Referer spam (also known as log spam or referer bombing) is a kind of spamdexing (spamming aimed at search engines). The technique involves making repeated web site requests using a fake referer URL that points to the site the spammer wishes to advertise. Sites that publicize 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.
This benefits the spammer because of the free link, which gives the spammer's site improved search engine ranking due to link-counting algorithms that search engines use.
Technical solutions 
As with e-mail spam, web site operators who receive unwanted referer spam may respond using filtering and blocking. A simple attempt at rendering this form of spam ineffective is to prevent the 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.
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