How to configure TLS encryption in Dovecot

DovecotLike most other internet services, Dovecot can be configured to use TLS encryption — and, unlike some others (such as web servers or SMTP servers), there’s little reason to not enforce it.

Notes:

  • Like other recent TLS tutorials on this blog, this is, of course, not a full Dovecot guide — that would be far too complex for a blog post. Instead, it’s just focused on enabling/configuring TLS encryption. I’ll be assuming you already have Dovecot configured, and able to access mailboxes on your server.
  • I’ll be focusing on IMAP only, not POP3 or anything else.
  • The server will be configured to require encryption, for both privacy and security reasons. I don’t know of any modern email client that doesn’t allow encrypted connections (and even then, you might work around it by configuring an encrypted tunnel, but that falls out of this post’s scope).

1. Getting a certificate

While I believe most modern email clients will support ECDSA certificates, such clients are not a known quantity like, say, web browsers are, so I suggest that you create an RSA certificate, unless you already have one whose Common Name (CN) matches your server’s public name (e.g. mail.domain.com). Dovecot versions 2.2.31 and newer support configuring alternative certificates so that you could support both kinds at the same time, but there’s probably little gain in doing that, since most clients will likely default to RSA anyway (and, unless you’re using Let’s Encrypt, certificates are not free).

So, put your certificate and private key in /etc/dovecot/. Let’s assume the certificate is called myserver-full.crt 1, and the private key is myserver.key. You should protect the private key from any users on your server, so do, for instance,

chown root:dovecot /etc/dovecot/myserver.key
chmod 640 /etc/dovecot/myserver.key

2. Configuring Dovecot for TLS

I’m assuming your Dovecot installation has a basic configuration file in /etc/dovecot/dovecot.conf, but the real “meat” of the configuration is in included files in /etc/dovecot/conf.d/. Your system may be slightly different, but I’m sure you can adapt. 🙂 Also, the numbers in the beginning of the file names may differ in your system.

This one isn’t really related to TLS, but it’s a good idea: edit /etc/dovecot/conf.d/20-imap.conf, and look for a line like this:

mail_plugins = $mail_plugins

and add to it (separated by a space):

imap_zlib

After all, there’s no reason not to use compression here, and bandwidth (especially on mobile) is still precious.

Now, edit /etc/dovecot/conf.d/10-ssl.conf, and add 2:

ssl = required
ssl_cert = </etc/dovecot/myserver-full.crt
ssl_key = </etc/dovecot/myserver.key
ssl_cipher_list = ECDHE-RSA-CHACHA20-POLY1305:ALL:!LOW:!SSLv2:!EXP:!aNULL
ssl_protocols = !SSLv2 !SSLv3
ssl_prefer_server_ciphers = yes

(The ssl_cipher_list line, besides setting secure defaults, sets the ChaCha20 protocol as the first one to be tried, since it’s considered one of the fastest and most secure. Note that it’ll require Dovecot linked to LibreSSL or OpenSSL 1.1.x to use that cipher, though Dovecot won’t complain if it doesn’t have access to it; it’ll just use the normal, secure defaults.)

If you don’t change anything else from the default configuration, the server will be listening on port 143 (IMAP) and port 993 (IMAPS). “But,” you ask, “isn’t standard IMAP unencrypted? I thought you were enforcing encryption…” Yes, but the standard IMAP port can be “upgraded” to TLS by entering the STARTTLS command, and email clients not only support that, but typically default to it (if not, just make sure you enable it). The server will refuse to authenticate any users on port 143 before they’ve “STARTTLSed”.

If you wanted to use only IMAP+STARTTLS, or only IMAPS, just edit /etc/dovecot/conf.d/10-master.conf,  look for the “service” configuration, and disable the one you don’t want. But I see no problem with keeping both enabled.

Thoughts? Questions?

(To do the same to SMTP / Postfix, please see How to configure TLS encryption in Postfix.)

How to create an RSA certificate

(Note: most of this post’s content originally appeared in a previous one.  The difference is that that one refers to ECDSA certificates, while this one is about the older, more common RSA certificates.)

Whether it’s a web server, an email server (two, in fact, assuming you’re using one for SMTP and another for IMAP, as is common), or other types of applications, to have encrypted connections a TLS certificate 1 is required. It can be self-signed (say, if you’ll be the only one needing to access that server), but browsers and email clients will complain (loudly!), therefore, if you want your server to be universally accessed, a “real” certificate is needed, so let’s show how to get one.

This recipe creates an RSA certificate (strongly suggested for, say, Postfix, since you can’t control what other email servers support, so you should go for the most common option); for web servers (accessed by standard browsers), an ECDSA certificate might be a good alternative.

A few notes:

  • There are alternatives (e.g. key sizes, etc.) for basically every parameter I’m using, but I’m not going into those. This is supposed to be as quick and easy as possible, after all.
  • Similarly, I’m not going into Let’s Encrypt; instead, I’m assuming a “normal” certificate authority such as Comodo. If you do use Let’s Encrypt, just use it to create the certificate instead of the “send the CSR to the certification authority” part.
  • The certificate (and server) will be compatible with most browsers and applications (email clients, etc.), but that “most” won’t include any Microsoft browsers on Windows XP (Firefox or Chrome on that abomination of an OS will still work).

Generating the key and the Certificate Signing Request (CSR):

openssl req -nodes -newkey rsa:4096 -keyout myserver.key -out myserver.csr

The command will ask you for details about your server/company (location, etc.). You should fill in every field, although the only mandatory one is “Common Name” (CN), which must match your server’s public name (not necessarily the machine’s name, but the host name people will type in the browser/email client, such as “zurgl.com” or “mail.something.com“. Note that a certificate for “domain.com” also includes “www.domain.com” (so don’t include the “www.” in the CN), but if the server is reached at “subdomain.domain.com“, then that’s what the CN needs to be.

Ordering and receiving the new certificate:

Now go to a certification authority (CA), order a new certificate, and when asked for a CSR, send them (usually you can just copy and paste it into a text entry window) that myserver.csr file.

If everything went well, then the CA should email you the new certificate in a short while. Typically they send you two files: the certificate itself, and a couple of “intermediate” certificates. Only the first is really needed, but I’ve had best results with concatenating your certificate (first) and the intermediate certs (last) into a single file, which you might call myserver-full.crt .

That file is, to all intents and purposes, your new certificate, and it’s ready to be used in application servers. You can use the same certificate for several services (e.g. an HTTPS web site, an SMTP server, an IMAP server), as long as the host name matches the certificate’s CN (so a certificate for “mail.myserver.com” won’t work for “www.myserver.com“, and vice-versa, but that’s because the names don’t match — not because they’re different services).

Adding TLS encryption (with your shiny new certificate) to internet services:

Here’s a (growing) list of tutorials: