SSL/TLS Configuration

Get the Certificate

We are using Dovecot for IMAP/POP3 (receiving mails) and exim4 for SMTP (sending mails). Both server applications should support TLS for client to server and server to server transfer to occur encrypted and secure. Thus, we first need an SSL/TLS certificate for our mail server domain mail.liberale-demokraten.de. Please follow the following steps to achieve that:

  1. Login as the admin user to the control panel
  2. Point the A and AAAA records for the subdomain to the respective IP address of the server
  3. Add the subdomain as a web domain (just Web, no E-Mail and DNS)
  4. Choose "redirect-LD-DE" as proxy template (it will redirect all calls to liberale-demokraten.de)
  5. Enable Let's Encrypt certificates for the subdomain

Set the proper Permissions

Now, we have obtained a valid SSL/TLS certificate from Let's Encrypt and vesta will automatically update the certificate before expiry. However, we are not done yet. We need to copy the certificates to another location and set the ownership and permissions accordingly. To do so, go through the following steps:

  1. Log in as root via SSH
  2. cd /usr/local/vesta/bin
  3. vim v-update-host-certificate
  4. Modify the opened file in a way that the certificate for the mail subdomain is in /usr/local/vesta/ssl as mail.crt and mail.key with the same permissions as for certificate.crt and certificate.key there (just copy, paste and modify)
  5. run ./v-update-host-certificate in the shell

Generate DH params

To ensure the so called Forward Secrecy, we will need to utilize DH params. That means that we first have to generate it. To do so, follow these steps:

  1. Login via SSH as root user
  2. cd /usr/local/vesta/ssl
  3. openssl dhparam -out /usr/local/vesta/ssl/dhparam.pem 4096
  4. Get yourself a cup of coffee and some cookies. It will most likely take quite a while

Configuring the Server

We are done as far as the preperations go. Now, we have to configure dovecot and exim.

Dovecot

Go to the control panel and enter the server settings on the top of the screen. Now search for dovecot and press the "configure" button. Seach for the editable file ending to ssl.conf and enter the following:

ssl = yes
ssl_cert = </usr/local/vesta/ssl/mail.crt
ssl_key = </usr/local/vesta/ssl/mail.key
ssl_min_protocol = TLSv1.2
ssl_cipher_list = EECDH+AESGCM:EDH+AESGCM
ssl_prefer_server_ciphers = yes
ssl_dh = </usr/local/vesta/ssl/dhparam.pem

Exim

Go to the control panel and enter the server settings on the top of the screen. Now search for exim and press the "configure" button. Now search for "tls_" and modify the settings as follows:

tls_advertise_hosts = *
tls_certificate = /usr/local/vesta/ssl/mail.crt
tls_privatekey = /usr/local/vesta/ssl/mail.key
# tls_require_ciphers = NORMAL:%LATEST_RECORD_VERSION:-VERS-SSL3.0:-VERS-TLS1.0:-VERS-TLS1.1
tls_dhparam = /usr/local/vesta/ssl/dhparam.pem

Note that exim in Debian comes with GnuTLS as per default instead of with OpenSSL. So we cannot modify alot, unfortunately. Especially as GnuTLS tends to be buggy. If it works, uncomment the tls_require_ciphers part, but make sure you test sending and receiving emails afterwards.