Postfix TLS with free CAcert.org certificates

1. Install Postfix with TLS

Since Postfix 2.2 release TLS support is included if compiled in. See: http://www.postfix.org/TLS_README.html.

2. Get familiar with CAcert.org website

Just go to https://www.cacert.org/ and join CAcert.org and fill in your details. There is also short process description available on their site. You'll get mail back to verify that you can read email on address you provided. After login, add domain and service will try to verify that you can read mail on one of following accounts: root, hostmaster, postmaster, admin, webmaster or email addresses that can be found on whois data of domain that you provided. Remember that only after you have verified your domain you can start adding servers that work on that domain.

3. Create private TLS key and certificate request

Certificate request is your public key to be signed by certificate authority. Creating it goes like this:

$ su -
Password:

mkdir /etc/postfix/tls
chown root:root /etc/postfix/tls
chmod 0500 /etc/postfix/tls
cd /etc/postfix/tls
openssl req -nodes -newkey rsa:2048 -keyout privatekey.pem -out csr.pem

Generating a 2048 bit RSA private key
...++++++
........++++++
writing new private key to 'privatekey.pem'
-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:FI
State or Province Name (full name) [Some-State]:Uusimaa
Locality Name (eg, city) []:Porvoo
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Petri Koistinen Home
Organizational Unit Name (eg, section) []:Desktop
Common Name (eg, YOUR name) []:joo.ath.cx
Email Address []:petri.koistinen@iki.fi

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:

chown root:root /etc/postfix/tls/privatekey.pem
chmod 0400 /etc/postfix/tls/privatekey.pem
chown root:root /etc/postfix/tls/csr.pem
chmod 0400 /etc/postfix/tls/csr.pem

Private keys should belong to "root" and be readable only by root. Postfix loads private keys before dropping superuser privileges.

4. Send your public key to be singed by CAcert.org authority

Request new server certificate from CAcert.org web site. This howto assume that you select Class 1 certificate. When you are asked for CSR paste content of /etc/postfix/tls/req.pem to box. It should look like this:

You should also verify on content of request with

openssl req -in /etc/postfix/tls/csr.pem -text -verify -noout

before sending it.

5. Install server certificate

Copy certificate from web page and put in /etc/postfix/tls/cert.pem file. Remember to do

chown root:postfix /etc/postfix/tls/cert.pem
chmod a=r /etc/postfix/tls/cert.pem

so that everybody can read it. You check contents of this file with

openssl x509 -in /etc/postfix/tls/cert.pem -text -noout

command, Validity and Subject fields should be checked at least.

6. Get CAcert.org root certificate

It you run Debian or Ubuntu and have installed ca-certificates package and chosen CAcert.org root certificate to be used you can skip this part.

wget -nv https://www.cacert.org/certs/root.crt -O /etc/postfix/tls/root.crt
chown root:postfix /etc/postfix/tls/root.crt
chmod a=r /etc/postfix/tls/root.crt

Here is the CAcert.org root certificate

You can examine root certificate in detail with this command

openssl x509 -in /etc/postfix/tls/root.crt -text -noout

It is important that root certificate is good and not tampered. You should see exactly this as output of that command:

