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Referrer spam (also known as referral spam, log spam or referrer bombing) is a kind of spamdexing (spamming aimed at search engines). The technique involves making repeated web site requests using a fake referrer URL to the site the spammer wishes to advertise. Sites that publish their access logs, including referer statistics, will then inadvertently link back to the spammer's site. These links will be indexed by search engines as they crawl the access logs, improving the spammer's search engine ranking. Except for polluting their statistics, the technique does not harm the affected sites.
At least since 2014(), a new variation of this form of spam occurs on Google Analytics. Spammers send fake visits to Google Analytics, often without ever accessing the affected site. The technique is used to have the spammers' URLs appear in the site statistics, inducing the site owner to visit the spam URLs. When the spammer never visited the affected site, the fake visits are also called Ghost Spam.
As with e-mail spam, referrer spam may be filtered or blocked. A website operator may mitigate referrer spam by preventing search engine spiders from crawling the site logs by moving them to a non-public area such as a password-protected area, by using a robot exclusion file, or by appending the nofollow value to the links.
It is found that while some referrer spam pollutes analytics statistics directly, some spammers actually hit the web server. Filtering referrer spam from analytics tools will hide it from the reports but can still continue to consume server bandwidth. To save server bandwidth spammers can be blocked using for example a .htaccess file.
Because the htaccess solutions require technical expertise, several developers have created tools for non-technical people to block the spam more automatically, specifically for Google Analytics.
- "Referrer Bombing has caused some of my sites to instantly disappear from google SERP". 22 September 2009. Retrieved 2013-11-19.
- "How to Remove Semalt from Google Analytics". 12 May 2014. Retrieved 2016-02-22.
- Analytics Referrer Spam blacklist on GitHub - a Community-contributed list of referrer spammers under Public Domain (550+ domains, Jan. 2016).
- Semalt, Darodar and other Referrer Spam Explained - Technical overview of referrer spam in Google Analytics: how it happens and what you can do