Failure mode is an important consideration in any security mechanism. Circuit gateways use access control rules to determine which host may use the gateway and can log information about the host name, identity of the user, number of bytes transferred, time and remote host. Circuit gateways do not understand the applications they support, so they can't log the names of files transferred using FTP or prevent users from sending files. You Can See Through It Circuit gateways and packet filters are, by their nature, transparent. One disadvantage of total transparency is that such products cannot authenticate users.
Some transparent solutions attempt to identify users with a protocol known as identd or authd .
The identity daemon can report the user name related to a particular network connection. Application gateways are considered the most secure firewall technology. They recognize the contents of each packet and can provide fine-grained control and logging of each application. For example, each request to download a file from an FTP server or a Web page can be logged, and requests to send files with FTP can be blocked. The main disadvantage of application gateways is the lack of flexibility.