Sysmon On RHEL 9: TCP Events Missing?

by Alex Johnson 38 views

Are you experiencing issues with Sysmon 1.4.0 on your RHEL 9 server, specifically the frustrating absence of TCP events in your logs? You're not alone! Many users have encountered this problem, where UDP events are logged without a hitch, but TCP connections seem to vanish into thin air. Let's dive deep into this issue, explore possible causes, and discuss potential solutions to get your Sysmon setup working as expected. We'll examine the configuration, the operating system, and the Sysmon version to pinpoint the root of the problem.

The Problem: TCP Events Not Logging

The core of the issue is quite straightforward: Sysmon 1.4.0 on RHEL 9 isn't capturing TCP events. This means that when a TCP connection is established, closed, or otherwise interacted with on your server, there's no corresponding log entry generated by Sysmon. However, UDP events, which represent a different type of network traffic, are being logged correctly. This behavior is peculiar, as Sysmon is designed to monitor both TCP and UDP traffic. This discrepancy makes it difficult to monitor the network on RHEL 9. The user has a wide-open Sysmon configuration file. It is the same configuration working on Ubuntu 24.04.3 LTS. This is an important troubleshooting factor. This suggests the issue is specific to the RHEL 9 environment rather than the configuration itself. The user has confirmed the Sysmon version is 1.4.0 and is running on RHEL 9.6 with kernel version 5.14.0-611.9.1.el9_7.x86_64.

Impact of Missing TCP Events

The lack of TCP event logging can severely hinder your ability to monitor and secure your server. TCP events are crucial for understanding network activity, including:

  • Identifying malicious connections: TCP is used for the majority of the malicious software and hackers on the internet. Without these logs, it's difficult to spot unauthorized attempts to connect to your server, such as brute-force attacks on SSH or web server exploits.
  • Troubleshooting network issues: If you're experiencing connectivity problems, TCP logs can help you trace the source of the issue by showing which connections are failing or timing out.
  • Auditing network usage: TCP logs provide valuable insight into how your server is being used, including which services are being accessed and by whom. This helps you ensure compliance with security policies and identify potential misuse.
  • Security Investigations: TCP logs provide key information that can be leveraged during a security investigation. These logs include source and destination IPs, ports, and timestamps.

Investigating the Root Cause

To resolve the missing TCP events, we need to carefully investigate the potential causes. Here are the key areas to focus on:

Configuration Review

Although the user has indicated that the same configuration file works on Ubuntu, it's still worth reviewing the Sysmon configuration file (sysmon.xml). Ensure that the configuration includes the appropriate rules to log TCP events. Look for <NetworkConnect event types, and verify that the rules are not inadvertently filtering out the traffic you're interested in. Also, look for errors within the Sysmon configuration file, which will prevent Sysmon from logging.

<Sysmon schemaversion="4.82">
  <EventFiltering>
    <NetworkConnect onmatch="include">
      <DestinationPort condition="is">80</DestinationPort>
    </NetworkConnect>
  </EventFiltering>
</Sysmon>

Compatibility with RHEL 9

Sysmon for Linux, while continually improving, may have compatibility quirks with specific Linux distributions and kernel versions. Double-check that Sysmon 1.4.0 is officially supported on RHEL 9.6 with the kernel version in use. Consult the official Sysmon documentation or community forums for any known compatibility issues or workarounds.

Kernel and Systemd

  • Kernel Modules: Sysmon relies on kernel modules to capture network events. Verify that the necessary kernel modules are loaded and functioning correctly on RHEL 9. Check the system logs for any errors related to kernel module loading or Sysmon. The kernel version is a key factor to consider, as the sysmon kernel module may not be compatible with the current version.
  • Systemd Configuration: Sysmon often integrates with systemd for startup, logging, and other system-level tasks. Review the Sysmon systemd service file to ensure it's configured correctly and that there are no errors preventing it from starting or functioning properly. Any issues with the systemd configuration could prevent Sysmon from capturing network events.

