Domain name and port based proxy - nginx

Question

I think I finally grasped how Docker works, so I am getting ready for the next step: cramming a whole bunch of unrelated applications into a single server with a single public IP. Say, for example, that I have a number of legacy Apache2-VHost-based web-sites, so the best I could figure was to run a LAMP container to replicate the current situation, and improve later. For argument sake, here is what I have a container at 172.17.0.2:80 that serves
http://www.foo.com
http://blog.foo.com
http://www.bar.com
Quite straightforward: publishing port 80 lets me correctly access all those sites. Next, I have two services that I need to run, so I built two containers
service-a -> 172.17.0.3:3000
service-b -> 172.17.0.4:5000
and all is good, I can privately access those services from my docker host. The trouble comes when I want to publicly restrict access to service-a through service-a.bar.com:80 only, and to service-b through www.foo.com:5000 only. A lot of reading after, it would seem that I have to create a dreadful artefact called a proxy, or reverse-proxy, to make things more confusing. I have no idea what I'm doing, so I dove nose-first into nginx -- which I had never used before -- because someone told me it's better than Apache at dealing with lots of small tasks and requests -- not that I would know how to turn Apache into a proxy, mind you. Anyway, nginx sounded perfect for a thing that has to take a request a pass it onto another server, so I started reading docs and I produced the following (in addition to the correctly working vhosts):
upstream service-a-bar-com-80 {
server 172.17.0.3:3000;
}
server {
server_name service-a.bar.com;
listen 80;
location / {
proxy_pass http://service-a-bar-com-80;
proxy_redirect off;
}
}
upstream www-foo-com-5000 {
server 172.17.0.4:5000;
}
server {
server_name www.foo.com;
listen 5000;
location / {
proxy_pass http://www-foo-com-5000;
proxy_redirect off;
}
}
Which somewhat works, until I access http://blog.bar.com:5000 which brings up service-b. So, my question is: what am I doing wrong?

Answer 1

nginx (like Apache) always has a default server for a given ip+port combination. You only have one server listening on port 5000, so it is your defacto default server for services on port 5000.
So blog.bar.com (which I presume resolves to the same IP address as www.foo.com) will use the default server for port 5000.
If you want to prevent that server block being the default server for port 5000, set up another server block using the same port, and mark it with the default_server keyword, as follows:
server {
listen 5000 default_server;
root /var/empty;
}
You can use a number of techniques to render the server inaccessible.
See this document for more.