Resolving the SSL_ERROR_SYSCALL Issue During File Uploads with curl and Java HTTP Client

Discover how to troubleshoot and resolve the `SSL_ERROR_SYSCALL` and `SSLHandshakeException` errors when using `curl` and Java HTTP Client for file uploads.
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Troubleshooting SSL_ERROR_SYSCALL During File Uploads

When working with file uploads to external APIs, you may encounter errors that can halt your progress. This particular guide addresses a common problem: the SSL_ERROR_SYSCALL and the SSLHandshakeException. Both of these errors can occur when attempting to upload a file using the curl command or Java's HTTP client, particularly after a system upgrade, such as the transition to Red Hat Enterprise Linux 8 (RHEL 8). Let's explore the issue and how to effectively troubleshoot and resolve it.

The Problem

Imagine you are trying to upload a file to an external API using the curl command, and suddenly, you run into the following error:

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Symptoms of the Issue

The errors indicate a failure to establish a secure connection to the server.

The issue arises specifically when using curl with the command:

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Logs indicate successful connection establishment but then the connection is terminated unexpectedly.

Diagnosis: What Went Wrong?

After further examination of the logs, it appears that the root cause of the connection failure can be traced back to firewall configurations blocking outgoing connections. This issue surfaced after upgrading to RHEL 8, as previous configurations may not have adjusted to new firewall rules or network policies.

Solution: Whitelisting the IP

Step 1: Identify the Firewall Settings

Check if the firewall is configured to block outgoing connections.

Use commands such as iptables or firewalld to examine the current firewall rules.

Step 2: Whitelist the Required IP

If the IP you are trying to connect to is blocked, you will need to whitelist it. Here’s how you can do that:

For firewalld:

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For iptables:

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Step 3: Reload Firewall Settings

After making the changes, ensure you reload or restart the firewall to apply new rules:

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Step 4: Retest the curl Command

Re-attempt your file upload with the same curl command to see if the issue is resolved.

Additional Considerations

If you continue to face issues, ensure that TLS/SSL libraries are up-to-date.

Make sure that any intermediate proxies (e.g., Zscaler mentioned in the logs) are not blocking your connection either.

Double-check the API endpoint for any updates to its SSL requirements.

Wrapping Up

In conclusion, the SSL_ERROR_SYSCALL and SSLHandshakeException error messages can often lead to confusion, especially after system upgrades. However, by diagnosing firewall settings and whitelisting the required IP addresses, you can resolve these issues and restore connectivity for file uploads.

Should you continue to encounter difficulties, consider reaching out to your systems administrator or network team for additional assistance. Your troubleshooting journey may open new avenues of learning and help you fine-tune your network configurations for future tasks.