A HTTP referrer is defined as the origin of the visitor to a website, usually a URL. The term "Referer" is used due to a spelling error in the original HTTP specification, and this misspelling has now become the default.
A referrer is always transmitted when a user accesses a website via a link on another page. When the link is clicked on, the URL of the "other" page is sent to the server on which the website to be reached is hosted. The server keeps a log file that records all website visits. A line is dedicated to each retrieval, in which, for example, the visitor's IP address or details of the User Agent are recorded. The referrer is now also logged here.
A referrer is not transferred if a user has typed in a URL manually or accessed it via their bookmarks – in this case, there is no referring website. The referrer may also be missing if the transmission is suppressed.
The referrer can be used to carry out evaluations regarding the origin of visitors. For example, site operators can determine the main source of where the visitors come from. Advertising measures such as advertising banners, affiliate links or purchased entries in classified directories can be checked for their effectiveness and thus evaluated.
Web analysis tools such as Google Analytics usually have the ability to read the referrer and process it in reports for such evaluations. This can also be referred to as "referring websites". In web analysis tools, it is often possible to extract from the referrer the search terms that the user used when searching with Google to find the website.
There are Internet users who do not want to have their path tracked on the Internet via referrers. They have the option of preventing this.
For this, a so-called "Dereferrer" is installed in the browser for example via an add-on. The latter executes each link call via a dereferrer service. For example, the user is currently surfing Website A and accessing Website B via a link. Instead of going the direct way, the dereferrer service C is called first. This is used to access Website B. In the log file of Website B, Website A does not appear as a referrer, but rather the dereferer service C. The actual origin is therefore concealed.
If a user uses such a dereferrer in his browser, he usually has the choice of whether to prevent the transmission of referrers in principle, or whether he wants to be asked for his permission every time. Many Internet users take advantage of this possibility for data protection reasons to prevent their IP address and referrer from creating a kind of "movement protocol" on the Internet and linking it to them.
In some cases, the transmitting of referrers is already blocked by default without the user having to do anything explicitly. This is the case, for example, when employees of companies surf the Internet via a company computer that sits behind a company proxy server and prevents the transmission. Internet security packages installed by private users are often responsible for the blockade.
Referrers have been abused for years as spam measures. The "attackers" often call up a website as many as several hundred of thousands of times, depending on the size of the page. This is an automated process. The referrer leaves the URL of the website after each call, which is therefore strengthened in the rankings (cf. Black Hat SEO). Within a few days, this page thus turns into the supposed visitor origin No. 1 and is listed at the top of the referrer statistics.
In the past, many blogs published their referrer statistics, which allowed them to be read by search engines including spam links. Referrer spam was used especially for advertising pornographic web offers.
The practical importance of referrers is especially important in web analytics. If page operators can analyze which search terms are used to find their own website, important starting points for on-page and off-page optimization can be derived. Referrers are therefore very significant for search engine optimization.
However, with referrals, it is not possible to draw conclusions regarding which website caused the user to become aware of an offer originally. For example, if the user visits an online shop five times and chooses to buy the last time, the conversion is assigned to the last referrer registered. However, the website which drew the user’s attention to an offer in the first place was actually more significant.