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Proxy server - Wikipedia

Proxy server – Wikipedia

For Wikipedia’s policy on editing from open proxies, please see Wikipedia:Open proxies. For other uses, see Proxy.
Communication between two computers (shown in grey) connected through a third computer (shown in red) which acts as a proxy server. Bob does not know to whom the information is going, which is the reason that proxies can be used to protect privacy.
In computer networking, a proxy server is a server application that acts as an intermediary between a client requesting a resource and the server providing that resource. [1]
Instead of connecting directly to a server that can fulfill a requested resource, such as a file or web page, the client directs the request to the proxy server, which evaluates the request and performs the required network transactions. This serves as a method to simplify or control the complexity of the request, or provide additional benefits such as load balancing, privacy, or security. Proxies were devised to add structure and encapsulation to distributed systems. [2] A proxy server thus functions on behalf of the client when requesting service, potentially masking the true origin of the request to the resource server.
Types[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 and caching. [3]
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. In 2008, network security expert Gordon Lyon estimates that “hundreds of thousands” of open proxies are operated on the Internet. [4]
Anonymous proxy – This server reveals its identity as a proxy server but does not disclose the originating IP address of the client. Although this type of server can be discovered easily, it can be beneficial for some users as it hides the originating IP address.
Transparent proxy – This server not only identifies itself as a proxy server but with the support of HTTP header fields such as X-Forwarded-For, the originating IP address can be retrieved as well. The main benefit of using this type of server is its ability to cache a website for faster retrieval.
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 that 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 websites 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 defense 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 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. 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 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, devices may be configured to trust a root certificate whose private key is known to the proxy. In such situations, proxy analysis of the contents of an 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. [6]: 3 
Web proxies are the most common means of bypassing government censorship, although no more than 3% of Internet users use any circumvention tools. [6]: 7 
Some proxy service providers allow businesses access to their proxy network for rerouting traffic for business intelligence purposes. [7]
In some cases, users can circumvent proxies which filter using blacklists using services designed to proxy information from a non-blacklisted location. [8]
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 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 the 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 privacy.
Improving performance[edit]
A caching proxy server accelerates service requests by retrieving the 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. Web proxies are commonly used to cache web pages from a web server. [9] Poorly implemented caching proxies can cause problems, such as an inability to use user authentication. [10]
A proxy that is designed to mitigate specific link related issues or degradation is a Performance Enhancing Proxy (PEPs). These are typically used to improve TCP performance in the presence of high round-trip times or high packet loss (such as wireless or mobile phone networks); or highly asymmetric links featuring very different upload and download rates. PEPs can make more efficient use of the network, for example, by merging TCP ACKs (acknowledgements) or compressing data sent at the application layer. [11]
Translation[edit]
A translation proxy is a proxy server that is used to localize a website experience for different markets. Traffic from the global audience is routed through the translation proxy to the source website. As visitors browse the proxied site, requests go back to the source site where pages are rendered. The original language content in the response is replaced by the translated content as it passes back through the proxy. The translations used in a translation proxy can be either machine translation, human translation, or a combination of machine and human translation. Different translation proxy implementations have different capabilities. Some allow further customization of the source site for the local audiences such as excluding the source content or substituting the source content with the original local content.
Repairing errors[edit]
A proxy can be used to automatically repair errors in the proxied content. For instance, the BikiniProxy system instruments JavaScript code on the fly in order to detect and automatically repair errors happening in the browser. [12] Another kind of repair that can be done by a proxy is to fix accessibility issues. [13]
Accessing services anonymously[edit]
An anonymous proxy server (sometimes called a web proxy) generally attempts to anonymize web surfing. Anonymizers may be differentiated into several varieties. 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. The requests are not anonymous to the anonymizing proxy server, however, and so a degree of trust is present between the proxy server and the user. Many proxy servers 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, make it appear that the proxy server is the client. A website could still suspect a proxy is being used if the client sends packets that 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.
QA geotargeted advertising[edit]
Advertisers use proxy servers for validating, checking and quality assurance of geotargeted ads. A geotargeting ad server checks the request source IP address and uses a geo-IP database to determine the geographic source of requests. [14] Using a proxy server that is physically located inside a specific country or a city gives advertisers the ability to test geotargeted ads.
Security[edit]
A proxy can keep the internal network structure of a company secret by using network address translation, which can help the security of the internal network. [15] This makes requests from machines and users on the local network anonymous. Proxies can also be combined with firewalls.
An incorrectly configured proxy can provide access to a network otherwise isolated from the Internet. [4]
Cross-domain resources[edit]