Sysmon Service Status

Make sure the Sysmon service is running and hasn't encountered any errors. Use systemctl to check the service status.

sudo systemctl status sysmon

Examine the service logs for any clues about why TCP events aren't being logged. Pay close attention to any error messages or warnings that might indicate a problem. Also, verify that the Sysmon service is properly enabled and set to start on boot. A misconfigured service can lead to missing events.

Logging Levels

Check the logging levels in your Sysmon configuration. Make sure that the logging level is set to capture the network events you need. If the logging level is set too high, it might filter out important information. Ensure that your logging configuration is not inadvertently filtering out TCP traffic.

SELinux Interference

If SELinux is enabled on your RHEL 9 server, it could potentially be interfering with Sysmon's ability to capture network events. Check the SELinux audit logs (/var/log/audit/audit.log) for any denied events related to Sysmon. If you find SELinux denials, you might need to adjust the SELinux policy to allow Sysmon to function correctly. This is one of the more common causes of Sysmon misconfiguration.

Packet Filtering

Examine the firewall rules on your RHEL 9 server. If the firewall is blocking TCP traffic, Sysmon won't be able to log it. Verify your firewall configuration to ensure that the necessary TCP traffic is allowed. Make sure the firewall isn't inadvertently blocking traffic that Sysmon needs to monitor. If you suspect firewall issues, try temporarily disabling the firewall to see if it resolves the problem.

Troubleshooting Steps

Here's a structured approach to troubleshoot the missing TCP events:

  1. Verify Sysmon Installation: Ensure Sysmon is correctly installed and running on your RHEL 9 server. Use systemctl status sysmon to check the service status. Confirm that Sysmon is running without errors.
  2. Configuration Check: Carefully review your sysmon.xml configuration file, paying close attention to the <NetworkConnect> event type rules. Make sure the rules are designed to capture the TCP traffic you're interested in. Double-check for any typos or misconfigurations.
  3. Check Logs: Examine the Sysmon logs (usually in /var/log/sysmon.log or your configured log location) and system logs (/var/log/messages or similar) for any error messages or warnings related to Sysmon or network events.
  4. Test with a Simple Configuration: Start with a very basic Sysmon configuration that includes only the essential TCP event logging rules. This can help you determine whether the issue is related to a specific configuration setting. A minimal configuration will help determine if the problem is specific to your configuration.
  5. Kernel Module Check: Verify that the necessary kernel modules for Sysmon are loaded and working correctly. Use lsmod to list loaded modules, and look for any Sysmon-related modules. If modules aren't loading properly, this indicates the potential of compatibility issues.
  6. SELinux and Firewall Check: Check your SELinux and firewall settings to ensure they are not blocking TCP traffic or interfering with Sysmon. Temporarily disable these features to see if the issue resolves. The problem may stem from a conflict with SELinux or the firewall.
  7. Version Compatibility: Confirm that Sysmon 1.4.0 is compatible with RHEL 9.6 and the specific kernel version you're using. Check the official Sysmon documentation or community forums for compatibility information.
  8. Update Sysmon: Consider upgrading to a newer version of Sysmon if available, as newer versions may include fixes for compatibility issues or other bugs. The current version may have a known issue that has been resolved in a later release.
  9. Community Support: Seek help from the Sysmon community forums or other online resources. Other users may have encountered and solved the same problem, and you can leverage their knowledge to find a solution.

Conclusion

Troubleshooting missing TCP events in Sysmon on RHEL 9 can be a complex process, but by systematically checking the configuration, compatibility, system services, and potential interference from SELinux and firewalls, you can increase your chances of finding a solution. Remember to carefully examine the logs, test with a simple configuration, and consult the Sysmon documentation and community resources.

By following these steps, you should be able to identify the root cause and get Sysmon logging TCP events on your RHEL 9 server, providing you with the necessary visibility into your network traffic. Remember that persistence and a methodical approach are key to resolving this issue. Good luck!

External Resource:

If you're looking for more in-depth information about Sysmon and its configuration, I recommend checking out the official Microsoft documentation on Sysmon.