<a href="http://www.nagios.org/faqs/viewfaq.php?faq_id=191">http://www.nagios.org/faqs/viewfaq.php?faq_id=191</a> I was having this error and solved it a different way than described in the FAQ above. Don't know who to contact about getting that updated, so I thought I'd at least post it here. The installation of NRPE I was running (2.10) was emerged in Gentoo, and the default config file comes with "allowed_hosts" set to <a href="http://127.0.0.1">127.0.0.1</a>. Once I changed that, all sorts of handshaking ensued. It's not a difficult fix, but if your search for answers lands you at that FAQ page, you won't find it there (and maybe you should). -Frank