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:
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 (usinggethostbyname()) and compares to ensure it is the original IP address.If the operating system supports the thread-safe
gethostbyaddr_r()andgethostbyname_r()calls, the thread uses them to perform host name resolution.If the operating system does not support the thread-safe calls, the thread locks a mutex and calls
gethostbyaddr()andgethostbyname()instead. In this case, no other thread can resolve host names that are not in the host name cache until the first thread unlocks the mutex.
You can disable DNS host name lookups by starting
mysqld with the
--skip-name-resolve option.
However, in this case, you can use only IP addresses 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 host name cache by starting the server
with the --skip-host-cache
option. To clear the host name 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.