You can use OpenSSL, which is commonly installed on Mac OS X and Linux systems and which is also available for other operating systems (the source code can be downloaded from OpenSSL Downloads and a Windows implementation is available at OpenSSL for Windows), to check the security certificate of a website using an openssl command in the form
openssl s_client -showcerts -connect fqdn:port where
fqdn is the
fully qualified domain name (FQDN) of the website and
port is the
port that the website is listening on for
HTTPS
connections, which is usually
well-known port 443, though it may sometimes be another port, such as the
registered port 8443. The showcerts option instructs openssl
to show all certificates in the
public key certificate chain.
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