How to Fix curl: (35) SSL Connect Error – Step-by-Step Guide

When using the curl command-line tool to send HTTP requests, you may encounter the error message curl: (35) SSL connect error. This error typically indicates a problem with the SSL/TLS handshake between your system and the server. It can occur due to outdated SSL versions, incorrect cURL configurations, or server-side issues.

Understanding and resolving this error is crucial, as it can prevent secure connections to APIs, websites, and cloud services. This guide will walk you through troubleshooting and fixing the curl: (35) SSL connect error effectively.

Common Causes of curl: (35) SSL Connect Error

Before diving into solutions, it is essential to understand the common reasons behind this error:

Outdated cURL version that lacks support for modern SSL/TLS protocols

Incorrect SSL/TLS configuration on the client or server

Expired or invalid SSL certificates

Firewall or network restrictions blocking SSL connections

Unsupported or deprecated SSL ciphers

Step-by-Step Solutions to Fix curl: (35) SSL Connect Error

Step 1: Check Your cURL Version

Outdated cURL versions may not support modern SSL/TLS protocols. To check your cURL version, run:

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curl --version

If you see an old version, update it using:

For Ubuntu/Debian:

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sudo apt update && sudo apt upgrade curl -y

For CentOS/RHEL:

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sudo yum update curl -y

For macOS:

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brew update && brew upgrade curl

After updating, verify the version again to ensure the latest release is installed.

Step 2: Force the Use of TLS 1.2 or 1.3

Older SSL/TLS protocols are often disabled for security reasons. If the server does not support older protocols, forcing TLS 1.2 or 1.3 might resolve the issue.

Try running the following command:

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curl --tlsv1.2 -I https://your-url.com

Or for TLS 1.3:

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curl --tlsv1.3 -I https://your-url.com

If one of these works, it indicates that the server does not support older SSL versions.

Step 3: Verify SSL Certificates

SSL certificate issues can cause cURL to fail. To check the SSL certificate of a website, use:

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curl -v https://your-url.com

If you notice certificate warnings, try downloading and updating your CA certificates:

For Ubuntu/Debian:

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sudo apt install --reinstall ca-certificates

For CentOS/RHEL:

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sudo yum reinstall ca-certificates

For macOS:

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brew install ca-certificates

If the certificate is self-signed, you may need to pass the certificate explicitly using:

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curl --cacert /path/to/certificate.pem https://your-url.com

Step 4: Disable SSL Verification (Temporary Fix)

If you suspect that the issue is due to SSL verification, you can temporarily disable SSL verification to check whether the connection works:

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curl -k https://your-url.com

However, this should only be used for testing, as it makes connections vulnerable to security threats.

Step 5: Check Firewall and Network Restrictions

Firewalls or network configurations may block SSL connections. If you are behind a corporate firewall, ensure that SSL traffic is allowed. You can test network connectivity using:

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telnet your-url.com 443

or

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openssl s_client -connect your-url.com:443 -servername your-url.com

If the connection fails, adjust your firewall rules to allow HTTPS traffic.

Step 6: Use a Different cURL Library or Rebuild cURL

If the issue persists, try using cURL with a different SSL library, such as OpenSSL. Some Linux distributions compile cURL with alternative SSL backends like GnuTLS, which might not be compatible with all servers.

You can check which SSL backend is being used with:

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curl --version

If it is not OpenSSL, you may need to recompile cURL with OpenSSL support.

Conclusion

The curl: (35) SSL connect error can be frustrating, but by systematically checking cURL versions, SSL protocols, certificates, and network configurations, you can resolve the issue efficiently. Ensuring that your system supports modern TLS protocols and keeping SSL certificates updated is key to avoiding connection problems in the future.

For a seamless and secure cloud experience, consider using Cyfuture Cloud. With cutting-edge security features, reliable SSL/TLS support, and 24/7 expert assistance, Cyfuture Cloud ensures your applications run smoothly and securely.