Connection method, communication system, device, and program

Abstract

A VPN connection method and a communication system are provided, each which can omit the setting of a firewall and the installation of a relay server and can establish communications from the outside of a firewall to the inside thereof. The internal networkis connected to the external networkvia the firewall. The internal networkenables communications by an e-mail from the external network(outside the firewall) to the mail serverin the internal network(inside the firewall) or communications by telephone or radio, which is not via the firewall. The feature of the firewallis utilized that access from the outside to the inside of the firewall through VPN connection is very difficult but the access in the reverse stream can be simply performed. Using the mail server, telephone line or radio line, the internal networkreceives an e-mail, which requires for VPN connection from the external network, or control information through the telephone line. Thus, the internal networksends callback for VPN connection to the e-mail transmission source. As a result, the callback allows the external networkto pass through the firewall