Proxy server
Encyclopedia
In computer network
s, a proxy server is a server
(a computer system or an application) that acts as an intermediary for requests from clients
seeking resources from other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource available from a different server. The proxy server evaluates the request according to its filtering rules. For example, it may filter traffic by IP address
or protocol. If the request is validated by the filter, the proxy provides the resource by connecting to the relevant server and requesting the service on behalf of the client. A proxy server may optionally alter the client's request or the server's response, and sometimes it may serve the request without contacting the specified server. In this case, it 'cache
s' responses from the remote server, and returns subsequent requests for the same content directly.
The proxy concept was invented in the early days of distributed systems as a way to simplify and control their complexity. Today, most proxies are a web proxy, allowing access to content on the World Wide Web
.
A proxy server that passes requests and responses unmodified is usually called a gateway or sometimes tunneling proxy.
A proxy server can be placed in the user's local computer or at various points between the user and the destination servers on the Internet.
A reverse proxy
is (usually) an Internet-facing proxy used as a front-end to control and protect access to a server on a private network, commonly also performing tasks such as load-balancing, authentication, decryption or caching.
The terms "forward proxy" and "forwarding proxy" are a general description of behavior (forwarding traffic) and thus ambiguous. Except for Reverse proxy, the types of proxies described in this article are more specialized sub-types of the general forward proxy concept.
while browsing the Web or using other Internet services. There are varying degrees of anonymity however, as well as a number of methods of 'tricking' the client into revealing itself regardless of the proxy being used.
Reverse proxies are installed in the neighborhood of one or more web servers. All traffic coming from the Internet and with a destination of one of the neighborhood's web servers goes through the proxy server. The use of "reverse" originates in its counterpart "forward proxy" since the reverse proxy sits closer to the web server and serves only a restricted set of websites.
There are several reasons for installing reverse proxy servers:
web proxy server provides administrative control over the content that may be relayed through the proxy. It is commonly used in both commercial and non-commercial organizations (especially schools) to ensure that Internet usage conforms to acceptable use policy
. In some cases users can circumvent the proxy, since there are services designed to proxy information from a filtered website through a non filtered site to allow it through the user's proxy.
A content filtering proxy will often support user authentication
, to control web access. It also usually produces logs
, either to give detailed information about the URLs accessed by specific users, or to monitor bandwidth usage statistics. It may also communicate to daemon
-based and/or ICAP
-based antivirus software
to provide security against virus and other malware
by scanning incoming content in real time before it enters the network.
Many work places, schools, and colleges restrict the web sites and online services that are made available in their buildings. This is done either with a specialized proxy, called a content filter (both commercial and free products are available), or by using a cache-extension protocol such as ICAP
, that allows plug-in extensions to an open caching architecture.
Some common methods used for content filtering include: URL or DNS blacklists
, URL regex filtering, MIME
filtering, or content keyword filtering. Some products have been known to employ content analysis techniques to look for traits commonly used by certain types of content providers.
Requests made to the open internet must first pass through an outbound proxy filter. The web-filtering company provides a database of URL patterns (regular expressions) with associated content attributes. This database is updated weekly by site-wide subscription, much like a virus filter subscription. The administrator instructs the web filter to ban broad classes of content (such as sports, pornography, online shopping, gambling, or social networking). Requests that match a banned URL pattern are rejected immediately.
Assuming the requested URL is acceptable, the content is then fetched by the proxy. At this point a dynamic filter may be applied on the return path. For example, JPEG
files could be blocked based on fleshtone matches, or language filters could dynamically detect unwanted language. If the content is rejected then an HTTP fetch error is returned and nothing is cached.
Extranet Publishing: a reverse proxy server facing the Internet can be used to communicate to a firewalled server internal to an organization, providing extranet access to some functions while keeping the servers behind the firewalls. If used in this way, security measures should be considered to protect the rest of your infrastructure in case this server is compromised, as its web application is exposed to attack from the Internet
Most web filtering companies use an internet-wide crawling robot that assesses the likelihood that a content is a certain type. The resultant database is then corrected by manual labor based on complaints or known flaws in the content-matching algorithms.
Web filtering proxies are not able to peer inside secure sockets HTTP transactions, assuming the chain-of-trust of SSL/TLS has not been tampered with. As a result, users wanting to bypass web filtering will typically search the internet for an open and anonymous HTTPS transparent proxy. They will then program their browser to proxy all requests through the web filter to this anonymous proxy. Those requests will be encrypted with https. The web filter cannot distinguish these transactions from, say, a legitimate access to a financial website. Thus, content filters are only effective against unsophisticated users.
As mentioned above, the SSL/TLS chain-of-trust does rely on trusted root certificate authorities; in a workplace setting where the client is managed by the organization, trust might be granted to a root certificate whose private key is known to the proxy. Concretely, a root certificate generated by the proxy is installed into the browser CA list by IT staff. In such scenarios, proxy analysis of the contents of a SSL/TLS transaction becomes possible. The proxy is effectively operating a man-in-the-middle attack
, allowed by the client's trust of a root certificate the proxy owns.
A special case of web proxies is "CGI proxies". These are web sites that allow a user to access a site through them. They generally use PHP
or CGI
to implement the proxy functionality. These types of proxies are frequently used to gain access to web sites blocked by corporate or school proxies. Since they also hide the user's own IP address
from the web sites they access through the proxy, they are sometimes also used to gain a degree of anonymity
, called "Proxy Avoidance".
Some poorly-implemented caching proxies have had downsides (e.g., an inability to use user authentication). Some problems are described in RFC 3143 (Known HTTP Proxy/Caching Problems).
