Communication between two computers (shown in grey) connected through a third computer (shown in red) acting as a proxy. Bob does not know to whom the information is going, which is why proxies can be used to protect privacy.
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- Free Internet Proxy Server Software
In computer networks, 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.[1] 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 and the proxy server evaluates the request as a way to simplify and control its complexity.[2] Proxies were invented to add structure and encapsulation to distributed systems.[3]
- 1Types of proxy servers
- 2Uses
- 2.1Monitoring and filtering
- 2.5Accessing services anonymously
- 2.6Security
- 2.7Malicious usages
- 3Implementations of proxies
- 3.3Transparent proxy
- 4See also
Types of proxy servers[edit]
A proxy server may reside on the user's local computer, or at any point between the user's computer and destination servers on the Internet.
- A proxy server that passes unmodified requests and responses is usually called a gateway or sometimes a tunneling proxy.
- A forward proxy is an Internet-facing proxy used to retrieve data from a wide range of sources (in most cases anywhere on the Internet).
- A reverse proxy is usually an internal-facing proxy used as a front-end to control and protect access to a server on a private network. A reverse proxy commonly also performs tasks such as load-balancing, authentication, decryption or caching.
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Open proxies[edit]
An open proxy forwarding requests from and to anywhere on the Internet.
An open proxy is a forwarding proxy server that is accessible by any Internet user. As of 2008, Gordon Lyon estimates there are 'hundreds of thousands' of open proxies on the Internet.[4] An anonymous open proxy allows users to conceal their IP address 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.
- Anonymous Proxy – Thіs server reveаls іts іdentіty as а server but does not dіsclose the іnіtіаl IP аddress. Though thіs server cаn be dіscovered easіly, іt cаn be benefіcіаl for some users as іt hіdes the Internet Protocol аddress.
- Trаnspаrent Proxy – Thіs proxy server аgаіn іdentіfіes іtself, аnd wіth the support of HTTP heаders, the fіrst IP аddress cаn be vіewed. The mаіn benefіt of usіng thіs sort of server іs іts аbіlіty to cаche the websіte.
Reverse proxies[edit]
A reverse proxy taking requests from the Internet and forwarding them to servers in an internal network. Those making requests connect to the proxy and may not be aware of the internal network.
A reverse proxy (or surrogate) is a proxy server that appears to clients to be an ordinary server. Reverse proxies forward requests to one or more ordinary servers which handle the request. The response from the proxy server is returned as if it came directly from the original server, leaving the client with no knowledge of the original server.[5] 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 Secure Sockets Layer (SSL) encryption is often not done by the web server itself, but by a reverse proxy that is equipped with SSL acceleration hardware. 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.509 certificates.
- Load balancing: 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 defence and can protect against some OS and Web Server specific attacks. However, it does not provide any protection from 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 firewall 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.
Uses[edit]
Monitoring and filtering[edit]
Content-control software[edit]
A content-filtering web proxy server provides administrative control over the content that may be relayed in one or both directions 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.
Content filtering proxy servers 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 workplaces, schools and colleges restrict the web sites and online services that are accessible and available in their buildings. Governments also censor undesirable content. 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.
Websites commonly used by students to circumvent filters and access blocked content often include a proxy, from which the user can then access the websites that the filter is trying to block.
Requests may be filtered by several methods, such as a 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.[citation needed] Blacklists are often provided and maintained by web-filtering companies, often grouped into categories (pornography, gambling, shopping, social networks, etc.).
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 may be returned to the requester.
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.
Some proxies scan outbound content, e.g., for data loss prevention; or scan content for malicious software.
Filtering of encrypted data[edit]
Web filtering proxies are not able to peer inside secure sockets HTTP transactions, assuming the chain-of-trust of SSL/TLS (Transport Layer Security) has not been tampered with.
The SSL/TLS chain-of-trust relies 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. Consequently, a root certificate generated by the proxy is installed into the browser CA list by IT staff.
In such situations, 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.
Bypassing filters and censorship[edit]
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 IP-based geolocation to restrict its service to a certain country can be accessed using a proxy located in that country to access the service.[citation needed]
Web proxies are the most common means of bypassing government censorship, although no more than 3% of Internet users use any circumvention tools.[6]
There are a few main Proxy services providers who allow businesses access to their proxy network, in order to reroute traffic via a proxy server or residential IP network for business intelligence purposes.[citation needed]
In some cases, users can circumvent proxies which filter using blacklists using services designed to proxy information from a non-blacklisted location.[7]
Many schools block access to popular websites such as Facebook. Students can use proxy servers to circumvent this security. However, by connecting to proxy servers, they might be opening themselves up to danger by passing sensitive information such as personal photos and passwords through the proxy server. Some content filters block proxy servers in order to keep users from using them to bypass the filter.
Logging and eavesdropping[edit]
Proxies can be installed in order to eavesdrop upon the. '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'.
TCP Intercept is a traffic filtering security feature that protects TCP servers from TCP SYN flood attacks, which are a type of denial-of-service attack. TCP Intercept is available for IP traffic only.
