TLS/SSL related stuff

Prerequisites

If needed, in Debian or Ubutnu gnutls-cli can be installed using

apt install gnutls-bin

Check certificate of a Web server

Using OpenSSL:

openssl s_client -verify_return_error -connect <HOST>:443

Using GnuTLS:

gnu-tls-cli <HOST>

Check certificate of an SMTP server

Using OpenSSL:

openssl s_client -verify_return_error -starttls smtp -connect <HOST>:25

Using GnuTLS

gnutls-cli --starttls-proto smtp <HOST>