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Request a free cert from Let's Encrypt (for iRedMail Enterprise Edition)

iRedMail EE generates a self-signed SSL certificate during installation, it's strongly recommended to use a valid one.

You can either request free cert from vendors like Let's Encrypt, or buy one from other vendors. In this tutorial, we show you how to request a free cert for hostname mail.mydomain.com from Let's Encrypt, and update related configurations to use the cert/key files.

Let's Encrypt supports wildcard host names, but it's not covered in this tutorial, please read its User Guide instead.

Before requesting a cert

Which host names should be supported in the SSL cert?

You must understand which host names you need to support in the SSL cert:

1. The full hostname of your mail server.

   Server hostname is usually used as SMTP/IMAP/POP3 server address in user's mail client application like Outlook, Thunderbird.

   You can get full hostname with command hostname -f on Linux, or hostname on OpenBSD. We use mail.mydomain.com for example in this tutorial.

2. The web host names you need to access via https.

   For example, https://mydomain.com, https://support.mydomain.com, then you need to support mydomain.com and support.mydomain.com in the cert.

3. No need to support mail domain name in SSL cert, unless it's also a web host name.

One cert for all domain names, or one cert for each domain name?

Dovecot and Nginx support reading/loading multiple ssl certs (for different host names), but old Postfix software doesn't, so we recommend to use one cert for all host names for easier management.

Make sure you have correct DNS record for the host names

The way we request free Let's Encrypt cert requires correct A type DNS record for the host name, because Let's Encrypt organization needs to make sure you actually own or control the domain name.

To check the DNS record, you can use dig command like below:

dig +short -t a mail.mydomain.com

It should return the (public) IP address of your server.

Request cert

• Install the certbot package first:

  • on CentOS/Rocky/AlmaLinux: yum install -y certbot (offered by EPEL repo)
  • on Debian/Ubuntu: apt install -y certbot

• Let's Encrypt has request rate limit control, you can request limited times for same domain in one day, but the verification process doesn't have such limit. We suggest run verification process first to make sure it fully matches the requirements.

  Run command below as root user to start the verification (Note: --dry-run argument is important here). It will ask few simple questions on console, please read carefully and answer them.

  certbot certonly --webroot --dry-run -w /opt/www/well_known/ -d mail.mydomain.com

  If you need to support multiple domain names in one cert, please append them with additional -d arguments like this:

  certbot certonly --webroot --dry-run -w /opt/www/well_known/ -d mail.mydomain.com -d 2nd.com -d 3rd.com -d 4th.com

• If everything went well and no error was reported by certbot, that means verification succeeds, it's ok to actually request the cert by running above command again without --dry-run argument:

certbot certonly --webroot -w /opt/www/well_known/ -d mail.mydomain.com

If succeeds, it stores the cert and key files under /etc/letsencrypt/live/mail.mydomain.com/ (You may have different host name instead of mail.mydomain.com in this sample path).

• cert.pem: Server certificate.
• chain.pem: Additional intermediate certificate or certificates that web browsers will need in order to validate the server certificate.
• fullchain.pem: All certificates, including server certificate (aka leaf certificate or end-entity certificate). The server certificate is the first one in this file, followed by any intermediates.
• privkey.pem: Private key for the certificate.

Directory /etc/letsencrypt/live/ and /etc/letsencrypt/archive/ are owned by root user and group, with permission 0700 (set by certbot program) by default, it means other users can not access them -- including the daemon users used to run network services like Postfix/Dovecot/OpenLDAP/MariaDB/PostgreSQL. It's necessary to set the permission to 0755 for other applications to access them.

chmod 0755 /etc/letsencrypt/{live,archive}

Use Let's Encrypt cert

The easiest and quickest way to use Let's Encrypt cert is creating symbol links to the self-signed SSL cert generated by iRedMail Easy, then restart services which use the cert files:

• /opt/iredmail/ssl/key.pem: private key
• /opt/iredmail/ssl/cert.pem: certificate
• /opt/iredmail/ssl/combined.pem: full chain

Run commands below to backup old cert/key files and create symbol links of Let's Encrypt cert:

cd /opt/iredmail/ssl/
mv cert.pem cert.pem.bak
mv key.pem key.pem.bak
mv combined.pem combined.pem.bak
ln -s /etc/letsencrypt/live/<domain>/fullchain.pem combined.pem
ln -s /etc/letsencrypt/live/<domain>/fullchain.pem cert.pem
ln -s /etc/letsencrypt/live/<domain>/privkey.pem key.pem

Now restart Postfix / Dovecot / Nginx services to use the cert:

systemctl restart postfix dovecot nginx

Verify the cert

• To verify ssl cert used in Postfix (SMTP server) and Dovecot, please launch a mail client application (MUA, e.g. Outlook, Thunderbird) and create an email account, make sure you correctly configured the MUA to connect to mail server. If SSL cert is not valid, MUA will show you a warning or error message.
• For web service, please visit your website with a web browser, the browser should show you the ssl cert status.

Renew the cert automatically

Cert offered by Let's Encrypt will expire in 90 days, you can renew it manually with command certbot renew, or run same command in a daily or weekly cron job to renew it automatically. Only those certs which expires in less than 30 days will be renewed. Background services which use the cert must be restarted or reloaded to load renewed cert files.

If cert was renewed, private key /etc/letsencrypt/live/<domain>/privkey.pem is re-created and linked to new file under /etc/letsencrypt/archive/<domain>/privkey<X>.pem (<X> is a digit number), but all files linked to /etc/letsencrypt/live/<domain>/privkey.pem were left to the old one, so we must update all files linked to /etc/letsencrypt/live/<domain>/privkey.pem after renewed.

Here's a sample cron job that runs at 3:01AM everyday, it runs certbot certificates command to print current cert info, then tries to renew the cert, and restart postfix/nginx/dovecot services after succeeded:

1 3 * * * certbot certificates; certbot renew --post-hook 'ln -sf /etc/letsencrypt/live/<domain>/privkey.pem /opt/iredmail/ssl/key.pem; /usr/bin/systemctl restart postfix dovecot nginx'

References

• Let's Encrypt: How it works

See Also

• Use a bought SSL certificate