Understanding cURL Error 35

Overview of cURL Error 35

cURL is a powerful command-line tool used for transferring data with URLs, supporting various protocols. However, during its operations, users might encounter errors that hinder the successful execution of their tasks. One such error is curl error 35, which signifies an SSL/TLS connection problem. Understanding the root causes and potential solutions for this error is crucial for developers and system administrators to ensure smooth and secure data transfers.

Common Causes of cURL Error 35

1. SSL Certificate Issues

SSL certificates are vital for establishing secure connections between clients and servers. Various problems related to SSL certificates can trigger curl error 35:

• Expired Certificates: Certificates have a validity period. If a certificate has expired, cURL cannot establish a secure connection.
• Self-Signed Certificates: Servers using self-signed certificates may not be trusted by the client, leading to handshake failures.
• Certificate Chain Validation Failure: If the server's certificate chain is incomplete or improperly configured, cURL may fail to validate the SSL certificate.
• Incorrect Certificate Formats: Certificates must be in the correct format (e.g., PEM). Incorrect formats can cause parsing errors.
• Incorrect Certificate Paths or Permissions: If cURL cannot access the certificate files due to incorrect paths or insufficient permissions, the SSL handshake will fail.

2. Outdated cURL or OpenSSL Versions

Using outdated versions of cURL or OpenSSL can lead to compatibility issues, especially if the server enforces newer SSL/TLS protocols. Older versions may not support the required encryption standards, resulting in connection failures.

3. Incorrect SSL/TLS Version Specifications

Servers may require connections over specific SSL/TLS versions for enhanced security. If cURL is not explicitly configured to use a compatible version, the connection attempt might fail.

4. Network or Security Configuration Problems

Issues within the network environment can obstruct SSL connections:

• Firewall or Proxy Interference: Firewalls or proxy servers might block or interfere with SSL connections, preventing cURL from establishing a secure link.
• Incorrect Network Settings: Misconfigured network settings can disrupt the SSL handshake process.
• Unstable Internet Connection: A weak or intermittent network connection can cause SSL connection attempts to fail.

5. System Date and Time Issues

SSL/TLS handshakes rely on accurate system time for certificate validation. If the system clock is significantly skewed, certificates may appear invalid, leading to connection failures.

6. Incorrect URL or Endpoint

Typographical errors in the URL or using an incorrect endpoint can result in cURL attempting to connect to the wrong server, which may not support the expected SSL configuration.

Troubleshooting and Resolving cURL Error 35

1. Verify SSL Certificate Validity

Ensure that the server's SSL certificate is valid and has not expired. You can check the certificate details by accessing the URL in a web browser or using the openssl command:

openssl s_client -connect example.com:443

This command retrieves the SSL certificate and displays its details. Look for the validity period and any potential errors in the certificate chain.

2. Update cURL and OpenSSL

Keeping cURL and OpenSSL up to date ensures compatibility with the latest SSL/TLS protocols and security standards. To update cURL and OpenSSL:

• For Linux: Use the package manager (e.g., apt, yum) to update both packages.
• For macOS: Use Homebrew:

  brew update
  brew upgrade curl openssl

• For Windows: Download the latest versions from the official websites or use package managers like choco.

3. Specify the SSL/TLS Version

If you suspect that the SSL/TLS version is causing the issue, you can explicitly specify the protocol version in your cURL command:

curl --tlsv1.2 https://example.com

Replace --tlsv1.2 with the required version as needed.

4. Enable Verbose Mode for Detailed Debugging

Activating verbose mode can provide insights into where the SSL handshake is failing:

curl -v https://example.com

Analyze the output to identify specific errors or stages where the connection process breaks down.

5. Temporarily Bypass SSL Verification (For Testing Purposes)

To determine if SSL verification is the root cause, you can bypass it using the --insecure or -k flag:

curl -k https://example.com

**Warning:** This approach is not recommended for production environments as it compromises security by not verifying the server's SSL certificate.

6. Check Network and Proxy Settings

Ensure that your network connection is stable and that no firewall or proxy settings are blocking SSL connections. You can test connectivity by accessing the URL in a web browser or using other network diagnostic tools like ping or traceroute.

7. Validate System Date and Time

Confirm that your system's date and time are accurate. Incorrect system time can lead to SSL certificate validation failures.

• For Windows: Check the date and time settings in the Control Panel.
• For macOS/Linux: Use the date command to verify and adjust if necessary.

8. Verify the Correctness of the URL

Double-check the URL for any typographical errors. Ensure that the endpoint you are trying to reach is correct and supports SSL/TLS connections.

9. Consult Server Logs

If you have access to the server, review the server logs to identify any issues related to SSL/TLS handshakes or certificate validations. Server-side logs can provide detailed information that might not be apparent from the client-side perspective.

10. Contact Hosting Provider or Support

If all else fails, reaching out to your hosting provider or the server administrator can help identify and resolve configuration issues that may be causing the SSL connection problems.

Best Practices to Prevent cURL Error 35

1. Regularly Update Software

Keeping cURL, OpenSSL, and other related software up to date ensures compatibility with the latest security protocols and reduces the likelihood of encountering SSL-related errors.

