Поменял на маршрутизаторе порт для подключений к серверу по ssh после такого лога на 20 тысяч строк:

player701server login failures:
Sep 25 00:20:54 player701server sshd[1499]: Invalid user server from 195.154.136.230
Sep 25 00:20:54 player701server sshd[1499]: input_userauth_request: invalid user server [preauth]
Sep 25 00:20:56 player701server sshd[1503]: Invalid user user from 195.154.136.230
Sep 25 00:20:56 player701server sshd[1503]: input_userauth_request: invalid user user [preauth]
Sep 25 00:20:56 player701server sshd[1505]: Invalid user git from 195.154.136.230
Sep 25 00:20:56 player701server sshd[1505]: input_userauth_request: invalid user git [preauth]
Sep 25 00:20:57 player701server sshd[1507]: Invalid user taras from 195.154.136.230
Sep 25 00:20:57 player701server sshd[1507]: input_userauth_request: invalid user taras [preauth]
Sep 25 00:20:58 player701server sshd[1509]: Invalid user oracle from 195.154.136.230
Sep 25 00:20:58 player701server sshd[1509]: input_userauth_request: invalid user oracle [preauth]
Sep 25 00:20:59 player701server sshd[1513]: Invalid user test from 195.154.136.230
Sep 25 00:20:59 player701server sshd[1513]: input_userauth_request: invalid user test [preauth]
Sep 25 00:21:00 player701server sshd[1515]: Invalid user student from 195.154.136.230
Sep 25 00:21:00 player701server sshd[1515]: input_userauth_request: invalid user student [preauth]
Sep 25 00:21:00 player701server sshd[1517]: Invalid user suporte from 195.154.136.230
Sep 25 00:21:00 player701server sshd[1517]: input_userauth_request: invalid user suporte [preauth]
Sep 25 00:21:01 player701server sshd[1519]: Invalid user postgres from 195.154.136.230
Sep 25 00:21:01 player701server sshd[1519]: input_userauth_request: invalid user postgres [preauth]
Sep 25 00:21:02 player701server sshd[1521]: Invalid user nagios from 195.154.136.230
Sep 25 00:21:02 player701server sshd[1521]: input_userauth_request: invalid user nagios [preauth]
Sep 25 00:21:02 player701server sshd[1523]: Invalid user ubuntu from 195.154.136.230
Sep 25 00:21:02 player701server sshd[1523]: input_userauth_request: invalid user ubuntu [preauth]
Sep 25 00:21:03 player701server sshd[1525]: Invalid user ubuntu from 195.154.136.230
Sep 25 00:21:03 player701server sshd[1525]: input_userauth_request: invalid user ubuntu [preauth]
Sep 25 00:21:04 player701server sshd[1527]: Invalid user ubuntu from 195.154.136.230
Sep 25 00:21:04 player701server sshd[1527]: input_userauth_request: invalid user ubuntu [preauth]
Sep 25 00:21:04 player701server sshd[1529]: Invalid user ubuntu from 195.154.136.230
Sep 25 00:21:04 player701server sshd[1529]: input_userauth_request: invalid user ubuntu [preauth]
Sep 25 00:21:05 player701server sshd[1531]: Invalid user ubuntu from 195.154.136.230
Sep 25 00:21:05 player701server sshd[1531]: input_userauth_request: invalid user ubuntu [preauth]
Sep 25 00:21:06 player701server sshd[1533]: Invalid user server1 from 195.154.136.230
Sep 25 00:21:06 player701server sshd[1533]: input_userauth_request: invalid user server1 [preauth]
Sep 25 00:21:06 player701server sshd[1535]: Invalid user server2 from 195.154.136.230
Sep 25 00:21:06 player701server sshd[1535]: input_userauth_request: invalid user server2 [preauth]
Sep 25 00:21:07 player701server sshd[1537]: Invalid user server1 from 195.154.136.230
Sep 25 00:21:07 player701server sshd[1537]: input_userauth_request: invalid user server1 [preauth]
Sep 25 00:21:08 player701server sshd[1539]: Invalid user server2 from 195.154.136.230
Sep 25 00:21:08 player701server sshd[1539]: input_userauth_request: invalid user server2 [preauth]
Sep 25 00:21:08 player701server sshd[1541]: Invalid user python from 195.154.136.230
Sep 25 00:21:08 player701server sshd[1541]: input_userauth_request: invalid user python [preauth]
Sep 25 00:21:09 player701server sshd[1543]: Invalid user python from 195.154.136.230
Sep 25 00:21:09 player701server sshd[1543]: input_userauth_request: invalid user python [preauth]
Sep 25 00:21:10 player701server sshd[1545]: Invalid user kasa1 from 195.154.136.230
Sep 25 00:21:10 player701server sshd[1545]: input_userauth_request: invalid user kasa1 [preauth]
Sep 25 00:21:10 player701server sshd[1547]: Invalid user kasa1 from 195.154.136.230
Sep 25 00:21:10 player701server sshd[1547]: input_userauth_request: invalid user kasa1 [preauth]
Sep 25 00:21:11 player701server sshd[1549]: Invalid user hadoop from 195.154.136.230
Sep 25 00:21:11 player701server sshd[1549]: input_userauth_request: invalid user hadoop [preauth]

Это, как говорится, только верхушка айсберга. Жесть, в общем. И это за один день! Хорошо, что я отключил ежедневные сообщения от сервера по e-mail, оставил только раз в неделю, а то бы вот каждый день такое наблюдал.