:7000
...
nginx:
ports:
# HOST:CONTAINER
- 80:80
- 443:443
...
Inside my nginx service's container:
$ cat /etc/hosts
127.0.0.1 localhost
::1 localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
192.168.144.4 790c0822b887
$ curl http://web:7000
<h1>Bad Request (400)</h1>
$ traceroute web
traceroute to web (192.168.144.3), 30 hops max, 46 byte packets
1 ec2-user_web_1.ec2-user_default (192.168.144.3) 0.008 ms 0.007 ms 0.006 ms
So I can lookup the web hostname and it's mapped to the IP address 192.168.144.3, but I don't see it in the /etc/hosts file.
How docker-compose specifically set up this container such that curl knows the IP address?
Docker provides a DNS service (search for “automatic DNS resolution between containers”), and Docker Compose provides a default “user-defined” network that uses it. If you look at /etc/resolv.conf inside your container, it should point at a Docker-private IP addresses which provides hostname resolution for both the general Internet and for other containers.
More specifically in a Docker Compose context, the name of each entry in the services: block is resolvable as a host name by other containers launched from the same docker-compose.yml file. You do not need to set container_name:, hostname:, links:, or networks:; this is all configured automatically for you.