Mysql 大量 unauthenticated user

1 When a new client connects to mysqld, mysqld spawns a new thread to handle the request. This thread first checks whether the host name is in the host name cache. If not, the thread attempts to resolve the host name: 
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3 The thread takes the IP address and resolves it to a host name (using gethostbyaddr()). It then takes that host name and resolves it back to the IP address (using gethostbyname()) and compares to ensure it is the original IP address. 

1 eg: If your client machine is called 'foo.domain.com' with IP 1.2.3.4, you might set up grants on your remote MySQL server like this:
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3 grant select on dbname.* to [email]username@foo.domain.com[/email] identified by 'somepassword'
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5 But when you try to connect from foo.domain.com you find you get the error "Host '1.2.3.4' is not allowed to connect to this MySQL server"
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7 This is because a reverse lookup of 1.2.3.4 may in fact return a fully qualified domain name of 4-3-2-1.isp.otherdomain.com, ie something *other* than foo.domain.com.
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9 So stick to granting access by IP, not hostname, unless your forward and reverse lookups match.