msnbot is a web-crawling robot (type of internet bot), deployed by Microsoft to supply Bing. It collects documents from the web to build a searchable index for the MSN Search engine, which went into beta in 2004, and had full public release in 2005. It performs the same function as Google's Googlebot, and Yahoo!'s slurp.
A typical user agent string for msnbot is "msnbot/1.1 (+http://search.msn.com/msnbot.htm)". This appears in the web server logs to tell the webmaster who is requesting a file. Each webmaster is able to use the included agent identifier, "msnbot", to deny or allow access to their site (by default access is allowed). If they don't want to grant access they can use the Robots Exclusion Standard to block it (relying on the assumed good behaviour of msnbot), or use other server specific means (relying on the web server to do the blocking).
Microsoft is going to replace MSNBot with Bingbot
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