net/haproxy: add HAProxy load balancer to plugins

[fraenki]

This adds the popular HAProxy load balancer to OPNsense. It allows for far more complex configurations and has a superior feature-set when compared to the (included) relayd load balancer.

Wording/language improvements, code reviews and suggestions are very very welcome!
Feel free to assign any HAProxy issue to me from now on.

Sponsored by: markt.de

LIMITATIONS

• HAProxy must be build with LUA option
• not compatible with the pf***** HAProxy plugin (i.e. you can't just import your pf***** HAProxy config)
• currently does not cleanup/wipe old certificates and lua scripts from disk if they become obsolete
• currently it's not possible to use (host, port) aliases with this plugin
• adopts HAProxy's naming conventions, so don't be confused ;-)

Besides that some more complex configurations may be a bit tricky on first attempt (especially ACLs/Actions): All possible parameters are always visible, even if they are not usable in this combination, which can be quite confusing. I hope this will be fixed alongside opnsense/core#870.

WARNING

I consider this plugin to be ALPHA quality software. While the code was tested fairly well, the resulting HAProxy configuration may contain errors or non-functional entries. The plugin should be ready for production use alongside the 16.7 release of OPNsense. (Please ship this plugin with 16.1.x to get some feedback and fix critical bugs before 16.7.)

ACKNOWLEDGEMENT

Thanks to @fichtner and @AdSchellevis for their tremendous support! This plugin uses a lot existing OPNsense code, which is reflected by the copyright notes. Besides that it was apparently heavily inspired by the pf***** HAProxy plugin (but no original code was included).

SCREENSHOTS

Some screenshots to demonstrate how this plugin integrates with the HAProxy framework and GUI.

Add servers once, but use them in multiple configurations:

Define your own (complex) health checks:

Upload and manage your Lua scripts:

Define your own HTTP error pages:

Get useful hints on syntax errors or other configuration issues:

[fichtner]

Hi Frank,

This is amazing work, thank you very much!!

I'll have some minor follow ups and we'll discuss the integration plan/timeline this week (some commits still need to go on the release in core.git).

Cheers,
Franco