This documentation is for Dovecot v2.x, see wiki1 for v1.x documentation.
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The most important SSL settings are: The most important SSL settings are (in {{{conf.d/10-ssl.conf}}}):
Line 144: Line 144:
ssl_require_client_cert = yes auth_ssl_require_client_cert = yes

Dovecot SSL configuration

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 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 intent 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:

  • 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

TLS SNI (Server Name Indication) support required

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.

local_name {
  ssl_cert = </etc/ssl/certs/
  ssl_key = </etc/ssl/private/
local_name {
  ssl_cert = </etc/ssl/certs/
  ssl_key = </etc/ssl/private/
# ..etc..

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

Different certificates per IP and protocol

local {
  protocol imap {
    ssl_cert = </etc/ssl/dovecot/
    ssl_key  = </etc/ssl/dovecot/

  protocol pop3 {
    ssl_cert = </etc/ssl/dovecot/
    ssl_key  = </etc/ssl/dovecot/

local {
  protocol imap {
    ssl_cert = </etc/ssl/dovecot/
    ssl_key  = </etc/ssl/dovecot/

  protocol pop3 {
    ssl_cert = </etc/ssl/dovecot/
    ssl_key  = </etc/ssl/dovecot/

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 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

auth_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. 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).


Try out your new setup:

openssl s_client -connect

You should see something like this:

depth=2 /O=Root CA/OU= Cert Signing Authority/
verify error:num=19:self signed certificate in certificate chain
verify return:0
Certificate chain
 0 s:/
   i:/O=CAcert Inc./OU= Class 3 Root
 1 s:/O=CAcert Inc./OU= Class 3 Root
   i:/O=Root CA/OU= Cert Signing Authority/
 2 s:/O=Root CA/OU= Cert Signing Authority/
   i:/O=Root CA/OU= Cert Signing Authority/
Server certificate
issuer=/O=CAcert Inc./OU= 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
    Protocol  : TLSv1
    Cipher    : DHE-RSA-AES256-SHA
    Session-ID: 114A22BE4625B33F6893124ACF640AE0628B48B5039E90B3B9A20ADF7FA691F3
    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.

None: SSL/DovecotConfiguration (last edited 2021-03-26 12:44:41 by 2a00:1190:c02a:131::1000)