I've found these six lines repeated over and over, easily hundreds of times, in the /var/logs/auth.log file of my Ubuntu server. I'm no sys-admin, so I'm not really sure what's going on, but it seems like my server is under attack.Is that what's going on? Is there anything I can do to prevent it?Oct 31 10:43:48 HKS sshd[5414]: error: Could not load host key: /etc/ssh/ssh_host_ed25519_keyOct 31 10:43:50 HKS sshd[5414]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=221.229.172.110  user=rootOct 31 10:43:52 HKS sshd[5414]: Failed password for root from 221.229.172.110 port 11962 ssh2Oct 31 10:43:57 HKS sshd[5414]: message repeated 2 times: [ Failed password for root from 221.229.172.110 port 11962 ssh2]Oct 31 10:43:57 HKS sshd[5414]: Received disconnect from 221.229.172.110: 11:  [preauth]Oct 31 10:43:57 HKS sshd[5414]: PAM 2 more authentication failures; logname= uid=0 euid=0 tty=ssh ruser= rhost=221.229.172.110  user=rootI started looking into this because the server mysteriously rebooted, and I'm still not sure why. It's probably related to this, right? I'm pretty sure this part of auth.log is related to the reboot:Oct 30 19:44:32 HKS sshd[29546]: error: Could not load host key: /etc/ssh/ssh_host_ed25519_keyOct 30 19:44:33 HKS sshd[29546]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=221.229.172.110  user=rootOct 30 19:44:36 HKS sshd[29546]: Failed password for root from 221.229.172.110 port 48035 ssh2Oct 30 19:44:41 HKS sshd[29546]: message repeated 2 times: [ Failed password for root from 221.229.172.110 port 48035 ssh2]Oct 30 19:44:41 HKS sshd[29546]: Received disconnect from 221.229.172.110: 11:  [preauth]Oct 30 19:44:41 HKS sshd[29546]: PAM 2 more authentication failures; logname= uid=0 euid=0 tty=ssh ruser= rhost=221.229.172.110  user=rootOct 30 20:05:32 HKS systemd-logind[723]: New seat seat0.Oct 30 20:05:32 HKS systemd-logind[723]: Watching system buttons on /dev/input/event0 (Power Button)Oct 30 20:05:33 HKS sshd[844]: Server listening on 0.0.0.0 port 22.Oct 30 20:05:33 HKS sshd[844]: Server listening on :: port 22.Oct 30 20:05:33 HKS sshd[887]: error: Could not load host key: /etc/ssh/ssh_host_ed25519_keyOct 30 20:05:34 HKS sshd[887]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=221.229.172.110  user=rootOct 30 20:05:36 HKS sshd[887]: Failed password for root from 221.229.172.110 port 16680 ssh2Oct 30 20:05:40 HKS sshd[887]: message repeated 2 times: [ Failed password for root from 221.229.172.110 port 16680 ssh2]Oct 30 20:05:41 HKS sshd[887]: Received disconnect from 221.229.172.110: 11:  [preauth]Oct 30 20:05:41 HKS sshd[887]: PAM 2 more authentication failures; logname= uid=0 euid=0 tty=ssh ruser= rhost=221.229.172.110  user=root
What do these lines in my auth.log mean?
ubuntu