Another important use of the proxy server is to reduce the hardware cost. An organization may have many systems on the same network or under control of a single server, prohibiting the possibility of an individual connection to the Internet for each system. In such a case, the individual systems can be connected to one proxy server, and the proxy server connected to the main server. An example of a software caching proxy is Squid.
-based geolocation
to restrict its service to a certain country can be accessed using a proxy located in that country to access the service.
Likewise, a badly configured proxy can provide access to a network otherwise isolated from the Internet.
upon the data-flow between client machines and the web. All content sent or accessed – including passwords submitted and cookies
used – can be captured and analyzed by the proxy operator. For this reason, passwords to online services (such as webmail and banking) should always be exchanged over a cryptographically secured connection, such as SSL.
By chaining proxies which do not reveal data about the original requester, it is possible to obfuscate activities from the eyes of the user's destination. However, more traces will be left on the intermediate hops, which could be used or offered up to trace the user's activities. If the policies and administrators of these other proxies are unknown, the user may fall victim to a false sense of security just because those details are out of sight and mind.
In what is more of an inconvenience than a risk, proxy users may find themselves being blocked from certain Web sites, as numerous forums and Web sites block IP addresses from proxies known to have spammed
or trolled
the site. Proxy bouncing can be used to maintain your privacy.
in linking two networks.
s. The destination server (the server that ultimately satisfies the web request) receives requests from the anonymizing proxy server, and thus does not receive information about the end user's address. However, the requests are not anonymous to the anonymizing proxy server, and so a degree of trust is present between the proxy server and the user. Many of them are funded through a continued advertising link to the user.
Access control: Some proxy servers implement a logon requirement. In large organizations, authorized users must log on to gain access to the web
. The organization can thereby track usage to individuals.
Some anonymizing proxy servers may forward data packets with header lines such as HTTP_VIA, HTTP_X_FORWARDED_FOR, or HTTP_FORWARDED, which may reveal the IP address of the client. Other anonymizing proxy servers, known as elite or high anonymity proxies, only include the REMOTE_ADDR header with the IP address of the proxy server, making it appear that the proxy server is the client. A website could still suspect a proxy is being used if the client sends packets which include a cookie from a previous visit that did not use the high anonymity proxy server. Clearing cookies, and possibly the cache, would solve this problem.
RFC 2616 (Hypertext Transfer Protocol—HTTP/1.1) offers standard definitions:
In 2009 security a flaw in the way that transparent proxies operate was published by Robert Auger and an advisory issued by the Computer Emergency Response Team listing dozens of affected transparent, and intercepting proxy servers.
Intercepting proxies are also commonly used by ISPs in some countries to save upstream bandwidth and improve customer response times by caching. This is more common in countries where bandwidth is more limited (e.g. island nations) or must be paid for.
which depend on certain behaviour of intercepting proxies that do not check or have access to information about the original (intercepted) destination. This problem can be resolved by using an integrated packet-level and application level appliance or software which is then able to communicate this information between the packet handler and the proxy.
Intercepting also creates problems for HTTP authentication, especially connection-oriented authentication such as NTLM
, since the client browser believes it is talking to a server rather than a proxy. This can cause problems where an intercepting proxy requires authentication, then the user connects to a site which also requires authentication.
Finally intercepting connections can cause problems for HTTP caches, since some requests and responses become uncacheble by a shared cache.
Therefore intercepting connections is generally discouraged. However due to the simplicity of deploying such systems, they are in widespread use.
(Web Cache Control Protocol). This proprietary protocol resides on the router and is configured from the cache, allowing the cache to determine what ports and traffic is sent to it via transparent redirection from the router. This redirection can occur in one of two ways: GRE Tunneling (OSI Layer 3) or MAC rewrites (OSI Layer 2).
Once traffic reaches the proxy machine itself interception is commonly performed with NAT (Network Address Translation). Such setups are invisible to the client browser, but leave the proxy visible to the web server and other devices on the internet side of the proxy. Recent Linux and some BSD releases provide TPROXY (transparent proxy) which performs IP-level (OSI Layer 3) transparent interception and spoofing of outbound traffic, hiding the proxy IP address from other network devices.
The Tor anonymity network
('Tor' for short) is a system aiming at online anonymity. Tor is an implementation of onion routing
. It works by relaying communications through a network of systems run by volunteers in various locations. By keeping some of the network entry points hidden, Tor is also able to evade internet censorship
. Tor is intended to protect users' personal freedom, privacy, and ability to conduct confidential business.
Users of a Tor network run an onion proxy software on their computer. The Tor software periodically negotiates a virtual circuit
through the Tor network. At the same time, the onion proxy software presents a SOCKS
interface to its clients or users. SOCKS-ifying applications like Polipo
may be linked with the Tor onion proxy software, which then multiplexes
the traffic through a Tor virtual circuit.
Tor is referred to as Onion Routing because as the packets are passed between each node, an additional layer of encryption is 'peeled back' like an onion. And by the time the packet reaches it's destination, all of the layers of encryption have been removed.
The software is open source
and the network is free of charge to use.
Vidalia
is a cross-platform controller GUI
for Tor
.
('I2P') is a proxy network aiming at online anonymity. It implements garlic routing
, which is an enhancement of Tor
's onion routing
. I2P is fully distributed and works by encrypting all communications in various layers and relaying them through a network of routers run by volunteers in various locations. By keeping the source of the information hidden, I2P offers censorship resistance. The goals of I2P are to protect users' personal freedom, privacy, and ability to conduct confidential business.
Each user of I2P runs an I2P router on their computer (node). The I2P router takes care of finding other peers and building anonymizing tunnels through them. I2P provides proxies for all protocols (HTTP, irc, SOCKS, ...).
