The MySQL server maintains a host cache in memory that
contains information about clients: IP address, host name, and
error information. The Performance Schema
host_cache table exposes the
contents of the host cache so that it can be examined using
SELECT statements. This may
help you diagnose the causes of connection problems. See
Section 126.96.36.199, “The host_cache Table”.
The server uses the host cache only for nonlocal TCP
connections. It does not use the cache for TCP connections
established using a loopback interface address (for example,
for connections established using a Unix socket file, named
pipe, or shared memory.
The server uses the host cache for several purposes:
By caching the results of IP-to-host name lookups, the server avoids doing a Domain Name System (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 host. The
max_connect_errorssystem variable determines the permitted number of successive errors before blocking occurs (see Section B.4.2.6, “Host 'host_name' is blocked”).
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 so, the server refuses or continues to process the connection request depending on whether or not the host is blocked. If the host is not in the cache, 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 handles entries in the host cache like this:
When the first TCP client connection reaches the server from a given IP address, a new cache entry is created to record the client IP, host name, and client lookup validation flag. Initially, the host name is set to
NULLand the flag is false. This entry is also used for subsequent client TCP connections from the same originating IP.
If the validation flag for the client IP entry is false, the server attempts an IP-to-host name-to-IP DNS resolution. If that is successful, the host name is updated with the resolved host name and the validation flag is set to true. If resolution is unsuccessful, the action taken depends on whether the error is permanent or transient. For permanent failures, the host name remains
NULLand the validation flag is set to true. For transient failures, the host name and validation flag remain unchanged. (In this case, another DNS resolution attempt occurs the next time a client connects from this IP.)
If an error occurs while processing an incoming client connection from a given IP address, the server updates the corresponding error counters in the entry for that IP. For a description of the errors recorded, see Section 188.8.131.52, “The host_cache Table”.
The server performs host name resolution using the
gethostbyname() system calls.
To unblock blocked hosts, flush the host cache by executing
FLUSH HOSTS statement, a
TRUNCATE TABLE statement that
truncates the Performance Schema
host_cache table, or a
mysqladmin flush-hosts command.
FLUSH HOSTS and
mysqladmin flush-hosts require the
TRUNCATE TABLE requires the
DROP privilege for the
It is possible for a blocked host to become unblocked even without flushing the host cache 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 unblocked.
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). For information about these errors, check the
status variables (see
Section 5.1.9, “Server Status Variables”).
The host cache is enabled by default. The
variable controls its size, as well as the size of the
table that exposes the cache contents. The cache size can be
set at server startup and changed at runtime. For example,
to set the size to 100 at startup, put these lines in the
To change the size to 300 at runtime, do this:
SET GLOBAL host_cache_size=300;
host_cache_size to 0, either at
server startup or at runtime, disables the host cache. With
the cache disabled, the server performs a DNS lookup every
time a client connects.
option is similar to setting the
variable to 0, but
host_cache_size is more
flexible because it can also be used to resize, enable, and
disable the host cache at runtime, not just at server
startup. Starting the server with
--skip-host-cache does not
prevent changes to the value of
host_cache_size, but such
changes have no effect and the cache is not re-enabled even
host_cache_size is set
larger than 0 at runtime.
To disable DNS host name lookups, start the server with the
variable enabled. 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. (A client may
not be able to connect if no account exists that specifies
the client IP address.)
If you have a very slow DNS and many hosts, you might be
able to improve performance either by disabling DNS lookups
+ by enabling
skip_name_resolve or by
increasing the value of
host_cache_size to make the
host cache larger.
To disallow TCP/IP connections entirely, start the server
system variable enabled.