问题
It has nothing to do file-descriptors. Is it some sort of connection between different protocols? Does there exist more like that? Reverse -proxy? Direct -proxy? Indirect -proxy? Does proxy mean 3-layer, 7-layer or different layer in OSI reference model? If you have NAT, you have 3-layer while 7-layer is the common proxy according to Wikipedia here. The Wikipedia continues "Because NAT operates at layer-3, it is less resource-intensive than the layer-7 proxy, but also less flexible" -- there are different kind of ways of doing the proxy:
So now a very stupid and irrogant question "What is a proxy in Apache?"
Other ignorant Questions by which I try to understand the proxies deeper
https://stackoverflow.com/questions/12397242/explain-apache-mod-proxy-module-is-it-overused-and-many-times-a-red-herring-w
Explain CouchDB's serving of websites, is CouchDB bundled somehow with Apache and how does it work?
回答1:
Apache is a layer-7 proxy (as far as OSI is concerned), it doesn't use network address translation or any type of packet mangling/rewriting. It receives a request and based on some rules/configuration, makes a request on behalf of the client. Apache can act as a forward proxy and/or reverse proxy. In your images above, apache would be running on the blob that is red.
In the first image, apache would be acting as a reverse proxy, it receives an HTTP request from the internet, and proxies it to a specific place internally.
In the second image, apache acts as a forward proxy. Local users are using it to request anything on the internet (within the rules/config).
In a reverse proxy, a request for a specific resource is received, e.g.
http://my.homepage.com/, and apache, knowing that the content is actually internally located athttp://192.168.2.45/my.homepage/, proxies the request to the internal location.In a forward proxy, a user on a LAN requests
http://www.google.com/, and either the browser or OS knows to proxy the request to a local proxy server (apache, the red blob in the image), and apache then makes the request to www.google.com on the user's behalf.
回答2:
There are different kinds of proxies! The key is a middleman, it is somehow in the middle of things A and B. I will use now terminology of Tanenbaum (more here). He defines for example in the context of the Globus security model two different proxies: user proxy and resource proxy. Then he defines object proxy that is an interface in object-distributed systems. Then he defines a web-proxy that is some sort of ancient idea when client-side web-browsers missed features such as ftp-support.
Now according to Jon Lin, reverse/forward proxies are similar to resource/user, respectively. Object proxy and web-proxy are special kind of implementations. I think they can be either resource -proxy or user -proxy, actually. If you have object -proxy, it could be implemented in different ways: you could could implement it so that user gives rights to use it, hence user proxy, or more global activity where it has different methods by which it co-operates with local environment from some global setting, hence a resource-proxy.
Related
- https://stackoverflow.com/questions/12398389/different-definitions-of-the-term-proxy/12398390#12398390
来源：https://stackoverflow.com/questions/12397349/what-is-a-proxy-what-is-it-in-apache-does-it-have-many-different-meanings