A proxy server is a computer network service which allows clients to make indirect network connections to other network services. A client connects to the proxy server, then requests a connection, file, or other resource available on a different server. The proxy provides the resource, possibly by connecting to the specified server, or by serving it from a cache. In some cases, the proxy may alter the client's request or the server's response for various purposes.

A common proxy application is a caching Web proxy. This provides a nearby cache of Web pages and files available on remote Web servers, allowing local network clients to access them more quickly or reliably.

When it receives a request for a Web resource (specified by a URL), a caching proxy looks for the resulting URL in its local cache. If found, it returns the document immediately. Otherwise it fetches it from the remote server, saves a copy in the cache and returns it to the requester. The cache will usually have an expiry algorithm which flushes documents according to their age, size, and access history. Two very simple algorithms include LRU and LFU. LRU will remove the least-recently-used documents, and LFU that removes the least-frequently-used documents.

Web proxies can also filter the content of Web pages served. Some censorware applications -- which attempt to block offensive Web content -- are implemented as Web proxies. Network operators can also deploy proxies to intercept computer viruses and other hostile content served from remote Web pages.

Proxies can also operate at a lower level on the protocol stack. Network address translation, or NAT, is a method for proxying TCP connections and UDP datagram exchanges. NAT is also known as IP masquerading.

Many organizations — including corporations, schools, and families — use proxy servers to enforce network use policies (see censorware) or provide security and caching services. A normal Web proxy, or other application proxy, is not transparent to the client application: the client must be configured to use it, manually or with a configuration script. Thus, it can be evaded by simply resetting the client configuration. A transparent proxy or transproxy combines a proxy server with NAT so that connections are routed into the proxy without client-side configuration.

Both NAT and transproxies are somewhat controversial in the Internet technical community, since both violate the end-to-end principle upon which TCP/IP was designed.

The term proxy is also used in a different sense in SIP, the Session Initiation Protocol used in many modern voice over IP systems. A SIP Proxy, unlike a Web proxy, does not handle the content of client data.

Open proxies, abuse, and detection

Both Web and other network proxies have been abused by spammers and other network abusers. An open proxy is a proxy server which will accept client connections from any IP address and make connections to any Internet resource. Abuse of open proxies is currently implicated in a significant portion of email spam delivery. Spammers frequently install open proxies on unwitting end users' Microsoft Windows computers by means of computer viruses designed for this purpose. Internet Relay Chat (IRC) abusers also frequently use open proxies to cloak their identities.

Because proxies are implicated in abuse, system administrators have developed a number of ways to refuse service to open proxies. IRC networks such as the Blitzed network automatically test client systems for known types of open proxy. Likewise, a mail server may be configured to automatically test mail senders for open proxies, using software such as Michael Tokarev's proxycheck.

Groups of IRC and electronic mail operators run DNSBLs publishing lists of the IP addresses of known open proxies, such as Blitzed OPM and CBL .

The ethics of automatically testing clients for open proxies are controversial. Some experts, such as Vernon Schryver, consider such testing to be equivalent to an attacker portscanning the client host.  Others consider the client to have solicited the scan by connecting to a server whose terms of service include testing.