MySQL的常见错误的解决方法(英文)

Common Errors When Using MySQL Programs

This section lists some errors that users frequently encounter when running MySQL programs. Although the problems show up when you try to run client programs, the solutions to many of the problems involves changing the configuration of the MySQL server.

1. Access denied

An Access denied error can have many causes. Often the problem is related to the MySQL accounts that the server allows client programs to use when connecting. See Section 5.7.8, “Causes of Access denied Errors”. See Section 5.7.2, “How the Privilege System Works”.

2. Can't connect to [local] MySQL server

A MySQL client on Unix can connect to the mysqld server in two different ways: By using a Unix socket file to connect through a file in the filesystem (default /tmp/mysql.sock), or by using TCP/IP, which connects through a port number. A Unix socket file connection is faster than TCP/IP, but can be used only when connecting to a server on the same computer. A Unix socket file is used if you don't specify a hostname or if you specify the special hostname localhost.

If the MySQL server is running on Windows 9x or Me, you can connect only via TCP/IP. If the server is running on Windows NT, 2000, XP, or 2003 and is started with the --enable-named-pipe option, you can also connect with named pipes if you run the client on the host where the server is running. The name of the named pipe is MySQL by default. If you don't give a hostname when connecting to mysqld, a MySQL client first tries to connect to the named pipe. If that doesn't work, it connects to the TCP/IP port. You can force the use of named pipes on Windows by using . as the hostname.

The error (2002) Can't connect to ... normally means that there is no MySQL server running on the system or that you are using an incorrect Unix socket filename or TCP/IP port number when trying to connect to the server.

Start by checking whether there is a process named mysqld running on your server host. (Use ps xa | grep mysqld on Unix or the Task Manager on Windows.) If there is no such process, you should start the server. See Section 2.9.2.3, “Starting and Troubleshooting the MySQL Server”.

If a mysqld process is running, you can check it by trying the following commands. The port number or Unix socket filename might be different in your setup. host_ip represents the IP number of the machine where the server is running.

shell> mysqladmin version

shell> mysqladmin variables

shell> mysqladmin -h `hostname` version variables

shell> mysqladmin -h `hostname` --port=3306 version

shell> mysqladmin -h host_ip version

shell> mysqladmin --protocol=socket --socket=/tmp/mysql.sock version

Note the use of backticks rather than forward quotes with the hostname command; these cause the output of hostname (that is, the current hostname) to be substituted into the mysqladmin command. If you have no hostname command or are running on Windows, you can manually type the hostname of your machine (without backticks) following the -h option. You can also try -h 127.0.0.1 to connect with TCP/IP to the local host.

Here are some reasons the Can't connect to local MySQL server error might occur:

mysqld is not running. Check your operating system's process list to ensure the mysqld process is present.

You're running a MySQL server on Windows with many TCP/IP connections to it. If you're experiencing that quite often your clients get that error, you can find a workaround here: Section 2.1, “Connection to MySQL Server Failing on Windows”.

You are running on a system that uses MIT-pthreads. If you are running on a system that doesn't have native threads, mysqld uses the MIT-pthreads package. See Section 2.1.1, “Operating Systems Supported by MySQL”. However, not all MIT-pthreads versions support Unix socket files. On a system without socket file support, you must always specify the hostname explicitly when connecting to the server. Try using this command to check the connection to the server:

shell> mysqladmin -h `hostname` version

Someone has removed the Unix socket file that mysqld uses (/tmp/mysql.sock by default). For example, you might have a cron job that removes old files from the /tmp directory. You can always run mysqladmin version to check whether the Unix socket file that mysqladmin is trying to use really exists. The fix in this case is to change the cron job to not remove mysql.sock or to place the socket file somewhere else. See Section A.4.5, “How to Protect or Change the MySQL Socket File /tmp/mysql.sock”.

You have started the mysqld server with the --socket=/path/to/socket option, but forgotten to tell client programs the new name of the socket file. If you change the socket pathname for the server, you must also notify the MySQL clients. You can do this by providing the same --socket option when you run client programs. You also need to ensure that clients have permission to access the mysql.sock file. To find out where the socket file is, you can do:

shell> netstat -ln | grep mysql

See Section A.4.5, “How to Protect or Change the MySQL Socket File /tmp/mysql.sock”.

You are using Linux and one server thread has died (dumped core). In this case, you must kill the other mysqld threads (for example, with kill or with the mysql_zap script) before you can restart the MySQL server. See Section A.4.2, “What to Do If MySQL Keeps Crashing”.

