我使用 freenas 服务器已经快一年了，这是本月内我第三次收到安全电子邮件，其中包含大量失败的 ssh 登录尝试，而这些尝试不是来自我或我认识的任何人。
freenas.local login failures:
May 3 00:00:38 freenas sshd[40708]: Invalid user postgres from 222.138.139.252
May 3 00:00:38 freenas sshd[40708]: input_userauth_request: invalid user postgres [preauth]
May 3 00:00:38 freenas sshd[40708]: Failed password for invalid user postgres from 222.138.139.252 port 9224 ssh2
May 3 00:13:20 freenas sshd[40981]: Invalid user zabbix from 154.66.147.4
May 3 00:13:20 freenas sshd[40981]: input_userauth_request: invalid user zabbix [preauth]
May 3 00:13:20 freenas sshd[40981]: Failed password for invalid user zabbix from 154.66.147.4 port 58938 ssh2
May 3 00:21:41 freenas sshd[41198]: Failed password for root from 222.138.139.252 port 9224 ssh2
May 3 00:43:53 freenas sshd[41773]: Invalid user admin from 222.138.139.252
May 3 00:43:53 freenas sshd[41773]: input_userauth_request: invalid user admin [preauth]
May 3 00:43:53 freenas sshd[41773]: Failed password for invalid user admin from 222.138.139.252 port 9224 ssh2
May 3 00:43:54 freenas sshd[41773]: Failed password for invalid user admin from 222.138.139.252 port 9224 ssh2
May 3 01:05:44 freenas sshd[42330]: Failed password for root from 222.138.139.252 port 9224 ssh2
May 3 01:26:47 freenas sshd[42820]: Invalid user guest from 222.138.139.252
May 3 01:26:47 freenas sshd[42820]: input_userauth_request: invalid user guest [preauth]
May 3 01:26:47 freenas sshd[42820]: Failed password for invalid user guest from 222.138.139.252 port 9224 ssh2
May 3 01:47:48 freenas sshd[43330]: Invalid user test from 222.138.139.252
May 3 01:47:48 freenas sshd[43330]: input_userauth_request: invalid user test [preauth]
May 3 01:47:48 freenas sshd[43330]: Failed password for invalid user test from 222.138.139.252 port 9224 ssh2
May 3 01:53:07 freenas sshd[43460]: Invalid user guest from 31.184.198.95
May 3 01:53:07 freenas sshd[43460]: input_userauth_request: invalid user guest [preauth]
May 3 01:53:07 freenas sshd[43460]: Failed password for invalid user guest from 31.184.198.95 port 51000 ssh2
May 3 01:54:03 freenas sshd[43497]: Invalid user guest from 31.184.198.95
May 3 01:54:03 freenas sshd[43497]: input_userauth_request: invalid user guest [preauth]
May 3 01:54:03 freenas sshd[43497]: Failed password for invalid user guest from 31.184.198.95 port 41555 ssh2
May 3 01:54:04 freenas sshd[43499]: Invalid user guest from 31.184.198.95
May 3 01:54:04 freenas sshd[43499]: input_userauth_request: invalid user guest [preauth]
May 3 01:54:04 freenas sshd[43499]: Failed password for invalid user guest from 31.184.198.95 port 41915 ssh2
May 3 02:08:41 freenas sshd[43880]: Invalid user info from 222.138.139.252
May 3 02:08:41 freenas sshd[43880]: input_userauth_request: invalid user info [preauth]
May 3 02:08:41 freenas sshd[43880]: Failed password for invalid user info from 222.138.139.252 port 9224 ssh2
May 3 02:12:58 freenas sshd[43989]: Invalid user admin from 31.184.198.95
May 3 02:12:58 freenas sshd[43989]: input_userauth_request: invalid user admin [preauth]
May 3 02:12:58 freenas sshd[43989]: Failed password for invalid user admin from 31.184.198.95 port 50234 ssh2
May 3 02:30:36 freenas sshd[44406]: Invalid user ubnt from 222.138.139.252
May 3 02:30:36 freenas sshd[44406]: input_userauth_request: invalid user ubnt [preauth]
May 3 02:30:36 freenas sshd[44406]: Failed password for invalid user ubnt from 222.138.139.252 port 9224 ssh2
May 3 02:30:43 freenas sshd[44408]: Invalid user admin from 31.184.198.95
May 3 02:30:43 freenas sshd[44408]: input_userauth_request: invalid user admin [preauth]
May 3 02:30:43 freenas sshd[44408]: Failed password for invalid user admin from 31.184.198.95 port 38066 ssh2
May 3 02:53:19 freenas sshd[44953]: Invalid user PlcmSpIp from 222.138.139.252
May 3 02:53:19 freenas sshd[44953]: input_userauth_request: invalid user PlcmSpIp [preauth]
May 3 02:53:19 freenas sshd[44953]: Failed password for invalid user PlcmSpIp from 222.138.139.252 port 9224 ssh2
May 3 03:18:35 freenas sshd[46224]: Failed password for root from 222.138.139.252 port 9224 ssh2
May 3 03:18:36 freenas sshd[46224]: Failed password for root from 222.138.139.252 port 9224 ssh2
May 3 03:18:36 freenas sshd[46224]: Failed password for root from 222.138.139.252 port 9224 ssh2
May 3 03:23:59 freenas sshd[46343]: Failed password for root from 220.124.151.130 port 53283 ssh2
May 3 03:24:02 freenas sshd[46345]: Failed password for root from 220.124.151.130 port 54390 ssh2
May 3 03:24:04 freenas sshd[46347]: Failed password for root from 220.124.151.130 port 55220 ssh2
May 3 03:24:07 freenas sshd[46349]: Invalid user syslog from 220.124.151.130