2. Implement Proper Certificate Management

Ensure that SSL certificates are correctly installed, regularly renewed before expiration, and properly configured to maintain a valid certificate chain.

3. Monitor System Time

Implement monitoring solutions to ensure that system clocks remain accurate, preventing SSL certificate validation issues due to time discrepancies.

4. Use Trusted Certificate Authorities

Obtain SSL certificates from trusted Certificate Authorities (CAs) to avoid trust issues associated with self-signed certificates, especially in production environments.

5. Configure Firewalls and Proxies Appropriately

Ensure that security appliances like firewalls and proxy servers are configured to allow SSL/TLS traffic without unnecessary restrictions that could impede cURL operations.

6. Test SSL Configurations Regularly

Periodically test your SSL configurations using tools like openssl or online SSL checkers to identify and rectify potential issues proactively.

Example Scenarios and Solutions

Scenario 1: Expired SSL Certificate

If the server's SSL certificate has expired, cURL will throw error 35. To resolve this:

1. Renew the SSL certificate through your Certificate Authority.
2. Install the updated certificate on the server.
3. Verify the installation by accessing the URL via a browser or using openssl.

Scenario 2: Outdated OpenSSL Library

An outdated OpenSSL library might not support the latest TLS protocols required by the server. To fix this:

1. Check the current OpenSSL version:

   openssl version

2. Update OpenSSL to the latest version using your system's package manager or by compiling from source.
3. Restart any services that depend on OpenSSL to apply the updates.

Scenario 3: Self-Signed Certificate in Development Environment

When working in a development environment with self-signed certificates, cURL may not trust the certificate by default. For testing purposes:

• Use the --insecure flag to bypass SSL verification:

  curl -k https://localhost

• Alternatively, add the self-signed certificate to your trusted certificate store.

Scenario 4: Firewall Blocking SSL Traffic

If a firewall is blocking SSL traffic, cURL cannot establish a secure connection, resulting in error 35. To address this:

1. Check firewall settings to ensure that outbound SSL/TLS traffic on port 443 is allowed.
2. Configure proxy settings if your network uses a proxy server.
3. Test the connection from a different network to determine if the issue is network-specific.

Scenario 5: Incorrect cURL Command Usage

Mistakes in crafting the cURL command, such as incorrect URLs or missing options, can lead to error 35. Ensure that:

• The URL is correctly typed and accessible.
• All necessary command options are included and correctly specified.
• Use verbose mode to identify where the command might be failing:

  curl -v https://example.com

Advanced Troubleshooting Techniques

1. Analyzing SSL Handshake with Wireshark

Wireshark is a powerful network protocol analyzer that can capture and display the SSL handshake process. By analyzing this handshake, you can pinpoint where the SSL connection is failing:

1. Install and open Wireshark.
2. Start capturing network traffic.
3. Initiate the cURL command that triggers error 35.
4. Stop the capture and filter the results for SSL/TLS traffic.
5. Examine the handshake steps to identify any failures or mismatches.

2. Using cURL's SFTP Verbosity Options

cURL offers verbosity options that provide detailed logs of the connection process:

curl --verbose https://example.com

This command outputs detailed information about the SSL handshake, certificate validation, and any errors encountered.

3. Testing with Alternative Tools

Utilizing alternative tools like wget or Postman can help determine if the issue is specific to cURL or a broader SSL problem:

wget https://example.com

If alternative tools also fail to establish a secure connection, the issue is likely server-side or network-related.

4. Reviewing Server Configuration Files

For those managing the server, reviewing configuration files (e.g., nginx.conf for Nginx or httpd.conf for Apache) can help identify misconfigurations related to SSL/TLS settings:

• Ensure that the correct SSL certificate and key files are specified.
• Verify that the SSL protocols and ciphers are properly configured.
• Check for any syntax errors or misconfigurations that could affect SSL handshakes.

Preventive Measures and Best Practices

1. Automate Certificate Renewals

Use tools like Let's Encrypt with automated renewal scripts to ensure that SSL certificates are always up to date, eliminating the risk of expiration-related errors.

2. Implement Robust Monitoring Systems

Deploy monitoring solutions that alert you to SSL certificate issues, system time discrepancies, and software updates, allowing for proactive maintenance and timely resolution of potential problems.

3. Standardize Development Environments

Maintain consistent configurations across development, testing, and production environments to minimize discrepancies that could lead to SSL/TLS issues.

4. Educate Team Members

Ensure that all team members are knowledgeable about SSL/TLS principles, certificate management, and cURL usage to foster a collaborative environment for troubleshooting and maintaining secure connections.

5. Utilize Configuration Management Tools

Tools like Ansible, Chef, or Terraform can help manage and automate configurations, ensuring consistency and reducing the likelihood of manual errors that might affect SSL connections.

Conclusion

cURL error 35, stemming from SSL/TLS connection issues, can be a complex challenge involving multiple facets of system and network configurations. By systematically addressing the common causes—ranging from certificate validity and software versions to network settings and server configurations—you can effectively troubleshoot and resolve this error. Employing best practices such as regular updates, automated certificate management, and robust monitoring can significantly reduce the occurrence of SSL-related issues, ensuring secure and reliable data transfers with cURL.

Further Resources