Proxies allow web sites 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. Proxies also allow the browser to make web requests to externally hosted content on behalf of a website when cross-domain restrictions (in place to protect websites from the likes of data theft) prohibit the browser from directly accessing the outside domains.
Malicious usages[edit]
Secondary market brokers[edit]
Secondary market brokers use web proxy servers to buy large stocks of limited products such as limited sneakers[16] or tickets.
Implementations of proxies[edit]
Web proxy servers[edit]
Web proxies forward HTTP requests. The request from the client is the same as a regular HTTP request except the full URL is passed, instead of just the path. [17]
GET HTTP/1. 1
Proxy-Authorization: Basic encoded-credentials
Accept: text/html
This request is sent to the proxy server, the proxy makes the request specified and returns the response.
HTTP/1. 1 200 OK
Content-Type: text/html; charset UTF-8
Some web proxies allow the HTTP CONNECT method to set up forwarding of arbitrary data through the connection; a common policy is to only forward port 443 to allow HTTPS traffic.
Examples of web proxy servers include Apache (with mod_proxy or Traffic Server), HAProxy, IIS configured as proxy (e. g., with Application Request Routing), Nginx, Privoxy, Squid, Varnish (reverse proxy only), WinGate, Ziproxy, Tinyproxy, RabbIT and Polipo.
For clients, the problem of complex or multiple proxy-servers is solved by a client-server Proxy auto-config protocol (PAC file).
SOCKS proxy[edit]
SOCKS also forwards arbitrary data after a connection phase, and is similar to HTTP CONNECT in web proxies.
Transparent proxy[edit]
Also known as an intercepting proxy, inline proxy, or forced proxy, a transparent proxy intercepts normal application layer 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. [18]
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”.
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, [19] and the Computer Emergency Response Team issued an advisory listing dozens of affected transparent and intercepting proxy servers. [20]
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. island nations) or must be paid for.
Issues[edit]
The diversion/interception of a TCP connection creates several issues. First, 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 behavior 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.
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 that 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]
Several methods may 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 on 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 (port 80) with that for a non-proxied protocol such as SMTP (port 25). [21]
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 behavior 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. [22]
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”, [23] was developed by American computer scientist James Marshall in early 1996 for an article in “Unix Review” by Rich Morin. [24]
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, [25] whilst PHProxy still receives hundreds of downloads per week. [26] Despite waning in popularity[27] due to VPNs and other privacy methods, as of September 2021 there are still a few hundred CGI proxies online. [28]
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. [29]
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. “”). 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]
Tor is a system intended to provide online anonymity. [30] Tor client software routes Internet traffic through a worldwide volunteer network of servers for concealing a user’s computer location or usage from someone conducting network surveillance or traffic analysis. Using Tor makes tracing Internet activity more difficult, [30] and is intended to protect users’ personal freedom, privacy.
“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. [31]
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,… ).
Comparison to network address translators[edit]
The proxy concept refers to a layer 7 application in the OSI reference model. Network address translation (NAT) is similar to a proxy but operates in layer 3.
In the client configuration of layer-3 NAT, configuring the gateway is sufficient. However, for the 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]
Some client programs “SOCKS-ify” requests, [32] which allows adaptation of any networked software to connect to external networks via certain types of proxy servers (mostly SOCKS).
Residential proxy[edit]
A residential proxy is an intermediary that uses a real IP address provided by an Internet Service Provider (ISP) with physical devices such as mobiles and computers of end-users. Instead of connecting directly to a server, residential proxy users connect to the target through residential IP addresses. The target then identifies them as organic internet users. It does not let any tracking tool identify the reallocation of the user. Any residential proxy can send any number of concurrent requests and IP addresses are directly related to a specific region. [33]
See also[edit]
Darknet
SMTP proxy
Web accelerator which discusses host-based HTTP acceleration
Web cache
Application firewall
Captive portal
Distributed Checksum Clearinghouse
Internet privacy
Proxy list
SOCKS an alternative firewall traversal protocol supported by many applications
References[edit]
^ World-Wide Web Proxies, Ari Luotonen, April 1994
^ [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. (ICDCS).
^ “Proxy servers and tunneling”. MDN Web Docs. Retrieved 6 December 2020.
^ a b Lyon, Gordon (2008). Nmap network scanning. US: Insecure. p. 270. ISBN 978-0-9799587-1-7.
^ “Forward and Reverse Proxies”. d mod_proxy. Apache. Retrieved 20 December 2010.
^ a b “2010 Circumvention Tool Usage Report” (PDF). The Berkman Center for Internet & Society at Harvard University. October 2010.
^ “How to Check if Website is Down or Working Worldwide”. Hostinger. 19 November 2019. Retrieved 14 December 2019.
^ “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. ISBN 978-1-59059-627-2. 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. June 2001. p. 4. sec. 2. 1. 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. arXiv:1803. 08725. 1109/ISSRE. 2018. 00012. ISBN 978-1-5386-8321-7. S2CID 4268784.
^ 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. 1145/3025453. 3025846. ISBN 9781450346559. S2CID 20937177.
^ “Hot Tactics For Geo-Targeted Ads on Google & Bing”. Retrieved 7 February 2014.
^ “Firewall and Proxy Server HOWTO”. 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. June 2014. p. 41. sec. 5. 3. 2. 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”. 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_”. Retrieved 16 November 2014.
^ Wessels, Duane (2004). Squid The Definitive Guide. O’Reilly. pp. 130. ISBN 978-0-596-00162-9.
^ Marshall, James. “CGIProxy”. Retrieved 12 November 2018.
^ “The Limit
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What is a Transparent Proxy | Client vs. Server Side Use Cases | Imperva

