MySQL :: Security in MySQL :: 4.17 Troubleshooting Problems Connecting to MySQL

Make sure that the server is running. If it is not, clients cannot connect to it. For example, if an attempt to connect to the server fails with a message such as one of those following, one cause might be that the server is not running:

$> mysql
ERROR 2003: Can't connect to MySQL server on 'host_name' (111)
$> mysql
ERROR 2002: Can't connect to local MySQL server through socket
'/tmp/mysql.sock' (111)

It might be that the server is running, but you are trying to connect using a TCP/IP port, named pipe, or Unix socket file different from the one on which the server is listening. To correct this when you invoke a client program, specify a --port option to indicate the proper port number, or a --socket option to indicate the proper named pipe or Unix socket file. To find out where the socket file is, you can use this command:

$> netstat -ln | grep mysql

Make sure that the server has not been configured to ignore network connections or (if you are attempting to connect remotely) that it has not been configured to listen only locally on its network interfaces. If the server was started with the skip_networking system variable enabled, it does not accept TCP/IP connections at all. If the server was started with the bind_address system variable set to 127.0.0.1, it listens for TCP/IP connections only locally on the loopback interface and does not accept remote connections.

Check to make sure that there is no firewall blocking access to MySQL. Your firewall may be configured on the basis of the application being executed, or the port number used by MySQL for communication (3306 by default). Under Linux or Unix, check your IP tables (or similar) configuration to ensure that the port has not been blocked. Under Windows, applications such as ZoneAlarm or Windows Firewall may need to be configured not to block the MySQL port.

The grant tables must be properly set up so that the server can use them for access control. For some distribution types (such as binary distributions on Windows, or RPM and DEB distributions on Linux), the installation process initializes the MySQL data directory, including the mysql system database containing the grant tables. For distributions that do not do this, you must initialize the data directory manually. For details, see Chapter 3, Postinstallation Setup and Testing.

To determine whether you need to initialize the grant tables, look for a mysql directory under the data directory. (The data directory normally is named data or var and is located under your MySQL installation directory.) Make sure that you have a file named user.MYD in the mysql database directory. If not, initialize the data directory. After doing so and starting the server, you should be able to connect to the server.

After a fresh installation, if you try to log on to the server as root without using a password, you might get the following error message.

$> mysql -u root 
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)

It means a root password has already been assigned during installation and it has to be supplied. See Section 3.4, “Securing the Initial MySQL Account” on the different ways the password could have been assigned and, in some cases, how to find it. If you need to reset the root password, see instructions in How to Reset the Root Password. After you have found or reset your password, log on again as root using the --password (or -p) option:

$> mysql -u root -p
Enter password:

However, the server is going to let you connect as root without using a password if you have initialized MySQL using mysqld --initialize-insecure (see Section 3.1, “Initializing the Data Directory” for details). That is a security risk, so you should set a password for the root account; see Section 3.4, “Securing the Initial MySQL Account” for instructions.

If you have updated an existing MySQL installation to a newer version, did you perform the MySQL upgrade procedure? If not, do so. The structure of the grant tables changes occasionally when new capabilities are added, so after an upgrade you should always make sure that your tables have the current structure. For instructions, see Upgrading MySQL.

If a client program receives the following error message when it tries to connect, it means that the server expects passwords in a newer format than the client is capable of generating:

$> mysql
Client does not support authentication protocol requested
by server; consider upgrading MySQL client

For information on how to deal with this, see Section 6.1.3, “Migrating Away from Pre-4.1 Password Hashing and the mysql_old_password Plugin”.

Remember that client programs use connection parameters specified in option files or environment variables. If a client program seems to be sending incorrect default connection parameters when you have not specified them on the command line, check any applicable option files and your environment. For example, if you get Access denied when you run a client without any options, make sure that you have not specified an old password in any of your option files!

You can suppress the use of option files by a client program by invoking it with the --no-defaults option. For example:

$> mysqladmin --no-defaults -u root version

The option files that clients use are listed in Using Option Files. Environment variables are listed in Environment Variables.

If you get the following error, it means that you are using an incorrect root password:

$> mysqladmin -u root -pxxxx ver
Access denied for user 'root'@'localhost' (using password: YES)

If the preceding error occurs even when you have not specified a password, it means that you have an incorrect password listed in some option file. Try the --no-defaults option as described in the previous item.

For information on changing passwords, see Section 4.10, “Assigning Account Passwords”.

If you have lost or forgotten the root password, see How to Reset the Root Password.

localhost is a synonym for your local host name, and is also the default host to which clients try to connect if you specify no host explicitly.

You can use a --host=127.0.0.1 option to name the server host explicitly. This makes a TCP/IP connection to the local mysqld server. You can also use TCP/IP by specifying a --host option that uses the actual host name of the local host. In this case, the host name must be specified in a user table row on the server host, even though you are running the client program on the same host as the server.

The Access denied error message tells you who you are trying to log in as, the client host from which you are trying to connect, and whether you were using a password. Normally, you should have one row in the user table that exactly matches the host name and user name that were given in the error message. For example, if you get an error message that contains using password: NO, it means that you tried to log in without a password.

If you get an Access denied error when trying to connect to the database with mysql -u user_name, you may have a problem with the user table. Check this by executing mysql -u root mysql and issuing this SQL statement:

SELECT * FROM user;

The result should include a row with the Host and User columns matching your client's host name and your MySQL user name.

If the following error occurs when you try to connect from a host other than the one on which the MySQL server is running, it means that there is no row in the user table with a Host value that matches the client host:

Host ... is not allowed to connect to this MySQL server

You can fix this by setting up an account for the combination of client host name and user name that you are using when trying to connect.

