See all SSH login attempts

Debian
View all failed login attempts
cat /var/log/auth.log | grep 'sshd.*Invalid'

View all successful logins
cat /var/log/auth.log | grep 'sshd.*opened'

RedHat/Centos
View all failed login attempts
cat /var/log/secure | grep 'sshd.*Invalid'

View all successful logins
cat /var/log/secure | grep 'sshd.*opened'