Proxy server some special types of applications

Application-Level and Loop-Level Agents

An application-level agent is a proxy for which application is known to be a proxy service. It can understand and interpret commands in the application protocol, while loop-level agents are on the client and server. A connection loop is established without interpreting the commands in the application protocol. The latest version of most application-level agents is an application like sendmail that does the store-and-forward protocol. Most of the latest loop-level agents are a new type of proxy gateway. This gateway acts like a proxy externally and acts as a filtering router.

The application-level agent uses the modified process, and the loop-level agent uses the modified client. This is related to the practicality of the agent. In order to implement a proxy connection, you must know the direction of the connection. A hybrid gateway can easily block connections, but a proxy host can only receive connections and determine from the information it is going to continue to connect there. A loop-level proxy cannot interpret the application protocol and needs to provide information to it in other ways. Because the capabilities of client programs are very efficient, application-level agents are often used to understand the benefits of application protocols, so they can use the modified process. Loop-level agents, usually cannot use the modified process, can only use modified client programs.

Although there are no application-level agents that have been modified, there are indeed modified loop-level agents, such as plug-gw, which is a modified process and a loop-level proxy. The destination address it connects to depends entirely on the source address and the source and destination ports to which it is connected.

The advantage of a loop-level proxy is that it can serve a variety of different protocols. Most loop-level proxy servers are also public proxy servers, which are supported for almost any protocol, but not every protocol can be easily implemented by loop-level proxies, as is the case with the FTP protocol. It requires a connection from the client's data port to the server and requires protocol-level tuning and application-level knowledge. The disadvantage of a loop-level proxy is that it has little control over the events that result from the proxy. Like packet filtering, it provides a connection for the source and destination addresses, but it cannot determine whether the commands passing through it are secure or beyond the scope of the protocol. The loop level agent can easily be fooled by the port number set by the server and assigned to other servers.

Public and Dedicated Proxy Servers

While "application level" and "loop level" are common terms, we are more concerned with the difference between "public" and "dedicated" proxy servers. A dedicated proxy server is only available for a single protocol, and a common proxy server is for multiple protocols. In fact, the dedicated proxy server is application-level, and the public proxy server is loop-level. Because there is a public application-level proxy server that understands many protocols, or a dedicated loop-level proxy server (providing only one service but understanding multiple protocols), the terms "dedicated" and "public" are used. "Application level" and "loop level" are well understood.

Intelligent Proxy Server

If a proxy server is not only forwarding requests, but also can do many other things, such a proxy server is called a smart proxy server, such as CERN HTTP proxy server. Ability to save data in the cache so that the same data no longer has to be downloaded from the Internet. Proxy servers (especially application-level proxy servers) provide better logging and access control than others. The capabilities of proxy servers are evolving rapidly, and many proxy servers are now adding new features in addition to basic functionality. It is easy to upgrade to a smart proxy server for a dedicated application-level proxy, but it is more difficult for a loop-level proxy.