The software is free and open-source
, and the network is free of charge to use.
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
s, a proxy server is a server
Server (computing)
In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...
(a computer system or an application) that acts as an intermediary for requests from clients
Client (computing)
A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....
seeking resources from other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource available from a different server. The proxy server evaluates the request according to its filtering rules. For example, it may filter traffic by IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
or protocol. If the request is validated by the filter, the proxy provides the resource by connecting to the relevant server and requesting the service on behalf of the client. A proxy server may optionally alter the client's request or the server's response, and sometimes it may serve the request without contacting the specified server. In this case, it 'cache
Cache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
s' responses from the remote server, and returns subsequent requests for the same content directly.
The proxy concept was invented in the early days of distributed systems as a way to simplify and control their complexity. Today, most proxies are a web proxy, allowing access to content on the World Wide Web
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
.
Uses
A proxy server has a large variety of potential purposes, including:- To keep machines behind it anonymous, mainly for securityComputer securityComputer security is a branch of computer technology known as information security as applied to computers and networks. The objective of computer security includes protection of information and property from theft, corruption, or natural disaster, while allowing the information and property to...
. - To speed up access to resources (using caching). Web proxies are commonly used to cacheCacheIn computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
web pages from a web server. - To apply access policy to network services or content, e.g. to block undesired sites.
- To access sites prohibited or filtered by your ISP or institution.
- To log / audit usage, i.e. to provide company employee Internet usage reporting.
- To bypass security / parental controls.
- To circumvent Internet filtering to access content otherwise blocked by governments.
- To scan transmitted content for malware before delivery.
- To scan outbound content, e.g., for data loss prevention.
- To allow a web site to make web requests to externally hosted resources (e.g. images, music files, etc.) when cross-domain restrictions prohibit the web site from linking directly to the outside domains.
A proxy server that passes requests and responses unmodified is usually called a gateway or sometimes tunneling proxy.
A proxy server can be placed in the user's local computer or at various points between the user and the destination servers on the Internet.
A reverse proxy
Reverse proxy
In computer networks, a reverse proxy is a type of proxy server that retrieves resources on behalf of a client from one or more servers. These resources are then returned to the client as though it originated from the reverse proxy itself...
is (usually) an Internet-facing proxy used as a front-end to control and protect access to a server on a private network, commonly also performing tasks such as load-balancing, authentication, decryption or caching.
Forward proxies
Forward proxies are proxies where the client server names the target server to connect to. Forward proxies are able to retrieve from a wide range of sources (in most cases anywhere on the Internet).The terms "forward proxy" and "forwarding proxy" are a general description of behavior (forwarding traffic) and thus ambiguous. Except for Reverse proxy, the types of proxies described in this article are more specialized sub-types of the general forward proxy concept.
Open proxies
An open proxy is a forwarding proxy server that is accessible by any Internet user. Gordon Lyon estimates there are "hundreds of thousands" of open proxies on the Internet. An anonymous open proxy allows users to conceal their IP addressIP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
while browsing the Web or using other Internet services. There are varying degrees of anonymity however, as well as a number of methods of 'tricking' the client into revealing itself regardless of the proxy being used.
Reverse proxies
A reverse proxy (or surrogate) is a proxy server that appears to clients to be an ordinary server. Requests are forwarded to one or more origin servers which handle the request. The response is returned as if it came directly from the proxy server.Reverse proxies are installed in the neighborhood of one or more web servers. All traffic coming from the Internet and with a destination of one of the neighborhood's web servers goes through the proxy server. The use of "reverse" originates in its counterpart "forward proxy" since the reverse proxy sits closer to the web server and serves only a restricted set of websites.
There are several reasons for installing reverse proxy servers:
- Encryption / SSL acceleration: when secure web sites are created, the SSL encryption is often not done by the web server itself, but by a reverse proxy that is equipped with SSL acceleration hardware. See Secure Sockets Layer. Furthermore, a host can provide a single "SSL proxy" to provide SSL encryption for an arbitrary number of hosts; removing the need for a separate SSL Server Certificate for each host, with the downside that all hosts behind the SSL proxy have to share a common DNS name or IP address for SSL connections. This problem can partly be overcome by using the SubjectAltName feature of X.509X.509In cryptography, X.509 is an ITU-T standard for a public key infrastructure and Privilege Management Infrastructure . X.509 specifies, amongst other things, standard formats for public key certificates, certificate revocation lists, attribute certificates, and a certification path validation...
certificates. - Load balancingLoad balancing (computing)Load balancing is a computer networking methodology to distribute workload across multiple computers or a computer cluster, network links, central processing units, disk drives, or other resources, to achieve optimal resource utilization, maximize throughput, minimize response time, and avoid...
: the reverse proxy can distribute the load to several web servers, each web server serving its own application area. In such a case, the reverse proxy may need to rewrite the URLs in each web page (translation from externally known URLs to the internal locations). - Serve/cache static content: A reverse proxy can offload the web servers by caching static content like pictures and other static graphical content.
- Compression: the proxy server can optimize and compress the content to speed up the load time.
- Spoon feeding: reduces resource usage caused by slow clients on the web servers by caching the content the web server sent and slowly "spoon feeding" it to the client. This especially benefits dynamically generated pages.
- Security: the proxy server is an additional layer of defense and can protect against some OS and WebServer specific attacks. However, it does not provide any protection to attacks against the web application or service itself, which is generally considered the larger threat.
- Extranet Publishing: a reverse proxy server facing the Internet can be used to communicate to a firewalled server internal to an organization, providing extranet access to some functions while keeping the servers behind the firewalls. If used in this way, security measures should be considered to protect the rest of your infrastructure in case this server is compromised, as its web application is exposed to attack from the Internet.
