A botnet is a number of Internet- connected devices, each of which is running one or more bots. Botnets can be used to perform distributed denial- of- service attack ( DDoS attack), steal data, send spam, and allows the attacker to access the device and its connection. The owner can control the botnet using command and control ( C& C) software. The word " botnet" is a combination of the words " robot. A Web crawler, sometimes called a spider or spiderbot and often shortened to crawler, is an Internet bot that systematically browses the World Wide Web, typically for the purpose of Web indexing ( web spidering).
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Web search engines and some other sites use Web crawling or spidering software to update their web content or indices of others sites' web content.