URL shortening

For information about human generated short URLs on Wikipedia, see WP:WP and mw:Manual:Short URL.

URL shortening is a technique on the World Wide Web in which a Uniform Resource Locator (URL) may be made substantially shorter in length and still direct to the required page. This is achieved by using an HTTP Redirect on a domain name that is short, which links to the web page that has a long URL. For example, the URL "http://en.wikipedia.org/wiki/URL_shortening" can be shortened to "http://bit.ly/urlwiki", "http://tinyurl.com/urlwiki", "http://is.gd/urlwiki" or "http://goo.gl/Gmzqv". This is especially convenient for messaging technologies such as Twitter and Identi.ca which severely limit the number of characters that may be used in a message. Short URLs allow otherwise long web addresses to be referred to in a tweet. In November 2009, the shortened links of the URL shortening service Bitly were accessed 2.1 billion times.

Other uses of URL shortening are to "beautify" a link, track clicks or disguise the underlying address. Although disguising of the underlying address may be desired for legitimate business or personal reasons, it is open to abuse and for this reason, some URL shortening service providers have found themselves on spam blacklists, because of the use of their redirect services by sites trying to bypass those very same blacklists. Some websites prevent short, redirected URLs from being posted.

Purposes

There are several reasons to use URL shortening. Often regular unshortened links may be aesthetically unpleasing. Many web developers pass descriptive attributes in the URL to represent data hierarchies, command structures, transaction paths or session information. This can result in URLs that are hundreds of characters long and that contain complex character patterns. Such URLs are difficult to memorize, type-out and distribute. As a result, long URLs must be copied-and-pasted for reliability. Thus, short URLs may be more convenient for websites or hard copy publications (e.g. a printed magazine or a book), the latter often requiring that very long strings be broken into multiple lines (as is the case with some e-mail software or internet forums) or truncated.

On Twitter and some instant-messaging services, there is a limit to the number of characters a message can carry. Using a URL shortener can allow linking to web pages which would otherwise violate this constraint. Some shortening services, such as tinyurl.com, and bit.ly, can generate URLs that are human-readable, although the resulting strings are longer than those generated by a length-optimized service. Finally, URL shortening sites provide detailed information on the clicks a link receives, which can be simpler than setting up an equally powerful server-side analytics engine.

URLs encoded in two-dimensional barcodes such as QR code are often shortened by a URL shortener in order to reduce the printed area of the code or allow printing at lower density in order to improve scanning reliability.

Registering a short URL

An increasing number of websites are registering their own short URLs to make sharing via Twitter and SMS easier. This can normally be done online, at the web pages of a URL shortening service. Short URLs often circumvent the intended use of top-level domains for indicating the country of origin; domain registration in many countries requires proof of physical presence within that country, although a redirected URL has no such guarantee.

Techniques

See also: URL redirection

In URL shortening, every long URL is associated with a unique key, which is the part after http://top-level domain name/, for example http://tinyurl.com/m3q2xt has a key of m3q2xt. Not all redirection is treated equally; the redirection instruction sent to a browser can contain in its header the HTTP status 301 (permanent redirect) 302, or 307 (temporary redirect).

There are several techniques to implement a URL shortening. Keys can be generated in base 36, assuming 26 letters and 10 numbers. In this case, each character in the sequence will be 0, 1, 2, ..., 9, a, b, c, ..., y, z. Alternatively, if uppercase and lowercase letters are differentiated, then each character can represent a single digit within a number of base 62 (26 + 26 + 10). In order to form the key, a hash function can be made, or a random number generated so that key sequence is not predictable. Or users may propose their own keys. For example, http://en.wikipedia.org/w/index.php?title=TinyURL&diff=283621022&oldid=283308287 can be shortened to http://bit.ly/tinyurlwiki.

Not all protocols are capable of being shortened, as of 2011, although protocols such as http, https, ftp, ftps, mailto, news, mms, rtmp, rtmpt, ed2k, pop, imap, nntp, news, ldap, gopher, dict and dns are being addressed by such services as URL Shortener. Typically, data: and javascript: URLs are not supported for security reasons. Some URL shortening services support the forwarding of mailto URLs, as an alternative to address munging, to avoid unwanted harvest by web crawlers or bots. This may sometimes be done using short, CAPTCHA-protected URLs, but this is not common.

