openssl: connexion SMTP avec TLS

Il est possible d'utiliser la commande openssl pour se connecter à un serveur STMP en utilisant la sécurité TLS,

Dans l'exemple suivant, l'option -CAfile /etc/ssl/certs/ca-certificates.crt permet d'indiquer l'autorité de certification dans le cas d'un certificat auto signé.

# openssl s_client -starttls smtp -crlf -CAfile /etc/ssl/certs/ca-certificates.crt -connect smtp.server.fr:25

CONNECTED(00000003)
depth=0 CN = localhost.localdomain
verify return:1
---
Certificate chain
 0 s:/CN=localhost.localdomain
   i:/CN=localhost.localdomain
---
Server certificate
-----BEGIN CERTIFICATE-----
MIIBtzCCASACCQCupdgFIYzFVDANBgkqhkiG9w0BAQUFADAgMR4w
b2NhbGhvaW4wHhcNMTEwNDI2MTkxMDM1WhcNMjEwNDIzMTkx
MDM1WjAgMR4wHAYDVQQDExVsb2NhbGhvc3QubG9jYWxkb21ha
hvcNAQEBBQADgY0AMIGJZS3OcCHjbQNa++F/J4WGFk6r7ICl9Ls0
ybT+1CzbFG1xE71NeRhbHkLGigZa5422afsqq/b3LBIpMX17B/gtuo+
QKRckuo+20BUqHFye7ZAVXSsw+bqmm9D0vB4uw9IwyE75jEAqU1
QX7RAgMBAAEwDQYJKoZIhvcNAQEFBQADgYEAe1iHUXoGSTj3Fv3
2Yg1EFtNm/RSYxdHV8jY6W6CtwNatN+EQqmWlfhtOSCWgXriUjX5
E4uEocf6x/vDrVkpU11u+0xDUYshmrLyk2nmX6wbopwQttAE9PLw
ahQBiuj+sz38OW8=
-----END CERTIFICATE-----
subject=/CN=localhost.localdomain
issuer=/CN=localhost.localdomain
---
No client certificate CA names sent
---
SSL handshake has read 1021 bytes and written 456 bytes
---
New, TLSv1/SSLv3, Cipher is ECDHE-RSA-AES256-GCM-SHA384
Server public key is 1024 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : ECDHE-RSA-AES256-GCM-SHA384
    Session-ID: D2A784125E749B8C74015978412B9B6E47DDA37BFB83FD4FEE8E6ED21BE79C92
    Session-ID-ctx:
    Master-Key: 631649B5A6A2F5272137C421AFB2F2E86CD483ED2FE8099BCDC9068A11EE7087F
    Key-Arg   : None
    PSK identity: None
    PSK identity hint: None
    SRP username: None
    Start Time: 1478726847
    Timeout   : 300 (sec)
    Verify return code: 0 (ok)
---
250 DSN
mail from: one@mail.fr
250 2.1.0 Ok
rcpt to: two@mail.fr
250 2.1.5 Ok
data
354 End data with <CR><LF>.<CR><LF>
from: one@mail.fr
to: two@mail.fr
subject: send mail with TLS connection

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.
250 2.0.0 Ok: queued as 145A53700157
quit
221 2.0.0 Bye
closed

Très utile pour tester un serveur STMP