Retrieve remote FTP or SMTP TLS server certificate

February 26, 2013

With SSL protected resources it is easy to find a command line to retrieve the certificate. Following is documented at many places:

openssl s_client -connect www.google.com:443

But if you use use this with a TLS server, be it SMTP or FTP you’ll get

CONNECTED(00000003)
527654:error:140770FC:SSL routines:SSL23_GET_SERVER_HELLO:unknown protocol:s23_clnt.c:607:

The solution ins quite easy (if you know it) .. use following command line:

openssl s_client -connect ftp.xxxx.at:21 -starttls ftp

or

openssl s_client -connect smtp.xxxx.at:25 -starttls smtp

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