In 2009 a security flaw in the way that transparent proxies operate was published by Robert Auger,[18] and the Computer Emergency Response Team issued an advisory listing dozens of affected transparent and intercepting proxy servers.[19]
Purpose[edit]
Intercepting proxies are commonly used in businesses to enforce acceptable use policy, and to ease administrative overheads, 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[edit]
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 attacks that depend on certain behaviour of intercepting proxies that do not check or have access to information about the original (intercepted) destination. This problem may 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.
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Intercepting also creates problems for HTTP authentication, especially connection-oriented authentication such as NTLM, as 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, as some requests and responses become uncacheable by a shared cache.
Implementation methods[edit]
In integrated firewall / proxy servers where the router/firewall is on the same host as the proxy, communicating original destination information can be done by any method, for example Microsoft TMG or WinGate.
Interception can also be performed using Cisco's WCCP (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[edit]
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. Google also returns the IP address as seen by the page if the user searches for 'IP'.
- By comparing the result of online IP checkers when accessed using https vs http, as most intercepting proxies do not intercept SSL. If there is suspicion of SSL being intercepted, one can examine the certificate associated with any secure web site, the root certificate should indicate whether it was issued for the purpose of intercepting.
- By comparing the sequence of network hops reported by a tool such as traceroute for a proxied protocol such as http (port 80) with that for a non proxied protocol such as SMTP (port 25).[20]
- 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.[21]
- By serving the end-user specially programmed Adobe Flash SWF applications or Sun Java applets that send HTTP calls back to their server.
CGI proxy[edit]
A CGI web proxy accepts target URLs using a Web form in the user's browser window, processes the request, and returns the results to the user's browser. Consequently, it can be used on a device or network that does not allow 'true' proxy settings to be changed. The first recorded CGI proxy, named 'rover' at the time but renamed in 1998 to 'CGIProxy'[22], was developed by American computer scientist James Marshall in early 1996 for an article in 'Unix Review' by Rich Morin.[23]
The majority of CGI proxies are powered by one of CGIProxy (written in the Perl language), Glype (written in the PHP language), or PHProxy (written in the PHP language). As of April 2016, CGIProxy has received about 2 million downloads, Glype has received almost a million downloads,[24] whilst PHProxy still receives hundreds of downloads per week.[25] Despite waning in popularity [26] due to VPNs and other privacy methods, there are still several thousand CGI proxies online.[27]
Some CGI proxies were set up for purposes such as making websites more accessible to disabled people, but have since been shut down due to excessive traffic, usually caused by a third party advertising the service as a means to bypass local filtering. Since many of these users don't care about the collateral damage they are causing, it became necessary for organizations to hide their proxies, disclosing the URLs only to those who take the trouble to contact the organization and demonstrate a genuine need.[28]
Suffix proxy[edit]
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 they do not offer high levels of anonymity and their primary use is for bypassing web filters. However, this is rarely used due to more advanced web filters.
Tor onion proxy software[edit]
The Vidalia Tor-network map.
Tor (short for The Onion Router) is a system intended to enable online anonymity.[29] 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. Using Tor makes it more difficult to trace Internet activity, including 'visits to Web sites, online posts, instant messages and other communication forms', back to the user.[29] It is intended to protect users' personal freedom, privacy, and ability to conduct confidential business by keeping their internet activities from being monitored.
'Onion routing' refers to the layered nature of the encryption service: The original data are encrypted and re-encrypted multiple times, then sent through successive Tor relays, each one of which decrypts a 'layer' of encryption before passing the data on to the next relay and ultimately the destination. This reduces the possibility of the original data being unscrambled or understood in transit.[30]
The Tor client is free software, and there are no additional charges to use the network.
I2P anonymous proxy[edit]
The I2P anonymous network ('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.
Proxy vs. NAT[edit]
'Proxy' refers to a layer-7 application on the OSI reference model. Network Address Translation (NAT) is similar to a proxy, but operates in layer-3.
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In client configuration of layer-3 NAT, configuring the gateway is sufficient. However, for client configuration of a layer-7 proxy, the destination of the packets that the client generates must always be the proxy server (layer-7), then the proxy server reads each packet and finds out the true destination.
Because NAT operates at layer-3, it is less resource-intensive than the layer-7 proxy, but also less flexible. As we compare these two technologies, we might encounter a terminology known as 'transparent firewall'. Transparent firewall means that the proxy uses the layer-7 proxy advantages without the knowledge of the client. The client presumes that the gateway is a NAT in layer-3, and it does not have any idea about the inside of the packet, but through this method the layer-3 packets are sent to the layer-7 proxy for investigation.
DNS proxy[edit]
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.
Proxifiers[edit]
There are client programs that 'SOCKS-ify',[31] which allows adaptation of any networked software to connect to external networks via certain types of proxy servers (mostly SOCKS).
