Performance Schema host_cache Table

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:

Column	Description
`IP`	Client IP address.
`HOST`	IP's resolved DNS host name, or `NULL` if unknown.
`HOST_VALIDATED`	YES 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_ERRORS`	Number of connection errors. Counts only protocol handshake errors for hosts that passed validation. These errors count towards max_connect_errors.
`COUNT_HOST_BLOCKED_ERRORS`	Number of blocked connections because `SUM_CONNECT_ERRORS` exceeded the max_connect_errors system variable.
`COUNT_NAMEINFO_TRANSIENT_ERRORS`	Number of transient errors during IP-to-host DNS lookups.
`COUNT_NAMEINFO_PERMANENT_ERRORS`	Number of permanent errors during IP-to-host DNS lookups.
`COUNT_FORMAT_ERRORS`	Number of host name format errors, for example a numeric host column.
`COUNT_ADDRINFO_TRANSIENT_ERRORS`	Number of transient errors during host-to-IP reverse DNS lookups.
`COUNT_ADDRINFO_PERMANENT_ERRORS`	Number of permanent errors during host-to-IP reverse DNS lookups.
`COUNT_FCRDNS_ERRORS`	Number of forward-confirmed reverse DNS errors, which occur when IP-to-host DNS lookup does not match the originating IP address.
`COUNT_HOST_ACL_ERRORS`	Number 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_ERRORS`	Number 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_ERRORS`	Number 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_ERRORS`	Number of errors detected at the wire protocol level.
`COUNT_PROXY_USER_ERRORS`	Number of errors detected when a proxy user is proxied to a user that does not exist.
`COUNT_PROXY_USER_ACL_ERRORS`	Number 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_ERRORS`	Number of errors where authentication failed.
`COUNT_SSL_ERRORS`	Number of errors due to TLS problems.
`COUNT_MAX_USER_CONNECTIONS_ERRORS`	Number of errors due to the per-user quota being exceeded.
`COUNT_MAX_USER_CONNECTIONS_PER_HOUR_ERRORS`	Number of errors due to the per-hour quota being exceeded.
`COUNT_DEFAULT_DATABASE_ERRORS`	Number of errors due to the user not having permission to access the specified default database, or it not existing.
`COUNT_INIT_CONNECT_ERRORS`	Number of errors due to statements in the init_connect system variable.
`COUNT_LOCAL_ERRORS`	Number of local server errors, such as out-of-memory errors, unrelated to network, authentication, or authorization.
`COUNT_UNKNOWN_ERRORS`	Number of unknown errors that cannot be allocated to another column.
`FIRST_SEEN`	Timestamp of the first connection attempt by the IP.
`LAST_SEEN`	Timestamp of the most recent connection attempt by the IP.
`FIRST_ERROR_SEEN`	Timestamp of the first error seen from the IP.
`LAST_ERROR_SEEN`	Timestamp 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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https://mariadb.com/kb/en/performance-schema-host_cache-table/