May 3 03:24:07 freenas sshd[46349]: input_userauth_request: invalid user syslog [preauth]
May 3 03:24:07 freenas sshd[46349]: Failed password for invalid user syslog from 220.124.151.130 port 56104 ssh2
May 3 03:24:09 freenas sshd[46374]: Invalid user syslog from 220.124.151.130
May 3 03:24:09 freenas sshd[46374]: input_userauth_request: invalid user syslog [preauth]
May 3 03:24:09 freenas sshd[46374]: Failed password for invalid user syslog from 220.124.151.130 port 57054 ssh2
May 3 03:42:50 freenas sshd[46890]: Failed password for root from 222.138.139.252 port 9224 ssh2
May 3 03:42:50 freenas sshd[46890]: Failed password for root from 222.138.139.252 port 9224 ssh2
May 3 04:04:26 freenas sshd[47508]: Invalid user ftpuser from 222.138.139.252
May 3 04:04:26 freenas sshd[47508]: input_userauth_request: invalid user ftpuser [preauth]
May 3 04:04:26 freenas sshd[47508]: Failed password for invalid user ftpuser from 222.138.139.252 port 9224 ssh2
May 3 04:28:17 freenas sshd[48044]: Invalid user support from 222.138.139.252
May 3 04:28:17 freenas sshd[48044]: input_userauth_request: invalid user support [preauth]
May 3 04:28:17 freenas sshd[48044]: Failed password for invalid user support from 222.138.139.252 port 9224 ssh2
May 3 04:51:52 freenas sshd[48631]: Invalid user debian from 222.138.139.252
May 3 04:51:52 freenas sshd[48631]: input_userauth_request: invalid user debian [preauth]
May 3 04:51:52 freenas sshd[48631]: Failed password for invalid user debian from 222.138.139.252 port 9224 ssh2
May 3 05:14:51 freenas sshd[49227]: Invalid user ubuntu from 222.138.139.252
May 3 05:14:51 freenas sshd[49227]: input_userauth_request: invalid user ubuntu [preauth]
May 3 05:14:51 freenas sshd[49227]: Failed password for invalid user ubuntu from 222.138.139.252 port 9224 ssh2
May 3 05:14:52 freenas sshd[49227]: Failed password for invalid user ubuntu from 222.138.139.252 port 9224 ssh2
May 3 06:26:16 freenas sshd[50976]: Invalid user ubnt from 146.0.79.253
May 3 06:26:16 freenas sshd[50976]: input_userauth_request: invalid user ubnt [preauth]
May 3 06:26:16 freenas sshd[50976]: Failed password for invalid user ubnt from 146.0.79.253 port 41046 ssh2
May 3 12:45:27 freenas sshd[17582]: Failed password for root from 176.122.49.3 port 51726 ssh2
May 3 12:45:27 freenas sshd[17584]: Invalid user naseej from 176.122.49.3
May 3 12:45:27 freenas sshd[17584]: input_userauth_request: invalid user naseej [preauth]
May 3 12:45:27 freenas sshd[17584]: Failed password for invalid user naseej from 176.122.49.3 port 52087 ssh2
May 3 12:45:29 freenas sshd[17586]: Failed password for root from 176.122.49.3 port 52432 ssh2
May 3 12:45:33 freenas sshd[17588]: Failed password for root from 176.122.49.3 port 53080 ssh2
May 3 12:45:37 freenas sshd[17590]: Invalid user syslog from 176.122.49.3
May 3 12:45:37 freenas sshd[17590]: input_userauth_request: invalid user syslog [preauth]
May 3 12:45:37 freenas sshd[17590]: Failed password for invalid user syslog from 176.122.49.3 port 54596 ssh2
May 3 12:45:38 freenas sshd[17592]: Invalid user shadmin from 176.122.49.3
May 3 12:45:38 freenas sshd[17592]: input_userauth_request: invalid user shadmin [preauth]
May 3 12:45:38 freenas sshd[17592]: Failed password for invalid user shadmin from 176.122.49.3 port 56197 ssh2
May 3 12:45:39 freenas sshd[17594]: Invalid user icnanker from 176.122.49.3
May 3 12:45:39 freenas sshd[17594]: input_userauth_request: invalid user icnanker [preauth]
May 3 12:45:39 freenas sshd[17594]: Failed password for invalid user icnanker from 176.122.49.3 port 56891 ssh2
May 3 12:45:40 freenas sshd[17596]: Failed password for root from 176.122.49.3 port 57199 ssh2
May 3 12:45:40 freenas sshd[17598]: Invalid user nano from 176.122.49.3
May 3 12:45:40 freenas sshd[17598]: input_userauth_request: invalid user nano [preauth]
May 3 12:45:40 freenas sshd[17598]: Failed password for invalid user nano from 176.122.49.3 port 57535 ssh2
May 3 12:45:42 freenas sshd[17600]: Failed password for root from 176.122.49.3 port 57814 ssh2
May 3 12:45:46 freenas sshd[17602]: Failed password for root from 176.122.49.3 port 58502 ssh2
May 3 12:45:49 freenas sshd[17604]: Failed password for root from 176.122.49.3 port 59876 ssh2
May 3 12:45:51 freenas sshd[17606]: Invalid user syslog from 176.122.49.3
May 3 12:45:51 freenas sshd[17606]: input_userauth_request: invalid user syslog [preauth]
May 3 12:45:51 freenas sshd[17606]: Failed password for invalid user syslog from 176.122.49.3 port 32837 ssh2
May 3 12:45:53 freenas sshd[17608]: Failed password for root from 176.122.49.3 port 33403 ssh2
-- End of security output --
那么，在哪里或如何我可以查看是否有我不认识的人能够登录我的服务器，如果可以，我应该采取什么措施来升级我的服务器的安全性？