OpenWRT – Redirect All Subdomains to Same Host for Reverse Proxy

Solution

Here is what I finally came up with after being set in the right direction by Miles Erickson. I wanted the address bar to reflect the original subdomain/domain of the request and not the redirected server and port, but he put me on the right path to Google up a solution using VirtualHost and I finally found a solution that included the use of mod_proxy.

First, make sure mod_proxy is enabled:

sudo a2enmod proxy
sudo a2enmod proxy_http
sudo a2enmod proxy_balancer
sudo a2enmod lbmethod_byrequests
sudo systemctl restart apache2

Next, add the following to your site config (e.g., /etc/apache2/sites-available/000-default.conf):

<VirtualHost *:80>
    ServerAdmin me@mydomain.com
    ServerName dev.mydomain.com
    ProxyPreserveHost On

    # setup the proxy
    <Proxy *>
        Order allow,deny
        Allow from all
    </Proxy>
    ProxyPass / http://localhost:8888/
    ProxyPassReverse / http://localhost:8888/
</VirtualHost>

Which DNS providers allow me to do this?

All competent ones. It doesn't even need to be a DNS-specific providing, most good hosts will help you with this too. There are two stages:

1. A DNS entry is added for *.example.com
2. The host's web server (e.g. Apache) is setup to respond to *.example.com by routing it to example.com's VirtualHost.

Then, at your end, you add a .htaccess entry that uses a RewriteRule to catch each subdomain, and redirect it to a folder using the HTTP_HOST variable.

Untested, but something like this:

RewriteEngine on
RewriteCond   %{HTTP_HOST}                 ^[^.]+\.example\.com$
RewriteRule   ^(.+)                        %{HTTP_HOST}$1          [C]
RewriteRule   ^([^.]+)\.example\.com(.*)   http://example.com/$1/$2

Suppose that the user requests app1.example.com/bob.

Line 2

This will catch app1.example.com in the HTTP_HOST - it'll match the RewriteCond.

Line 3

Normally, Apache will only pass /bob to be checked against the RewriteRule. To use the subdomain, we exploit Chaining by forcing the next rule to match

${HTTP_HOST}$1
|           ^ /bob
^ app1.example.com

Line 4

The final RewriteRule is now matching against app1.example.com/bob and not just /bob. As a result, it can have a 'full' regular expression to match both this subdomain and the path, and perform the desired redirect.

If you don't want this feature, you can simplify the rules.

Are these alternatives feasible? Redirect them all to a special webapp with a static IP that redirects to the proper subfolder. How can I know from which subdomain they came from?

There is no sure-fire way of doing this. You could try the HTTP Referer, but it's not always sent by all browsers, some firewalls remove the header, etc.

Programatically create each rule when I need it. Which DNS providers have API access to add rules? I think Amazon Route 53 might be the answer here.

This would actually result in a more complex solution to the above, so I wouldn't recommend it. It would also create a delay: DNS caches would need to update and certain visitors wouldn't be able to use the subdomain immediately until they do. At least with my suggestion above, they'd be able to get going instantly.