Abstract: A security gateway, for use in a network system for linking at least a client end and a server end, includes a user interface, a SSL VPN driver, a connection interface and an IPSEC VPN driver. The security gateway supports IPSEC and SSL protocols. Before establishing an IPSEC VPN between a client end and a server end, the security gateway will perform ID authentication for the user of the client end with a widely-used SSL protocol, so as to establish a SSL VPN between a server end and a client end. When the ID of the client end is authorized, a configuration file comprising the SA is generated and then safely sent to the client end through the SSL VPN tunnel. After the client end receives and executes the configuration file having the SA, an IPSEC VPN tunnel between the server end and the client end is established.