Only allow required users to login.

Mac: in “System Preferences/Sharing/Remote Login”, choose users in the “Allow access for:” box.

First: deny connection from all in /etc/hosts.deny

# /etc/hosts.deny
# deny from all
sshd: ALL

Second: specify the allowed IPs in /etc/hosts.allow

# /etc/hosts.allow
# allow intranet IPs (192.168.1.*)
# allow IPMB IPs (172.*.*.*)
# allow Academia Sinica IPs (140.109.*.*)
sshd: 192.168.1., 172., 140.109.: allow

Mac:

grep sshd /var/log/system.log

Ubuntu:

grep sshd /var/log/auth.log

Install fail2ban

Ubuntu:

# install
sudo apt install fail2ban
# edit the conf file /etc/fail2ban/jail.conf as necessary
# for example, increase 'bantime' or reduce 'maxretry' to enhance the security level
# restart the service
sudo /etc/init.d/fail2ban restart 
# regex test
sudo fail2ban-regex /var/log/auth.log /etc/fail2ban/filter.d/sshd.conf
# check status
sudo /etc/init.d/fail2ban status
# check client status
sudo fail2ban-client status
sudo fail2ban-client status sshd
# check log
cat /var/log/fail2ban.log
# unban
sudo fail2ban-client set sshd unbanip xxx.xxx.xxx.xxx

If registered at http://www.blocklist.de, edit /etc/fail2ban/jail.conf to add:

destemail = fail2ban@blocklist.de
sendermail = fail2ban@your-server.tld