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Dependency deadlock



I just came across an interesting issue with deadlocked dependencies. Case as follows:

Remote site with dynamic IP address, gateway router configured with DDNS (No-IP). PRTG has a ping sensor for the gateway router to check if the site is operational.

IP address changed at the remote site, PRTG's ping sensor went down, and paused the parent device. The parent device then seems to have paused the ping sensor.

Log entries show that PRTG did ask DNS to see if the IP address had changed, and the query was forwarded by the local DNS to the upstream (external) DNS server. Problem seems to be related to that the the upstream DNS didn't refresh the dynamic DNS entry and still forwarded the old IP. Nevertheless, the external DNS returned the correct IP a minute after the last check by PRTG. When the sensor and device was paused, no more DNS queries were sent for that device.

Is there a way to have PRTG to avoid this deadlock situation? Example recheck the device every hour or similar?

ddns deadlock dependency dns

Created on Jan 12, 2023 4:32:31 PM

Last change on Jan 16, 2023 7:38:46 AM by  Felix Wiesneth [Paessler Support]

3 Replies




could the issue in this that PRTG caches resolved IPs?

If a ping sensor turns red and pauses its parent device object using the setting "Master object for parent", then the ping sensor does continue to scan.

Created on Jan 16, 2023 2:06:39 PM by  Arne Seifert [Paessler Support]

Last change on Jan 16, 2023 2:07:30 PM by  Arne Seifert [Paessler Support]



I should add to this issue that I have another branch of devices depending on this sensor - these are the internal devices just reachable by VPN when the gateway is up. From what I recall, the ping sensor and it's parent device both showed a status of "paused by dependency", and there was never any Down sensor notification email on it.

Sensor log shows two messages "Destination host unreachable (ICMP error #11003)" before going to "paused by dependency" Trying to simulate a situation like this by tweaking my internal DNS server triggered 2 x "Socket error Host not found" before the sensor went to "Paused by dependency". I will try to do some other tests of device not reachable - no ping reply vs. dns not resolved.

Any other suggestions are appreciated...

Created on Jan 16, 2023 4:48:36 PM



PRTG uses the Windows API calls to resolve DNS. So it's always used directly from the hosts file or the DNS and should not be cached separately. Is PRTG querying the correct device after using the "ipconfig /flushdns" command on the PRTG (Probe) Server?

Created on Jan 17, 2023 12:27:47 PM by  Timo Dambach [Paessler Support]

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