Performance Enhancing Proxies
A proxy that is designed to mitigate specific link related issues or degradations. PEPs (Performance Enhancing Proxies) are typically used to improve TCP performance in the presence of high Round Trip Times (RTTs) and wireless links with high packet loss. They are also frequently used for highly asynchronous links featuring very different upload and download rates.Filtering
A content-filteringContent filtering
Content filtering is the technique whereby content is blocked or allowed based on analysis of its content, rather than its source or other criteria. It is most widely used on the internet to filter email and web access.- Content filtering of email :...
web proxy server provides administrative control over the content that may be relayed through the proxy. It is commonly used in both commercial and non-commercial organizations (especially schools) to ensure that Internet usage conforms to acceptable use policy
Acceptable use policy
An acceptable use policy is a set of rules applied by the owner/manager of a network, website or large computer system that restrict the ways in which the network site or system may be used...
. In some cases users can circumvent the proxy, since there are services designed to proxy information from a filtered website through a non filtered site to allow it through the user's proxy.
A content filtering proxy will often support user authentication
Authentication
Authentication is the act of confirming the truth of an attribute of a datum or entity...
, to control web access. It also usually produces logs
Server log
A server log is a log file automatically created and maintained by a server of activity performed by it.A typical example is a web server log which maintains a history of page requests. The W3C maintains a standard format for web server log files, but other proprietary formats exist...
, either to give detailed information about the URLs accessed by specific users, or to monitor bandwidth usage statistics. It may also communicate to daemon
Daemon (computer software)
In Unix and other multitasking computer operating systems, a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user...
-based and/or ICAP
Internet Content Adaptation Protocol
The Internet Content Adaptation Protocol is a lightweight HTTP-like protocol specified in RFC 3507 which is used to extend transparent proxy servers, thereby freeing up resources and standardizing the way in which new features are implemented. ICAP is generally used to implement virus scanning,...
-based antivirus software
Antivirus software
Antivirus or anti-virus software is used to prevent, detect, and remove malware, including but not limited to computer viruses, computer worm, trojan horses, spyware and adware...
to provide security against virus and other malware
Malware
Malware, short for malicious software, consists of programming that is designed to disrupt or deny operation, gather information that leads to loss of privacy or exploitation, or gain unauthorized access to system resources, or that otherwise exhibits abusive behavior...
by scanning incoming content in real time before it enters the network.
Many work places, schools, and colleges restrict the web sites and online services that are made available in their buildings. This is done either with a specialized proxy, called a content filter (both commercial and free products are available), or by using a cache-extension protocol such as ICAP
Internet Content Adaptation Protocol
The Internet Content Adaptation Protocol is a lightweight HTTP-like protocol specified in RFC 3507 which is used to extend transparent proxy servers, thereby freeing up resources and standardizing the way in which new features are implemented. ICAP is generally used to implement virus scanning,...
, that allows plug-in extensions to an open caching architecture.
Some common methods used for content filtering include: URL or DNS blacklists
DNSBL
A DNSBL is a list of IP addresses published through the Internet Domain Name Service either as a zone file that can be used by DNS server software, or as a live DNS zone that can be queried in real-time...
, URL regex filtering, MIME
MIME
Multipurpose Internet Mail Extensions is an Internet standard that extends the format of email to support:* Text in character sets other than ASCII* Non-text attachments* Message bodies with multiple parts...
filtering, or content keyword filtering. Some products have been known to employ content analysis techniques to look for traits commonly used by certain types of content providers.
Requests made to the open internet must first pass through an outbound proxy filter. The web-filtering company provides a database of URL patterns (regular expressions) with associated content attributes. This database is updated weekly by site-wide subscription, much like a virus filter subscription. The administrator instructs the web filter to ban broad classes of content (such as sports, pornography, online shopping, gambling, or social networking). Requests that match a banned URL pattern are rejected immediately.
Assuming the requested URL is acceptable, the content is then fetched by the proxy. At this point a dynamic filter may be applied on the return path. For example, JPEG
JPEG
In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....
files could be blocked based on fleshtone matches, or language filters could dynamically detect unwanted language. If the content is rejected then an HTTP fetch error is returned and nothing is cached.
Extranet Publishing: a reverse proxy server facing the Internet can be used to communicate to a firewalled server internal to an organization, providing extranet access to some functions while keeping the servers behind the firewalls. If used in this way, security measures should be considered to protect the rest of your infrastructure in case this server is compromised, as its web application is exposed to attack from the Internet
Most web filtering companies use an internet-wide crawling robot that assesses the likelihood that a content is a certain type. The resultant database is then corrected by manual labor based on complaints or known flaws in the content-matching algorithms.
Web filtering proxies are not able to peer inside secure sockets HTTP transactions, assuming the chain-of-trust of SSL/TLS has not been tampered with. As a result, users wanting to bypass web filtering will typically search the internet for an open and anonymous HTTPS transparent proxy. They will then program their browser to proxy all requests through the web filter to this anonymous proxy. Those requests will be encrypted with https. The web filter cannot distinguish these transactions from, say, a legitimate access to a financial website. Thus, content filters are only effective against unsophisticated users.
As mentioned above, the SSL/TLS chain-of-trust does rely on trusted root certificate authorities; in a workplace setting where the client is managed by the organization, trust might be granted to a root certificate whose private key is known to the proxy. Concretely, a root certificate generated by the proxy is installed into the browser CA list by IT staff. In such scenarios, proxy analysis of the contents of a SSL/TLS transaction becomes possible. The proxy is effectively operating a man-in-the-middle attack
Man-in-the-middle attack
In cryptography, the man-in-the-middle attack , bucket-brigade attack, or sometimes Janus attack, is a form of active eavesdropping in which the attacker makes independent connections with the victims and relays messages between them, making them believe that they are talking directly to each other...
