My challenge is to maximise a set of IP Addresses allocated to us serving a group of servers - issue is that we now have more servers than IP addresses. However the clients only really need access to port 80, so the idea is to use application layer routing, so the destination URL can be recognised, and data routed as required to the appropriate internal server on an internal network.
I know I can do this with LDirectorD on a linux box, but would prefer to do this in our Mikrotik router.
Has anyone done this, or know of how best to implement this feature on RouterOS?
I wouldn’t recommend trying. A few have talked about this using the Web Proxy, but this is more suited for a small linux box - I do this same thing using Nginx. I have a CCR1016 with two very large ESXi servers each with about virtual servers. One VM is Nginx that just routes web traffic to the proper server. As an added bonus, it also doubles as an SSL Off-loader.
RouterOS cannot do this, because it does not work on the application level. So you will need an additional
reverse-proxy box to do this, like what you describe.
Definitely use a purpose-built system for this task. RouterOS simply does not have the tools to do any serious work at layer 7.
You could also offer IPv6 so your customers can have real addresses to use. . .