Openssl S_client Not Showing Cert With -showcert

OpenSSL s_client Not Showing Certificate with -showcert: Troubleshooting and Solutions

OpenSSL's s_client command is a powerful tool for testing SSL/TLS connections. The -showcert option is intended to display the server's certificate details after a successful connection. However, sometimes you might encounter situations where the certificate information isn't displayed, even with -showcert specified. This article explores common reasons for this issue and provides practical solutions. This guide is for troubleshooting and understanding the behavior of openssl s_client, not for malicious purposes.

Why isn't my certificate showing up? This problem often stems from issues with the server's configuration, the client's environment, or network complexities. Let's dive into the most frequent causes.

1. Server-Side Issues: The Root of the Problem

• Self-signed certificates: The most common culprit is a self-signed certificate on the server. While s_client will connect, it might not display the certificate details because it doesn't trust the certificate authority (CA). The server's certificate isn't signed by a trusted root CA in the client's trust store.
• Certificate chain issues: The server might not send the complete certificate chain, including intermediate certificates. s_client needs this complete chain to verify the server's identity. A missing intermediate certificate will prevent proper display.
• Server configuration errors: Incorrect SSL/TLS configuration on the server (misconfigured virtual hosts, wrong port, etc.) can prevent the certificate from being properly sent. Check the server's configuration files for any errors.
• Server certificate problems: The server's certificate itself might be invalid (expired, revoked, improperly generated).

2. Client-Side Issues: Checking Your Setup

• Missing or incorrect CA certificates: Your system might lack the necessary CA certificates to verify the server's certificate. This is particularly relevant for self-signed certificates or certificates issued by a CA not included in your system's default trust store.
• Network problems: Network issues like firewalls, proxies, or DNS resolution problems can interfere with the connection and prevent the certificate from being retrieved correctly. Check network connectivity and any intervening systems.
• OpenSSL version: Older versions of OpenSSL might have bugs or limitations affecting certificate display. Updating to the latest stable version is always recommended.

3. Troubleshooting Steps: Practical Solutions

1. Verify Server Certificate: First, independently verify the server's certificate using a browser. If the browser doesn't show the certificate, the problem originates on the server.

2. Check the Server's Configuration: Ensure that the server is properly configured to use SSL/TLS, that the certificate and key are correct, and that the certificate chain is complete.

3. Inspect the openssl s_client Output: Even if the certificate isn't fully displayed, examine the openssl s_client output carefully. Look for error messages that might indicate connection or certificate validation problems.

4. Use -debug Flag: The -debug flag provides more verbose output, potentially revealing the cause of the certificate display issue. This can be useful for identifying low-level problems. openssl s_client -connect example.com:443 -showcerts -debug

5. Import the Certificate: If you're working with a self-signed certificate, you'll need to import the CA certificate into your system's trust store. The method depends on your operating system.

6. Bypass Certificate Verification (For Testing Only): Use the -ign_eof and -verify 0 options only for testing purposes in secure environments. This bypasses certificate verification, allowing you to see the certificate even if it's not trusted. Never use this in production. openssl s_client -connect example.com:443 -showcerts -ign_eof -verify 0

7. Update OpenSSL: Ensure that you are using an up-to-date version of OpenSSL.

8. Check Network Connectivity: Rule out network-related problems that might be preventing the connection or the retrieval of certificate information.

By systematically investigating these potential causes and implementing the suggested troubleshooting steps, you should be able to resolve the issue and successfully display the server's certificate using openssl s_client with the -showcert option. Remember to prioritize security and only use options like -verify 0 for testing in controlled environments.