, allowed by the client's trust of a root certificate the proxy owns.
A special case of web proxies is "CGI proxies". These are web sites that allow a user to access a site through them. They generally use PHP
PHP
PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...
or CGI
Common Gateway Interface
The Common Gateway Interface is a standard method for web servers software to delegate the generation of web pages to executable files...
to implement the proxy functionality. These types of proxies are frequently used to gain access to web sites blocked by corporate or school proxies. Since they also hide the user's own IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
from the web sites they access through the proxy, they are sometimes also used to gain a degree of anonymity
Anonymity
Anonymity is derived from the Greek word ἀνωνυμία, anonymia, meaning "without a name" or "namelessness". In colloquial use, anonymity typically refers to the state of an individual's personal identity, or personally identifiable information, being publicly unknown.There are many reasons why a...
, called "Proxy Avoidance".
Caching
A caching proxy server accelerates service requests by retrieving content saved from a previous request made by the same client or even other clients. Caching proxies keep local copies of frequently requested resources, allowing large organizations to significantly reduce their upstream bandwidth usage and costs, while significantly increasing performance. Most ISPs and large businesses have a caching proxy. Caching proxies were the first kind of proxy server.Some poorly-implemented caching proxies have had downsides (e.g., an inability to use user authentication). Some problems are described in RFC 3143 (Known HTTP Proxy/Caching Problems).
Another important use of the proxy server is to reduce the hardware cost. An organization may have many systems on the same network or under control of a single server, prohibiting the possibility of an individual connection to the Internet for each system. In such a case, the individual systems can be connected to one proxy server, and the proxy server connected to the main server. An example of a software caching proxy is Squid.
DNS proxy
A DNS proxy server takes DNS queries from a (usually local) network and forwards them to an Internet Domain Name Server. It may also cache DNS records.Bypassing filters and censorship
If the destination server filters content based on the origin of the request, the use of a proxy can circumvent this filter. For example, a server using IPInternet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
-based geolocation
Geolocation
Geolocation is the identification of the real-world geographic location of an object, such as a radar, mobile phone or an Internet-connected computer terminal...
to restrict its service to a certain country can be accessed using a proxy located in that country to access the service.
Likewise, a badly configured proxy can provide access to a network otherwise isolated from the Internet.
Logging and eavesdropping
Proxies can be installed in order to eavesdropEavesdropping
Eavesdropping is the act of secretly listening to the private conversation of others without their consent, as defined by Black's Law Dictionary...
upon the data-flow between client machines and the web. All content sent or accessed – including passwords submitted and cookies
HTTP cookie
A cookie, also known as an HTTP cookie, web cookie, or browser cookie, is used for an origin website to send state information to a user's browser and for the browser to return the state information to the origin site...
used – can be captured and analyzed by the proxy operator. For this reason, passwords to online services (such as webmail and banking) should always be exchanged over a cryptographically secured connection, such as SSL.
By chaining proxies which do not reveal data about the original requester, it is possible to obfuscate activities from the eyes of the user's destination. However, more traces will be left on the intermediate hops, which could be used or offered up to trace the user's activities. If the policies and administrators of these other proxies are unknown, the user may fall victim to a false sense of security just because those details are out of sight and mind.
In what is more of an inconvenience than a risk, proxy users may find themselves being blocked from certain Web sites, as numerous forums and Web sites block IP addresses from proxies known to have spammed
Spam (electronic)
Spam is the use of electronic messaging systems to send unsolicited bulk messages indiscriminately...
or trolled
Troll (Internet)
In Internet slang, a troll is someone who posts inflammatory, extraneous, or off-topic messages in an online community, such as an online discussion forum, chat room, or blog, with the primary intent of provoking readers into an emotional response...
the site. Proxy bouncing can be used to maintain your privacy.
Gateways to private networks
Proxy servers can perform a role similar to a network switchNetwork switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
in linking two networks.
Accessing services anonymously
An anonymous proxy server (sometimes called a web proxy) generally attempts to anonymize web surfing. There are different varieties of anonymizerAnonymizer
An anonymizer or an anonymous proxy is a tool that attempts to make activity on the Internet untraceable.It is a proxy server computer that acts as an intermediary and privacy shield between a client computer and the rest of the Internet...
s. The destination server (the server that ultimately satisfies the web request) receives requests from the anonymizing proxy server, and thus does not receive information about the end user's address. However, the requests are not anonymous to the anonymizing proxy server, and so a degree of trust is present between the proxy server and the user. Many of them are funded through a continued advertising link to the user.
Access control: Some proxy servers implement a logon requirement. In large organizations, authorized users must log on to gain access to the web
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
. The organization can thereby track usage to individuals.
Some anonymizing proxy servers may forward data packets with header lines such as HTTP_VIA, HTTP_X_FORWARDED_FOR, or HTTP_FORWARDED, which may reveal the IP address of the client. Other anonymizing proxy servers, known as elite or high anonymity proxies, only include the REMOTE_ADDR header with the IP address of the proxy server, making it appear that the proxy server is the client. A website could still suspect a proxy is being used if the client sends packets which include a cookie from a previous visit that did not use the high anonymity proxy server. Clearing cookies, and possibly the cache, would solve this problem.
