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How MySQL uses DNS
When a new threads connects to mysqld, mysqld will span a new thread to handle the request. This thread will first check if
the hostname is in the hostname cache. If not the thread will call gethostbyaddr_r() and gethostbyname_r() to resolve the
hostname.
If the operating system doesn't support the above thread-safe calls, the thread will lock a mutex and call gethostbyaddr()
and gethostbyname() instead. Note that in this case no other thread can resolve other hostnames that is not in the hostname
cache until the first thread is ready.
You can disable DNS host lookup by starting mysqld with --skip-name-resolve. In this case you can however only use IP names
in the MySQL privilege tables.
If you have a very slow DNS and many hosts, you can get more performance by either disabling DNS lookop with
--skip-name-resolve or by increasing the HOST_CACHE_SIZE define (default: 128) and recompile mysqld.
You can disable the hostname cache with --skip-host-cache. You can clear the hostname cache with FLUSH HOSTS or mysqladmin
flush-hosts.
If you don't want to allow connections over TCP/IP, you can do this by starting mysqld with --skip-networking.
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