The Cisco Systems logo is seen as part of a display at the Microsoft Ignite technology conference in ChicagoCisco Systems Inc said on Tuesday it would buy OpenDNS, a privately held cloud-based security firm, for $635 million, the latest move to boost its security business as cyber attacks increase in number and sophistication. Cisco has been buying a number of security companies, which has made its relatively tiny security business one of its fastest growing in the past two years. OpenDNS uses predictive intelligence to block malware, botnets and phishing threats that antivirus and firewalls miss.