Web proxy
A web proxy passes along http protocol requests like any other proxy server. However, the web proxy accepts target URLs within a user's browser window, processes the request, and then displays the contents of the requested URL immediately back within the users browser. This is generally quite different than a corporate intranet proxy which some people mistakenly refer to as a web proxy.Suffix proxy
A suffix proxy allows a user to access web content by appending the name of the proxy server to the URL of the requested content (e.g. "en.wikipedia.org.SuffixProxy.com"). Suffix proxy servers are easier to use than regular proxy servers. But do not offer anonymity and the primary use is bypassing web filters; however, this is rarely used due to more advanced web filters.Transparent proxy
Also known as an intercepting proxy or forced proxy, a transparent proxy intercepts normal communication without requiring any special client configuration. Clients need not be aware of the existence of the proxy. A transparent proxy is normally located between the client and the Internet, with the proxy performing some of the functions of a gateway or router.RFC 2616 (Hypertext Transfer Protocol—HTTP/1.1) offers standard definitions:
- "A 'transparent proxy' is a proxy that does not modify the request or response beyond what is required for proxy authentication and identification".
- "A 'non-transparent proxy' is a proxy that modifies the request or response in order to provide some added service to the user agent, such as group annotation services, media type transformation, protocol reduction, or anonymity filtering".
In 2009 security a flaw in the way that transparent proxies operate was published by Robert Auger and an advisory issued by the Computer Emergency Response Team listing dozens of affected transparent, and intercepting proxy servers.
Purpose
Intercepting proxies are commonly used in businesses to prevent avoidance of acceptable use policy, and to ease administrative burden, since no client browser configuration is required. This second reason however is mitigated by features such as Active Directory group policy, or DHCP and automatic proxy detection.Intercepting proxies are also commonly used by ISPs in some countries to save upstream bandwidth and improve customer response times by caching. This is more common in countries where bandwidth is more limited (e.g. island nations) or must be paid for.
Issues
The diversion / interception of a TCP connection creates several issues. Firstly the original destination IP and port must somehow be communicated to the proxy. This is not always possible (e.g. where the gateway and proxy reside on different hosts). There is a class of cross site attacksCross-site scripting
Cross-site scripting is a type of computer security vulnerability typically found in Web applications that enables attackers to inject client-side script into Web pages viewed by other users. A cross-site scripting vulnerability may be used by attackers to bypass access controls such as the same...
which depend on certain behaviour of intercepting proxies that do not check or have access to information about the original (intercepted) destination. This problem can be resolved by using an integrated packet-level and application level appliance or software which is then able to communicate this information between the packet handler and the proxy.
Intercepting also creates problems for HTTP authentication, especially connection-oriented authentication such as NTLM
NTLM
In a Windows network, NTLM is a suite of Microsoft security protocols that provides authentication, integrity, and confidentiality to users....
, since the client browser believes it is talking to a server rather than a proxy. This can cause problems where an intercepting proxy requires authentication, then the user connects to a site which also requires authentication.
Finally intercepting connections can cause problems for HTTP caches, since some requests and responses become uncacheble by a shared cache.
Therefore intercepting connections is generally discouraged. However due to the simplicity of deploying such systems, they are in widespread use.
Implementation methods
Interception can be performed using Cisco's WCCPWeb Cache Communication Protocol
Web Cache Communication Protocol is a Cisco-developed content-routing protocol that provides a mechanism to redirect traffic flows in real-time. It has built-in load balancing, scaling, fault tolerance, and service-assurance mechanisms...
(Web Cache Control Protocol). This proprietary protocol resides on the router and is configured from the cache, allowing the cache to determine what ports and traffic is sent to it via transparent redirection from the router. This redirection can occur in one of two ways: GRE Tunneling (OSI Layer 3) or MAC rewrites (OSI Layer 2).
Once traffic reaches the proxy machine itself interception is commonly performed with NAT (Network Address Translation). Such setups are invisible to the client browser, but leave the proxy visible to the web server and other devices on the internet side of the proxy. Recent Linux and some BSD releases provide TPROXY (transparent proxy) which performs IP-level (OSI Layer 3) transparent interception and spoofing of outbound traffic, hiding the proxy IP address from other network devices.
Detection
There are several methods that can often be used to detect the presence of an intercepting proxy server:- By comparing the client's external IP address to the address seen by an external web server, or sometimes by examining the HTTP headers received by a server. A number of sites have been created to address this issue, by reporting the user's IP address as seen by the site back to the user in a web page.http://www.dslreports.com/ip
- By comparing the sequence of network hops reported by a tool such as tracerouteTraceroutetraceroute is a computer network diagnostic tool for displaying the route and measuring transit delays of packets across an Internet Protocol network. Traceroute is available on most operating systems....
for a proxied protocol such as http (port 80) with that for a non proxied protocol such as SMTP (port 25). http://tracetcp.sourceforge.net/usage_proxy.html,http://svn.haxx.se/dev/archive-2003-02/0257.shtml - By attempting to make a connection to an IP address at which there is known to be no server. The proxy will accept the connection and then attempt to proxy it on. When the proxy finds no server to accept the connection it may return an error message or simply close the connection to the client. This difference in behaviour is simple to detect. For example most web browsers will generate a browser created error page in the case where they cannot connect to an HTTP server but will return a different error in the case where the connection is accepted and then closed.
- By serving the end-user specially programmed flash files that send HTTP calls back to their server.
Tor onion proxy software
Tor (anonymity network)
Tor is a system intended to enable online anonymity. Tor client software routes Internet traffic through a worldwide volunteer network of servers in order to conceal a user's location or usage from someone conducting network surveillance or traffic analysis...
('Tor' for short) is a system aiming at online anonymity. Tor is an implementation of onion routing
Onion routing
Onion routing is a technique for anonymous communication over a computer network. Messages are repeatedly encrypted and then sent through several network nodes called onion routers. Like someone unpeeling an onion, each onion router removes a layer of encryption to uncover routing instructions, and...
