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// Tales from software development

More NETBIOS name lookup problems and port 137 weirdness

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Back in May last year I blogged about why port 137 needs to be open for UDP requests if you want NETBIOS name lookups to work.

I bought a new laptop earlier this year that I use every day for software development. Right from the start I’ve never been able to connect to its network shares or to RDP to it. This has never been a problem because I usually work on this machine and connect to other machines rather than the other way around. I’d briefly tried to find the cause of the problem a couple of times without any success. It wasn’t much of an inconvenience but, out of curiosity, I kept meaning to find out what the problem was.

Over the Christmas holiday I had the time to do this so I started looking. I expected to find that port 137 was blocked and that I simply needed to set up an exception for it. It was blocked but in a rather strange way…

I found that File and Printer Sharing was enabled on the Exceptions tab of the Windows Firewall configuration applet. As this includes an exception for port 137 I looked for the cause of the problem elsewhere but couldn’t find anything. Eventually, I came back to the Exceptions tab again, selected File and Printer Sharing item, and then clicked the Edit button. It was immediately obvious that something was amiss:

Windows Firewall - Port 137 

Two questions: Why was the Scope for UDP 137 different from the other exceptions ? And, how was it different ?

The second question was easier to answer. Clicking the Change Scope button displayed this:

Windows Firewall - Port 137 Custom

Instead of being set to My network (subnet) only as I’d expect the scope was set to what looked like a specific IP address. On closer inspection though, that address is actually a range of addresses and they’re not even on my local subnet. It’s the subnet of my company’s VPN.

I can’t understand how this happened. I certainly wouldn’t have deliberately configured it like this. At first I wondered if it was some weirdness that happened during Windows installation. But, the more I thought about it, the more I began to suspect that it’s the result of a poor choice by me to a prompt by Windows. I’m guessing here, but I think Windows must have prompted me at some point to allow a connection request while I was connected to the office VPN and offered a choice of any computer, local network, or a custom list, and I selected the last option thinking that this was a single request and not realising that it was going to set an exception policy that I’d have to live with.

Of course, this is easily corrected (by clicking the My network (subnet) only option and clicking OK) but finding the cause of the problem was less obvious than I’d expected.

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Written by Sea Monkey

January 12, 2010 at 8:00 am

Posted in Environments

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