Open proxy

An open proxy is a proxy server that is accessible by any Internet user. Generally, a proxy server only allows users within a network group (i.e. a closed proxy) to store and forward Internet services such as DNS or web pages to reduce and control the bandwidth used by the group. With an open proxy, however, any user on the Internet is able to use this forwarding service.

An anonymous open proxy is useful to those seeking online anonymity and privacy as it can help users hide their IP address from web servers since the server requests appear to originate from the proxy server. It makes it harder to reveal their identity and thereby helps preserve their perceived security while browsing the web or using other internet services. Real anonymity and extensive internet security might not be achieved by this measure alone as website operators can use client-side scripts to determine the browser’s real IP address and the open proxy may be keeping logs of all connections.

It is possible for a computer to run as an open proxy server without the computer's owner knowing it. This can result from misconfiguration of proxy software running on the computer, or from infection with malware (viruses, trojans or worms) designed for this purpose. If it is caused by malware, the infected computer is known as a zombie computer.

An open proxy is a high risk for the server operator:

Many open proxies run very slowly, sometimes below 14.4 kbit/s, or even below 300 bit/s, while other times the speed may change from fast to slow every minute.

Because open proxies are often implicated in abuse, a number of methods have been developed to detect them and to refuse service to them. IRC networks with strict usage policies automatically test client systems for known types of open proxies. Likewise, a mail server may be configured to automatically test mail senders for open proxies, using software such as proxycheck.

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