How do I use OpenSSL to Retrieve a Certificate from a Server?

To retrieve a certificate from a server using OpenSSL, you can use the s_client command. This command connects to the server, initiates an SSL/TLS handshake, and outputs the server’s certificate chain. Here’s a step-by-step guide:

Steps to Retrieve a Certificate

1. Open a Terminal or Command Prompt:
   • Ensure OpenSSL is installed on your system and accessible from the terminal.
2. Run the s_client Command: Use the following syntax to connect to the server:
   bash

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   openssl s_client -connect <server>:<port>

   • Replace <server> with the server’s domain name or IP address.
   • Replace <port> with the server’s SSL/TLS port (usually 443 for HTTPS).

   Example:

   bash

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   openssl s_client -connect www.example.com:443

3. View the Certificate:
   • Look for the certificate in the output, typically between the lines:
     css

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     -----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----

4. Save the Certificate:
   • If you need to save the certificate, redirect the output to a file and extract the certificate portion:
     bash

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     openssl s_client -connect www.example.com:443 > server_cert.pem

   • Edit the file to keep only the certificate block (from -----BEGIN CERTIFICATE----- to -----END CERTIFICATE-----).

Options for Fine-Tuning the Command

• Specify Protocol: Use -starttls followed by the protocol name for services like SMTP or IMAP:
  bash

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  openssl s_client -connect mail.example.com:587 -starttls smtp

• Limit Depth of Output: Use the -showcerts flag to display all certificates in the chain:
  bash

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  openssl s_client -connect www.example.com:443 -showcerts

• Verify the Certificate: Add the -verify flag to validate the certificate:
  bash

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  openssl s_client -connect www.example.com:443 -verify 5


Common Issues

1. Firewall/Network Restrictions:
   • Ensure the server and port are reachable from your network.
2. Expired Certificates:
   • If the certificate is expired, OpenSSL may still retrieve it but display a warning.
3. Untrusted Certificate Authorities:
   • If the certificate isn’t issued by a trusted authority, OpenSSL will still retrieve it, but it won’t verify the trust chain.

This method allows you to retrieve and inspect certificates for troubleshooting, validation, or extraction purposes.