DNS resolves to different machine for the same IP

Question

I am using a Windows server running IIS within a domain network. When accessing the website hosted on the server, i able to open the correct page. Refreshing the page within a few seconds shows a web page from a Apache webserver running on a linux machine(with same ip address).

In short, the same IP address connects to different servers within a matter of seconds intermittently. How is this possible? My stream of thoughts so far: - DNS resolves the url to same ip address correctly, but the machine corresponding to the IP address is switching. Is it possible to have two machines with same IP address on a network? - The call to one server could automatically redirect to another url. I used Curl to check redirects. But, there aren't any.

I am trying to think of any other diagnostics i could run to determine the root cause.

Any thoughts/suggestions would be helpful. Questions are most welcome!

Answer 1

If both machines are using same IP address... you're running into a classic collision. Every machine on the network should have a different IP address. (excluding situations where you're clustering with specialized software) If they don't, your machine may connect to each server randomly.

What happens behind the scenes is known as arp-caching. Computers in the same local network (broadcast domain) do not talk to each other by IP address. They use the physical address (mac address). Your PC does a broadcast (arp request) asking "who has IP x.x.x.x?" At which point the machine that has the address should respond. When two machines have the same IP... it becomes a battle of which responded first. Once that address is found... it's stored temporarily in an "arp-cache"... until a set period of time expires (it can vary between machines)... at which point it will start the lookup process again... which could end up with getting the same or the other machine's response first.

Answer 2

The first thing you could do is to turn the windows server off. Then use another machine to ping the IP address. If you get a response then yes, you have two machines with the same address on the same network.

If this is the case I can think of a few reasons:

1. Configuration error (either the linux host or the windows host has the wrong IP).
2. Documentation error (IP used but not documented that it was in use. Differs from point 1 that 1 is most likely a typo when configuring one of the hosts).
3. Someone is trying to intentionally mess with the network.
4. Someone is unintentionally interfering with the network.

I have point four in action when a linux laptop got booted at home and got a IP address based on the home DHCP server. That laptop got suspended, carried to work and turned back on where it conflicted with the IP of one of the local servers.

Note1: Turning the server off while others are using it is obviously a bad idea. :)

Note2: You could just [temporarily] change the IP of the windows server to test. Please consider how you connect to the server while doing this.

Answer 3

How is this possible? ... any other diagnostics

There's nothing to diagnose. your network is mis-configured. Fix it.

Is it possible to have two machines with same IP address on a network?

Obviously it is possible - but not on a working network.

Answer 4

if that linux machine is ubuntu or similar distributions, it will not bother checking the ip conflicts and will grab the ip whether it has been used or not by another server.

windows may be doing the same thing (i have less experience there so am only guessing).

you should contact your network administrator to sort this out, as neither servers will be working correctly.