DNS packets with 3G USB dongle tethered to a PC

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Hi,

I used the sniffer application on a PC connected to a 3G USB dongle for internet access.
I have found that there are no DNS packets (port 53) associated with either TCP or UDP packets. Are you able to tell me why, I think i'm missing a simple setting to monitor the traffic on the IP address that has been allocated to the tethered device.

Using LAN and WiFi works like a charm.
Any suggestions would be greatly appreciated

Regards
Ralph

What I have tried:

Gone through all the ip header, TCP and UDP docs. Checked all byte offsets.

Posted 25-Jun-20 7:21am

Member 1628554

Updated 25-Jun-20 7:33am

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Dave Kreskowiak · Answer 1 · 2020-06-25T07:33:00

Solution 1

The machine may be caching the DNS resolution results from previous requests.

Open a CMD prompt and type ipconfig /displaydns to show what's in the resolver cache.

Posted 25-Jun-20 7:33am

Dave Kreskowiak

Comments

Member 1628554 25-Jun-20 14:33pm

Hi Dave thank you so much for your quick reply. I flushed the DNS cache before running the application. If I run the app on a LAN with an internet connection or use WiFi with internet, the app works like a bomb, lots of ip packets with DNS. The minute I remove the PC from the LAN and plug in the USB dongle that has a 3g connection to the internet, I stop receiving DNS packets. My browser is rendering web pages properly, who or what is resolving the A address.

Dave Kreskowiak 25-Jun-20 14:36pm

I that case, it's entirely possible the work is being done on the service provider side and not the PC.

You can clear the DNS cache, hit a couple of web pages, then dump the cache as see if anything is there. If this is the case, i have no idea why you're not seeing the DNS conversation.

Member 1628554 25-Jun-20 14:50pm

Hi Dave, did as you suggested and the DNS entries I need are there. Any reason you can think from an application perspective (written in c#) that I am unable to discover them. I am monitoring port 53 on both TCP and UDP protocols. Once again thank you so much for your time in trying to help me.

Dave Kreskowiak 25-Jun-20 15:04pm

I have no idea why it's doing this.

Member 1628554 25-Jun-20 15:06pm

Thanks again Dave your help was really appreciated. You gave me somewhere to look :)