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My HTTP sensors don't work. What can I do?

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What can I do when my HTTP sensors in PRTG show errors? What are troubleshooting methods to fix these issues?

error http-sensor prtg troubleshooting

Created on May 16, 2019 12:35:16 PM by  Maike Behnsen [Paessler Support]

Last change on Sep 19, 2019 11:51:38 AM by  Maike Behnsen [Paessler Support]



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HTTP Sensors: Overview of Possible Errors and Suggestions for Troubleshooting

Error TypesError Message(s)Troubleshooting Solutions (Short Version)
General Application Protocol Errors(403) Forbidden

(400) Bad Request
Solution 1: Check the target server’s virtual host configuration (https://www.example.com and https://example.com can use different SNI settings, as they are two different virtual host configurations).

Solution 2: Check if a redirect to a different host address is used by your endpoint.

For more information, see My HTTP sensors fail to monitor websites which use SNI. What can I do?
General SSL/TLS Protocol ErrorsFailed to establish secure connection

Error connecting with SSL

SSL handshake failure
Solution 1: Check the Server Name Indication (SNI) field in your sensor’s settings:
Change the host address of the sensor’s parent device or disable SNI Inheritance by selecting the option Do not inherit SNI from parent device so that the SNI is determined only from the sensor’s target URL, or
Change the target URL of the sensor to match the configuration of the target server.
For more information, see My HTTP sensors fail to monitor websites which use SNI. What can I do?

Solution 2: Check if the server that you monitor supports the required Diffie-Hellman (D-H) key length, which needs to be longer than 768 Bits. If not, upgrade the affected server’s security. For more information, see Some HTTP sensors show an SSL error after a PRTG update. What can I do?

Solution 3: If you are running PRTG on a virtual machine, make sure that the system time of VMware is synchronized with the time of the host system or an NTP server, but not both. Check that the host correctly synchronizes time as well. For more information, see I run PRTG on VMware. How can I obtain best performance?
Error connecting with SSL. EOF was observed that violates the protocolSolution: This error occurs when you monitor via HTTPS and the target URL redirects to another URL and sets a cookie at the same time. Check if there is a respective entry in the local hosts file of the machine that is hosting the probe and correct it, if necessary.
Errors with HTTP XML/REST Value sensorsCould not create SSL/TLS secure channelSolution 1: Do not request a URL secured with SSL via HTTP.

Solution 2: Check if the URL that you monitor only accepts ciphers for SSL connections that allow Perfect Forward Secrecy. If yes, set the option Use SSLv3 if available under SSLv3 Connection in the sensor’s settings.

For more information, see My http sensors could not create an SSL secure channel and are down. What can I do?
Protocol Error: The remote server returned an error.
(404) Not Found.
Solution: Check if the URL that is monitored contains specific URL encoded characters. If yes, create a new configuration file that forces the .NET XmlValue.exe file of PRTG to interpret special characters correctly. For detailed steps and the required file content, see Why does my HTTP XML/REST Value sensor return a 404 error?
The server committed a protocol violation.
Section=ResponseStatusLine
Solution: Configure the sensor to use a more tolerant behavior with regards to web requests. For implementation steps and the required file content, see HTTP XML/REST Value Sensor shows protocol violation. What can I do?
Errors with HTTP Full Webpage sensorstemp_dir_.CreateUniqueTempDir() (with the Chromium browser engine)Solution 1: In the sensor’s settings, use one of the other browser engines.

Solution 2: Select the option Use Windows credentials of parent device in the sensor’s Security Context settings. Make sure that valid credentials are entered in the Settings tab of the PRTG device that your sensor runs on.

For more information, see What to do when I see a CreateUniqueTempDir() error message for my HTTP Full Webpage Sensor?
Unable to navigate
(ERR_NAME_NOT_RESOLVED)
Solution 1: Check if PRTG can reach the URL that is monitored.

Solution 2: Check your graphics card settings. Disable power-saving graphics processing units and use high-performance processors only.

Solution 3: Make sure that the NLA (Network Location Awareness) service is running under services.msc.

Solution 4: If your PRTG probe runs on a Windows 7 machine, scan the computer for the required Windows Experience Index.

For more information, see HTTP Full Web Page sensor is “unable to navigate”. What can I do?

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Created on May 16, 2019 12:48:07 PM by  Maike Behnsen [Paessler Support]

Last change on May 17, 2019 12:55:52 PM by  Florian Weik [Paessler Support]



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Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.