DNS Questions

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rkylemyers

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I just have a few questions that I hope everyone can help me with. There are no problems really, just more of an informative post...

I believe there is a user at my workplace who is stealing internet. He had a buddy who used to work for comcast that has set him up. He never actually said this to me (I am a help desk extraordinare).

He uses a computer (a portable WM5 handheld device) to transfer information via FTP everyday to our server. Normally this is via WiFi (Internet). Well, he has been using another method of communicating recently, which is the Cellular method, using AT&Ts network. We provide that service for them if they need it. He just called in today and tried to get set back up on transferring data thru his WiFi connection at his house, as he just purchased a Wireless Router, and had his friend set it up for him.

This is where I start to not understand...

We use a program called SOTI MobiControl to remote his computer that we provide for him. This remote access to his computer does in fact use port 8080. So he called in with his problem that he can not communicate with our server (which would use FTP port 21). I was able to remote his machine, and see it fail.

So I dug a little deeper..

After verifying that our server was not blocking him, and our firewall was not blocking him, I started asking him questions. I sent him the "TeamViewer" program link, so I could remote his computer (his personal laptop). I was looking around and noticed that he set his DNS Server to a public DNS IP address. Now I know there are benefits when doing this, so I didn't look at it too hard.

What was weird is that he could access our FTP Server thru his Laptop (who he set his DNS to a public DNS) but when the handheld computer tries to access it, it just times out (which can only receive IP and DNS information dynamically from his linksys router).

Public DNS resolution gives him access...
Private DNS resolution gives him jack squat...

So my question is: If you purchase a modem online for cheap, and set it up for internet access, would you have to use Public DNS for domain resolution? Because the way I see it you wouldn't be able to with the ISP as you do not have an account with them. Therefore if you try to resolve DNS privately (using the ISP) then you wouldn't have internet access at all? I have heard other stories similar to this when I was first hired, but not quite the same. If I am wrong in all this type of thinking, please just shoot me down now. I just need to understand what may be going on here.

Thanks in advance all!

KYLE
 

TerryNet

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Mar 23, 2005
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Terry
Using a public DNS server or the ISP's DNS server would make no difference, except that any given DNS server can be out of date or otherwise messed up and not resolving everything it should. Also, he can set his router to use a public DNS server instead of whatever would be assigned by the ISP.

I cannot offer any more than this, but I'm going to move your thread to Networking, where you are more likely to catch the attention of the right folks.
 

rkylemyers

Thread Starter
Joined
Aug 3, 2009
Messages
17
Thanks for the response! Does anyone else have an idea why this would be happening then?

So by your advice, if I make his router resolve all devices thru a public IP address, all devices connected to that router in turn would be doing the same? So this handheld device would then resolve to a public DNS?
 

TerryNet

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Terry
For a device connected to a router

a. if its DNS server address is a "real" one, such as the ISP's or a public DNS server, that will be used;

b. if its DNS server address is the router's LAN address then the DNS server actually used is whatever is assigned in the router's WAN.
 
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