The Ultimate Guide to High Availability with MariaDB

Performance Schema host_cache Table

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The host_cache table contains host and IP information from the host_cache, used for avoiding DNS lookups for new client connections.

The host_cache table contains the following columns:

ColumnDescription
IPClient IP address.
HOSTIP's resolved DNS host name, or NULL if unknown.
HOST_VALIDATEDYES if the IP-to-host DNS lookup was successful, and the HOST column can be used to avoid DNS calls, or NO if unsuccessful, in which case DNS lookup is performed for each connect until either successful or a permanent error.
SUM_CONNECT_ERRORSNumber of connection errors. Counts only protocol handshake errors for hosts that passed validation. These errors count towards max_connect_errors.
COUNT_HOST_BLOCKED_ERRORSNumber of blocked connections because SUM_CONNECT_ERRORS exceeded the max_connect_errors system variable.
COUNT_NAMEINFO_TRANSIENT_ERRORSNumber of transient errors during IP-to-host DNS lookups.
COUNT_NAMEINFO_PERMANENT_ERRORSNumber of permanent errors during IP-to-host DNS lookups.
COUNT_FORMAT_ERRORSNumber of host name format errors, for example a numeric host column.
COUNT_ADDRINFO_TRANSIENT_ERRORSNumber of transient errors during host-to-IP reverse DNS lookups.
COUNT_ADDRINFO_PERMANENT_ERRORSNumber of permanent errors during host-to-IP reverse DNS lookups.
COUNT_FCRDNS_ERRORSNumber of forward-confirmed reverse DNS errors, which occur when IP-to-host DNS lookup does not match the originating IP address.
COUNT_HOST_ACL_ERRORSNumber of errors occurring because no user from the host is permitted to log in. These attempts return error code 1130 ER_HOST_NOT_PRIVILEGED and do not proceed to username and password authentication.
COUNT_NO_AUTH_PLUGIN_ERRORSNumber of errors due to requesting an authentication plugin that was not available. This can be due to the plugin never having been loaded, or the load attempt failing.
COUNT_AUTH_PLUGIN_ERRORSNumber of errors reported by an authentication plugin. Plugins can increment COUNT_AUTHENTICATION_ERRORS or COUNT_HANDSHAKE_ERRORS instead, but, if specified or the error is unknown, this column is incremented.
COUNT_HANDSHAKE_ERRORSNumber of errors detected at the wire protocol level.
COUNT_PROXY_USER_ERRORSNumber of errors detected when a proxy user is proxied to a user that does not exist.
COUNT_PROXY_USER_ACL_ERRORSNumber of errors detected when a proxy user is proxied to a user that exists, but the proxy user doesn't have the PROXY privilege.
COUNT_AUTHENTICATION_ERRORSNumber of errors where authentication failed.
COUNT_SSL_ERRORSNumber of errors due to TLS problems.
COUNT_MAX_USER_CONNECTIONS_ERRORSNumber of errors due to the per-user quota being exceeded.
COUNT_MAX_USER_CONNECTIONS_PER_HOUR_ERRORSNumber of errors due to the per-hour quota being exceeded.
COUNT_DEFAULT_DATABASE_ERRORSNumber of errors due to the user not having permission to access the specified default database, or it not existing.
COUNT_INIT_CONNECT_ERRORSNumber of errors due to statements in the init_connect system variable.
COUNT_LOCAL_ERRORSNumber of local server errors, such as out-of-memory errors, unrelated to network, authentication, or authorization.
COUNT_UNKNOWN_ERRORSNumber of unknown errors that cannot be allocated to another column.
FIRST_SEENTimestamp of the first connection attempt by the IP.
LAST_SEENTimestamp of the most recent connection attempt by the IP.
FIRST_ERROR_SEENTimestamp of the first error seen from the IP.
LAST_ERROR_SEENTimestamp of the most recent error seen from the IP.

The host_cache table, along with the host_cache, is cleared with FLUSH HOSTS, TRUNCATE TABLE host_cache or by setting the host_cache_size system variable at runtime.

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