How to delete cookies with random names?

Make sure to unset logoff or set it to 0 if you're setting it in HTTP_REQUEST to avoid removing the cookies on subsequent requests on the same TCP connection:

when HTTP_REQUEST {
    if { [HTTP::uri] equals "/logoff.php" } {
        set logoff 1
        set incoming_cookies [HTTP::cookie names]
    } else {
        set logoff 0    
    }
}   
when HTTP_RESPONSE {
    if { $logoff==0 } {
        foreach x $incoming_cookies {
            HTTP::header insert Set-Cookie "$x=deleted; path=/; expires=Thu, 01 Jan 1970 00:00:00 GMT"
        }
    }
}

Aaron