If you do not know the IP address or host name of the machine from which you are connecting, you should put a row with '%' as the Host column value in the user table. After trying to connect from the client machine, use a SELECT USER() query to see how you really did connect. Then change the '%' in the user table row to the actual host name that shows up in the log. Otherwise, your system is left insecure because it permits connections from any host for the given user name.

On Linux, another reason that this error might occur is that you are using a binary MySQL version that is compiled with a different version of the glibc library than the one you are using. In this case, you should either upgrade your operating system or glibc, or download a source distribution of MySQL version and compile it yourself. A source RPM is normally trivial to compile and install, so this is not a big problem.

If you specify a host name when trying to connect, but get an error message where the host name is not shown or is an IP address, it means that the MySQL server got an error when trying to resolve the IP address of the client host to a name:

$> mysqladmin -u root -pxxxx -h some_hostname ver
Access denied for user 'root'@'' (using password: YES)

If you try to connect as root and get the following error, it means that you do not have a row in the user table with a User column value of 'root' and that mysqld cannot resolve the host name for your client:

Access denied for user ''@'unknown'

These errors indicate a DNS problem. To fix it, execute mysqladmin flush-hosts to reset the internal DNS host cache. See DNS Lookups and the Host Cache.

Some permanent solutions are:

Determine what is wrong with your DNS server and fix it.
Specify IP addresses rather than host names in the MySQL grant tables.
Put an entry for the client machine name in /etc/hosts on Unix or \windows\hosts on Windows.
Start mysqld with the skip_name_resolve system variable enabled.
Start mysqld with the --skip-host-cache option.
On Unix, if you are running the server and the client on the same machine, connect to localhost. For connections to localhost, MySQL programs attempt to connect to the local server by using a Unix socket file, unless there are connection parameters specified to ensure that the client makes a TCP/IP connection. For more information, see Connecting to the MySQL Server Using Command Options.
On Windows, if you are running the server and the client on the same machine and the server supports named pipe connections, connect to the host name . (period). Connections to . use a named pipe rather than TCP/IP.

If mysql -u root works but mysql -h your_hostname -u root results in Access denied (where your_hostname is the actual host name of the local host), you may not have the correct name for your host in the user table. A common problem here is that the Host value in the user table row specifies an unqualified host name, but your system's name resolution routines return a fully qualified domain name (or vice versa). For example, if you have a row with host 'pluto' in the user table, but your DNS tells MySQL that your host name is 'pluto.example.com', the row does not work. Try adding a row to the user table that contains the IP address of your host as the Host column value. (Alternatively, you could add a row to the user table with a Host value that contains a wildcard (for example, 'pluto.%'). However, use of Host values ending with % is insecure and is not recommended!)

If mysql -u user_name works but mysql -u user_name some_db does not, you have not granted access to the given user for the database named some_db.

If mysql -u user_name works when executed on the server host, but mysql -h host_name -u user_name does not work when executed on a remote client host, you have not enabled access to the server for the given user name from the remote host.

If you cannot figure out why you get Access denied, remove from the user table all rows that have Host values containing wildcards (rows that contain '%' or '_' characters). A very common error is to insert a new row with Host='%' and User='some_user', thinking that this enables you to specify localhost to connect from the same machine. The reason that this does not work is that the default privileges include a row with Host='localhost' and User=''. Because that row has a Host value 'localhost' that is more specific than '%', it is used in preference to the new row when connecting from localhost! The correct procedure is to insert a second row with Host='localhost' and User='some_user', or to delete the row with Host='localhost' and User=''. After deleting the row, remember to issue a FLUSH PRIVILEGES statement to reload the grant tables. See also Section 4.5, “Access Control, Stage 1: Connection Verification”.

If you are able to connect to the MySQL server, but get an Access denied message whenever you issue a SELECT ... INTO OUTFILE or LOAD DATA statement, your row in the user table does not have the FILE privilege enabled.

If you change the grant tables directly (for example, by using INSERT, UPDATE, or DELETE statements) and your changes seem to be ignored, remember that you must execute a FLUSH PRIVILEGES statement or a mysqladmin flush-privileges command to cause the server to reload the privilege tables. Otherwise, your changes have no effect until the next time the server is restarted. Remember that after you change the root password with an UPDATE statement, you do not need to specify the new password until after you flush the privileges, because the server does not yet know that you have changed the password.

If your privileges seem to have changed in the middle of a session, it may be that a MySQL administrator has changed them. Reloading the grant tables affects new client connections, but it also affects existing connections as indicated in Section 4.9, “When Privilege Changes Take Effect”.

If you have access problems with a Perl, PHP, Python, or ODBC program, try to connect to the server with mysql -u user_name db_name or mysql -u user_name -ppassword db_name. If you are able to connect using the mysql client, the problem lies with your program, not with the access privileges. (There is no space between -p and the password; you can also use the --password=password syntax to specify the password. If you use the -p or --password option with no password value, MySQL prompts you for the password.)

For testing purposes, start the mysqld server with the --skip-grant-tables option. Then you can change the MySQL grant tables and use the SHOW GRANTS statement to check whether your modifications have the desired effect. When you are satisfied with your changes, execute mysqladmin flush-privileges to tell the mysqld server to reload the privileges. This enables you to begin using the new grant table contents without stopping and restarting the server.

If everything else fails, start the mysqld server with a debugging option (for example, --debug=d,general,query). This prints host and user information about attempted connections, as well as information about each command issued. See The DBUG Package.

If you have any other problems with the MySQL grant tables and ask on the MySQL Community Slack, always provide a dump of the MySQL grant tables. You can dump the tables with the mysqldump mysql command. To file a bug report, see the instructions at How to Report Bugs or Problems. In some cases, you may need to restart mysqld with --skip-grant-tables to run mysqldump.