. It works by relaying communications through a network of systems run by volunteers in various locations. By keeping some of the network entry points hidden, Tor is also able to evade internet censorship
Internet censorship
Internet censorship is the control or suppression of the publishing of, or access to information on the Internet. It may be carried out by governments or by private organizations either at the behest of government or on their own initiative...
. Tor is intended to protect users' personal freedom, privacy, and ability to conduct confidential business.
Users of a Tor network run an onion proxy software on their computer. The Tor software periodically negotiates a virtual circuit
Virtual circuit
In telecommunications and computer networks, a virtual circuit , synonymous with virtual connection and virtual channel, is a connection oriented communication service that is delivered by means of packet mode communication...
through the Tor network. At the same time, the onion proxy software presents a SOCKS
SOCKS
SOCKS is an Internet protocol that routes network packets between a client and server through a proxy server. SOCKS5 additionally provides authentication so only authorized users may access a server...
interface to its clients or users. SOCKS-ifying applications like Polipo
Polipo
Polipo is a fast and lightweight, forwarding and caching proxy server and computer software daemon.By virtue of being a compliant HTTP 1.1 proxy, Polipo has all the uses of traditional Web proxies. It features HTTP 1.1, IPv4 & IPv6, traffic filtering and privacy-enhancement. Polipo supports HTTP...
may be linked with the Tor onion proxy software, which then multiplexes
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
the traffic through a Tor virtual circuit.
Tor is referred to as Onion Routing because as the packets are passed between each node, an additional layer of encryption is 'peeled back' like an onion. And by the time the packet reaches it's destination, all of the layers of encryption have been removed.
The software is open source
Free software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...
and the network is free of charge to use.
Vidalia
Vidalia project
Vidalia is a cross-platform controller GUI for Tor, built using the Qt. It allows the user to start, stop, and view the status of Tor; monitor bandwidth usage; view, filter, and search log messages; and configure some aspects of Tor...
is a cross-platform controller GUI
Gui
Gui or guee is a generic term to refer to grilled dishes in Korean cuisine. These most commonly have meat or fish as their primary ingredient, but may in some cases also comprise grilled vegetables or other vegetarian ingredients. The term derives from the verb, "gupda" in Korean, which literally...
for Tor
Tor (anonymity network)
Tor is a system intended to enable online anonymity. Tor client software routes Internet traffic through a worldwide volunteer network of servers in order to conceal a user's location or usage from someone conducting network surveillance or traffic analysis...
.
I2P anonymous proxy
The I2P anonymous networkI2P
I2P is a mixed-license, free and open source project building an anonymous network .The network is a simple layer that applications can use to anonymously and securely send...
('I2P') is a proxy network aiming at online anonymity. It implements garlic routing
Garlic routing
Garlic routing is a variant of onion routing that encrypts multiple messages together to make it more difficult for attackers to perform traffic analysis....
, which is an enhancement of Tor
Tor (anonymity network)
Tor is a system intended to enable online anonymity. Tor client software routes Internet traffic through a worldwide volunteer network of servers in order to conceal a user's location or usage from someone conducting network surveillance or traffic analysis...
's onion routing
Onion routing
Onion routing is a technique for anonymous communication over a computer network. Messages are repeatedly encrypted and then sent through several network nodes called onion routers. Like someone unpeeling an onion, each onion router removes a layer of encryption to uncover routing instructions, and...
. I2P is fully distributed and works by encrypting all communications in various layers and relaying them through a network of routers run by volunteers in various locations. By keeping the source of the information hidden, I2P offers censorship resistance. The goals of I2P are to protect users' personal freedom, privacy, and ability to conduct confidential business.
Each user of I2P runs an I2P router on their computer (node). The I2P router takes care of finding other peers and building anonymizing tunnels through them. I2P provides proxies for all protocols (HTTP, irc, SOCKS, ...).
The software is free and open-source
Free software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...
, and the network is free of charge to use.
Overview & Discussions
- Web acceleratorWeb acceleratorA web accelerator is a proxy server that reduces web site access times. They can be a self-contained hardware appliance or installable software....
which discusses host-based HTTP acceleration - Transparent SMTP proxyTransparent SMTP proxySMTP proxies are specialized Mail Transfer Agents that, similar to other types of proxy servers, pass SMTP sessions through to other MTAs without using the store-and-forward approach of a typical MTA. When an SMTP proxy receives a connection, it initiates another SMTP session to a destination MTA...
- Reverse proxyReverse proxyIn computer networks, a reverse proxy is a type of proxy server that retrieves resources on behalf of a client from one or more servers. These resources are then returned to the client as though it originated from the reverse proxy itself...
which discusses origin-side proxies - Comparison of web serversComparison of web servers-Overview:-Features:- Operating system support :...
- Comparison of lightweight web serversComparison of lightweight web serversLightweight web servers are web servers which have been designed to run with very small resource overhead because of hardware, environment, or simply for the challenge of it....
Proxy-Servers
- Apache HTTP ServerApache HTTP ServerThe Apache HTTP Server, commonly referred to as Apache , is web server software notable for playing a key role in the initial growth of the World Wide Web. In 2009 it became the first web server software to surpass the 100 million website milestone...
- Apache Traffic ServerTraffic ServerThe Apache Traffic Server is a modular, high-performance reverse proxy and forward proxy server, generally comparable to Nginx and Squid. It was created by Inktomi, and distributed as a commercial product called the Inktomi Traffic Server, before Inktomi was acquired by Yahoo!...
