mbedtls linking error with mbedtls_test_cas_pem

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01-10-2024

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Troubleshooting mbedTLS Linking Errors: "undefined reference to `mbedtls_test_cas_pem'"

Have you encountered a linking error like "undefined reference to `mbedtls_test_cas_pem'" while working with mbedTLS? This error typically arises when your project doesn't link correctly with the mbedTLS library, particularly its CA certificate functionality. Let's delve into this problem and explore solutions for fixing it.

Scenario:

You're building a project using mbedTLS, aiming to perform certificate verification. You've included the necessary mbedTLS headers, but during compilation, you hit the error:

undefined reference to `mbedtls_test_cas_pem'

Explanation:

The error "undefined reference to mbedtls_test_cas_pem'" indicates that your linker cannot find the definition for the mbedtls_test_cas_pem` variable. This variable holds the PEM-encoded data of a predefined set of CA certificates that mbedTLS uses for testing and demonstration purposes.

Causes:

• Missing Library Files: The most likely cause is that your project is not linking with the mbedTLS library correctly. The mbedtls_test_cas_pem variable is usually defined within the library, and the linker needs to access it during the linking phase.
• Incorrect Library Linking: You might have specified the mbedTLS library in your project's build system, but the linker might not be able to find the correct library files.
• Missing Definitions: Occasionally, the mbedTLS library might not have the necessary definitions for mbedtls_test_cas_pem in the specific configuration you are using. This could happen if you are using a custom build of mbedTLS or have disabled certain features.

Solutions:

1. Ensure Proper Linking:

   • Build System: Make sure you have correctly configured your build system (like CMake, Make, etc.) to link with the mbedTLS library.
   • Library Path: Provide the correct path to the mbedTLS library files in your linker settings. This often involves adding the library directory to the linker's search path.
   • Library Name: Verify that you are using the correct name for the mbedTLS library file (usually libmbedtls.a or libmbedtls.so depending on your operating system and build configuration).

2. Verify the mbedTLS Configuration:

   • CA Certificates: Ensure that the mbedTLS library is configured to include the CA certificates. Sometimes, the mbedtls_test_cas_pem variable might be omitted in a custom build or when certain features are disabled.
   • Predefined CA Certificates: Review the documentation of your chosen mbedTLS version to understand whether it contains predefined CA certificates and whether the mbedtls_test_cas_pem variable is available.

3. Alternative CA Sources:

   • Custom CA Certificates: Instead of relying on mbedtls_test_cas_pem, you can load your own custom CA certificates for verification. This approach gives you more control and flexibility. You'll need to create a separate PEM-encoded certificate file and provide it to mbedTLS using the appropriate functions.
   • Third-Party CA Sources: If you are working with a specific certificate authority, you can directly obtain their CA certificates from their websites.

Example:

#include <mbedtls/x509.h> 
#include <mbedtls/ssl.h>

int main(void) {
    // Load your custom CA certificates
    mbedtls_x509_crt ca_cert;
    mbedtls_x509_crt_init(&ca_cert);
    if (mbedtls_x509_crt_parse_file(&ca_cert, "path/to/your/ca.pem") != 0) {
        // Error loading CA certificate
        return 1;
    }
    
    // ... perform other mbedTLS operations ... 
}

Additional Tips:

• Debugging: Enable verbose logging in mbedTLS to get more information about the error and pinpoint the root cause.
• Documentation: Consult the official mbedTLS documentation for detailed information on configuring and using CA certificates.
• Community Forums: If you face difficulties, reach out to the mbedTLS community forums or ask for assistance on dedicated platforms like Stack Overflow.

Resources:

• mbedTLS Documentation
• mbedTLS GitHub Repository
• Stack Overflow

By carefully examining your project configuration, library linking, and CA certificate management, you can effectively resolve the "undefined reference to `mbedtls_test_cas_pem'" error and unlock the full potential of mbedTLS for secure communication in your applications.