ClientIPForwarder is a simple ISAPI plug-in for Microsoft ISA server that
uses pass-through technology to control incoming requests on Microsoft
ISA server.

ClientIPForwarder modifies http(s) request header and adds
a variable IPREMOTEADDR containing client IP address to it. It allows to monitor real client IP addresses in your asp or java application behind Microsoft ISA server. Without the plug-in your application can only access ServerVariables.REMOTE_ADDR (or REMOTE_HOST) containing IP address of ISA firewall.

Variable IPREMOTEADDR can be accessed for example by ASP object Request
and collection GetServerVariables:

response.write("HTTP_IPREMOTEADDR:" & Request.ServerVariables("HTTP_IPREMOTEADDR:"))

Fully functional trial version free.