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SSL, HTTPS, HSTS and additional security measures

It's highly recommended to enable SSL/TLS on your server, both for the web app and email server.

Using Certbot to get a certificate

This doc will use https://letsencrypt.org to get a free SSL certificate for app.mydomain.com that's used by both Postfix and Nginx. Let's Encrypt provides Certbot, a tool to obtain and renew SSL certificates.

To install Certbot, please follow instructions on https://certbot.eff.org

Then obtain a certificate for Nginx, use the following command. You'd need to provide an email so Let's Encrypt can send you notifications when your domain is about to expire.

sudo certbot --nginx

After this step, you should see some "managed by Certbot" lines in /etc/nginx/sites-enabled/simplelogin

Securing Postfix

Now let's use the new certificate for our Postfix.

Replace these lines in /etc/postfix/main.cf

smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key

with

smtpd_tls_cert_file = /etc/letsencrypt/live/app.mydomain.com/fullchain.pem
smtpd_tls_key_file = /etc/letsencrypt/live/app.mydomain.com/privkey.pem

Make sure to replace app.mydomain.com with your own domain.

Updating simplelogin.env

Make sure to change the URL in simplelogin.env to https://app.mydomain.com, otherwise not all page assets will load securely, and some functionality (e.g. Webauthn) will break. You will need to reload the docker containers for this to take effect.

HTTP Strict Transport Security (HSTS)

HSTS is an extra step you can take to protect your web app from certain man-in-the-middle attacks. It does this by specifying an amount of time (usually a really long one) for which you should only accept HTTPS connections, not HTTP ones. Because of this you should only enable HSTS once you know HTTPS is working correctly, as otherwise you may find your browser blocking you from accessing your own web app.

To enable HSTS, add the following line to the server block of the Nginx configuration file:

add_header Strict-Transport-Security "max-age: 31536000; includeSubDomains" always;

(The max-age is the time in seconds to not permit a HTTP connection, in this case it's one year.)

Now, reload Nginx:

sudo systemctl reload nginx

Additional security measures

For additional security, we recommend you take some extra steps.

Enable Certificate Authority Authorization (CAA)

Certificate Authority Authorization is a step you can take to restrict the list of certificate authorities that are allowed to issue certificates for your domains.

Use SSLMate’s CAA Record Generator to create a CAA record with the following configuration:

  • flags: 0
  • tag: issue
  • value: "letsencrypt.org"

To verify if the DNS works, the following command

dig @1.1.1.1 mydomain.com caa

should return:

mydomain.com.	3600	IN	CAA	0 issue "letsencrypt.org"

SMTP MTA Strict Transport Security (MTA-STS)

MTA-STS is an extra step you can take to broadcast the ability of your instance to receive and, optionally enforce, TSL-secure SMTP connections to protect email traffic.

Enabling MTA-STS requires you serve a specific file from subdomain mta-sts.domain.com on a well-known route.

Create a text file /var/www/.well-known/mta-sts.txt with the content:

version: STSv1
mode: testing
mx: app.mydomain.com
max_age: 86400

It is recommended to start with mode: testing for starters to get time to review failure reports. Add as many mx: domain entries as you have matching MX records in your DNS configuration.

Create a TXT record for _mta-sts.mydomain.com. with the following value:

v=STSv1; id=UNIX_TIMESTAMP

With UNIX_TIMESTAMP being the current date/time.

Use the following command to generate the record:

echo "v=STSv1; id=$(date +%s)"

To verify if the DNS works, the following command

dig @1.1.1.1 _mta-sts.mydomain.com txt

should return a result similar to this one:

_mta-sts.mydomain.com.	3600	IN	TXT	"v=STSv1; id=1689416399"

Create an additional Nginx configuration in /etc/nginx/sites-enabled/mta-sts with the following content:

server {
	server_name mta-sts.mydomain.com;
	root /var/www;
	listen 80;

	location ^~ /.well-known {}
}

Restart Nginx with the following command:

sudo service nginx restart

A correct configuration of MTA-STS, however, requires that the certificate used to host the mta-sts subdomain matches that of the subdomain referred to by the MX record from the DNS. In other words, both mta-sts.mydomain.com and app.mydomain.com must share the same certificate.

The easiest way to do this is to expand the certificate associated with app.mydomain.com to also support the mta-sts subdomain using the following command:

certbot --expand --nginx -d app.mydomain.com,mta-sts.mydomain.com

SMTP TLS Reporting

TLSRPT is used by SMTP systems to report failures in establishing TLS-secure sessions as broadcast by the MTA-STS configuration.

Configuring MTA-STS in mode: testing as shown in the previous section gives you time to review failures from some SMTP senders.

Create a TXT record for _smtp._tls.mydomain.com. with the following value:

v=TSLRPTv1; rua=mailto:YOUR_EMAIL

The TLSRPT configuration at the DNS level allows SMTP senders that fail to initiate TLS-secure sessions to send reports to a particular email address. We suggest creating a tls-reports alias in SimpleLogin for this purpose.

To verify if the DNS works, the following command

dig @1.1.1.1 _smtp._tls.mydomain.com txt

should return a result similar to this one:

_smtp._tls.mydomain.com.	3600	IN	TXT	"v=TSLRPTv1; rua=mailto:[email protected]"