iRule to randomly change source IP

And maybe you find these useful as well:

when CLIENT_ACCEPTED {     
    snat 10.10.10.[expr ( [getfield [IP::client_addr] "." 4] % 32 ) + 1]
   snat 10.10.[getfield [IP::client_addr] "." 3].[getfield [IP::client_addr] "." 4]
}

The following one simulates varying clients by inserting a randomized X-Forwarded-For and uses another internal virtual server for processing:

when HTTP_REQUEST {
    snat automap
    HTTP::header insert X-Forwarded-For [expr int(rand()*128) + 32].[expr int(rand()*254)].[expr int(rand()*254)].[expr int(rand()*254)]    
    virtual vs_internal
}