Troubleshooting MRTG: Common Issues and Quick Fixes

Troubleshooting MRTG: Common Issues and Quick Fixes

1. Graphs not updating

• Cause: mrtg process/crontab not running or incorrect permissions.
• Fix: Ensure mrtg is scheduled (crontab or systemd timer). Run mrtg manually:

  bash
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  /usr/bin/mrtg /etc/mrtg.cfg 

  Check file ownership and permissions for generated HTML/PNG files; adjust with chown/chmod.

2. Incorrect or flatlined data

• Cause: Wrong SNMP community, host, interface OID, or SNMP version mismatch.
• Fix: Test SNMP with snmpwalk/snmpget:

  bash
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  snmpwalk -v2c -c public router.example.com .1.3.6.1.2.1.2.2.1.10 

  Update MRTG target lines to use correct OIDs and SNMP version.

3. High CPU or slow rendering

• Cause: Large number of targets or frequent polling interval.
• Fix: Increase polling interval, stagger cron jobs, or run multiple mrtg instances on different threads/hosts. Enable rrdtool backend to reduce load.

4. Permission denied saving graphs

• Cause: Web server user can’t write output directory.
• Fix: Set outputdir writable by web server (e.g., chown www-data:www-data /var/www/mrtg -R) and secure permissions.

5. Time/date on graphs wrong

• Cause: Server timezone mismatch or incorrect system time.
• Fix: Sync time with NTP/chrony and ensure PHP/HTTP server and mrtg use same timezone.

6. Missing PNG generation

• Cause: rrdtool or GD library missing, or mrtg compiled without PNG support.
• Fix: Install rrdtool and required image libs; rebuild mrtg if necessary. Verify mrtg output mentions rrdtool usage.

7. Alerts/thresholds not triggering

• Cause: Wrong mrtg.cfg threshold settings or missing cfg include.
• Fix: Verify AbsMax, MaxBytes, and Target[…] options; test with known values and check log output.

8. Duplicate or overlapping graphs

• Cause: Multiple cfgs referencing same target or include order issues.
• Fix: Consolidate targets, remove duplicates, and ensure unique SetEnv/Target lines.

9. SSL/HTTPS access problems for generated pages

• Cause: Web server misconfiguration or mixed-content blocking images.
• Fix: Serve images over HTTPS, update base URLs, and fix webserver virtual host settings.

10. Debugging tips and logs

• Run mrtg in debug/verbose mode:

  bash
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  mrtg –logging /var/log/mrtg.log /etc/mrtg.cfg 

• Check syslog, web server logs, and SNMP agent logs. Use snmpwalk/snmpget to confirm device responses.

If you want, I can generate a checklist customized to your mrtg.cfg or help debug specific errors—paste the relevant config lines or log output.