Dovecot SSL configuration
=========================

Contents


 1. Dovecot SSL configuration

     1. Multiple SSL certificates

         1. Different certificates per algorithm

         2. Different certificates per IP and protocol

         3. With client TLS SNI (Server Name Indication) support

     2. Password protected key files

     3. Chained SSL certificates

     4. SSL security settings

     5. SSL verbosity

     6. Client certificate verification/authentication

     7. Testing

     8. Testing CA

         1. Testing CA On Debian

         2. Testing CA On RHEL

         3. Testing CA Success

     9. Client connections

The most important SSL settings are (in 'conf.d/10-ssl.conf'):

---%<-------------------------------------------------------------------------
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 '<' is mandatory. It indicates that the variable should contain *contents*
of the file, instead of the file name. Not using it will cause an error.

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. However its
important to note that "ssl = yes" must be set globally if you require SSL for
any protocol (or dovecot will not listen on the SSL ports), which in turn
requires that a certificate and key are specified globally even if you intend
to specify certificates per protocol. The per protocol certificate settings
override the global setting.:

---%<-------------------------------------------------------------------------
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:

 * 'ssl=no': SSL/TLS is completely disabled.
 * 'ssl=yes' and 'disable_plaintext_auth=no': SSL/TLS is offered to the client,
   but the client isn't required to use it. The client is allowed to login with
   plaintext authentication even when SSL/TLS isn't enabled on the connection.
   This is insecure, because the plaintext password is exposed to the internet.
 * 'ssl=yes' and 'disable_plaintext_auth=yes': SSL/TLS is offered to the
   client, but the client isn't required to use it. The client isn't allowed to
   use plaintext authentication, unless SSL/TLS is enabled first. However, if
   <non-plaintext authentication mechanisms> [Authentication.Mechanisms.txt]
   are enabled they are still allowed even without SSL/TLS. Depending on how
   secure they are, the authentication is either fully secure or it could have
   some ways for it to be attacked.
 * 'ssl=required': SSL/TLS is always required, even if non-plaintext
   authentication mechanisms are used. Any attempt to authenticate before
   SSL/TLS is enabled will cause an authentication failure.
 * NOTE: If you have only plaintext mechanisms enabled (e.g. 'auth { mechanisms
   = plain login }'), 'ssl=yes' and 'ssl=required' are completely equivalent
   because in either case the authentication will fail unless SSL/TLS is
   enabled first.
 * NOTE2: With both 'ssl=yes' and 'ssl=required' it's still possible that the
   client attempts to do a plaintext authentication before enabling SSL/TLS,
   which exposes the plaintext password to the internet. Dovecot attempts to
   indicate this to the IMAP clients via the LOGINDISABLED capability, but many
   clients still ignore it and send the password anyway. There is unfortunately
   no way for Dovecot to prevent this behavior. The POP3 standard doesn't have
   an equivalent capability at all, so the POP3 clients can't even know if the
   server would accept a plaintext authentication.
 * The main difference between 'ssl=required' and 'disable_plaintext_auth=yes'
   is that if 'ssl=required', it guarantees that the entire connection is
   protected against eavesdropping (SSL/TLS encrypts the rest of the
   connection), while 'disable_plaintext_auth=yes' only guarantees that the
   password is protected against eavesdropping (SASL mechanism is encrypted,
   but no SSL/TLS is necessarily used). Nowadays you most likely should be
   using SSL/TLS anyway for the entire connection, since the cost of SSL/TLS is
   cheap enough. Using both SSL/TLS and non-plaintext authentication would be
   the ideal situation since it protects the plaintext password even against
   man-in-the-middle attacks.

Note that plaintext authentication is always allowed (and SSL not required) for
connections from localhost, as they're assumed to be secure anyway. This
applies to all connections where the local and the remote IP addresses are
equal. Also IP ranges specified by 'login_trusted_networks' setting are assumed
to be secure.

Multiple SSL certificates
-------------------------

Different certificates per algorithm
------------------------------------

Since v2.2.31+ you can specify alternative ssl certificate that will be used if
the algorithm differs from the primary certificate. This is useful when
migrating to e.g. ECDSA certificate.

---%<-------------------------------------------------------------------------
ssl_alt_cert=</path/to/alternative/cert.pem
ssl_alt_key=</path/to/alternative/key.pem
---%<-------------------------------------------------------------------------

Different certificates per IP and protocol
------------------------------------------

If you have multiple IPs available, this method is guaranteed to work with all
clients.