Makers of URL Shorteners Usually register domain names with less popular or esoteric Top-level domains in order to achieve a short URL and a catchy name, often using domain hacks. This results in registration of different URL shorteners with a myriad of different countries, leaving no relation between the country where the domain has been registered and the URL shortener itself or the shortened links. Top-level domains of countries such as Libya (.ly), Samoa (.ws), Mongolia (.mn), Malaysia (.my) and Lichtenstein (.li) have been used as well as many others. In some cases, The Political or cultural aspects of the country In charge of the Top-level domain may become an issue for users and owners, but this is not usually the case.

Tinyarro.ws, urlrace.com, and qoiob.com use Unicode characters to achieve the shortest URLs possible, since more condensed URLs are possible with a given number of characters compared to those using a standard Latin alphabet.^{[citation needed]}

Statistics

Services may record inbound statistics, which may be viewed publicly by others.

History

An early reference is US Patent 6957224, which describes

...a system, method and computer program product for providing links to remotely located information in a network of remotely connected computers. A uniform resource locator (URL) is registered with a server. A shorthand link is associated with the registered URL. The associated shorthand link and URL are logged in a registry database. When a request is received for a shorthand link, the registry database is searched for an associated URL. If the shorthand link is found to be associated with an URL, the URL is fetched, otherwise an error message is returned.

The patent was filed in September 2000; while the patent was issued in 2005, patent applications are made public within 18 months of filing.

Another reference to URL shortening was in 2001. The first notable URL shortening service, TinyURL, was launched in 2002. Its popularity influenced the creation of at least 100 similar websites, although most are simply domain alternatives. Initially Twitter automatically translated long URLs using TinyURL, although it began using bit.ly in 2009.

In May 2009, the service .tk, which previously generated memorable domains via URL redirection, launched tweak.tk, which generates very short URLs. On 14 August 2009, WordPress announced the wp.me URL shortener for use when referring to any WordPress.com blog post. In November 2009, shortened links on bit.ly were accessed 2.1 billion times. Around that time, bit.ly and TinyURL were the most widely used URL-shortening services.

On 10 August 2009, however, tr.im, announced that it was curtailing the generation of new shortened URLs, but assured that existing tr.im short URLs would "continue to redirect, and will do so until at least December 31, 2009". A blog post on the site attributed this move to several factors, including a lack of suitable revenue-generating mechanisms to cover ongoing hosting and maintenance costs, a lack of interest among possible purchasers of the service and Twitter's default use of the bit.ly shortener. This blog post also questioned whether other shortening services can successfully make money from URL shortening in the longer term. A few days later, tr.im appeared to alter its stance, announcing that it would resume all operations "going forward, indefinitely, while we continue to consider our options in regards to tr.im's future" but, as of 11 July 2011, the tr.im service failed.

In December 2009, the URL shortener TO./ NanoURL was launched by .TO. This service creates a URL address which looks like http://to./xxxx, where xxxx represents a combination of random numbers and letters. NanoURL currently generates the shortest URLs of all URL shortening services, because it is hosted on a top-level domain (the one of Tonga). This rare form of URL may cause problems with some browsers, however, where the string is interpreted as a search term and passed to a search engine, instead of being opened. As of 2011, the service is no longer available.

On 14 December 2009, Google announced a service called Google URL Shortener at goo.gl, which originally was only available for use through Google products (such as Google Toolbar and FeedBurner). It does, however, have two extensions (Standard and Lite versions) for Google Chrome. On 21 December 2009, Google also announced a service called YouTube URL Shortener, youtu.be, and since September 2010, Google URL Shortener has become available via a direct interface, Google's direct link (goo.gl) will ask you to prove you're not a robot with CAPTCHA (May 2012). Additional features of the goo.gl service include analytics details and a QR code generator.

Shortcomings

Abuse

URL shortening may be utilized by spammers or for illicit internet activities. As a result, many have been removed from online registries or shut down by web hosts or internet service providers.

According to Tonic Corporation, the registry for .to domains, it is "very serious about keeping domains spam free" and may remove URL shortening services from their registry if the service is abused.

In addition, "u.nu" made the following announcement upon closing operations:

The last straw came on September 3, 2010, when the server was disconnected without notice by our hosting provider in response to reports of a number of links to child pornography sites. The disconnection of the server caused us serious problems, and to be honest, the level and nature of the abuse has become quite demoralizing. Given the choice between spending time and money to find a different home, or just giving up, the latter won out.

