Tag Archives: Windows

CLI

Use batch script to continually check site status

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Recently my blog went down (the ISP running it had downtime.) I wanted to see when it came back up. As a result I wrote a little Windows batch script to continually poll my address in order to do just that.

The script issues a query to the default DNS server as well as pings the address of the blog. I used both since sometimes in Windows a ping will simply use internal system cache, which may be wrong if the IP address hosting my blog changes (it’s address is dynamic.)

The script is below:

@ECHO OFF
:loop
 cls
 nslookup jeppson.org | findstr "Address" | findstr /V 10.97.160.160
 ping -n 1 jeppson.org
 timeout /t 3
goto loop

I use the /V argument to take out the first bit spit out from the nslookup command, namely the IP address of the nameserver being used.

A simpler version of the script only issues one ping, waits a second, and then repeats the command. This is different from doing ping -t because it forces ping to do a new lookup for the domain name, whereas ping -t only resolves the IP once, then just pings the IP address. That wouldn’t work in my case as the IP of the domain name changes when it comes back online.

@ECHO OFF
:loop
 ping -n 1 jeppson.org
 timeout /t 1
goto loop

Thanks to Stack Overflow for educating me on how to write a quick loop to emulate the Linux Watch command,  ping only once, and use an application similar to grep to clean up output.

 

Virtualization

Xen HVM domU doesn’t synchronize with dom0 clock

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After much research I’ve discovered that Xen does not synchronize the clock between dom0 and its HVM domUs. This poses a problem when you implement S3 sleep. Upon resume,  dom0 realizes how much time has passed but none of the domUs do. I realized this after a few days of successfully putting my Xen machine to sleep with running DomU virtual machines. 

The DomU in my case is a Windows 8.1 virtual machine. At first I thought that the standard Windows time service would take care of any clock discrepancies – it doesn’t. If your clock gets too far behind it simply refuses to update. My solution to this problem is two fold:

  1. Configure Windows to use my NTP server for clock updates
  2. Force Windows to check with the NTP server every minute and update its clock accordingly.

Fortunately the later Windows versions have an NTP client built in. Simply open an administrator command prompt and issue two commands:

w32tm /config /syncfromflags:manual /manualpeerlist:<hostname>

schtasks /create /sc minute /mo 1 /tn "NTP clock update" /tr "%WINDIR%\system32\w32tm.exe /resync /force" /RU SYSTEM

The first command configures your system with your NTP server of choice. Replace <hostname> with your desired hostname or IP address, minus the brackets. The second command creates a task which executes a command to force an NTP check every minute as the SYSTEM user (non-privileged users get an access denied message.) You can do it all with a GUI but the command line is so much more efficient 🙂

It works perfectly. My DomU now automatically checks if it has the correct time – very important if you ever put your dom0 to sleep while DomUs are running.