The rapid growth of the Internet in the number of users and web content has fueled extensive efforts to improve the user's overall Internet experiences. A growing number of providers deliver content from multiple servers or proxies to reduce response time by moving content closer to end users. An increasingly popular mechanism to direct client requests towards one of the replica servers is the DNS-based redirection. In his mechanism, the Authoritative Domain Name Server (ADNS) of the web site takes the request dispatcher role by mapping the URL host name into the IP address of one of the replica servers (both locally and geographically distributed). However, the ADNS controls only a very small fraction of the requests reaching the web site because the address mapping is not requested for each client access. Indeed, to reduce Internet traffic due to DNS requests, address resolution is cashed at various name servers for a time to live (TTL) period, opening a new set of problems not addressed in classical centralized shedulers of parallel and/or distributed systems. In this paper we present an approximate Generalized Stochastic Petri Nets (GSPN) model for the analysis of the workload oofered to replica servers ina DNS based redirection architecture. Different dispatching algorithms as well as different strategies to manage TTL assignment at the ADNS will be considered to show the potential of the GSPN based analysis.