Google's url-shortener discussion group has frequently included messages from frustrated users reporting that specific shortened URLs have been disabled after they were reported as spam.

A study in May 2012 showed that 61% of URL shorteners had shut down (614 of 1002). The most common cause cited was abuse by spammers.

Linkrot

The convenience offered by URL shortening also introduces potential problems, which have led to criticism of the use of these services. Short URLs, for example, will be subject to linkrot if the shortening service stops working; all URLs related to the service will become broken. It is a legitimate concern that many existing URL shortening services may not have a sustainable business model in the long term. This worry was highlighted by a statement from tr.im in August 2009 (see above). In late 2009, the Internet Archive started the "301 Works" projects, together with twenty collaborating companies (initially), whose short URLs will be preserved by the project. The URL shortening service ur1.ca provides its entire database as a file download, so if its website stops working, other websites may be able to provide ways to correct broken links to URLs shortened with its service. A circumvention could be that a website provided its own shortlinks instead of relying on a shortening service – but this is not common.

Transnational law

Shortened internet links typically use foreign country domain names, and are therefore under the jurisdiction of that nation. Libya, for instance, exercised its control over the .ly domain in October 2010 to shut down vb.ly for violating Libyan pornography laws. Failure to predict such problems with URL shorteners and investment in URL shortening companies may reflect a lack of due diligence.

Blocking

Some websites prevent short, redirected URLs from being posted.

In 2009, the Twitter network replaced TinyURL with Bit.ly as its default shortener of links longer than twenty-six characters. In April 2009, TinyURL was reported to be blocked in Saudi Arabia. Yahoo! Answers blocks postings that contain TinyURLs and Wikipedia does not accept links by any URL shortening services in its articles.

Privacy and security

Users may be exposed to privacy issues through a URL shortening service's ability to track a user's behavior across many domains.

On the security side, a short URL obscures the target address, and as a result, can be used to redirect to an unexpected site. Examples of this are rickrolling, redirecting to shock sites, or to affiliate websites. Short URLs can also unexpectedly redirect a user to scam pages or pages containing malware or XSS attacks, which use the redirect to bypass URL blacklists. TinyURL tries to disable spam-related links from redirecting. ZoneAlarm, however, has warned its users: "TinyURL may be unsafe. This website has been known to distribute spyware." TinyURL countered this problem by offering an option to preview a link before using a shortened URL. This ability is installed on the browser via the TinyURL website, however, and requires the use of cookies. However, a preview may also be obtained by simply prefixing the word "preview" to the front of the URL: for example, http://tinyurl.com/8kmfp could be retyped as http://preview.tinyurl.com/8kmfp to see where the link will lead. Other URL shortening services have also added support for preview of a link using different user interfaces. Security professionals suggest that users should always preview a short URL before accessing it, following an instance where the URL shortening service cli.gs was compromised, exposing millions of users to security uncertainties.

There are also several web applications that can expand a shortened URL to allow the user to check where it leads.

Some URL shortening services have started filtering their links through services like Google Safe Browsing. Many sites that accept user-submitted content block links, however, to certain domains in order to cut down on spam and for this reason, known URL redirection services are often themselves added to spam blacklists.

Additional layer of complexity

Short URLs, although making it easier to access what might otherwise be a very long URL or user-space on an ISP server, add an additional layer of complexity to the process of retrieving web pages. Every access requires more requests (at least one more DNS lookup and HTTP request), thereby increasing latency, the time taken to access the page, and also the risk of failure, since the shortening service may become unavailable. Another operational limitation of URL shortening services is that browsers do not resend POST bodies when a redirect is encountered. This can be overcome by making the service a reverse proxy, or by elaborate schemes involving cookies and buffered POST bodies, but such techniques present security and scaling challenges, and are therefore not used on extranets or Internet-scale services.

Notable URL shortening services

• bitly
• TinyURL
• goo.gl

See also

• Clean URL – http://example.com/index.asp?mod=profiles&id=193 becomes http://example.com/user/john-doe
• Country code top-level domain
• Domain name system
• Generic top-level domain
• Link rot
• List of Internet top-level domains
• Semantic URL
• Vanity domain
• Vanity URL

External links

• URL redirection and shortening at the Open Directory Project
• "Comparison of URL Shortening Services" SearchEngineLand. April 2009.
• Nesbitt, Scott (7 February 2010). "Shorter Is Sweeter: A Look at URL Shorteners". Geeks.com.