我为我的邮件服务器购买了一个证书，并在 Postfix 中对其进行了如下配置：
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
我创建chain.crt了Postfix 说明通过连接我的证书和两个中间 CA 证书。（准确地说，首先是我的证书，然后COMODORSADomainValidationSecureServerCA.crt是COMODORSAAddTrustCA.crt）根据日志，现在到我的邮件服务器的大多数连接都已加密，因此一切似乎都正常。
但是，当我使用连接到服务器时，openssl s_client -connect mail.example.com:25 -starttls smtp我得到以下输出，这似乎表明出现了问题：
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)
---
此外，运行时openssl verify chain.crt我收到相同的错误消息：
chain.crt: OU = Domain Control Validated, OU = PositiveSSL, CN = mail.example.com
error 20 at 0 depth lookup:unable to get local issuer certificate
我尝试AddTrustExternalCARoot.crt在末尾添加chain.crt，这会使错误消息更改为verify error:num=19:self signed certificate in certificate chain。此外，我连接的机器已经安装了此证书，因此不需要它。
所以我的问题是：我的邮件服务器有问题吗？还是我只是使用命令openssl不正确？我是否应该将根证书包含在链中？
答案1
听起来你已经正确安装了，但是你的验证步骤不正确。除非你明确告诉它，否则 OpenSSL 不知道在哪里查找根证书。
尝试openssl s_client -connect mail.example.com:25 -starttls smtp -CApath /etc/ssl/certs一下。那应该有效。