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Comment: I was running into this problem and took some time to find that hint on the mailing list.
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The most important SSL settings are: | The most important SSL settings are (in {{{conf.d/10-ssl.conf}}}): |
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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 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..
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 192.0.2.10 { 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 } }
Password protected key files
SSL key files may be password protected. There are two ways to provide Dovecot with the password:
Starting Dovecot with dovecot -p asks the password. It's not stored anywhere, so this method prevents Dovecot from starting automatically at startup.
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:
- Dovecot's public certificate
- TDC SSL Server CA
- TDC Internet Root CA
- 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 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:
- Issuing CA cert
- Issuing CA CRL
- Intermediate CA cert
- Intermediate CA CRL
- Root CA cert
- 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).
Testing
Try out your new setup:
openssl s_client -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----- MIIE1DCCArygAwIBAgIDAMBPMA0GCSqGSIb3DQEBBAUAMFQxFDASBgNVBAoTC0NB Y2VydCBJbmMuMR4wHAYDVQQLExVodHRwOi8vd3d3LkNBY2VydC5vcmcxHDAaBgNV BAMTE0NBY2VydCBDbGFzcyAzIFJvb3QwHhcNMTAxMjIwMTM1NDQ1WhcNMTIxMjE5 MTM1NDQ1WjAmMSQwIgYDjksadnjkasndjksandjksandjksandj5YXJlYS5vcmcw ggEiMA0GCSqGSIb3DQEBAQUAA4IBDwAwggEKAoIBAQC3jOX3FC8wVqnb2r65Sfvk cYUpJhlbhCfqPdN41c3WS0y1Jwwum1q4oMAJvdRnD5TMff1+fqTFy3lS1sYxIXiD kBRo478eNqzXHMpBOqbvKjYp/UZgWUNA9ebI1nQtwd7rnjmm/GrtyItjahCsgzDS qPAie+mXYzuT49ZoG+Glg7/R/jDcLMcJY0d5eJ7kufB1RLhvRitZD4FEbJVehqhY aevf5bLk1BNFhzRBfLXmv6u/kfvWf2HjGAf0aFhaQyiAldDgnZrvaZOFjkToJk27 p9MguvwGmbciao0DmMjcJhQ0smclFwy8Kj98Tz+nTkfAlU8jJdb1J/tIatJdpSRh AgMBAAGjgdwwgdkwDAYDVR0TAQH/BAIwADA0BgNVHSUELTArBggrBgEFBQcDAgYI KwYBBQUHAwEGCWCGSAGG+EIEAQYKKwYBBAGCNwoDAzALBgNVHQ8EBAMCBaAwMwYI KwYBBQUHAQEEJzAlMCMGCCsGAQUFBzABhadodHRwOi8vb2NzcC5jYWNlcnQub3Jn LzBRBgNVknsadkjasnjdksandjksandjsnNlY3VyaXR5YXJlYS5vcmegKQYIKwYB BQUHCAWgHQwbbWFpbC5qb2ludC5zZWN1cml0eWFyZWEub3JnMA0GCSqGSIb3DQEB BQUAA4ICAQAX8ceObvUZNKYTlNQ/cv0BiA1XweRsVNca1ILACNLdVPR9mvf+aXCh ODkHaZAmGngj1DfD4fJsTbaydGWSPeVH91Qi9F+Pi6szhsxylI83NKbuXihcenuG twnte8aIb5FelVHttLQPSKRR62E8YmDWk3KYivuFAuZqDaGnWc5yeneTBpsGter/ 4awqsgymBK2YEg1HIWMPaRBvwzCVN/yUyWhFH9Nj11f/xgZE87VXrjLHWT/73i2Z S4uIZ2KHQUYuxMGldgpXm+QxFM8DGA6z1T1oPCVfW85cezlfr8QVvX6SXZrAUNL0 3D5YPzQuevW+5CrqnGA+F5ff4mBMl8R8Sg0+0LoLqt5PbpGyTt9vS1INZCdfvtIA /d7Ae7Xp9W8FVRqd7tvNMIy3ZA0/wNMDUczkhC/YtvHfMELpjtMJAGF15OtO7Vik V+FZnBP1Yd7760dtEmd6bF8vjcXCvDdxwGtcAehAUpIgAWvkHHOt8+H56tkFENAP /ZpJ+Wr+K3lxkkG+BN1bucxMuAdVyTpFyZfKDHRXIO/5e0hpPOaTO+obD3kifzdh yy7KmdKvDclHTiPuonJBzEXeM3JQBjcDHbMSyA6+38yBcso27h9VqCQJB2cZmSlW 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.