What is a Transparent Proxy
A transparent proxy, also known as an inline proxy, intercepting proxy or forced proxy, is a server that intercepts the connection between an end-user or device and the internet. It is called “transparent” because it does so without modifying requests and responses. Squid Transparent Proxy Server is a popular open source transparent proxy tool.
For example, a user on a corporate network may be surfing the Internet. The user requests to view a news article on, and views the same content as they would on their local connection at home.
However, unbeknownst to the user, the news article is delivered not from the origin server, but rather from a transparent proxy running on the corporate network. The user’s experience is exactly the same. However, the user’s employer now has the ability to monitor their behavior, and also restrict access to certain websites.
Example of a transparent proxy deployment
Transparent Proxies and Forced Proxies
Transparent proxies are sometimes known as forced proxies because they can be applied to a user’s connection without any change to their computer’s proxy settings.
As a result, a transparent proxy can be “forced” on a user without their consent or knowledge (although in many cases users are informed about the presence of a proxy). Some websites maintain unofficial transparent proxy lists, to help users become aware they are monitored.
Transparent proxies, by definition, are set up by the operator of a network or a website, and not by the end-user.
Transparent Proxy Settings
When you set up a transparent proxy, some of the common proxy settings are:
Authentication—provides the server with the same credentials as the users behind the proxy
Interception—defines how the proxy should intercept traffic, at the operating system level or at the router level
Caching—defines whether the proxy server should cache content for returning users
Reverse proxy—you can place the proxy in front of a web server to accelerate performance for users (as opposed to setting it to intercept remote access)
Filtering chat, data streaming, torrent threads, etc—configure the transparent proxy not to allow users to access certain protocols or ports
Uses for Transparent Proxy on Client Side
You can deploy a transparent proxy on the client side, meaning that all traffic to and from a client endpoint is intercepted by the proxy. Use cases for client-side transparent proxies include:
Content Filtering
You can use a transparent proxy to filter out unwanted content, defined via proxy settings. For example, when a specific website is requested, the proxy can refrain from forwarding the request to the web server. Instead, it intercepts the connection and displays an error or notice to the user.
Gateway Proxies
You can use a gateway proxy to modify or block network traffic based on rules. For example, a firewall is a transparent proxy, which allows traffic to pass between an internal network and the Internet, but blocks traffic if it violates the firewall’s rule table.
Transparent Caching
If multiple people are accessing the same content from the same location—for example, many students viewing the same news site via their university network—it is more efficient to initially cache the content, and serve it from cache to subsequent users. A transparent proxy can do this for an organization, facility or neighborhood.
Traffic Monitoring
If you operate a network, you can set up a transparent proxy to monitor user traffic and behavior.
Traffic monitoring can also have illegitimate uses—for example, an unscrupulous public wifi operator can monitor user’s connections and steal data and credentials.
Authentication
Public wifi spots and cellular Internet operators sometimes use transparent proxies to force users to authenticate themselves on the network, and agree to terms of service. Only after a user authenticates and agrees, are they allowed to surf.
Users may not realize that even after the initial authentication screen, the entire connection is intercepted and could be monitored by the operator, via the transparent proxy.
Uses for Transparent Proxy on the Server Side
TCP Intercept for DoS Protection
TCP intercept is a type of transparent proxy which you can use to protect a server against a SYN-flood Denial of Service (DoS) attack. It intercepts all traffic to a web server, accepts client requests, and performs a three-way handshake. If successful, it performs a three-way handshake with the server, and joins the two half-connections between client and server.
The TCP intercept watches TCP requests, and waits (typically 30 seconds) for connections to be established. When the number of inactive connections exceeds a certain threshold, the TCP intercept enters “aggressive mode”. In this mode, each new arriving connection causes the oldest inactive connection to be deleted.
This technique is no longer effective against modern, large scale Distributed Denial of Service (DDoS) attacks. Attackers controlling high-powered servers, or millions of zombie computers, can create SYN floods that easily overwhelm a TCP intercept controller.
This is why many organizations are using cloud-based services like Imperva’s DDoS Protection. Cloud-based DDoS services are able to scale up on-demand to handle large scale attacks, and can also protect against other types of DDoS. For example, DDoS services can prevent protocol attacks and application layer attacks, which do not occur at the TCP layer.
Transparent Proxy and CDN for Front-End Optimization
A Content Delivery Network (CDN) is a globally distributed network of proxy servers, which caches and serves content to users near their geographical location.
A CDN, such as Imperva’s Global Content Delivery Network, is a type of transparent proxy operating on the server side, whose purpose is to perform front-end optimization to improve the end-user experience. It intercepts traffic to a web server and instead of letting the user access the origin server directly, it offers the same content from its cache. This results in improved performance for user and reduced system resources required on the server.
Transparent Proxy vs. Non-Transparent Porxy - Network ...

