Using a proxy container with Docker for virtualhosts

I’ve been asked a few times over the last few weeks about how you can have multiple services (for example, Munki and Sal) running on the same port on the same server - how we used to do Virtual Hosting when we ran our apps on the host OS. My usual four word answer has been ‘use a proxy container’. How you actually do that has been undocumented - this post hopes to recitfy that.

Let’s consider a usual setup that has Sal and Munki running on the same box. We have two hostnames that both resolve to our Docker server - sal.example.com and munki.example.com, and we have our containers running in a manner similar to the below:

Each of our containers is linked to it’s parent:

# Munki server
docker run -d \
-v /usr/local/docker/munki:/munki_repo \
--restart="always" \
--name="munki" \
macadmins/munki

# Postgres container for Sal
docker run -d --name="postgres-sal" \
-v /usr/local/docker/sal/db:/var/lib/postgresql/data \
-e DB_NAME=sal \
-e DB_USER=admin \
-e DB_PASS=password \
--restart="always" \
grahamgilbert/postgres

# Sal Container
docker run -d \
-e ADMIN_PASS=pass \
-e DB_NAME=sal \
-e DB_USER=admin \
-e DB_PASS=password \
--name="sal" \
--link postgres-sal:db \
--restart="always" \
macadmins/sal

# Proxy
docker run -d --name="proxy" \
--link sal:sal \
--link munki:munki \
-v /usr/local/docker/proxy/sites-templates:/etc/nginx/sites-templates \
-v /usr/local/docker/proxy/keys:/etc/ssl/keys \
-p 443:443 \
-p 80:80 \
--restart="always" \
grahamgilbert/proxy

So how does the proxy container know about the other containers? When you link a Docker container to another, a few environment variables are made available to you - there are two we’re interested in:

• CONTAINERNAME_PORT_EXPOSEDPORTNUMBER_TCP_ADDR
• CONTAINERNAME_PORT_EXPOSEDPORTNUMBER_TCP_PORT

So in the case of the Sal container, it would be SAL_PORT_8000_TCP_ADDR and SAL_PORT_8000_TCP_PORT. The grahamgilbert/proxy image will simply run over any file named *.tmpl in /etc/nginx/sites-templates and replace the placeholders with the value that’s in the environment variable - here’s my nginx configuration file template for Sal:

{% codeblock nginx /etc/nginx/sites-templates/sal.example.com.tmpl %} server { listen 80; server_name sal.example.com; rewrite ^ https://$server_name$request_uri? permanent; }

server { listen 443 ssl; server_name sal.example.com; ssl_certificate /etc/ssl/keys/ssl-unified.crt; ssl_certificate_key /etc/ssl/keys/sal.key;

    location / {
        proxy_pass http://${SAL_PORT_8000_TCP_ADDR}:${SAL_PORT_8000_TCP_PORT}; # set to your own port
        proxy_redirect off;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

        client_max_body_size 10m;
        client_body_buffer_size 128k;

        proxy_connect_timeout 90;
        proxy_send_timeout 90;
        proxy_read_timeout 90;
        proxy_buffer_size 4k;
        proxy_buffers 4 32k;
        proxy_busy_buffers_size 64k;
        proxy_temp_file_write_size 64k;
    }
}

{% endcodeblock %}

Using this technique, you can have as many containers as you like all serving content on the same port number - just change line 14 above to reflect the exposed ports and names of your Docker containers.