If you are noticing a large spike in visitors for some unknown reason you might be seeing the false positives caused by semalt.com. Since the beginning of the year this website has been crawling websites and catorizing a lot of sites as competitors. The website is supposed to be some sort of SEO tool for webmasters. This service and the way it has appeared out of no where is something that makes webmasters uneasy.
We still are unsure if this is a valid service or not. So far the signs are pointing to a spam service. The domain is very new, and the homepage is basically just a signup form.
A problem with the way it’s crawling websites is that it is showing us higher visits. Real website crawlers work in the background and don’t report these extra visits.
This could just be a guerilla marketing campaign, and many of us are just waiting to see what comes from semalt.com
You can however block semalt.com with the htaccess file. Add the code below to your file, only if you are familiar with the htaccess file.
# block visitors referred from semalt.com
RewriteEngine on
RewriteCond %{HTTP_REFERER} semalt.com [NC]
RewriteRule .* - [F]
Source – http://logorrhoea.net/2014/01/how-to-block-semalt-com-referrer-traffic-using-htaccess/