See also[edit]
Overview and discussions[edit]
- Web accelerator which discusses host-based HTTP acceleration
Diverse topics[edit]
- SOCKS an alternative firewall traversal protocol supported by many applications
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References[edit]
- ^World-Wide Web Proxies, Ari Luotonen, April 1994
- ^'A Survey of Techniques for Improving Efficiency of Mobile Web Browsing', Concurrency and Computation: Practice and Experience, 2018
- ^[1], Marc Shapiro. Structure and Encapsulation in Distributed Systems: the Proxy Principle. Int. Conf. on Distr. Comp. Sys. (ICDCS), 1986, Cambridge, MA, USA, United States. pp.198--204, 1986, Int. Conf. on Distr. Comp. Sys. (ICDCS).
- ^ abLyon, Gordon (2008). Nmap network scanning. US: Insecure. p. 270. ISBN978-0-9799587-1-7.
- ^ ab'Forward and Reverse Proxies'. httpd mod_proxy. Apache. Retrieved 20 December 2010.
- ^'2010 Circumvention Tool Usage Report'(PDF). The Berkman Center for Internet & Society at Harvard University. October 2010.
- ^'Using a Ninjaproxy to get through a filtered proxy'. advanced filtering mechanics. TSNP. Archived from the original on 9 March 2016. Retrieved 17 September 2011.
- ^Thomas, Keir (2006). Beginning Ubuntu Linux: From Novice to Professional. Apress. ISBN978-1-59059-627-2.
A proxy server helps speed up Internet access by storing frequently accessed pages
- ^I. Cooper; J. Dilley (June 2001). Known HTTP Proxy/Caching Problems. IETF. doi:10.17487/RFC3143. RFC 3143. Retrieved 17 May 2019.
- ^'Layering'. Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations. IETF. June 2001. p. 4. sec. 2.1. doi:10.17487/RFC3135. RFC 3135. Retrieved 21 February 2014.
- ^Durieux, T.; Hamadi, Y.; Monperrus, M. (2018). Fully Automated HTML and Javascript Rewriting for Constructing a Self-Healing Web Proxy. 2018 IEEE 29th International Symposium on Software Reliability Engineering (ISSRE). pp. 1–12. doi:10.1109/ISSRE.2018.00012. ISBN978-1-5386-8321-7.
- ^Zhang, Xiaoyi; Ross, Anne Spencer; Caspi, Anat; Fogarty, James; Wobbrock, Jacob O. (2017). Interaction Proxies for Runtime Repair and Enhancement of Mobile Application Accessibility. Proceedings of the 2017 CHI Conference on Human Factors in Computing Systems. pp. 6024–6037. doi:10.1145/3025453.3025846. ISBN9781450346559.
- ^'Hot Tactics For Geo-Targeted Ads On Google & Bing'. Retrieved 7 February 2014.
- ^'Firewall and Proxy Server HOWTO'. tldp.org. Retrieved 4 September 2011.
The proxy server is, above all, a security device.
- ^'Sneaker Bot Supreme Proxy'. GeoSurf. Retrieved 24 September 2017.
- ^'absolute-form'. HTTP/1.1 Message Syntax and Routing. IETF. June 2014. p. 41. sec. 5.3.2. doi:10.17487/RFC7230. RFC 7230. Retrieved 4 November 2017.
a client MUST send the target URI in absolute-form as the request-target
- ^'Transparent Proxy Definition'. ukproxyserver.org. 1 February 2011. Archived from the original on 1 March 2013. Retrieved 14 February 2013.
- ^'Socket Capable Browser Plugins Result In Transparent Proxy Abuse'. The Security Practice. 9 March 2009. Retrieved 14 August 2010.
- ^'Vulnerability Note VU#435052'. US CERT. 23 February 2009. Retrieved 14 August 2010.
- ^'Subversion Dev: Transparent Proxy detection (was Re: Introduction_'. Tracetop.sourceforge.net. Retrieved 16 November 2014.
- ^Wessels, Duane (2004). Squid The Definitive Guide. O'Reilly. p. 130. ISBN978-0-596-00162-9.
- ^Marshall, James. 'CGIProxy'. Retrieved 12 November 2018.
- ^'The Limits of Control'. June 1996. Retrieved 12 November 2018.
- ^'Glype® Proxy Script'. glype.com. Archived from the original on 3 January 2013. Retrieved 17 May 2019.
- ^'PHProxy'.
- ^'Google Trends'.
- ^'Proxy Stats :: Get Proxi.es'.
- ^Estrada-Jiménez, José (March 2017). 'Online advertising: Analysis of privacy threats and protection approaches'. Computer Communications. 100: 32–51. doi:10.1016/j.comcom.2016.12.016.
- ^ abGlater, Jonathan (25 January 2006). 'Privacy for People Who Don't Show Their Navels'. The New York Times. Retrieved 4 August 2011.
- ^The Tor Project. 'Tor: anonymity online'. Retrieved 9 January 2011.
- ^Zwicky, Elizabeth D.; Cooper, Simon; Chapman, D. Brent (2000). Building Internet Firewalls (2nd ed.). p. 235. ISBN978-1-56592-871-8.
External links[edit]
- Proxy software and scripts at Curlie
- Free web-based proxy services at Curlie
- Free http proxy servers at Curlie
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