====== SSH ======
===== sshd_config =====
After change, restart sshd unit (systemd)\\
Edit /etc/ssh/sshd_config
==== no IPv6 ====
<code perl>
#AddressFamily any
AddressFamily inet
</code>

==== Root key only ====
<code perl>
PermitRootLogin prohibit-password
#PermitRootLogin yes
</code>
==== Users key only ====
<code perl>
PubkeyAuthentication yes
...
# To disable tunneled clear text passwords, change to no here!
PasswordAuthentication no
PermitEmptyPasswords no
</code>

==== Check ====
<code bash> sudo sshd -T | grep -E '^(passwordauthentication|pubkeyauthentication)'
</code>
===== .ssh/config =====

===== examples =====