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Tools for Techs: USB Dualcomm Port Mirroring Switch

Updated on February 9, 2013
Pcunix profile image

I was born in 1948 and spent most of my career as a self employed computer trouble shooter for Unix systems.

In the days of old, when networks were built with hubs rather than switches, it was much easier to trace network problems.   A hub exposes all network traffic to each of its ports; a switch only puts traffic on the ports it needs to be on.  You could put an analyzer on any old port on a hub and see every bit of network traffic, but with a switch, you can only see broadcasts and traffic that belongs to the analyzer.  

As switches started coming down in price (they were once pretty expensive for a small business to consider using), and became more common, I found myself needing to carry a five port hub with me to assist in network troubleshooting.

Over time, ports failed on that device and it became useless.  I could find replacements on E-Bay, but soon nobody was making anything but switches.

Smart Switch

A "smart switch" would sometimes let me have my way with network traffic, but those were a bit more expensive than plain old "dumb" switches, so I couldn't count on finding these at customer sites.

Dualcomm Port Mirroring Switch
Dualcomm Port Mirroring Switch

Dualcomm DCSW-1005 Switch

That why I was very happy to find this inexpensive USB powered port mirroring switch.

USB powered is nice because if I'm there to troubleshoot, of course I have my laptop, but you can't always count on finding a free power plug near the devices you want to snoop. I do carry an outlet bar with me, but often there is no place to plug that in, so the USB is appreciated.

Unix vs. Windows

I'd normally bring my MacBook Pro and use tcpdump and other Unix tools for this sort of work, but recently I contracted to show someone how to use this with Windows and Windump. I had him buy his own DualComm and met him on site where I helped him install Windump on his laptop. I had also brought an old Windows laptop of mine with Windump installed just in case.

I'd much rather carry a Mac or Linux laptop for this kind of work because there are many other tools that Windows doesn't have. You can get every single one of those tools for Windows, sometimes for free and sometimes not, but they don't necessarily work as well and why on earth would you? If you are serious about network troubleshooting, you are going to have some sort of a Unix box or you'll be missing a lot. However, this consultant was more comfortable with Windows, so I showed him what he needed to know using Windump.

On site, I connected my laptop to port 5 of his DualComm, took the patch cord that went to the ISP's router and put it in port 2, and then ran port 1 back to the customer's switch where I had unplugged the router cable. That forced all traffic heading to the router to be on port one, and as the laptop was on port 5, and 1 and 5 are mirrored by default, we could now see all outbound traffic.

I started up a CMD window and showed him that we could do things like:

windump  "tcp port 25 or tcp port 465"

That showed any traffic going out of the building on common mail ports. That's what he wanted to learn how to do as he had customers with possibly infected machines that were suspected of sending spam email.

At this customer, he had done a full virus sweep of all machines and had identified and cleaned out several problems, but the customer wanted to be sure that nothing remained.

We only saw traffic from his internal mailserver. His router had already been reprogrammed to only allow that machine outbound mail port access, so I wouldn't have expected to see anything else, but this exercise was just to make sure and so that he would know how to use the tools in the future.

Of course there is much more you can do and dedicated network analyzers can make that much easier.  You can do quite a lot with an inexpensive switch like this and free tools like Windump and tcpdump.   As you gain experience, you may add other tools to your skill set, but this is a good way to start.


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