5.1.11.2 DNS Lookups and the Host Cache

The MySQL server maintains an in-memory host cache 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 25.12.16.1, “The host_cache Table”.

The following sections discuss how the host cache works, as well as other topics such as how to configure and monitor the cache.

• Host Cache Operation

• Configuring the Host Cache

• Monitoring the Host Cache

• Flushing the Host Cache

• Dealing with Blocked Hosts

Host Cache Operation

The server uses the host cache only for non-localhost TCP connections. It does not use the cache for TCP connections established using a loopback interface address (for example, 127.0.0.1 or ::1), or 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 client 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_errors system variable determines the permitted number of successive errors before blocking occurs.

For each applicable 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 performs host name resolution using the getaddrinfo() system call.

The server handles entries in the host cache like this:

1. 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 NULL and the flag is false. This entry is also used for subsequent client TCP connections from the same originating IP.

2. 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 NULL and 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.)

3. 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 25.12.16.1, “The host_cache Table”.

To unblock blocked hosts, flush the host cache; see Dealing with Blocked Hosts.

It is possible for a blocked host to become unblocked even without flushing the host cache if activity from other hosts occurs:

• If the cache is full when a connection arrives from a client IP not in the cache, the server discards the least recently used cache entry to make room for the new entry.

• 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 Connection_errors_xxx status variables (see Section 5.1.9, “Server Status Variables”).

Configuring the Host Cache

The host cache is enabled by default. The host_cache_size system variable controls its size, as well as the size of the Performance Schema host_cache 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 server my.cnf file:

[mysqld]
host_cache_size=200

To change the size to 300 at runtime, do this:

SET GLOBAL host_cache_size=300;

Setting 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.

Changing the cache size at runtime causes an implicit host cache flushing operation that clears the host cache, truncates the host_cache table, and unblocks any blocked hosts; see Flushing the Host Cache.

Using the --skip-host-cache option is similar to setting the host_cache_size system 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 runtime changes to the value of host_cache_size, but such changes have no effect and the cache is not re-enabled even if host_cache_size is set larger than 0.

To disable DNS host name lookups, start the server with the skip_name_resolve system 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 enabling skip_name_resolve to disable DNS lookups, or by increasing the value of host_cache_size to make the host cache larger.

To disallow TCP/IP connections entirely, start the server with the skip_networking system variable enabled.

To adjust the permitted number of successive connection errors before host blocking occurs, set the max_connect_errors system variable. For example, to set the value at startup put these lines in the server my.cnf file:

[mysqld]
max_connect_errors=10000

To change the value at runtime, do this:

SET GLOBAL max_connect_errors=10000;

Monitoring the Host Cache

The Performance Schema host_cache table exposes the contents of the host cache. This table can be examined using SELECT statements, which may help you diagnose the causes of connection problems. The Performance Schema must be enabled or this table is empty. For information about this table, see Section 25.12.16.1, “The host_cache Table”.

Flushing the Host Cache

Flushing the host cache might be advisable or desirable under these conditions:

• Some of your client hosts change IP address.

• The error message Host 'host_name' is blocked occurs for connections from legitimate hosts. (See Dealing with Blocked Hosts.)

Flushing the host cache has these effects:

• It clears the in-memory host cache.

• It removes all rows from the Performance Schema host_cache table that exposes the cache contents.

• It unblocks any blocked hosts. This enables further connection attempts from those hosts.

To flush the host cache, use any of these methods:

• Change the value of the host_cache_size system variable. This requires the SUPER privilege.

• Execute a TRUNCATE TABLE statement that truncates the Performance Schema host_cache table. This requires the DROP privilege for the table.

• Execute a FLUSH HOSTS statement. This requires the RELOAD privilege.

• Execute a mysqladmin flush-hosts command. This requires the RELOAD privilege.

Dealing with Blocked Hosts

The server uses the host cache to track errors that occur during the client connection process. If the following error occurs, it means that mysqld has received many connection requests from the given host that were interrupted in the middle:

Host 'host_name' is blocked because of many connection errors.
Unblock with 'mysqladmin flush-hosts'

The value of the max_connect_errors system variable determines how many successive interrupted connection requests the server permits before blocking a host. After max_connect_errors failed requests without a successful connection, the server assumes that something is wrong (for example, that someone is trying to break in), and blocks the host from further connection requests.

To unblock blocked hosts, flush the host cache; see Flushing the Host Cache.

Alternatively, to avoid having the error message occur, set max_connect_errors as described in Configuring the Host Cache. The default value of max_connect_errors is 100. Increasing max_connect_errors to a large value makes it less likely that a host reaches the threshold and becomes blocked. However, if the Host 'host_name' is blocked error message occurs, first verify that there is nothing wrong with TCP/IP connections from the blocked hosts. It does no good to increase the value of max_connect_errors if there are network problems.