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 0 (0x0)
        Signature Algorithm: md5WithRSAEncryption
        Issuer: O=Root CA, OU=http://www.cacert.org, CN=CA Cert Signing Authority/emailAddress=support@cacert.org
        Validity
            Not Before: Mar 30 12:29:49 2003 GMT
            Not After : Mar 29 12:29:49 2033 GMT
        Subject: O=Root CA, OU=http://www.cacert.org, CN=CA Cert Signing Authority/emailAddress=support@cacert.org
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
            RSA Public Key: (4096 bit)
                Modulus (4096 bit):
                    00:ce:22:c0:e2:46:7d:ec:36:28:07:50:96:f2:a0:
                    33:40:8c:4b:f1:3b:66:3f:31:e5:6b:02:36:db:d6:
                    7c:f6:f1:88:8f:4e:77:36:05:41:95:f9:09:f0:12:
                    cf:46:86:73:60:b7:6e:7e:e8:c0:58:64:ae:cd:b0:
                    ad:45:17:0c:63:fa:67:0a:e8:d6:d2:bf:3e:e7:98:
                    c4:f0:4c:fa:e0:03:bb:35:5d:6c:21:de:9e:20:d9:
                    ba:cd:66:32:37:72:fa:f7:08:f5:c7:cd:58:c9:8e:
                    e7:0e:5e:ea:3e:fe:1c:a1:14:0a:15:6c:86:84:5b:
                    64:66:2a:7a:a9:4b:53:79:f5:88:a2:7b:ee:2f:0a:
                    61:2b:8d:b2:7e:4d:56:a5:13:ec:ea:da:92:9e:ac:
                    44:41:1e:58:60:65:05:66:f8:c0:44:bd:cb:94:f7:
                    42:7e:0b:f7:65:68:98:51:05:f0:f3:05:91:04:1d:
                    1b:17:82:ec:c8:57:bb:c3:6b:7a:88:f1:b0:72:cc:
                    25:5b:20:91:ec:16:02:12:8f:32:e9:17:18:48:d0:
                    c7:05:2e:02:30:42:b8:25:9c:05:6b:3f:aa:3a:a7:
                    eb:53:48:f7:e8:d2:b6:07:98:dc:1b:c6:34:7f:7f:
                    c9:1c:82:7a:05:58:2b:08:5b:f3:38:a2:ab:17:5d:
                    66:c9:98:d7:9e:10:8b:a2:d2:dd:74:9a:f7:71:0c:
                    72:60:df:cd:6f:98:33:9d:96:34:76:3e:24:7a:92:
                    b0:0e:95:1e:6f:e6:a0:45:38:47:aa:d7:41:ed:4a:
                    b7:12:f6:d7:1b:83:8a:0f:2e:d8:09:b6:59:d7:aa:
                    04:ff:d2:93:7d:68:2e:dd:8b:4b:ab:58:ba:2f:8d:
                    ea:95:a7:a0:c3:54:89:a5:fb:db:8b:51:22:9d:b2:
                    c3:be:11:be:2c:91:86:8b:96:78:ad:20:d3:8a:2f:
                    1a:3f:c6:d0:51:65:87:21:b1:19:01:65:7f:45:1c:
                    87:f5:7c:d0:41:4c:4f:29:98:21:fd:33:1f:75:0c:
                    04:51:fa:19:77:db:d4:14:1c:ee:81:c3:1d:f5:98:
                    b7:69:06:91:22:dd:00:50:cc:81:31:ac:12:07:7b:
                    38:da:68:5b:e6:2b:d4:7e:c9:5f:ad:e8:eb:72:4c:
                    f3:01:e5:4b:20:bf:9a:a6:57:ca:91:00:01:8b:a1:
                    75:21:37:b5:63:0d:67:3e:46:4f:70:20:67:ce:c5:
                    d6:59:db:02:e0:f0:d2:cb:cd:ba:62:b7:90:41:e8:
                    dd:20:e4:29:bc:64:29:42:c8:22:dc:78:9a:ff:43:
                    ec:98:1b:09:51:4b:5a:5a:c2:71:f1:c4:cb:73:a9:
                    e5:a1:0b
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Subject Key Identifier: 
                16:B5:32:1B:D4:C7:F3:E0:E6:8E:F3:BD:D2:B0:3A:EE:B2:39:18:D1
            X509v3 Authority Key Identifier: 
                keyid:16:B5:32:1B:D4:C7:F3:E0:E6:8E:F3:BD:D2:B0:3A:EE:B2:39:18:D1
                DirName:/O=Root CA/OU=http://www.cacert.org/CN=CA Cert Signing Authority/emailAddress=support@cacert.org
                serial:00

            X509v3 Basic Constraints: critical
                CA:TRUE
            X509v3 CRL Distribution Points: 
                URI:https://www.cacert.org/revoke.crl

