Openssl s client

• openssl s_client -showcerts -connect YOUR_DOMAIN.COM:443

• echo -n | openssl s_client -connect HOST:PORTNUMBER | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' > /tmp/$SERVERNAME.cert

See also

• openssl [ rand | s_client | passwd | openssl req | openssl rsa | openssl genrsa | openssl x509 | openssl ca | openssl verify | openssl ec | openssl dgst | openssl pkcs12 | openssl asn1parse | openssl help | .cer to .pem, openssl version
• OpenSSL: RSA, ECDSA, WolfSSL, AES, Diffie-Hellman (DH) key-exchange, /etc/ssl/openssl.cnf, OpenSSL v3

https://serverfault.com/questions/139728/how-to-download-the-ssl-certificate-from-a-website