- high-performance open-source HTTP proxy server. - DeleGateDeleGate (networking)DeleGate is a lightweight proxy server. It features support for HTTP, FTP, NNTP, SMTP, POP, IMAP, LDAP, Telnet, SOCKS, DNS, SFTP/SSH, and many more protocols. SSL/TLS and IPv6 are also supported. DeleGate has the ability to convert IPv4 to IPv6 and vice versa.The sources and binaries for...
- a multi-purpose application level gateway and proxy server supporting many protocols. - lighttpdLighttpdlighttpd is an open-source web server more optimized for speed-critical environments than common products while remaining standards-compliant, secure and flexible...
- open-source web server, optimized for speed-critical environments. - Microsoft Forefront Threat Management Gateway, (ISA), forward and reverse caching proxy and firewall.
- NginxNginxnginx is a Web server and a reverse proxy server for HTTP, SMTP, POP3 and IMAP protocols, with a strong focus on high concurrency, performance and low memory usage. It is licensed under a BSD-like license and it runs on Unix, Linux, BSD variants, Mac OS X, Solaris, and Microsoft Windows.- Overview...
- lightweight, high-performance web server, reverse proxy and e-mail proxy (IMAP/POP3). - PolipoPolipoPolipo is a fast and lightweight, forwarding and caching proxy server and computer software daemon.By virtue of being a compliant HTTP 1.1 proxy, Polipo has all the uses of traditional Web proxies. It features HTTP 1.1, IPv4 & IPv6, traffic filtering and privacy-enhancement. Polipo supports HTTP...
- lightweight pipelining, multiplexing, forwarding and caching proxy, SOCKS proxy and daemon. - Pound reverse proxyPound (networking)Pound is a lightweight open source reverse proxy program and application firewall suitable to be used as a web server load balancing solution. Developed by an IT security company, it has a strong emphasis on security. The original intent on developing Pound was to allow distributing the load among...
. - PrivoxyPrivoxyPrivoxy is a non-caching web proxy with filtering capabilities for enhancing privacy, modifying web page data and HTTP headers before the page is rendered by the browser. Privoxy is a "privacy enhancing proxy", filtering Web pages and removing advertisements...
- privacy enhancing proxy. - Squid cacheSquid cacheSquid is a proxy server and web cache daemon. It has a wide variety of uses, from speeding up a web server by caching repeated requests; to caching web, DNS and other computer network lookups for a group of people sharing network resources; to aiding security by filtering traffic...
- a proxy server and web cache daemon. - TinyproxyTinyproxyTinyproxy is a HTTP proxy server daemon for POSIX operating systems. Designed to be fast and small, it is useful when an HTTP/HTTPS proxy is required, but the system resources for a larger proxy are unavailable. Because of this reason, it has been put to uses such as a tether on the iPhone, and on...
- a fast and small HTTP proxy server daemon, which supports reverse proxying and transparent proxying. - TriangleBoyTriangleBoyTriangleBoy is a proxying tool designed to allow users to get around firewalls and censorship, and anonymously visit web sites. The tool was created by Stephen Hsu, founder of SafeWeb, which later stopped support and distribution of the tool...
- an anonymizing proxy service. - Varnish - a performance-focused open source reverse proxy.
- WinGateWingate-Places:In New Zealand:* Wingate, New Zealand, A suburb of Lower HuttIn the United Kingdom:* Wingate, County Durham* Old Wingate, County Durham* Wingates, Bolton, Greater ManchesterIn the United States:* Wingate, Indiana...
- multi-protocol forward/reverse/caching/intercepting proxy and packet firewall / NAT for Windows platforms. - ZiproxyZiproxyZiproxy is a forwarding, non-caching, HTTP proxy targeted for traffic optimization.The ziproxy software is regarded as lightweight in terms of memory and processing power consumption....
- lightweight forwarding, non-caching, HTTP proxy for traffic optimization. - SuffixProxy - open-source HTTP-Suffix proxy server. Easy to use.
- Glype - A free php based web proxy script. Easy to setup a proxy on a webserver quickly.
- GeoEdge - A professional proxy server provider with 102 locations worldwide.
Proxifiers
There are client programs that "SOCKS-ify", which allows adaptation of any networked software to connect to external networks via certain types of proxy servers (mostly SOCKS).- Comparison of proxifiersComparison of proxifiersA proxifier is a software program which can make other programs passing through a proxy server by intercepting and modifying their socket library calls...
Diverse Topics
- Application layer firewallApplication layer firewallAn application firewall is a form of firewall which controls input, output, and/or access from, to, or by an application or service. It operates by monitoring and potentially blocking the input, output, or system service calls which do not meet the configured policy of the firewall...
- Captive portalCaptive portalThe captive portal technique forces an HTTP client on a network to see a special web page before using the Internet normally. A captive portal turns a Web browser into an authentication device. This is done by intercepting all packets, regardless of address or port, until the user opens a browser...
- Distributed Checksum ClearinghouseDistributed Checksum ClearinghouseDistributed Checksum Clearinghouse is a hash sharing method of spam email detection.The basic logic in DCC is that most spam mails are sent to many recipients. The same message body appearing many times is therefore bulk email. DCC identifies bulk email by taking a checksum and sending that...
- Internet privacyInternet privacyInternet privacy involves the right or mandate of personal privacy concerning the storing, repurposing, providing to third-parties, and displaying of information pertaining to oneself via the Internet. Privacy can entail both Personally Identifying Information or non-PII information such as a...
- Proxy listProxy listGenerally spoken, a proxy list is a list of open HTTP/HTTPS/SOCKS proxy servers all on one website. Proxies allow users to make indirect network connections to other computer network services. Proxy lists include the IP addresses of computers hosting open proxy servers, meaning that these proxy...