Docker Networking Confusion

General Discussions
1

I have some confusion about Docker networking, and I was hoping that y’all could help clear me up. I presume I’m just not thinking about this the right way.

I am currently running a number of different services/appliances/etc, each in a different VM. We are trying to research and understand how to move these services to Docker in order to reduce our server overhead. Also, many of these VMs are “pets”, and Docker gives us a good way to move away from this model.

As a good example, we have our corporate wiki running in Mediawiki. On that VM, there are four services that support Mediawiki: Apache, MariaDB, Memcached, and Elastisearch. And all four have readily available container implementations, and I’ve tested them together, and it’s working fine.

The problem that I’m running into is networking from outside in the broader network into this cluster. And this is where I think my confusion is coming from.

When I publish the web ports on the Apache container, I want to be able to publish them on an arbitrary IP address different from the host. Similar to how I’m able to assign a VM an IP address different from the hypervisor. When I use a bridge network, the bridge allows the containers to talk to each other. I would assume there’s a way to assign a LAN IP address to the bridge, so that when I publish a port, it’s accessible on that IP.

As a more concrete example, I currently have 3 webservers, hosting a few different internal websites. Each webserver is a VM. Their IP addresses are 192.168.x.70, .x.71, and .x.72. If I move all three to containers on the same host, I want to be able to preserve this scheme: the containers support webserver 1 are in a bridge addressable at 192.168.x.70, webserver 2 at .x.71, etc.

But as it is currently, I can’t for the life of me figure out how to to this. In fact, it doesn’t appear that what I’m looking for is possible. But of course in my mind, what I’m asking for is obviously desirable, so I’m confused why I can’t figure this out.

Can anyone help my confusion?

2

The first step is to forget everything about VMs. Containers are not VMs, but isolated and ephemeral instances that run a single service. Second step, forget about IP addresses. If the containers have to communicated with each other, you can start them with the same compose file and they can immediately call each other. Take the compose file in the docs for the wordpress image as an example:

services:
  wordpress:
    image: wordpress
    ports:
      - 8080:80
    environment:
      WORDPRESS_DB_HOST: db
    ...
  db:
    image: mysql:5.7
    ...

Here the wordpress service addresses the database as db and its port 80 is published as 8080 on the host.
If you have several web servers, then you can run a proxy like traefik in front of them.