Proxy Servers

Purposes of proxy servers

Using a proxy means that instead of communicating with the remote end directly, there is a middleman transmitting everything back and forth. There is a long list of reasons to use proxies, including the following:
   > To prevent the remote end from knowing where the request came from (for security purposes).
   > For caching: some proxies store the information that passes through them and supply the same content for a while (to speed up data delivery).
   > To block unwanted resources based on keywords, URL's or other attributes, enforcing a policy.
   > For logging and auditing usage in order to provide detailed statistics and reports.
   > To bypass content filters (applied by other proxies).
   > For anti-virus purposes: proxies can scan files for malware before serving it to the client.
   > To prevent data leaks from behind the proxy sent outside.
   > For circumventing regional restrictions (make it look like resources were accessed from a different country).

Open proxy server

If a proxy is open, it means anyone may use it. The three most common reasons why open proxies exist are:
   > Accidental misconfigurations: they were originally set up for private use, but due to bad configuration allow anyone to use them.
   > Compromised machines: deliberately set up by a 'hacker' - a proxy server was installed without the knowledge and consent of the owner.
   > Honeypots: deliberately set up by the owner in order to monitor what people are using it for.

Since many 'hackers' scan the internet for open proxies, such servers are quickly found and shared publicly, leading to dozens or maybe hundreds of people using the connection for various purposes. These purposes may or may not be illegal, but in the end they eat up bandwidth of the host which the owner surely notices. If the owner doesn't notice, the provider does. For this reason, open proxies do not stay alive for very long in the wild.

HTTP proxy

It is recognized as an ipaddress:port (or hostname:port) combination. An HTTP proxy is a software running on a host that accepts connections from clients and acts as a proxy for them. By supporting the CONNECT method, it allows clients to not only use the HTTP protocol, but HTTPS (encrypted) and other arbitrary protocols as well. To make use of it, you need to configure the application (most often a web browser) to send queries through it instead of a direct connection.

Transparent HTTP proxy

It is a kind of HTTP proxy that is prepared to accept connections originally sent to a different host, communicating with the HTTP protocol. A firewall or networking entity is used to divert connections to go through the proxy server. This means that opposed to regular HTTP proxies, clients do not need to configure the applications to communicate through it. Websurfing is automatically sent through the proxy.

Reverse proxy

Reverse proxies are used by the servers themselves and not the clients. They are a buffer between the visitors' queries to the server. They help speed up the process by caching static content, load balancing when there are multiple web servers and increasing security by looking out for suspicious or malicious queries. To an end user, it is basically transparent.

Web proxy

Web proxy servers on the internet are typically sites that let you enter a URL and see it through the server running the proxy software. It is usually nothing more than a regular PHP script running on a shared hosting server. To use a web based proxy, all you need is a browser. Using a web based proxy from a technical point of view is the same as browsing a regular site. The contents of the destination webpage are shown as being part of the proxy website. Advantages of web proxies:
   > They are easy to find. There are many sites that are dedicated to list them.
   > If you can browse the web, you can use them - no special configuration or client side privileges necessary.
   > They are easy to use. Anyone can type the URL into a webpage, that is why web proxies are so popular.

However, there are many disadvantages that shift the balance way into the negative region:
   > They are often stuffed full of advertisements for profit.
   > Because of the complicated HTML rewriting used, there is a very high chance of your real IP address being leaked.
   > Very uncompatible with modern sites that use Javascript, AJAX and similar technologies.
   > Since they are regular sites, every request is logged. The addresses you visited through these proxies can be read from these logs (along with your real IP address).
   > Many of them are honeypots to steal and use information.

High anonymity proxy

Do not be confused by the name. This name comes from the wild and doesn't mean that the proxy actually gives you 'high anonymity'. The expression is used for web proxies that do not share the original IP address with the destination host as part of the HTTP headers. There is no guarantee that the proxy isn't a honeypot, has no logging, or doesn't have the logs shared with the public.