---%<-------------------------------------------------------------------------
local 192.0.2.10 { # instead of IP you can also use hostname, which will be
resolved
  protocol imap {
    ssl_cert = </etc/ssl/dovecot/imap-01.example.com.cert.pem
    ssl_key  = </etc/ssl/dovecot/imap-01.example.com.key.pem
  }

  protocol pop3 {
    ssl_cert = </etc/ssl/dovecot/pop-01.example.com.cert.pem
    ssl_key  = </etc/ssl/dovecot/pop-01.example.com.key.pem
  }
}

local 192.0.2.20 {
  protocol imap {
    ssl_cert = </etc/ssl/dovecot/imap-02.example.com.cert.pem
    ssl_key  = </etc/ssl/dovecot/imap-02.example.com.key.pem
  }

  protocol pop3 {
    ssl_cert = </etc/ssl/dovecot/pop-02.example.com.cert.pem
    ssl_key  = </etc/ssl/dovecot/pop-02.example.com.key.pem
  }
}
---%<-------------------------------------------------------------------------

Note that you will still need a top-level "default" 'ssl_key' and 'ssl_cert' as
well, or you will receive errors.

---%<-------------------------------------------------------------------------
# doveconf -n
doveconf: Error: ssl enabled, but ssl_cert not set
---%<-------------------------------------------------------------------------

With client TLS SNI (Server Name Indication) support
----------------------------------------------------

/It is important to note that having multiple SSL certificates per IP will not
be compatible with all clients, especially mobile ones. It is a TLS SNI
limitation./ See <SSL.SNIClientSupport.txt> for list of clients known to (not)
support SNI.

---%<-------------------------------------------------------------------------
local_name imap.example.org {
  ssl_cert = </etc/ssl/certs/imap.example.org.crt
  ssl_key = </etc/ssl/private/imap.example.org.key
}
local_name 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.dat'. Dovecot v2.1.x and older
regenerated them every week by default, but because the extra security gained
by the regeneration is quite small, Dovecot v2.2 disabled the regeneration
feature completely.

 * Since v2.3.3+ Diffie-Hellman parameters have been made optional, and you are
   encouraged to disable non-ECC DH algorithms completely.

From and up to version 2.2, you can specify the wanted DH parameters length
using:

---%<-------------------------------------------------------------------------
ssl_dh_parameters_length = 2048
---%<-------------------------------------------------------------------------

From version 2.3, you must specify path to DH parameters file using:

---%<-------------------------------------------------------------------------
ssl_dh=</path/to/dh.pem
---%<-------------------------------------------------------------------------

To generate new parameters file, you can use:

---%<-------------------------------------------------------------------------
# This might take a very long time. Run it on a machine with sufficient
entropy.
openssl dhparam 4096 > dh.pem
---%<-------------------------------------------------------------------------

You can also convert an old v2.2 parameters file with command:

---%<-------------------------------------------------------------------------
dd if=/path/to/ssl-parameters.dat bs=1 skip=88 | openssl dhparam -inform DER
---%<-------------------------------------------------------------------------

This should work most of the times. If not, generate new file.

By default Dovecot's allowed ciphers list contains:

---%<-------------------------------------------------------------------------
ssl_cipher_list =
ALL:!kRSA:!SRP:!kDHd:!DSS:!aNULL:!eNULL:!EXPORT:!DES:!3DES:!MD5:!PSK:!RC4:!ADH:!LOW@STRENGTH
---%<-------------------------------------------------------------------------

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.

You should usually prefer server ciphers and their order, so setting

---%<-------------------------------------------------------------------------
ssl_prefer_server_ciphers=yes
---%<-------------------------------------------------------------------------

is recommended.

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

auth_ssl_require_client_cert = yes
#auth_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. This could present a problem if you're using Dovecot to
provide SASL authentication for an MTA (such as Postfix) which is not capable
of supplying client certificates for SASL authentication. If you need Dovecot
to provide SASL authentication to an MTA without requiring client certificates
and simultaneously provide IMAP service to clients while requiring client
certificates, you can put 'auth_ssl_require_client_cert = yes' inside of a
protocol block as shown below to make an exemption for SMTP SASL clients (such
as Postfix).

---%<-------------------------------------------------------------------------
protocol !smtp {
  auth_ssl_require_client_cert = yes
}
---%<-------------------------------------------------------------------------

You may also force the username to be taken from the certificate by setting
'auth_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> [PasswordDatabase.txt] allow
logins using any password (typically requiring <"nopassword" extra field>
[PasswordDatabase.ExtraFields.txt] to be returned).

