Establishing a TLS client connection on the command line

To establish a TLS client connection, we will use the s_client subcommand of the openssl tool. Its documentation can be found on its man page:

$ man openssl-s_client

There is an HTTPS server on the internet to use as an example, https://example.org/. Let’s connect to it via TLS and get its home page:

$ openssl s_client -connect example.org:443

The openssl tool will output a lot of information about how the TLS handshaking has gone, which cryptographic algorithms were used, and even the base64-encoded server certificate will be printed.

We can also request verification of the server certificate and its hostname by adding the -verify_return_error and -verify_hostname command-line options:

$ openssl s_client \
    -connect example.org:443 \
    -verify_return_error \
    -verify_hostname example.org

If you want to verify the server certificate...