opensslコマンドを利用して、サーバーに設定されている証明書情報を確認する方法

$ openssl s_client -connect github.com:443 -showcerts < /dev/null
CONNECTED(00000003)
depth=2 C = US, O = DigiCert Inc, OU = www.digicert.com, CN = DigiCert High Assurance EV Root CA
verify return:1
depth=1 C = US, O = DigiCert Inc, OU = www.digicert.com, CN = DigiCert SHA2 Extended Validation Server CA
verify return:1
depth=0 businessCategory = Private Organization, jurisdictionC = US, jurisdictionST = Delaware, serialNumber = 5157550, street = "88 Colin P Kelly, Jr Street", postalCode = 94107, C = US, ST = California, L = San Francisco, O = "GitHub, Inc.", CN = github.com
verify return:1
---
Certificate chain
 0 s:/businessCategory=Private Organization/jurisdictionC=US/jurisdictionST=Delaware/serialNumber=5157550/street=88 Colin P Kelly, Jr Street/postalCode=94107/C=US/ST=California/L=San Francisco/O=GitHub, Inc./CN=github.com
   i:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 Extended Validation Server CA
-----BEGIN CERTIFICATE-----
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-----END CERTIFICATE-----
 1 s:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 Extended Validation Server CA
   i:/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert High Assurance EV Root CA
-----BEGIN CERTIFICATE-----
MIIEtjCCA56gAwIBAgIQDHmpRLCMEZUgkmFf4msdgzANBgkqhkiG9w0BAQsFADBs
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-----END CERTIFICATE-----
---
Server certificate
subject=/businessCategory=Private Organization/jurisdictionC=US/jurisdictionST=Delaware/serialNumber=5157550/street=88 Colin P Kelly, Jr Street/postalCode=94107/C=US/ST=California/L=San Francisco/O=GitHub, Inc./CN=github.com
issuer=/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 Extended Validation Server CA
---
No client certificate CA names sent
Peer signing digest: SHA512
Server Temp Key: ECDH, P-256, 256 bits
---
SSL handshake has read 3637 bytes and written 434 bytes
---
New, TLSv1/SSLv3, Cipher is ECDHE-RSA-AES128-GCM-SHA256
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : ECDHE-RSA-AES128-GCM-SHA256
    Session-ID: 2850806DDE9DCDB9F5DB4EEBCF2752BDA9DC7244907677EE6BDE6F401D76D67E
    Session-ID-ctx:
    Master-Key: 3DD6FE32CA6D5FDDACBC48465CB200FFF894C4601B978F7A666F3C72ECED6FA9D8745EECC3C3CD17C3C1149933B6582C
    Key-Arg   : None
    PSK identity: None
    PSK identity hint: None
    SRP username: None
    Start Time: 1490110988
    Timeout   : 300 (sec)
    Verify return code: 0 (ok)
---
DONE