mysql Error : Missing system table mysql.proxies_priv

To remove the error, run following command

sudo mysql_upgrade -u root -p

If your system does not identify mysql_upgrade, you can try these two commands.

mysqlcheck --no-defaults --check-upgrade --all-databases --auto-repair
mysql < fix_priv_tables

InnoDB: Database was not shut down normally!

mysql crashes frequently

Solution : 1
Restart your machine.

If it does not solve the problem and mysql crashes frequently with same error.

Solution : 2
Check attribute max_allowed_packet in the file my.cnf

max_allowed_packet=128M
OR
max_allowed_packet=256M

Restart mysql.
If it is not there, add it and restart mysql.

but if it does not solve the problem and mysql crashes frequently with same error.

Solution : 3
Repair all databases of mysql using following commands.

mysqlcheck --no-defaults --check-upgrade --all-databases --auto-repair
mysqlcheck --no-defaults --all-databases --fix-db-names --fix-table-names

If still your mysql crashes frequently.
Reinstall it again.

Allow / Block ssh / sftp for specific Users / Group

Allow ssh / sftp to selected users
If you want to give ssh / sftp privileges to certain users, here is the solution.
Add following line in the file /etc/sshd_config

AllowUsers username1 username2 username3

If you want to allow certain groups only, add following line in /etc/sshd_config

AllowGroups username1 username2 username3

Restart service ssh.

Block ssh / sftp for selected users
If you want to restrict certain users to access ssh / sftp, here is the solution.
Add following line in the file /etc/sshd_config

DenyUsers username1 username2 username3

If you want to block certain groups only, add following line in /etc/sshd_config

DenyGroups username1 username2 username3

Restart service ssh.