Why does DNS server entry in ipconfig break point-to-point ethernet?

[Jeff in Pleasanton]

My application uses a panel pc as the user interface for a cRIO controlling a medical device.  The panle pc and cRIO communicate using shared variables.  During development, both devices (panel pc and cRIO) were on our company network.  Curiously, when I connected them point-to-point with a crossover cable, they refused to communicate when running their LabVIEW application code.  Pings from the panel pc to cRIO were successful.  Much troubleshooting ensued.

Long story short, the fix was to simply delete the DNS server IP entry from both the panel pc and the cRIO ipconfig tables (see attached pic for the WinXP version of this table).  If either of them had our company's DNS server IP address fillied in, my LabVIEW application would fail.

This leads me to suspect that there is something in my LabVIEW executables that wants to touch the company network.  Apparently, if there is a DNS server entry in the ipconfig table, this something thinks it has a chance to "phone home" and it tries to do so.  When this happens on the panel pc end of the crossover cable, the machine acts like it's locked up, but the task manager shows the CPU to be ~98% idle.

If there is no entry for a DNS server, I guess this something realizes that there is no way to "phone home", so it doesn't try and my application works great.  Since the application works with no DNS server table entry, I think my crossover cable is working correctly.

Does anyone have any idea what this something might be?

[NathanT]

Jeff,
      I am glad you got your application working with just the cross-over cable.  The last line of this KB indicates that if your RT engine is not on the network, that the DNS server and Gateway should be left blank.

Thanks,

Nathan

[Ben]

"

Does anyone have any idea what this something might be?

"

Here is my read. I am not going to go reasearch any of this so if someone else knows better.... please correct me.

The DNS entry tells your machine that it should look for a node named"XXX' to get an IP from it. Without that entry, the PC assumes it has a fixed IP and will use whatever is specified.

DNS is a service that runs on a mahcine in a network and hands out IP- addresses to anyone it is configured to serve.

So....

With a valid entry in the config it attempts to contact that node to get an IP.

With no path to the DNS server, your machine has NO IP address.

NO IP NO talk.

Ben

[tbob]

You've hit the nail on the head Ben.  For point to point communications, the IP addresses should be fixed, therefore there is no need for DNS.  If a DNS is configured, the NIC (Network Interface Card) drivers will try to contact it.

In my test system, I need DNS for the test computer, but I am communicating with a dedicated Spectrum Analyzer over TCP/IP.  So I added a second NIC.  The main NIC is configured for DNS and all that jazz.  The second NIC (plugged into PCI slot) is configured with a hard coded address, no DNS, no Gateway, nothing else.  I connect that NIC to the spectrum analyzer using a crossover cable.  The analyzer is configured with a hard coded address also.  Now my computer can get on our company network, and the spectrum analyzer is isolated from the network, so it can't catch any viruses, etc., and it still talks to the computer.  When using a configuration such as this, it is best to use a dedicated address for the 2nd NIC and spectrum analyzer in the range of 192.168.100.0 to 254

[Jeff in Pleasanton]

Both of my boxes have had static IPs from the beginning of the project.  No DHCP.  The part that confuses me is that they communicate over the crossover just fine when their DNS server IP address entries are blank, but they won't speak to each other over the crossover cable if there is a valid DNS entry in either box.

[Justin D]

Jeff,

Regardless of whether the ip addresses are static, if a DNS is specified the cRIO will try to connect to it.  If you're not anticipating having your cRIO system connected to the network I would just leave the DNS blank.

Justin D