When you set up a blog or forum on a separate server, you still want to have it linked from the main site, typically using subdomains like forums.sitename.com or blog.sitename.com. The problem with this approach for SEO purposes is that search engines regularly treat each subdomain as a separate site when counting incoming links. The incoming link juice is therefore split among the domains. Google makes an exception only when displaying search results.
The single domain will benefit from higher rankings if links to the subdomains are funneled to the main one. Luckily, there is a technique to do this — reverse proxies.
A reverse proxy is a proxy server that is installed in a server network. Typically, reverse proxy sits in front of the web server, receives all requests, does some special processing (such as caching) and forwards the requests to the actual servers. Besides main security reason (the proxy server may provide an additional layer of defense) a reverse proxy can be used to map URLs to different servers, and this feature comes in very handy for SEO.
You can use Apache’s mod_proxy for this. Here is a sample configuration
ProxyRequests Off
<Proxy *>
Order deny,allow
Allow from all
</Proxy>
ProxyPass /blog http://blog.sitename.com
ProxyPassReverse /forum http://forum.sitename.com
Instead of directing users to blog.sitename.com, you write a reverse proxy rule to send requests for sitename.com/blog to the internal server blog.sitename.com. You can do the same for forums, chat, e-commerce systems, and so on. It is completely transparent to the user (and search engines) that the website is divided among multiple servers. Note that each web server will need to be isolated completely for the security to work. If someone breaks into the blog because the software hasn’t been updated, for instance, at least he won’t get to the e-commerce system.