What is referrer spam?

[craigwilson]

Referrer spam is when a spammer sends fake referrers to your server. They do this because they know most web stats lists referrers as hyperlinks. They then submit your stats to the search engines in the hopes that they will crawl your stats and find that link. They also hope you click on that link yourself.
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[aarisseo]

Referrer spam also known as 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 that points to the site the spammer wishes to advertise. Sites that publicize their access logs, including referrer statistics, will then end up linking to the spammer's site, which will in turn be indexed by the search engines as they crawl the access logs.