Dovecot SSL configuration

The most important SSL settings are:

ssl = yes
# Preferred permissions: root:root 0444
ssl_cert = </etc/ssl/certs/dovecot.pem
# Preferred permissions: root:root 0400
ssl_key = </etc/ssl/private/dovecot.pem

The certificate file can be world-readable, since it doesn't contain anything sensitive (in fact it's sent to each connecting SSL client). The key file's permissions should be restricted to only root (and possibly ssl-certs group or similar if your OS uses such). Dovecot opens both of these files while still running as root, so you don't need to give Dovecot any special permissions to read them (in fact: do not give dovecot user any permissions to the key file).

It's possible to keep the certificate and the key both in the same file:

# Preferred permissions: root:root 0400
ssl_cert = </etc/ssl/dovecot.pem
ssl_key = </etc/ssl/dovecot.pem

It's also possible to use different certificates for IMAP and POP3:

protocol imap {
  ssl_cert = </etc/ssl/certs/imap.pem
  ssl_key = </etc/ssl/private/imap.pem
}
protocol pop3 {
  ssl_cert = </etc/ssl/certs/pop3.pem
  ssl_key = </etc/ssl/private/pop3.pem
}

There are a couple of different ways to specify when SSL/TLS is required:

• disable_plaintext_auth=yes allows plaintext authentication only when SSL/TLS is used first.

• ssl = required requires SSL/TLS also for non-plaintext authentication.

• If you have only plaintext mechanisms enabled (auth { mechanisms = plain login } ), you can use either (or both) of the above settings. They behave exactly the same way then.

Multiple SSL certificates

local imap.example.org {
  ssl_cert = </etc/ssl/certs/imap.example.org.crt
  ssl_key = </etc/ssl/private/imap.example.org.key
}
local imap.example2.org {
  ssl_cert = </etc/ssl/certs/imap.example2.org.crt
  ssl_key = </etc/ssl/private/imap.example2.org.key
}
# ..etc..

Password protected key files

SSL key files may be password protected. There are two ways to provide Dovecot with the password:

1. Starting Dovecot with dovecot -p asks the password. It's not stored anywhere, so this method prevents Dovecot from starting automatically at startup.

2. ssl_key_password setting. Note that dovecot.conf is by default world-readable, so you probably shouldn't place it there directly. Instead you could store it in a different file, such as /etc/dovecot-private.conf containing:

    ssl_key_password = secret

   and then use !include_try /etc/dovecot-private.conf in the main dovecot.conf.

Chained SSL certificates

Put all the certificates in the ssl_cert file. For example when using a certificate signed by TDC the correct order is:

1. Dovecot's public certificate
2. TDC SSL Server CA
3. TDC Internet Root CA
4. Globalsign Partners CA

SSL security settings

When Dovecot starts up for the first time, it generates new 512bit and 1024bit Diffie Hellman parameters and saves them into <prefix>/var/lib/dovecot/ssl-parameters.ssl. After the initial creation they're by default regenerated every week. With newer computers the generation shouldn't take more than a few seconds, but with older computers it can take as long as half an hour. The extra security gained by the regeneration is quite small, so with slower computers you might want to disable it:

ssl_parameters_regenerate = 0

By default Dovecot's allowed ciphers list contains:

ssl_cipher_list = ALL:!LOW:!SSLv2:!EXP:!aNULL

Disallowing more won't really gain any security for those using better ciphers, but it does prevent people from accidentally using insecure ciphers. See http://www.openssl.org/docs/apps/ciphers.html for a list of the ciphers.

SSL verbosity

verbose_ssl = yes

This will make Dovecot log all the problems it sees with SSL connections. Some errors might be caused by dropped connections, so it could be quite noisy.

Client certificate verification/authentication

If you want to require clients to present a valid SSL certificate, you'll need these settings:

ssl_ca = </etc/ssl/ca.pem
ssl_verify_client_cert = yes

ssl_require_client_cert = yes
#ssl_username_from_cert = yes

The CA file should contain the certificate(s) followed by the matching CRL(s). Note that the CRLs are required to exist. For a multi-level CA place the certificates in this order:

1. Issuing CA cert
2. Issuing CA CRL
3. Intermediate CA cert
4. Intermediate CA CRL
5. Root CA cert
6. Root CA CRL

The certificates and the CRLs have to be in PEM format. To convert a DER format CRL (e.g. http://crl.cacert.org/class3-revoke.crl) into PEM format, use:

openssl crl -in class3-revoke.crl -inform DER -outform PEM > class3-revoke.pem

With the above settings if a client connects which doesn't present a certificate signed by one of the CAs in the ssl_ca file, Dovecot won't let the user log in.

You may also force the username to be taken from the certificate by setting ssl_username_from_cert = yes.

• The text is looked up from subject DN's specified field using OpenSSL's X509_NAME_get_text_by_NID() function.

• By default the CommonName field is used.

• You can change the field with ssl_cert_username_field = name setting (parsed using OpenSSL's OBJ_txt2nid() function). x500UniqueIdentifier is a common choice.

You may also want to disable the password checking completely. Doing this currently circumvents Dovecot's security model so it's not recommended to use it, but it is possible by making the passdb allow logins using any password (typically requiring "nopassword" extra field to be returned).