昔は SSL 証明書は購入することが必要でしたが、最近は Let’s Encrypt が普及したことで無料で https 対応できるようになりました。
ただこの証明書は有効期間が短いため、適切に設定しておかないとすぐに期限切れになってしまいます。 これは、 certbot を使って Ubuntu の Nginx の指定ドメインに対してセットアップするときの忘備録です。

certbot のリポジトリ追加とインストール

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$ sudo add-apt-repository ppa:certbot/certbot
$ sudo apt-get update
$ sudo apt-get install python-certbot-nginx

指定したドメインに certbot を設定する

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$ sudo certbot --nginx -d yourdomain.com

実行すると、下記のとおり出力されます。 途中訊かれるのは http のリクエストを https にリダイレクトするかどうかです。

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Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator nginx, Installer nginx
Obtaining a new certificate
Performing the following challenges:
http-01 challenge for yourdomain.com
Waiting for verification...
Cleaning up challenges
Deploying Certificate to VirtualHost /etc/nginx/sites-enabled/yourdomain.com

Please choose whether or not to redirect HTTP traffic to HTTPS, removing HTTP access.
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1: No redirect - Make no further changes to the webserver configuration.
2: Redirect - Make all requests redirect to secure HTTPS access. Choose this for
new sites, or if you're confident your site works on HTTPS. You can undo this
change by editing your web server's configuration.
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Select the appropriate number [1-2] then [enter] (press 'c' to cancel): 2
Redirecting all traffic on port 80 to ssl in /etc/nginx/sites-enabled/yourdomain.com

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Congratulations! You have successfully enabled https://yourdomain.com

You should test your configuration at:
https://www.ssllabs.com/ssltest/analyze.html?d=yourdomain.com
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IMPORTANT NOTES:
- Congratulations! Your certificate and chain have been saved at:
/etc/letsencrypt/live/yourdomain.com/fullchain.pem
Your key file has been saved at:
/etc/letsencrypt/live/yourdomain.com/privkey.pem
Your cert will expire on 2019-07-07. To obtain a new or tweaked
version of this certificate in the future, simply run certbot again
with the "certonly" option. To non-interactively renew *all* of
your certificates, run "certbot renew"
- If you like Certbot, please consider supporting our work by:

Donating to ISRG / Let's Encrypt: https://letsencrypt.org/donate
Donating to EFF: https://eff.org/donate-le

外部からの SSL 設定の確認

上記の You should test your configuration at: にある通り、
ブラウザで https://www.ssllabs.com/ssltest/analyze.html?d=_yourdomain.com_ を開いて適切かどうか確認しましょう。

自動更新の確認

続いて自動更新のドライランを試します。

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$ sudo certbot renew --dry-run

実行すると、下記のとおり出力されます。 Congratulations, all renewals succeeded. と成功しています。

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Saving debug log to /var/log/letsencrypt/letsencrypt.log

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Processing /etc/letsencrypt/renewal/yourdomain.com.conf
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Cert not due for renewal, but simulating renewal for dry run
Plugins selected: Authenticator nginx, Installer nginx
Renewing an existing certificate
Performing the following challenges:
http-01 challenge for yourdomain.com
Waiting for verification...
Cleaning up challenges

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new certificate deployed with reload of nginx server; fullchain is
/etc/letsencrypt/live/yourdomain.com/fullchain.pem
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** DRY RUN: simulating 'certbot renew' close to cert expiry
** (The test certificates below have not been saved.)

Congratulations, all renewals succeeded. The following certs have been renewed:
/etc/letsencrypt/live/yourdomain.com/fullchain.pem (success)
** DRY RUN: simulating 'certbot renew' close to cert expiry
** (The test certificates above have not been saved.)
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IMPORTANT NOTES:
- Your account credentials have been saved in your Certbot
configuration directory at /etc/letsencrypt. You should make a
secure backup of this folder now. This configuration directory will
also contain certificates and private keys obtained by Certbot so
making regular backups of this folder is ideal.

cron の確認

上記の自動更新は cron によって定期実行されます。

/etc/cron.d/certbot があることを確認します。

以上で完了です。