Testing
-------

Try out your new setup:

---%<-------------------------------------------------------------------------
openssl s_client -servername mail.sample.com -connect mail.sample.com:pop3s
---%<-------------------------------------------------------------------------

You should see something like this:

---%<-------------------------------------------------------------------------
CONNECTED(00000003)
depth=2 /O=Root CA/OU=http://www.cacert.org/CN=CA Cert Signing
Authority/emailAddress=support@cacert.org
verify error:num=19:self signed certificate in certificate chain
verify return:0
---
Certificate chain
 0 s:/CN=mail.example.com
   i:/O=CAcert Inc./OU=http://www.CAcert.org/CN=CAcert Class 3 Root
 1 s:/O=CAcert Inc./OU=http://www.CAcert.org/CN=CAcert Class 3 Root
   i:/O=Root CA/OU=http://www.cacert.org/CN=CA Cert Signing
Authority/emailAddress=support@cacert.org
 2 s:/O=Root CA/OU=http://www.cacert.org/CN=CA Cert Signing
Authority/emailAddress=support@cacert.org
   i:/O=Root CA/OU=http://www.cacert.org/CN=CA Cert Signing
Authority/emailAddress=support@cacert.org
---
Server certificate
-----BEGIN CERTIFICATE-----

ArS/9wt2X21KgeuGHlTZ/8z9gXAjQKXhDYECWWd6LkWl98ZDBihslQ==
-----END CERTIFICATE-----
subject=/CN=mail.example.com
issuer=/O=CAcert Inc./OU=http://www.CAcert.org/CN=CAcert Class 3 Root
---
No client certificate CA names sent
---
SSL handshake has read 5497 bytes and written 293 bytes
---
New, TLSv1/SSLv3, Cipher is DHE-RSA-AES256-SHA
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: zlib compression
Expansion: zlib compression
SSL-Session:
    Protocol  : TLSv1
    Cipher    : DHE-RSA-AES256-SHA
    Session-ID:
114A22BE4625B33F6893124ACF640AE0628B48B5039E90B3B9A20ADF7FA691F3
    Session-ID-ctx:
    Master-Key:
B8A55EC91A060575CFB29503FBF7160C2DC8BCBFE02D20A7F704882F72D8D00272D8D002CE5CCC4B94A492F43ED8F
    Key-Arg   : None
    TLS session ticket:
    0000 - 86 c7 46 63 a5 b6 48 74-16 d8 e0 a7 e2 64 e8 89   ..Fc..Ht.....d..
    0010 - 97 90 59 4b 57 f3 e2 b3-e2 d2 88 90 a8 aa b4 44   ..YKW..........D
    0020 - ea 24 08 5e b4 14 7f e1-2a 1a 1c 40 ca 85 e7 41   .$.^....*..@...A
    0030 - 9d 0d a8 4c f7 e3 db 1e-ef da 53 9c fe 43 cc 62   ...L......S..C.b
    0040 - 79 b6 ad ea 9d cf ca b2-37 41 b7 0f ea 7d 59 e8   y.......7A...}Y.
    0050 - 10 01 a0 eb dc c2 63 66-56 54 6a e8 3a 4b 93 49   ......cfVTj.:K.I
    0060 - 77 da e4 4b 21 e8 30 7e-bf 10 91 3a 2c f9 59 80   w..K!.0~...:,.Y.
    0070 - 01 1f 36 0b 92 85 67 55-c8 86 1d 44 b1 6f 0d ae   ..6...gU...D.o..
    0080 - 15 36 b6 49 3a ef 94 9a-ef 6d 27 f0 80 20 43 09   .6.I:....m'.. C.
    0090 - be 70 c5 30 15 3b 93 c6-c1 4c e9 7f 5c 34 98 dd   .p.0.;...L..\4..

    Compression: 1 (zlib compression)
    Start Time: 1292857721
    Timeout   : 300 (sec)
    Verify return code: 19 (self signed certificate in certificate chain)
---
+OK Dovecot ready.
---%<-------------------------------------------------------------------------

Testing CA
----------

The above test procedure returns:

---%<-------------------------------------------------------------------------
    Verify return code: 19 (self signed certificate in certificate chain)
---%<-------------------------------------------------------------------------

which is expected result since test command omits option to verify CA root
certificate.  The following commands will enable CA root certificate
validation.

Testing CA On Debian
--------------------

On Debian derived distributions try:

---%<-------------------------------------------------------------------------
openssl s_client -CApath /etc/ssl/certs -connect mail.sample.com:pop3s
---%<-------------------------------------------------------------------------

Testing CA On RHEL
------------------

On Red Hat Enterprise Linux derived distributions try:

---%<-------------------------------------------------------------------------
openssl s_client -CAfile /etc/pki/tls/cert.pem -connect mail.sample.com:pop3s
---%<-------------------------------------------------------------------------

Testing CA Success
------------------

---%<-------------------------------------------------------------------------
    Verify return code: 0 (ok)
---%<-------------------------------------------------------------------------

Client connections
------------------

Since v2.3.4 dovecot accepts default system CAs for outgoing connections.

---%<-------------------------------------------------------------------------
ssl_client_ca_dir = /path/to/pem/certificates
ssl_client_ca_file = /path/to/pem/bundle
---%<-------------------------------------------------------------------------

(This file was created from the wiki on 2019-06-19 12:42)
