“Proxy server”的意思、由来-开放百科全书

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]

Types of proxy servers

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.

Open proxies

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.

Reverse proxies

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 origin servers.^[4] 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:

Uses

Monitoring and filtering

Content-control software

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.

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 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 blacklist, 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|date=February 2014}} 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

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

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.

Web proxies are the most common means of bypassing government censorship, although no more than 3% of Internet users use any circumvention tools.^[5]

In some cases, users can circumvent proxies which filter using blacklists using services designed to proxy information from a non-blacklisted location.^[6]

Logging and eavesdropping

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

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.^[7] Poorly implemented caching proxies can cause problems, such as an inability to use user authentication.^[8]

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.^[9]

Translation

A translation proxy is a proxy server that is used to localize a website experience for different markets. Traffic from 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. 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 local audience such as excluding the source content or substituting the source content with the original local content.

Repairing errors

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.^[10]. Another kind of repair that can be done by a proxy is to fix accessibility issues.^[11]

Accessing services anonymously

An anonymous proxy server (sometimes called a web proxy) generally attempts to anonymize web surfing. There are different varieties of anonymizers. 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 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.

QA geotargeted advertising

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.^[12] Using a proxy server that is physically located inside a specific country or a city gives advertisers the ability to test geotargeted ads.

Security

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.^[13] 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.^[14]

Cross-domain resources

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

Secondary market brokers

Secondary market brokers use web proxy servers to buy large stocks of limited products such as limited sneakers^[15] or tickets.

Implementations of proxies

Web proxy servers

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.^[16]

This request is sent to the proxy server, the proxy makes the request specified and returns the response.

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, RabbIT4 and Polipo.

SOCKS proxy

SOCKS also forwards arbitrary data after a connection phase, and is similar to HTTP CONNECT in web proxies.

Transparent proxy

Also known as an intercepting proxy, inline proxy, or forced proxy, a transparent proxy intercepts normal communication at the network layer 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.^[17]

"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,^[18] and the Computer Emergency Response Team issued an advisory listing dozens of affected transparent and intercepting proxy servers.^[19]

Purpose

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

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.

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

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

There are several methods that can often be used to detect the presence of an intercepting proxy server:

CGI proxy

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

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

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

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, ...).

Proxy vs. NAT

'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.

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

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.

See also

Overview and discussions

Proxifiers

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).

