我是 的作者checkdmarc，这是一款用于检查 DMARC 和其他电子邮件安全标准的开源 CLI 工具。其中一项检查涉及测试域MX记录中列出的邮件服务器是否支持 TLS。我发现在许多情况下，域的 MX 记录中列出的主机名与该服务器使用的证书的备用名称值不匹配CN。这会导致与这些主机的 TLS 连接因主机名不匹配而失败。
例如，MX 记录为gmail.com：
gmail.com. 1550 IN MX 20 alt2.gmail-smtp-in.l.google.com.
gmail.com. 1550 IN MX 40 alt4.gmail-smtp-in.l.google.com.
gmail.com. 1550 IN MX 10 alt1.gmail-smtp-in.l.google.com.
gmail.com. 1550 IN MX 5 gmail-smtp-in.l.google.com.
gmail.com. 1550 IN MX 30 alt3.gmail-smtp-in.l.google.com.
然而，快速检查证书gmail-smtp-in.l.google.com显示，该主机提供的证书的 CN 是mx.google.com。我该如何解释这一点？邮件客户端是否只是忽略了 MX 服务器上的主机名匹配？这似乎很疯狂！
openssl s_client -connect gmail-smtp-in.l.google.com:25 -starttls smtp
CONNECTED(00000003)
depth=2 C = US, O = Google Trust Services LLC, CN = GTS Root R1
verify return:1
depth=1 C = US, O = Google Trust Services, CN = WR2
verify return:1
depth=0 CN = mx.google.com
verify return:1
---
Certificate chain
0 s:CN = mx.google.com
i:C = US, O = Google Trust Services, CN = WR2
a:PKEY: id-ecPublicKey, 256 (bit); sigalg: RSA-SHA256
v:NotBefore: Jun 24 07:37:53 2024 GMT; NotAfter: Sep 16 07:37:52 2024 GMT
1 s:C = US, O = Google Trust Services, CN = WR2
i:C = US, O = Google Trust Services LLC, CN = GTS Root R1
a:PKEY: rsaEncryption, 2048 (bit); sigalg: RSA-SHA256
v:NotBefore: Dec 13 09:00:00 2023 GMT; NotAfter: Feb 20 14:00:00 2029 GMT
2 s:C = US, O = Google Trust Services LLC, CN = GTS Root R1
i:C = BE, O = GlobalSign nv-sa, OU = Root CA, CN = GlobalSign Root CA
a:PKEY: rsaEncryption, 4096 (bit); sigalg: RSA-SHA256
v:NotBefore: Jun 19 00:00:42 2020 GMT; NotAfter: Jan 28 00:00:42 2028 GMT
---
Server certificate
-----BEGIN CERTIFICATE-----
MIIGwjCCBaqgAwIBAgIQZZLFVIncLgcKOAT21sXTxzANBgkqhkiG9w0BAQsFADA7
MQswCQYDVQQGEwJVUzEeMBwGA1UEChMVR29vZ2xlIFRydXN0IFNlcnZpY2VzMQww
CgYDVQQDEwNXUjIwHhcNMjQwNjI0MDczNzUzWhcNMjQwOTE2MDczNzUyWjAYMRYw
FAYDVQQDEw1teC5nb29nbGUuY29tMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE
EPY8iG+t2yuh+G2C7yURtlapDVtP5VYg0EifYWjDqqP0wUDTG82xOglMV1f5KGEy
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ghVhc3BteDIuZ29vZ2xlbWFpbC5jb22CFWFzcG14My5nb29nbGVtYWlsLmNvbYIV
YXNwbXg0Lmdvb2dsZW1haWwuY29tghVhc3BteDUuZ29vZ2xlbWFpbC5jb22CEWdt
ci1teC5nb29nbGUuY29tMBMGA1UdIAQMMAowCAYGZ4EMAQIBMDYGA1UdHwQvMC0w
K6ApoCeGJWh0dHA6Ly9jLnBraS5nb29nL3dyMi85VVZiTjB3NUU2WS5jcmwwggEC
BgorBgEEAdZ5AgQCBIHzBIHwAO4AdQDuzdBk1dsazsVct520zROiModGfLzs3sNR
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v2s/tbYin5vCu1xr6HCRcWy7UYSFNL2kPTBI1/urAAABkEllANgAAAQDAEYwRAIg
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MmtOZbkiDk9Ist2pmGRTP0Z7VpJy12aG/hUoeqcN1FmrWCNKG7ycMIVrairGrI4A
8wa5dUpDwezKaP3V3U8o9btRcuRD8doKFTPhzas+d1EZ1mtS/m9nKR1ssGfjtKFX
7zyNb+OoZOES1JpEjke3vc1iwkri+PkG+KuREsvFvb0sK6X6Gq0xE8EF25765UGz
vuIwywDO
-----END CERTIFICATE-----
subject=CN = mx.google.com
issuer=C = US, O = Google Trust Services, CN = WR2
---
No client certificate CA names sent
Peer signing digest: SHA256
Peer signature type: ECDSA
Server Temp Key: X25519, 253 bits
---
SSL handshake has read 5003 bytes and written 441 bytes
Verification: OK
---
New, TLSv1.3, Cipher is TLS_AES_256_GCM_SHA384
Server public key is 256 bit
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
Early data was not sent
Verify return code: 0 (ok)
