Communication system, reliable communication mechanism, and communication method used for the same

A communication system comprising:

• a DNS (Domain Name System) server which extracts or acquires, when access has been generated using a FQDN (Fully Qualified Domain Name) for accessing from a user terminal to a service providing sever, an IP (Internet Protocol) address corresponding to said FQDN, and transmits the IP address to said user terminal or performs access using the IP address;
• a mediation server device which first receives, when the access using the said IP address corresponding to said FQDN has been generated from said user terminal or said DNS server, the access on behalf of said service providing server, and performs first authentication for said user terminal;
• an intermediary server device which performs second authentication for the user terminal identified as a reliable terminal in said first authentication by said mediation server device;
• a traceable ID managing server which generates an IP address as a traceable private ID capable of specifying a transmission source by a destination address in correspondence with the user terminal identified as a reliable terminal in said second authentication by said intermediary server device;
• and
• a security gateway device which belongs to the same site as that of said service providing server,
• wherein said mediation server device provides an intermediary server device ID for connecting to said intermediary server device to said user terminal or said DNS server that has performed said access using said IP address corresponding to said FQDN when said user terminal has been identified as the reliable terminal in said first authentication,
• said intermediary server device transmits a notification indicating that access from the reliable terminal has been generated to said service providing server through said security gateway device when the user terminal has been identified as the reliable terminal in said second authentication,
• said service providing server instructs said traceable ID managing server to generate a traceable private ID upon receipt of said notification indicating that there is a connection request from the user terminal identified as the reliable terminal in said second authentication, so that said traceable ID managing server receiving the instruction generates and transmits the traceable private ID to the user terminal and said service providing server, and
• said security gateway device verifies that the access is access to said service providing server through a reliable path by said traceable private ID when access using said traceable private ID is generated from said user terminal, so that said service providing server provides a service to said user terminal,
• wherein said second authentication is authentication for said user terminal and said server, and is performed by at least authenticating a communication path of which security has been identified between said user terminal and said server.