            Netscape CA Revocation Url: 
                https://www.cacert.org/revoke.crl
            Netscape CA Policy Url: 
                http://www.cacert.org/index.php?id=10
            Netscape Comment: 
                To get your own certificate for FREE head over to http://www.cacert.org
    Signature Algorithm: md5WithRSAEncryption
        28:c7:ee:9c:82:02:ba:5c:80:12:ca:35:0a:1d:81:6f:89:6a:
        99:cc:f2:68:0f:7f:a7:e1:8d:58:95:3e:bd:f2:06:c3:90:5a:
        ac:b5:60:f6:99:43:01:a3:88:70:9c:9d:62:9d:a4:87:af:67:
        58:0d:30:36:3b:e6:ad:48:d3:cb:74:02:86:71:3e:e2:2b:03:
        68:f1:34:62:40:46:3b:53:ea:28:f4:ac:fb:66:95:53:8a:4d:
        5d:fd:3b:d9:60:d7:ca:79:69:3b:b1:65:92:a6:c6:81:82:5c:
        9c:cd:eb:4d:01:8a:a5:df:11:55:aa:15:ca:1f:37:c0:82:98:
        70:61:db:6a:7c:96:a3:8e:2e:54:3e:4f:21:a9:90:ef:dc:82:
        bf:dc:e8:45:ad:4d:90:73:08:3c:94:65:b0:04:99:76:7f:e2:
        bc:c2:6a:15:aa:97:04:37:24:d8:1e:94:4e:6d:0e:51:be:d6:
        c4:8f:ca:96:6d:f7:43:df:e8:30:65:27:3b:7b:bb:43:43:63:
        c4:43:f7:b2:ec:68:cc:e1:19:8e:22:fb:98:e1:7b:5a:3e:01:
        37:3b:8b:08:b0:a2:f3:95:4e:1a:cb:9b:cd:9a:b1:db:b2:70:
        f0:2d:4a:db:d8:b0:e3:6f:45:48:33:12:ff:fe:3c:32:2a:54:
        f7:c4:f7:8a:f0:88:23:c2:47:fe:64:7a:71:c0:d1:1e:a6:63:
        b0:07:7e:a4:2f:d3:01:8f:dc:9f:2b:b6:c6:08:a9:0f:93:48:
        25:fc:12:fd:9f:42:dc:f3:c4:3e:f6:57:b0:d7:dd:69:d1:06:
        77:34:0a:4b:d2:ca:a0:ff:1c:c6:8c:c9:16:be:c4:cc:32:37:
        68:73:5f:08:fb:51:f7:49:53:36:05:0a:95:02:4c:f2:79:1a:
        10:f6:d8:3a:75:9c:f3:1d:f1:a2:0d:70:67:86:1b:b3:16:f5:
        2f:e5:a4:eb:79:86:f9:3d:0b:c2:73:0b:a5:99:ac:6f:fc:67:
        b8:e5:2f:0b:a6:18:24:8d:7b:d1:48:35:29:18:40:ac:93:60:
        e1:96:86:50:b4:7a:59:d8:8f:21:0b:9f:cf:82:91:c6:3b:bf:
        6b:dc:07:91:b9:97:56:23:aa:b6:6c:94:c6:48:06:3c:e4:ce:
        4e:aa:e4:f6:2f:09:dc:53:6f:2e:fc:74:eb:3a:63:99:c2:a6:
        ac:89:bc:a7:b2:44:a0:0d:8a:10:e3:6c:f2:24:cb:fa:9b:9f:
        70:47:2e:de:14:8b:d4:b2:20:09:96:a2:64:f1:24:1c:dc:a1:
        35:9c:15:b2:d4:bc:55:2e:7d:06:f5:9c:0e:55:f4:5a:d6:93:
        da:76:ad:25:73:4c:c5:43

7. c_rehash will let OpenSSL know about CAcert.org root certificate

I hope CAcert.org root certificate will be shipped some day with OpenSSL by default, but in the meanwhile we need add it. Finding where OpenSSL root certificates are stored can be little bit tricky. With Debian (or Ubuntu) directory is /usr/lib/ssl/certs. You can install it like this: (Debian ca-certificates users must not do this.)

cp /etc/postfix/tls/root.crt /usr/lib/ssl/certs/CAcert.org_Root_Certificate.pem
c_rehash /usr/lib/ssl/certs

In case you don't have that c_rehash perl script you can download it here. I hope it works.

After installation of root certificate, you should test installation like this:

petri@dsl-prvgw1nf5:/etc/postfix/tls$ openssl verify /etc/postfix/tls/cert.pem 
/etc/postfix/tls/cert.pem: OK

If installation failed you probably get message like this:

petri@dsl-prvgw1nf5:/etc/postfix/tls$ openssl verify /etc/postfix/tls/cert.pem 
/etc/postfix/tls/cert.pem: /CN=joo.ath.cx
error 20 at 0 depth lookup:unable to get local issuer certificate

8. Configure Postfix main.cf file

Add this configuration to /etc/postfix/main.cf if that's where your main.cf is.

# TLS PART START

smtp_tls_CAfile = /etc/postfix/tls/root.crt
smtp_tls_cert_file = /etc/postfix/tls/cert.pem
smtp_tls_key_file = /etc/postfix/tls/privatekey.pem
smtp_tls_session_cache_database = btree:$data_directory/smtp_tls_session_cache
smtp_tls_security_level = may
smtp_tls_loglevel = 1

smtpd_tls_CAfile = /etc/postfix/tls/root.crt
smtpd_tls_cert_file = /etc/postfix/tls/cert.pem
smtpd_tls_key_file = /etc/postfix/tls/privatekey.pem
smtpd_tls_session_cache_database = btree:$data_directory/smtpd_tls_session_cache
smtpd_tls_security_level = may
smtpd_tls_loglevel = 1

smtpd_tls_received_header = yes
smtpd_tls_ask_ccert = yes

# TLS PART END

Debian ca-certificates users should alternatively do same except on these lines

...
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
...
smtpd_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
...

Make sure that $data_directory is set.

You (and I) probably should learn how to add other than CAcert.org root certificates too. By the way, btree databases need Berkeley DB support.

9. Test installation

First you could test your own server by "telnet localhost 25" and saying "EHLO localhost" checking if you can see "STARTTLS" greeting. If you do, just say "STARTTLS" and check what server replies back and check what Postfix log files says.

