The MySQL server maintains a host cache in memory that contains
information about clients: IP address, host name, and error
information. The server uses this cache for nonlocal TCP
connections. It does not use the cache for TCP connections
established using the loopback interface address
127.0.0.1), or for connections established
using a Unix socket file, named pipe, or shared memory.
For each new client connection, the server uses the client IP address to check whether the client host name is in the host cache. If not, the server attempts to resolve the host name. First, it resolves the IP address to a host name and resolves that host name back to an IP address. Then it compares the result to the original IP address to ensure that they are the same. The server stores information about the result of this operation in the host cache. If the cache is full, the least recently used entry is discarded.
The server performs host name resolution using the thread-safe
gethostbyname_r() calls if the operating
system supports them. Otherwise, the thread performing the
lookup locks a mutex and calls
gethostbyname() instead. In this case, no
other thread can resolve host names that are not in the host
cache until the thread holding the mutex lock releases it.
The server uses the host cache for several purposes:
By caching the results of IP-to-host name lookups, the server avoids doing a DNS lookup for each client connection. Instead, for a given host, it needs to perform a lookup only for the first connection from that host.
The cache contains information about errors that occur
during the connection process. Some errors are considered
“blocking.” If too many of these occur
successively from a given host without a successful
connection, the server blocks further connections from that
variable determines the number of permitted errors before
blocking occurs. See Section B.5.2.6, “Host 'host_name' is blocked”.
It is possible for a blocked host to become unblocked even
HOSTS if activity from other hosts has occurred since
the last connection attempt from the blocked host. This can
occur because the server discards the least recently used cache
entry to make room for a new entry if the cache is full when a
connection arrives from a client IP not in the cache. If the
discarded entry is for a blocked host, that host becomes
The host cache is enabled by default. To disable it, start the
server with the
To disable DNS host name lookups, start the server with the
--skip-name-resolve option. In
this case, the server uses only IP addresses and not host names
to match connecting hosts to rows in the MySQL grant tables.
Only accounts specified in those tables using IP addresses can
be used. (Be sure that an account exists that specifies an IP
address or you may not be able to connect.)
If you have a very slow DNS and many hosts, you might be able to
improve performance either by disabling DNS lookups with
--skip-name-resolve or by
(default value: 128) and recompiling the server
To disallow TCP/IP connections entirely, start the server with
Some connection errors are not associated with TCP connections, occur very early in the connection process (even before an IP address is known), or are not specific to any particular IP address (such as out-of-memory conditions).