iRule for logging x-forwarded-for header

Where do you want to log the values? What are the specifics regarding client IP-addresses?

want to log the values in ltm logs.

The simplest approach might be something like this:

when HTTP_REQUEST {
    if { [HTTP::header exists X-Forwarded-For] } {
        log local0. "Received XFF from [IP::client_addr]: [HTTP::header X-Forwarded-For]"    

        if { [class match [HTTP::header X-Forwarded-For] equals my_ip_dg] } {
            log local0. "Data group source match"
        }
    }
}