What is the debian-sys-maint MySQL user (and more)?

What is debian-sys-maint used for?

One major thing it is used for is telling the server to roll the logs. It needs at least the reload and shutdown privilege.

See the file /etc/logrotate.d/mysql-server

It is used by the /etc/init.d/mysql script to get the status of the server. It is used to gracefully shutdown/reload the server.

Here is the quote from the README.Debian

* MYSQL WON'T START OR STOP?:
=============================
You may never ever delete the special mysql user "debian-sys-maint". This user
together with the credentials in /etc/mysql/debian.cnf are used by the init
scripts to stop the server as they would require knowledge of the mysql root
users password else.

What is the easiest way to restore it after I've lost it?

The best plan is to simply not lose it. If you really lose the password, reset it, using another account. If you have lost all admin privileges on the mysql server follow the guides to reset the root password, then repair the debian-sys-maint.

You could use a command like this to build a SQL file that you can use later to recreate the account.

mysqldump --complete-insert --extended-insert=0 -u root -p mysql | grep 'debian-sys-maint' > debian_user.sql

Is the password in /etc/mysql/debian.cnf already hashed

The password is not hashed/encrypted when installed, but new versions of mysql now have a way to encrypt the credentials (see: https://serverfault.com/a/750363).

The debian-sys-maint user is by default a root equivalent. It is used by certain maintenance scripts on Debian systems, and as a side-effect, allows users with root access on the box to view the plaintext password in /etc/mysql/debian.cnf (good or bad?)

You can re-create the user by:

GRANT ALL PRIVILEGES on *.* TO `debian-sys-maint`@`localhost` IDENTIFIED BY 'your password' WITH GRANT OPTION;

Just make sure the password matches that in /etc/mysql/debian.cnf

I wanted to just comment, but I think correct syntax deserves it's own entry. This will create the debian-sys-maint user:

mysql> GRANT ALL PRIVILEGES on *.* TO 'debian-sys-maint'@'localhost' IDENTIFIED BY 'plaintextpassword' WITH GRANT OPTION; FLUSH PRIVILEGES;

If you still have the /etc/mysql/debian.cnf file, just use the password in there.

Feel free to come up with a more paranoid secure solution.

You could also:

sudo dpkg-reconfigure mysql-server-5.0

Which will give you the option to recreate the debian-sys-maint user. Existing users and databases are safe.

If you need to add the debian-sys-maint user just for logrotate.d purposes, you should not grant ALL PRIVILEGES or the GRANT OPTION -- this is an unnecessary giant security hole. Instead, you can just add the user with the RELOAD privilege like this (assuming you are accessing your db as root, and you replace xxxxxx with your password)

# add the user with the reload right
GRANT RELOAD on *.* TO 'debian-sys-maint'@'localhost' IDENTIFIED BY 'xxxxxx'; 

# reload the rights
FLUSH PRIVILEGES;

# double check
select * from mysql.user;

2019 Update

This answer may be out of date -- please see the strongly opinionated comments below.

Instead of

GRANT ALL PRIVILEGES on *.* TO 'debian-sys-maint'@'localhost' IDENTIFIED BY PASSWORD('your password') WITH GRANT OPTION; FLUSH PRIVILEGES;

I think

GRANT ALL PRIVILEGES on *.* TO 'debian-sys-maint'@'localhost' IDENTIFIED BY 'your password' WITH GRANT OPTION; FLUSH PRIVILEGES;

because the password is not hashed ...?

debian-sys-maint required permissions

Other answers have sufficiently addressed everything except the minimum set of permissions that are required for the debian-sys-maint user. Many of the answers here are simply wrong in that respect, and in fact dangerous. Do not reduce debian-sys-maint privileges (including the grant option) without reading and understanding below:

The Debian maintainer did not give all privileges to the user capriciously. Here is what is required, where and why. Some of these privileges are supersets of others, but I will list them independently in case you want to customize things and remove the requirement for them:

• shutdown and reload, required unsurprising enough, for shutting down or doing a database, done by /etc/init.d/mysql
• select on mysql.user, required for sanity checks done when the database is started, ensuring that there is a root user. Done each startup by /etc/mysql/debian-start (called by /etc/init.d/mysql) with the actual code in the function check_root_accounts in the file /usr/share/mysql/debian-start.inc.sh
• select on information_schema.tables, global select, required for checking for crashed tables. Done each startup by /etc/mysql/debian-start (called by /etc/init.d/mysql) with the actual code in the function check_for_crashed_tables in the file /usr/share/mysql/debian-start.inc.sh
• global all privileges, required for upgrading tables if/when a new version of MySQL is installed through an update or Debian upgrade. Done each startup by /etc/mysql/debian-start (called by /etc/init.d/mysql) with the actual code in the function upgrade_system_tables_if_necessary in the file /usr/share/mysql/debian-start.inc.sh - actually calls the MySQL binary mysql_upgrade - do not be fooled by the function name (upgrade_system_tables_if_necessary), this can potentially touch all tables - see below

The last one is, of course, the major requirement for privileges. The man page for mysql_upgrade states that:

mysql_upgrade examines all tables in all databases for incompatibilities with the current version of MySQL Server. mysql_upgrade also upgrades the system tables so that you can take advantage of new privileges or capabilities that might have been added.

If mysql_upgrade finds that a table has a possible incompatibility, it performs a table check and, if problems are found, attempts a table repair.

WARNING If you decide to cut down on the privileges that debian-sys-maint has, then make sure you are prepared to manually handle any future debian security updates and/or upgrades that touch MySQL. If you perform an update on the MySQL packages with a reduced debian-sys-maint privilege, and if mysql_upgrade cannot complete as a result, it may leave your database in an undefined (read broken) state. Reducing privileges may not have any apparent day-to-day issues until an update comes along, so do not go by the fact that you have already reduced privileges with no harmful effects as a basis for thinking it is safe.

As a side note to this, take a look at this mysqlperformanceblog post for reasons why you might want to disable the debian-specific stuff.

When using MySQL 5.6+, I would recommend using the mysql_config_editor command to create an entry for the user 'debian-sys-maint'@'localhost' using the relevant password, meaning the password does not need to be stored in plain text on the server.

mysql_config_editor set --login-path=debian-sys-maint --host=localhost --user=debian-sys-maint --password

Following this, the debian specific config file /etc/mysql/debian.cnf can be altered so the username and password details are not stored in the file.

Finally, alter the logrotate file for MySQL so that it uses the login details stored in the ~/.mylogin.cnf file instead of the debian specific file by replacing

/usr/bin/mysqladmin --defaults-file=/etc/mysql/debian.cnf

with

/usr/bin/mysqladmin --login-path=debian-sys-maint

Hope this helps :)

Dave