The server or client program might not have the proper access privileges for the directory that holds the Unix socket file or the socket file itself. In this case, you must either change the access privileges for the directory or socket file so that the server and clients can access them, or restart mysqld with a --socket option that specifies a socket filename in a directory where the server can create it and where client programs can access it.

If you get the error message Can't connect to MySQL server on some_host, you can try the following things to find out what the problem is:

Check whether the server is running on that host by executing telnet some_host 3306 and pressing the Enter key a couple of times. (3306 is the default MySQL port number. Change the value if your server is listening to a different port.) If there is a MySQL server running and listening to the port, you should get a response that includes the server's version number. If you get an error such as telnet: Unable to connect to remote host: Connection refused, then there is no server running on the given port.

If the server is running on the local host, try using mysqladmin -h localhost variables to connect using the Unix socket file. Verify the TCP/IP port number that the server is configured to listen to (it is the value of the port variable.)

Make sure that your mysqld server was not started with the --skip-networking option. If it was, you cannot connect to it using TCP/IP.

Check to make sure that there is no firewall blocking access to MySQL. Applications such as ZoneAlarm and the Windows XP personal firewall may need to be configured to allow external access to a MySQL server.

2.1. Connection to MySQL Server Failing on Windows

When you're running a MySQL server on Windows with many TCP/IP connections to it, and you're experiencing that quite often your clients get a Can't connect to MySQL server error, the reason might be that Windows doesn't allow for enough ephemeral (short-lived) ports to serve those connections.

By default, Windows allows 5000 ephemeral (short-lived) TCP ports to the user. After any port is closed it will remain in a TIME_WAIT status for 120 seconds. This status allows the connection to be reused at a much lower cost than re-initialising a brand new connection. However, the port will not be available again until this time expires.

With a small stack of available TCP ports (5000) and a high number of TCP ports being open and closed over a short period of time along with the TIME_WAIT status you have a good chance for running out of ports. There are two ways to address this problem:

Reduce the number of TCP ports consumed quickly by investigating connection pooling or persistent connections where possible

Tune some settings in the Windows registry (see below)

IMPORTANT: The following procedure involves modifying the Windows registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, view the following article in the Microsoft Knowledge Base: http://support.microsoft.com/kb/256986/EN-US/.

Start Registry Editor (Regedt32.exe).

Locate the following key in the registry:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

On the Edit menu, click Add Value, and then add the following registry value:

Value Name: MaxUserPort

Data Type: REG_DWORD

Value: 65534

This sets the number of ephemeral ports available to any user. The valid range is between 5000 and 65534 (decimal). The default value is 0x1388 (5000 decimal).

On the Edit menu, click Add Value, and then add the following registry value:

Value Name: TcpTimedWaitDelay

Data Type: REG_DWORD

Value: 30

This sets the number of seconds to hold a TCP port connection in TIME_WAIT state before closing. The valid range is between 0 (zero) and 300 (decimal). The default value is 0x78 (120 decimal).

Quit Registry Editor.

Reboot the machine.

Note: Undoing the above should be as simple as deleting the registry entries you've created.

3. Client does not support authentication protocol

MySQL 5.0 uses an authentication protocol based on a password hashing algorithm that is incompatible with that used by older (pre-4.1) clients. If you upgrade the server from 4.1, attempts to connect to it with an older client may fail with the following message:

shell> mysql

Client does not support authentication protocol requested

by server; consider upgrading MySQL client

To solve this problem, you should use one of the following approaches:

Upgrade all client programs to use a 4.1.1 or newer client library.

When connecting to the server with a pre-4.1 client program, use an account that still has a pre-4.1-style password.

Reset the password to pre-4.1 style for each user that needs to use a pre-4.1 client program. This can be done using the SET PASSWORD statement and the OLD_PASSWORD() function:

mysql> SET PASSWORD FOR

-> 'some_user'@'some_host' = OLD_PASSWORD('newpwd');

Alternatively, use UPDATE and FLUSH PRIVILEGES:

mysql> UPDATE mysql.user SET Password = OLD_PASSWORD('newpwd')

-> WHERE Host = 'some_host' AND User = 'some_user';

mysql> FLUSH PRIVILEGES;

Substitute the password you want to use for “newpwd” in the preceding examples. MySQL cannot tell you what the original password was, so you'll need to pick a new one.

Tell the server to use the older password hashing algorithm:

Start mysqld with the --old-passwords option.

Assign an old-format password to each account that has had its password updated to the longer 4.1 format. You can identify these accounts with the following query:

mysql> SELECT Host, User, Password FROM mysql.user

-> WHERE LENGTH(Password) > 16;

For each account record displayed by the query, use the Host and User values and assign a password using the OLD_PASSWORD() function and either SET PASSWORD or UPDATE, as described earlier.

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