• サーバへssh接続。

$ ssh root@xxxxxxxxx.ip-51-68-xxx.eu

• root パスワード変更

# passwd
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

• rootでのsshログイン不許可設定

# sed -i.org -e "s/^PermitRootLogin yes/#PermitRootLogin yes/" /etc/ssh/sshd_config
# diff /etc/ssh/sshd_config.org /etc/ssh/sshd_config
< PermitRootLogin yes
---
> #PermitRootLogin yes

• ユーザ作成とsudo設定

# useradd -d /home/<username> -s /bin/bash -m <username>
# passwd <username>
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
# usermod -aG sudo <username>

• アップグレードおよびリブート。

# apt-get update && apt-get -y upgrade && apt-get -y dist-upgrade && apt-get -y autoremove
# reboot

• root でsshできないことを確認。

$ ssh root@xxxxxxxx.ip-51-68-xxx.eu
root@xxxxxxxx.ip-51-68-xxx.eu's password:
Permission denied, please try again.

• 作成した一般ユーザでアクセスできることを確認

$ ssh -o "ServerAliveInterval 60" username@xxxxxxxx.ip-51-68-xxx.eu

• 環境確認

$ uname -a
Linux ns3130598 4.15.0-42-generic #45-Ubuntu SMP Thu Nov 15 19:32:57 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
$ cat /etc/issue

  Ubuntu 18.04.1 LTS

  Linux ubuntu 4.15.0-42-generic #45-Ubuntu SMP Thu Nov 15 19:32:57 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

...

$ sudo netstat -tulnp
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 127.0.0.1:53            0.0.0.0:*               LISTEN      1375/named
tcp        0      0 127.0.0.53:53           0.0.0.0:*               LISTEN      1231/systemd-resolv
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      1614/sshd
tcp        0      0 127.0.0.1:953           0.0.0.0:*               LISTEN      1375/named
tcp6       0      0 ::1:53                  :::*                    LISTEN      1375/named
tcp6       0      0 :::22                   :::*                    LISTEN      1614/sshd
tcp6       0      0 ::1:953                 :::*                    LISTEN      1375/named
udp        0      0 127.0.0.1:53            0.0.0.0:*                           1375/named
udp        0      0 127.0.0.53:53           0.0.0.0:*                           1231/systemd-resolv
udp6       0      0 ::1:53                  :::*                                1375/named