Then next send message between two servers that support TLS and have CAcert.org root certificates installed. Receiving server should have mail log looking like this:

Oct 16 17:18:23 dsl-prvgw1ib8 postfix/smtpd[2921]: connect from dsl-prvgw1nf5.dial.inet.fi[80.223.61.245]
Oct 16 17:18:23 dsl-prvgw1ib8 postfix/smtpd[2921]: setting up TLS connection from dsl-prvgw1nf5.dial.inet.fi[80.223.61.245]
Oct 16 17:18:24 dsl-prvgw1ib8 postfix/smtpd[2921]: fingerprint=C9:54:81:FB:D4:05:05:32:CA:1C:8D:0B:C8:7E:58:E2
Oct 16 17:18:24 dsl-prvgw1ib8 postfix/smtpd[2921]: Verified: subject_CN=joo.ath.cx, issuer=CA Cert Signing Authority
Oct 16 17:18:24 dsl-prvgw1ib8 postfix/smtpd[2921]: TLS connection established from dsl-prvgw1nf5.dial.inet.fi[80.223.61.245]: TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)
Oct 16 17:18:24 dsl-prvgw1ib8 postfix/smtpd[2921]: 19EB52019D88D: client=dsl-prvgw1nf5.dial.inet.fi[80.223.61.245]

And header of received message should have header looking like this:

Received: from dsl-prvgw1nf5.dial.inet.fi (dsl-prvgw1nf5.dial.inet.fi [80.223.61.245])
    (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits))
    (Client CN "joo.ath.cx", Issuer "CA Cert Signing Authority" (verified OK))
    by dsl-prvgw1ib8.dial.inet.fi (Postfix) with ESMTP id 19EB52019D88D
    for <petri@juu.ath.cx>; Sat, 16 Oct 2004 17:18:24 +0300 (EEST)

Everything should be done now. Please note, that if receiver's server don't have CAcert.org root certificate installed header will look like this:

    ...
    (Client CN "joo.ath.cx", Issuer "CA Cert Signing Authority" (not verified))
    ...

Perhaps you could ask nicely if CAcert.org root certificate could be installed on receiving server.

Even if your certificate is not verified, your mail will be delivered, if it's accepted by receiving server. Not verified means that servers don't trust each others via Certificate Authority (CA) that is common to both parties. Without verification, you can not know if your communication is tampered in between, but still the communication is encrypted between the servers. For more information see for example Wikipedia article on a href="http://en.wikipedia.org/wiki/Man-in-the-middle_attack"> Man in the middle attack.

10. Extra: Practical usage scenario

Here is a way to enable relay based on TLS certificate trust rather than usual IP address trust (see mynetworks option). The main.cf options:

smtpd_recipient_restrictions =
 permit_mynetworks,
 permit_tls_clientcerts,
 reject_unauth_destination

relay_clientcerts = dbm:/etc/postfix/relay_clientcerts

and content of that file:

$ cat /etc/postfix/relay_clientcerts
C9:54:81:FB:D4:05:05:32:CA:1C:8D:0B:C8:7E:58:E2 joo.ath.cx

(Remember to do postmap /etc/postfix/relay_clientcerts). Where do you get this fingerprint then? Either you look on receiving server mail.log or use this command:

$ openssl x509 -md5 -fingerprint -in /etc/postfix/tls/server.pem -noout
MD5 Fingerprint=C9:54:81:FB:D4:05:05:32:CA:1C:8D:0B:C8:7E:58:E2

With help of this you can enable email relay for certain users which present verifiable certificate with certain fingerprint. It doesn't matter on which IP address mail relay client is connecting from. This could be useful for mobile users.

You will most likely want to add following to master.cf, so that mobile users can avoid restrictions that are in place for outbound 25/tcp traffic.

submission   inet    n       -       n       -       -       smtpd
	-o smtpd_enforce_tls=yes

Submission is on port 587/tcp. See RFC 2476 for more info.

11. Please provide feedback about this document

You can provide feedback about this document to me: petri.koistinen@iki.fi. All feedback is welcome, whatever it's about tiny typos or major misconceptions. Don't be afraid to ask anything or don't worry about flooding my mailbox, so far I have only got 20 replies by email. If you just got TLS working with help of this document, drop me a line or send message me on IRC, details on front page of my home pages. You might also be interest about my spam filtering configuration for Postfix.

And remember that this doesn't provide end-to-end security for email, but it helps a bit.

Document author: Petri T. Koistinen. Document last modified: Sunday, 10-Mar-2013 00:47:33 EET
Please, use this permanent URL for linking to this document: http://www.iki.fi/petri.koistinen/postfix/postfix-tls-cacert.shtml