Postfix and OpenSSL: "Unable to get local issuer certificate"

Question

I bought a certicifate for my mail server and configured it in Postfix like so:

smtpd_tls_security_level=may
smtpd_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
smtpd_tls_cert_file = /somepath/chain.crt
smtpd_tls_key_file = /somepath/myserver.key

I created chain.crt per Postfix instructions by concatenating my certificate and the two intermediate CA certs. (To be precise, my own cert first, then COMODORSADomainValidationSecureServerCA.crt and then COMODORSAAddTrustCA.crt) According to the logs, most connections to my mail server are now encrypted so everything seems to work.

However, when I connect to the server using openssl s_client -connect mail.example.com:25 -starttls smtp I get the following output, which seems to indicate that something is wrong:

CONNECTED(00000003)
depth=2 C = GB, ST = Greater Manchester, L = Salford, O = COMODO CA Limited, CN = COMODO RSA Certification Authority
verify error:num=20:unable to get local issuer certificate
verify return:0
---
Certificate chain
 0 s:/OU=Domain Control Validated/OU=PositiveSSL/CN=mail.example.com
   i:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA
 1 s:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA
   i:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Certification Authority
 2 s:/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Certification Authority
   i:/C=SE/O=AddTrust AB/OU=AddTrust External TTP Network/CN=AddTrust External CA Root
---
Server certificate
-----BEGIN CERTIFICATE-----
8<
-----END CERTIFICATE-----
subject=/OU=Domain Control Validated/OU=PositiveSSL/CN=mail.example.com
issuer=/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=COMODO RSA Domain Validation Secure Server CA
---
No client certificate CA names sent
---
SSL handshake has read 5217 bytes and written 489 bytes
---
New, TLSv1/SSLv3, Cipher is ECDHE-RSA-AES256-GCM-SHA384
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: zlib compression
Expansion: zlib compression
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : ECDHE-RSA-AES256-GCM-SHA384
    Session-ID: B2736D2B1639153CB8379480ACDBC1F0B1DCC51DE7CC27DA59061740F0186EA1
    Session-ID-ctx: 
    Master-Key: 9EE658D6B1835F94D140E7BBF05A5A8960566584A8BC6FF40DD5D038C70127A63BAB580A41BC55DEEA3C031FCC7FA550
    Key-Arg   : None
    PSK identity: None
    PSK identity hint: None
    SRP username: None
    TLS session ticket lifetime hint: 3600 (seconds)
    TLS session ticket:
    0000 - 09 8a 6d 68 70 0c 64 c0-23 2e fc f2 d4 21 ca bd   ..mhp.d.#....!..
    0010 - 0a 15 4f 36 06 a8 ea f9-e2 84 de 18 cf 31 af 77   ..O6.........1.w
    0020 - 2a a1 98 d5 f8 df 54 48-82 68 cb c3 91 9b 38 60   *.....TH.h....8`
    0030 - dc 2e 18 f4 b9 bd 43 3e-26 cf 80 49 48 bb a4 98   ......C>&..IH...
    0040 - bc 94 7d 1e 5f 51 7e 52-d6 ad 7f 8b e5 36 ee 55   ..}._Q~R.....6.U
    0050 - 7a bb 15 d3 c2 7f b2 f4-5a 85 a5 80 1b 9e 67 98   z.......Z.....g.
    0060 - 3a 2f 5c d1 34 d5 bb 11-c7 8f fc 10 cb c3 b4 cb   :/\.4...........
    0070 - 49 61 41 ac aa b1 cb eb-6f 7a 22 e5 79 a2 c6 1f   IaA.....oz".y...
    0080 - 98 25 39 f0 f0 79 b4 ee-95 72 93 04 0f a2 f3 69   .%9..y...r.....i
    0090 - 8f 8a 86 c6 8f a5 38 ed-25 ba fc c8 db 6b 9a 8e   ......8.%....k..
    00a0 - 13 1e e5 c6 28 98 e4 17-ce 69 d5 c2 aa 1a de 4b   ....(....i.....K

    Compression: 1 (zlib compression)
    Start Time: 1400131328
    Timeout   : 300 (sec)
    Verify return code: 20 (unable to get local issuer certificate)
---

Also, when running openssl verify chain.crt I get the same error message:

chain.crt: OU = Domain Control Validated, OU = PositiveSSL, CN = mail.example.com
error 20 at 0 depth lookup:unable to get local issuer certificate

I have tried adding the AddTrustExternalCARoot.crt to the end of chain.crt which makes the error message change to verify error:num=19:self signed certificate in certificate chain. Besides, the machine I'm connecting from already has this certificate installed so there should be no need for it.

So my question is: Is there something wrong with my mailserver or am I just using the openssl command incorrectly? And should I include the root certificate in the chain or not?

Solution

It sounds like you've installed it correctly, but your verification step is incorrect. OpenSSL doesn't know where to look to find root certificates unless you explicitly tell it.

Try openssl s_client -connect mail.example.com:25 -starttls smtp -CApath /etc/ssl/certs instead. That should work.