Cauldron is a .jar file replacer that enables the use of Bukkit plugins on a Forge server. It can also be used to provide some stability to Forge servers, allowing you to configure chunk loading on your server, which cuts down on world leaks, one of the most common causes of excessive RAM use on FTB servers.
You can read more about Cauldron on their website - http://cauldron.minecraftforge.net/
Cauldron is widely compatible with a range of mods, but there are just a select few that do not work well with it.
There are currently known compatibility issues with Pixelmon, CoFHCore B8, NotEnoughResources and Fastcraft.
Following the DCMA takedown requests of Bukkit and Cauldron, the builds of Cauldron available for public use do not contain everything needed to properly install Cauldron on your server. Because of this, our Control Panel installs of Cauldron are currently not working.
NOTE - The original Cauldron author has moved over to Sponge, and so recent Cauldron updates have been unofficial in nature. This has left the software very buggy, and because of this, we're afraid we can no longer recommend or support its use. However, if you wish to install Cauldron, you can find various builds of it around the internet and can install it manually using the installation instructions provided on the page that supplies the build you find.
After installing, navigate to your Control Panel > Minecraft > Server Properties page and make sure the correct jar file is selected, and your server should start.
As mentioned above, Cauldron allows you to use Bukkit plugins with your Forge server. Please be aware though that most plugins weren't developed to be used on Forge servers, and you use them at your own risk. Using plugins that change blocks specifically can lead to corrupted worlds; for example, World Edit was built against vanilla block IDs and won't recognise Forge block IDs; using WorldEdit on a Forge server causes errors that will very likely corrupt your world file. Proceed with caution!