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There are a lot of code example out there that helps users do this. Eg: https://stackoverflow.com/questions/12693729/how-to-display-google-adsense-ads-to-search-engine-traffic-only
But I was looking authoritative links or answers, since there is no answer in Adsense TOS.
I don't see any reason that it wouldn't be OK with AdSense.
The only issue you might have is from the organic search side. If you are showing any kind of content only to search users (including ads) that could be considered "cloaking". To ensure that you don't get an organic ranking penalty, you would want to make sure you serve the same page to search engine robots (like Googlebot) that you serve to visitors from search engines.
If you consider the contributors on the AdSense forums to be authoritative, then they don't seem to find any problems with selecting what visitors to show Google ads to.
https://productforums.google.com/d/msg/adsense/caLd7wnkTW4/kCGsvS_aLIAJ
https://productforums.google.com/d/msg/adsense/JNPb6d4ShTU/VNtX5v_BbPcJ
Yes, it is safe.
Reason : Nowhere it is mentioned that it is unsafe. Giants like twitter do that. The only risk associated with cloaking is SEO issues, as cloaking is against the Google webmaster Guidelines.
