Reset SQL users, logs: info about tail

docs/u_and_e.md

@@ -215,6 +215,51 @@ source mailcow.conf
 docker-compose exec mysql-mailcow mysql -u${DBUSER} -p${DBPASS} ${DBNAME} < backup_file.sql
 ```
 
+### Reset MySQL passwords
+
+Stop the stack by running `docker-compose stop`.
+
+When the containers came to a stop, run this command:
+
+```
+docker-compose run --rm --entrypoint '/bin/sh -c "gosu mysql mysqld --skip-grant-tables & sleep 10 && mysql -hlocalhost -uroot && exit 0"' mysql-mailcow
+```
+
+**1\. Find database name**
+
+```
+MariaDB [(none)]> show databases;
++--------------------+
+| Database           |
++--------------------+
+| information_schema |
+| mailcow_database   | <=====
+| mysql              |
+| performance_schema |
++--------------------+
+4 rows in set (0.00 sec)
+```
+
+**2\. Reset one or more users
+
+Both "password" and "authentication_string" exist. Currently "password" is used, but better set both.
+
+```
+MariaDB [(none)]> SELECT user FROM mysql.user;
++--------------+
+| user         |
++--------------+
+| mailcow_user | <===== 
+| root         |
++--------------+
+2 rows in set (0.00 sec)
+
+MariaDB [(none)]> FLUSH PRIVILEGES;
+MariaDB [(none)]> UPDATE mysql.user SET authentication_string = PASSWORD('gotr00t'), password = PASSWORD('gotr00t') WHERE User = 'root' AND Host = '%';
+MariaDB [(none)]> UPDATE mysql.user SET authentication_string = PASSWORD('mookuh'), password = PASSWORD('mookuh') WHERE User = 'mailcow' AND Host = '%';
+MariaDB [(none)]> FLUSH PRIVILEGES;
+```
+
 ## Debugging
 
 You can use `docker-compose logs $service-name` for all containers.
@@ -223,6 +268,8 @@ Run `docker-compose logs` for all logs at once.
 
 Follow the log output by running docker-compose with `logs -f`.
 
+Limit the output by calling logs with `--tail=300` like `docker-compose logs --tail=300 mysql-mailcow`.
+
 ## Redirect port 80 to 443
 
 Since February the 28th 2017 mailcow does come with port 80 and 443 enabled.