Open proxy Closed proxy
An open proxy is a proxy server that is accessible by any Internet user. Generally, a proxy server allows users within a network group 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.
So-called “anonymous” open proxies allow a user to conceal his or her IP address from the accessed service, as it is possible to use someone else’s computer in order to hide one’s identity and/or location. This is sometimes used to abuse or interrupt the accessed service, potentially violating its terms of service or some laws; open proxies are therefore often seen as a problem. However, anonymous open proxies increase the user’s anonymity or security when browsing the web or using other Internet services: a user’s true IP address can be used to help deduce information about that user as part of an attempted computer crack. Furthermore, open proxies can be used to circumvent efforts at Internet censorship by governments or organizations. Several web sites provide constantly updated lists of open proxies.
It is possible for a computer to run as an open proxy server without the computer’s owner knowing this. 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. There are hundreds of thousands of open proxies online. This is rarely a matter of misconfiguration, in most cases an open proxy is unknown to the server owner. Using an open proxy can be legally dangerous: the current law in Germany, for example, makes it illegal to access a computer or network without the owner’s knowledge, even if the proxy is not password-protected. There are no reasons to run a clean open proxy for an organisation, so any open proxy is suspicious. The main sign of a legal open proxy is the fact that it is not anonymizing the connection.
Running an open proxy is a high risk for the server operator; providing an anonymous proxy server can cause real legal troubles to the owner. Such services are frequently used to break into foreign computer systems, child pornography is usually consumed through proxies, and illegal content is likely to be spread through such proxies. Also, such a proxy can cause a high bandwidth usage resulting in higher latency to the subnetwork and violation of bandwidth limits. A wrongly configured open proxy can also allow access to a private subnetwork or DMZ: this is a high security concern for any company or home network because computers that usually are out of risk or firewalled can be directly attacked.
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. Some, such as PlanetLab proxies, run faster and were intentionally set up for public use.
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. Increasingly, mail servers are configured out of the box to consult various DNSBL servers in order to block spam; some of those DNSBLs also list open proxies.
A closed proxy is a proxy server that is only accessible to specific individuals because it only allows users within its area connect to it and/or requires a username and password.