Dovecot: Missing dh.pem

Yesterday, I upgraded my mail server’s operating system. After upgrading, I encountered the following error in dovecot:

config: Warning: please set ssl_dh=</etc/dovecot/dh.pem

What’s dh.pem? I found the answer in the Dovecot 2.3 upgrade documentation. Apparently, the ssl-parameters.dat file is now obsolete. You should use ssl_dh setting instead by adding: ssl_dh=</etc/dovecot/dh.pem to the configuration.

But how do you get dh.pem?

One way is to can convert an existing/old ssl-parameters.dat to dh.pem:

dd if=/var/lib/dovecot/ssl-parameters.dat bs=1 skip=88 | openssl dhparam -inform der > /etc/dovecot/dh.pem

Another way, which I found here, is to generate a new dh.pem:

openssl dhparam -out /etc/dovecot/dh.pem 4096 -days 3650

It takes a LOOONG time. But once the file is ready, just add it to /etc/dovecot/conf.d/10-ssl.conf

ssl_cert = </etc/letsencrypt/live/
ssl_key = </etc/letsencrypt/live/
ssl_dh = </etc/dovecot/dh.pem

Restart dovecot and you’re back in business.

Enable HTTPS On Your Website With Let’s Encrypt

There’s no question that secure communications is critical. On the web, this is done using HTTPS. HTTPS is secure extension of the HTTP. In HTTPS, communications is encrypted using Transport Layer Security (TLS), or its deprecated predecessor, Secure Sockets Layer (SSL).

TLS uses a public key encryption scheme where you have a public and private key pair. The web server provides they public key which the web browser can use to encrypt communications with. The public key is signed to certify the identity of the web server owning the key. This gives you the public key certificate or just simply certificate.

You can self-sign (or self-certify) just so you can encrypt communications and that’s fine if your dealing with yourself or parties who trust you and your self-signed certificate (e.g. your own systems or employees). But if you deal with other parties (e.g. other systems or customers) you need a certificate from a certificate authority (CA), a trusted entity that signs keys and issues certificates. 

Let’s Encrypt is a free, automated, and open certificate authority brought to you by the non-profit Internet Security Research Group (ISRG). They provide certificates absolutely free. The certificates expire in 90 days, but they can be automatically renewed using Certbot. There’s simply no excuse not to have a secure site. And it’s so easy to boot. There are step-by-step instructions for almost every web server and operating system combination at the Certbot page.

Here are the steps for getting certificates using Ubuntu and Apache:

  1. Add the Certbot apt repository
    • sudo add-apt-repository ppa:certbot/certbot
  2. Update the repository
    • sudo apt-get update
  3. Install Certbot from the new repository with apt-get:
    • sudo apt-get install python-certbot-apache
  4. Obtain a certificate for your domain

This give your certificates for your new files and configures Apache automatically. But you should be able to find the certificate files for other purposes (see below) at /etc/letsencrypt/live/

The certificate only last for 90 days. However, Certbot takes care of this problem by running certbot renew twice a day via a systemd timer or cron. We can also manually test renewal:

  • sudo certbot renew –dry-run

BONUS: If you’re using Dovecot, you can also use the certificate:

  1. Edit /etc/dovecot/conf.d/10-ssl.conf:
    • ssl_cert = /etc/letsencrypt/live/
    • ssl_key = /etc/letsencrypt/live/
  2. Restart dovecot:
    • sudo service dovecot restart

That’s it! You now have a secure website and email server.