Transparent Proxy vs. Non-Transparent Porxy – Network …

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web proxy service can be configured to operate in either transparent or non-transparent mode – but what are the differences, and how should you choose between them?
In transparent mode, there are no special configuration steps needed to setup client browsers, thus allowing the proxy service to be activated and in-use almost immediately. Once activated, all traffic destined for the Internet arriving on port 80 is automatically redirected through the proxy. With the latest Guardian products you can even use NTLM with Active Directory in conjunction with transparent proxying allowing for single sign on and minimal network configuration.
Both transparent and non-transparent proxying can be used together at the same time. Enabling transparent does not stop non-transparent from working. In situations where transparent is the norm but a specific application requires non-transparent you can simply configure the proxy settings in that application. Both modes have pros and cons. This article explains how to decide on the most appropriate mode for your network.
When to avoid transparent proxying
Transparent mode should be avoided in the following situations:
When you want to filter HTTPS sites – Content filtering cannot be applied to HTTPS traffic because transparent proxying cannot redirect HTTPS. HTTPS pages are encrypted, so content filtering cannot be applied to HTTPS pages – only filtering on the unencrypted URL will work. In general, transparent proxying is not robust enough to guarantee restrictions on web downloads or access to inappropriate sites. What you can do, however, is block all HTTPS through traffic using outgoing firewall rules and then those few that require HTTPS can have their browser settings configured to manual proxy settings.
When using proxy authentication – Proxy authentication cannot be used when operating in transparent mode. This is because the browser does not know that a proxy is being used (i. e. the proxy is transparent) and consequently does not know how to respond to a proxy authentication request.
When using Ident for authentication – Ident with transparent proxying is possible, but all Ident servers must be configured to accept any request, not just one qualified by correct destination IP and ports.
When using web-enabled client applications – Applications that connect to the Internet are often confused by transparent proxying. This can normally only be resolved by configuring the client application with the proxy details.
When you want to use the SSL Login authentication method. – It is not possible to be redirected to the requested website once logged in – this requires the use of the Guardian proxy in non-transparent mode.
When exceptions are required – If a client needs to have direct access to a particular domain without going through the proxy, transparent mode should not be used as such a setup is very difficult to configure and manage.
When you have no local DNS server – If your computers are using transparent proxying and you have no local caching DNS server then all requests will require a DNS lookup to your ISP slowing down browsing. Using manual proxy settings will cause the proxy to do the DNS lookups, which it will cache, and speed up web browsing. An example might be in a basic Web Cafe with no firewall or router that has a caching DNS.
Why use non-transparent proxying?
The main reason to use non-transparent (or manual proxying) is so that the web browser and other client applications know that a proxy is being used, and so can act accordingly. Initial configuration of a non-transparent proxy might be trickier, but ultimately provides a much more powerful and flexible proxying service.
Another advantage of non-transparent proxying is that spyware and worms that use the web for transmission may not be able to function because they don’t know the proxy settings. This can reduce the spread of malicious software and prevent bandwidth from being wasted by infected systems.
Configuring proxy settings in non-transparent mode
When using non-transparent proxying, appropriate proxy settings must be configured on client machines and browsers. This can be achieved in a number of different ways:
Manually – Proxy settings can be entered manually in most web browsers and web-enabled applications. Usually such settings are entered as part of the applications Connection Settings or similar. The address of the proxy is required, along with the proxy port number. These settings are displayed on the “Services / web proxy” and “Guardian / web proxy” pages as part of the “Automatic configuration script” region.
Automatic configuration script – The Smoothwall proxy provides file that can be used to automatically configure proxy settings in most Internet browsers. To use the automatic configuration script, enter the URL displayed in the “Automatic configuration script” region of the “Services / web proxy” and “Guardian / web proxy” pages into your browser software.
