Ask the Engineer- Automatic reboot when a device fails to respond to a ping request?

Annie Paquette
December 30, 2009

Question from Carlton <omitted>: I have multiple remote DSL modems that each stop working two to three times a week. I've deployed 2-outlet Sentry Switched PDU units to allow me to remotely reboot them when they hang, and they work great!  But, I have to wait until someone notifies me of the problem, then I have to manually login to the PDU to initiate the reboot. I've noticed that the DSL modems fail to respond to ping requests when they hang. Is there a method available to automatically reboot the DSL modems when they stop responding to ping requests? 

Engineer Reply: Assuming you have a Windows machine available to constantly monitor the DSL modems, you can easily achieve your goal by obtaining two pieces of freeware and enabling SNMP within the Sentry units.  First, for constantly monitoring the DSL modems, I recommend Check Host v1.0.10 which can be found here. This wonderful utility is capable of constantly pinging multiple devices and, upon failing to receive a response, can send an email alert and/or start a program.  In addition to ping tests, Check Host also supports status checking using TCP or UDP socket connection tests.  When a check fails, Check Host can execute a command to send a SNMP SET instruction to the appropriate Sentry unit to initiate a reboot.  For generating the SNMP SET instruction, I recommend the Net-SNMP toolset available from  Specifically, download the v5.5 binary titled "net-snmp-5.5.0-1.x86.exe".  After installing Check Host and Net-SNMP v5.5, you will need to get the SNMP Management Information Base (MIB) for the Sentry products, found here:, and place the file into the \share\snmp\mibs subdirectory where you've chosen to install the Net-SNMP toolset.  Check Host is very intuitive and quite easy to set up, but SNMP can be daunting if you are not already familiar with the basics.  Check this blog again at a later date for a link to a future Application Note that will contain details, or check the Server Technology web site directly.  For additional and immediate assistance, please contact Server Technology's Technical Support department at Server Technology's headquarters in Reno, NV, USA.

Ask the Engineer- How do I calculate power from 3-phase LED readings?