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29.12.21.3 The host_cache Table

The MySQL server maintains an in-memory host cache that contains client host name and IP address information and is used to avoid Domain Name System (DNS) lookups. Thehost_cache table exposes the contents of this cache. Thehost_cache_size system variable controls the size of the host cache, as well as the size of thehost_cache table. For operational and configuration information about the host cache, seeSection 7.1.12.3, “DNS Lookups and the Host Cache”.

Because thehost_cache table exposes the contents of the host cache, it can be examined usingSELECT statements. This may help you diagnose the causes of connection problems.

Thehost_cache table has these columns:

  • IP

    The IP address of the client that connected to the server, expressed as a string.

  • HOST

    The resolved DNS host name for that client IP, orNULL if the name is unknown.

  • HOST_VALIDATED

    Whether the IP-to-host name-to-IP DNS resolution was performed successfully for the client IP. IfHOST_VALIDATED isYES, theHOST column is used as the host name corresponding to the IP so that additional calls to DNS can be avoided. WhileHOST_VALIDATED isNO, DNS resolution is attempted for each connection attempt, until it eventually completes with either a valid result or a permanent error. This information enables the server to avoid caching bad or missing host names during temporary DNS failures, which would negatively affect clients forever.

  • SUM_CONNECT_ERRORS

    The number of connection errors that are deemed“blocking” (assessed against themax_connect_errors system variable). Only protocol handshake errors are counted, and only for hosts that passed validation (HOST_VALIDATED = YES).

    OnceSUM_CONNECT_ERRORS for a given host reaches the value ofmax_connect_errors, new connections from that host are blocked. TheSUM_CONNECT_ERRORS value can exceed themax_connect_errors value because multiple connection attempts from a host can occur simultaneously while the host is not blocked. Any or all of them can fail, independently incrementingSUM_CONNECT_ERRORS, possibly beyond the value ofmax_connect_errors.

    Suppose thatmax_connect_errors is 200 andSUM_CONNECT_ERRORS for a given host is 199. If 10 clients attempt to connect from that host simultaneously, none of them are blocked becauseSUM_CONNECT_ERRORS has not reached 200. If blocking errors occur for five of the clients,SUM_CONNECT_ERRORS is increased by one for each client, for a resultingSUM_CONNECT_ERRORS value of 204. The other five clients succeed and are not blocked because the value ofSUM_CONNECT_ERRORS when their connection attempts began had not reached 200. New connections from the host that begin afterSUM_CONNECT_ERRORS reaches 200 are blocked.

  • COUNT_HOST_BLOCKED_ERRORS

    The number of connections that were blocked becauseSUM_CONNECT_ERRORS exceeded the value of themax_connect_errors system variable.

  • COUNT_NAMEINFO_TRANSIENT_ERRORS

    The number of transient errors during IP-to-host name DNS resolution.

  • COUNT_NAMEINFO_PERMANENT_ERRORS

    The number of permanent errors during IP-to-host name DNS resolution.

  • COUNT_FORMAT_ERRORS

    The number of host name format errors. MySQL does not perform matching ofHost column values in themysql.user system table against host names for which one or more of the initial components of the name are entirely numeric, such as1.2.example.com. The client IP address is used instead. For the rationale why this type of matching does not occur, seeSection 8.2.4, “Specifying Account Names”.

  • COUNT_ADDRINFO_TRANSIENT_ERRORS

    The number of transient errors during host name-to-IP reverse DNS resolution.

  • COUNT_ADDRINFO_PERMANENT_ERRORS

    The number of permanent errors during host name-to-IP reverse DNS resolution.

  • COUNT_FCRDNS_ERRORS

    The number of forward-confirmed reverse DNS errors. These errors occur when IP-to-host name-to-IP DNS resolution produces an IP address that does not match the client originating IP address.

  • COUNT_HOST_ACL_ERRORS

    The number of errors that occur because no users are permitted to connect from the client host. In such cases, the server returnsER_HOST_NOT_PRIVILEGED and does not even ask for a user name or password.

  • COUNT_NO_AUTH_PLUGIN_ERRORS

    The number of errors due to requests for an unavailable authentication plugin. A plugin can be unavailable if, for example, it was never loaded or a load attempt failed.

  • COUNT_AUTH_PLUGIN_ERRORS

    The number of errors reported by authentication plugins.

    An authentication plugin can report different error codes to indicate the root cause of a failure. Depending on the type of error, one of these columns is incremented:COUNT_AUTHENTICATION_ERRORS,COUNT_AUTH_PLUGIN_ERRORS,COUNT_HANDSHAKE_ERRORS. New return codes are an optional extension to the existing plugin API. Unknown or unexpected plugin errors are counted in theCOUNT_AUTH_PLUGIN_ERRORS column.

  • COUNT_HANDSHAKE_ERRORS

    The number of errors detected at the wire protocol level.

  • COUNT_PROXY_USER_ERRORS

    The number of errors detected when proxy user A is proxied to another user B who does not exist.

  • COUNT_PROXY_USER_ACL_ERRORS

    The number of errors detected when proxy user A is proxied to another user B who does exist but for whom A does not have thePROXY privilege.

  • COUNT_AUTHENTICATION_ERRORS

    The number of errors caused by failed authentication.

  • COUNT_SSL_ERRORS

    The number of errors due to SSL problems.

  • COUNT_MAX_USER_CONNECTIONS_ERRORS

    The number of errors caused by exceeding per-user connection quotas. SeeSection 8.2.21, “Setting Account Resource Limits”.

  • COUNT_MAX_USER_CONNECTIONS_PER_HOUR_ERRORS

    The number of errors caused by exceeding per-user connections-per-hour quotas. SeeSection 8.2.21, “Setting Account Resource Limits”.

  • COUNT_DEFAULT_DATABASE_ERRORS

    The number of errors related to the default database. For example, the database does not exist or the user has no privileges to access it.

  • COUNT_INIT_CONNECT_ERRORS

    The number of errors caused by execution failures of statements in theinit_connect system variable value.

  • COUNT_LOCAL_ERRORS

    The number of errors local to the server implementation and not related to the network, authentication, or authorization. For example, out-of-memory conditions fall into this category.

  • COUNT_UNKNOWN_ERRORS

    The number of other, unknown errors not accounted for by other columns in this table. This column is reserved for future use, in case new error conditions must be reported, and if preserving the backward compatibility and structure of thehost_cache table is required.

  • FIRST_SEEN

    The timestamp of the first connection attempt seen from the client in theIP column.

  • LAST_SEEN

    The timestamp of the most recent connection attempt seen from the client in theIP column.

  • FIRST_ERROR_SEEN

    The timestamp of the first error seen from the client in theIP column.

  • LAST_ERROR_SEEN

    The timestamp of the most recent error seen from the client in theIP column.

Thehost_cache table has these indexes:

  • Primary key on (IP)

  • Index on (HOST)

TRUNCATE TABLE is permitted for thehost_cache table. It requires theDROP privilege for the table. Truncating the table flushes the host cache, which has the effects described inFlushing the Host Cache.