As a first step for running asp.net applications using lighttpd webserver on FreeBSD you need to download source for xsp from mono project site. Here follow instructions for patching and compiling this source in order to run (hopefully) better on FreeBSD. These instructions apply to xsp-2.10.2
You need to patch following files
1. src/Mono.WebServer.FastCgi/Record.cs (patch Record.cs < Record.cs.diff)
2. src/Mono.WebServer.FastCgi/UnmanagedSocket.cs
3. src/Mono.WebServer.FastCgi/main.cs