Diverse topics

References

1. ^World-Wide Web Proxies, Ari Luotonen, April 1994
2. ^"[https://www.researchgate.net/publication/329502453_A_Survey_of_Techniques_for_Improving_Efficiency_of_Mobile_Web_Browsing A Survey of Techniques for Improving Efficiency of Mobile Web Browsing]", Concurrency and Computation: Practice and Experience, 2018
3. ^[https://hal.inria.fr/inria-00444651/document], 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).
4. ^{{cite web |url=http://httpd.apache.org/docs/2.0/mod/mod_proxy.html#forwardreverse |title=Forward and Reverse Proxies |work=httpd mod_proxy |publisher=Apache |accessdate=20 December 2010 }}
5. ^{{cite web |title=2010 Circumvention Tool Usage Report |url=http://cyber.law.harvard.edu/sites/cyber.law.harvard.edu/files/2010_Circumvention_Tool_Usage_Report.pdf |publisher=The Berkman Center for Internet & Society at Harvard University |date=October 2010}}
6. ^{{cite web |url=http://sitevana.com/webtech/ |title=Using a Ninjaproxy to get through a filtered proxy. |work=advanced filtering mechanics |publisher=TSNP |accessdate=17 September 2011 }}
7. ^{{cite book |quote=A proxy server helps speed up Internet access by storing frequently accessed pages |first=Keir |last=Thomas |title=Beginning Ubuntu Linux: From Novice to Professional |publisher=Apress |year=2006 |isbn=978-1-59059-627-2 }}
8. ^{{cite IETF |title=Known HTTP Proxy/Caching Problems |rfc=3143 |author=I. Cooper |author2=J. Dilley |date=June 2001 |publisher=IETF |accessdate=February 2014 }}
9. ^{{cite IETF |title=Performance Enhancing Proxies Intended to Mitigate Link-Related Degradations |rfc=3135 |sectionname=Layering |section=2.1 |page=4 |date=June 2001 |publisher=IETF |accessdate=21 February 2014 }}
10. ^{{cite book |last1=Durieux |first1=T. |last2=Hamadi |first2=Y. |last3=Monperrus |first3=M. |title=Fully Automated HTML and Javascript Rewriting for Constructing a Self-Healing Web Proxy |journal=2018 IEEE 29th International Symposium on Software Reliability Engineering (ISSRE) |pages=1–12 |date=2018 |doi=10.1109/ISSRE.2018.00012 |url=https://hal.archives-ouvertes.fr/hal-01746141/document|isbn=978-1-5386-8321-7 }}
11. ^{{Cite book|last=Zhang|first=Xiaoyi|last2=Ross|first2=Anne Spencer|last3=Caspi|first3=Anat|last4=Fogarty|first4=James|last5=Wobbrock|first5=Jacob O.|date=2017|title=Interaction Proxies for Runtime Repair and Enhancement of Mobile Application Accessibility|journal=Proceedings of the 2017 CHI Conference on Human Factors in Computing Systems|pages=6024–6037|doi=10.1145/3025453.3025846|isbn=9781450346559}}
12. ^{{cite web|title=Hot Tactics For Geo-Targeted Ads On Google & Bing|url=http://searchengineland.com/hot-tactics-for-geo-targeted-ads-on-google-bing-smx-east-173213|accessdate=7 February 2014}}
13. ^{{cite web |quote=The proxy server is, above all, a security device. |url=http://tldp.org/HOWTO/Firewall-HOWTO-11.html |publisher=tldp.org |title=Firewall and Proxy Server HOWTO |accessdate=4 September 2011 }}
14. ^¹{{cite book |title=Nmap network scanning |last=Lyon |first=Gordon |authorlink=Gordon Lyon |year=2008 |publisher=Insecure |location=US |isbn=978-0-9799587-1-7 |page=270 }}
15. ^{{cite web|title=Sneaker Bot Supreme Proxy|url=https://www.geosurf.com/sneaker-residential-ips-proxy/|publisher=GeoSurf|accessdate=24 September 2017}}
16. ^{{cite IETF |title=HTTP/1.1 Message Syntax and Routing |quote=a client MUST send the target URI in absolute-form as the request-target |rfc=7230 |sectionname=absolute-form |section=5.3.2 |page=41 |date=June 2014 |publisher=IETF |accessdate=4 November 2017 }}
17. ^{{cite web |url=http://www.ukproxyserver.org/transparent-proxy/ |archive-url=https://web.archive.org/web/20130301235707/http://www.ukproxyserver.org/transparent-proxy/ |dead-url=yes |archive-date=1 March 2013 |publisher=ukproxyserver.org |title=Transparent Proxy Definition |date=1 February 2011 |accessdate=14 February 2013 }}
18. ^{{cite web |url=http://www.thesecuritypractice.com/the_security_practice/2009/03/socket-capable-browser-plugins-result-in-transparent-proxy-abuse.html |title=Socket Capable Browser Plugins Result In Transparent Proxy Abuse |publisher=The Security Practice |date=9 March 2009 |accessdate=14 August 2010}}
19. ^{{cite web |url=http://www.kb.cert.org/vuls/id/435052|title=Vulnerability Note VU#435052 |publisher=US CERT |date=23 February 2009|accessdate=14 August 2010}}
20. ^{{cite web|url=http://svn.haxx.se/dev/archive-2003-02/0257.shtml|title=Subversion Dev: Transparent Proxy detection (was Re: Introduction_|publisher=Tracetop.sourceforge.net|accessdate=16 November 2014}}
21. ^{{cite book |last=Wessels |first=Duane |year=2004 |title=Squid The Definitive Guide |publisher=O'Reilly |isbn=978-0-596-00162-9 |pages=130}}
22. ^{{cite web |last1=Marshall |first1=James |title=CGIProxy |url=https://www.jmarshall.com/tools/cgiproxy/ |accessdate=12 November 2018}}
23. ^{{cite web |title=The Limits of Control |url=http://www.cfcl.com/rdm/Pubs/tin/P/199606.shtml |date=June 1996 |accessdate=November 12, 2018}}
24. ^https://www.glype.com/ - dead url, archived at https://archive.fo/P9rjE
25. ^{{cite web|url=https://sourceforge.net/projects/poxy/|title=PHProxy|publisher=}}
26. ^{{cite web|url=https://www.google.com/trends/explore?date=all&q=glype%20proxy|title=Google Trends|publisher=}}
27. ^{{cite web|url=http://getproxi.es/proxy-stats/|title=Proxy Stats :: Get Proxi.es|publisher=}}
28. ^{{Cite journal|last=Estrada-Jiménez|first=José|date=March 2017|title=Online advertising: Analysis of privacy threats and protection approaches|url=https://www.sciencedirect.com/science/article/pii/S0140366416307083?via%3Dihub|journal=Computer Communications|volume=100|pages=32-51|via=}}
29. ^¹{{cite news |url=https://www.nytimes.com/2006/01/25/technology/techspecial2/25privacy.html?_r=1 |title=Privacy for People Who Don't Show Their Navels |first=Jonathan |last=Glater |newspaper=The New York Times |date=25 January 2006 |accessdate=4 August 2011 }}
30. ^{{cite web| last = The Tor Project | title = Tor: anonymity online | url = https://www.torproject.org/ | accessdate=9 January 2011}}
31. ^{{cite book |last1=Zwicky |first1=Elizabeth D. |last2=Cooper |first2=Simon |last3=Chapman |first3=D. Brent |year=2000 |title=Building Internet Firewalls |edition=2nd |isbn=978-1-56592-871-8 |page=235 }}