Microsoft Windows 2000 domain – In a Windows 2000+ domain, proxy settings can be configured in the domain security policy. This eliminates the need to manually configure any part of the users system.
Automatic discovery – Many browsers support automatic discovery of proxy settings using the WPAD (Web Proxy Auto-Discovery) protocol. This is relatively easy to configure if you have a local DNS server.
Using DHCP to distrubute proxy settings – DHCP can also be used to set proxy settings. That might be a better method than using security policies. Currently the DHCP server on the Smoothwall firewalls cannot be used for giving out locations.
Microsoft Windows login script – The Windows login script can be used to import a registry file which will automatically configure the system wide proxy settings.
files – Browsers like Firefox can be configured automatically with ini files. Such files could be copied or modified as part of the login script on a Microsoft Windows or Linux network.
Third party solutions – Third party applications are available for Windows which can, at login, automatically configure web browser proxy settings. These range from simple programs designed specifically to automate proxy configuration, or more sophisticated applications that provide a range of services such as monitoring the users desktop.
When to use transparent proxying
When minimal or no network configuration is required.
Transparent proxying can be useful in mixed environments containing Unix, Linux, Apple Mac and Microsoft Windows systems. This allows quick access to the web proxy for everyone, without having to configure a multitude of different platform specific applications and browsers.
Transparent mode can be used for convenience if Guardian is being used to provide non-HTTPS filtering. However, if Guardian is being used to guarantee prevention of abuse, non-transparent proxying should be used or instead use transparent and have outgoing HTTPS blocked at the firewall.
Problem
What is the difference between transparent and non-transparent proxies, and how does the firewall handle them differently? Which proxies can the firewall handle non-transparently?
Solution
In a transparent proxy connection, the client sends all requests through its default gateway. The destination IP address in the packet from the client is the actual destination’s IP address (e. g., ’s IP address) and not the firewall. Since the firewall lies along the routing path to the client’s default gateway, or is the client’s default gateway, it is able to inspect the proxy application layer data as specified. After inspecting the data, the firewall passes the packet on. The client is responsible for its own DNS lookups. In a transparent connection, the client is unaware of the firewall.
In a non-transparent proxy connection, the client (e. g., a Web browser) sends all requests to the firewall. The client’s connections settings explicitly specify that all requests be sent to the firewall as a proxy. The destination IP address in the packet from the client is the firewall’s IP address, even though the site it wants to access is, for example, The firewall inspects the proxy application layer data as specified, NATs the packets, and passes them on to the final destination. The firewall is responsible for DNS lookups. In a non-transparent connection, the client is completely aware of the firewall.
Although a non-transparent connection may sound more complicated, it may be beneficial (or even necessary) depending on routing or if you use certain authentication methods or non-standard ports. The following proxies can be configured to be non-transparent:
FTP
HTTP
HTTPS
Oracle
Telnet

Frequently Asked Questions about non transparent proxy

What is transparent proxy?

A transparent proxy, also known as an inline proxy, intercepting proxy or forced proxy, is a server that intercepts the connection between an end-user or device and the internet. It is called “transparent” because it does so without modifying requests and responses.

What is the difference between transparent and non-transparent?

In a transparent connection, the client is unaware of the firewall. In a non-transparent proxy connection, the client (e.g., a Web browser) sends all requests to the firewall. … The firewall is responsible for DNS lookups. In a non-transparent connection, the client is completely aware of the firewall.Apr 28, 2014

How do you know if a proxy is transparent?

A simple way to check whether or not you’re behind a transparent proxy is to attempt to connect to a server you know does not exist. If there’s no proxy, you’ll see a typical error message in your browser. Chrome says, “This site can’t be reached,” because the IP address cannot be found.Feb 20, 2019

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