7.5.6. How MySQL Uses DNS
When a new client connects to mysqld,
mysqld spawns a new thread to handle the
request. This thread first checks whether the hostname is in the
hostname cache. If not, the thread attempts to resolve the
hostname:
If the operating system supports the thread-safe
gethostbyaddr_r() and
gethostbyname_r() calls, the thread uses
them to perform hostname resolution.
If the operating system does not support the thread-safe
calls, the thread locks a mutex and calls
gethostbyaddr() and
gethostbyname() instead. In this case, no
other thread can resolve hostnames that are not in the
hostname cache until the first thread unlocks the mutex.
You can disable DNS hostname lookups by starting
mysqld with the
--skip-name-resolve option. However, in this
case, you can use only IP numbers in the MySQL grant tables.
If you have a very slow DNS and many hosts, you can get more
performance by either disabling DNS lookups with
--skip-name-resolve or by increasing the
HOST_CACHE_SIZE define (default value: 128)
and recompiling mysqld.
You can disable the hostname cache by starting the server with
the --skip-host-cache option. To clear the
hostname cache, issue a FLUSH HOSTS statement
or execute the mysqladmin flush-hosts
command.
To disallow TCP/IP connections entirely, start
mysqld with the
--skip-networking option.
