Linux Sysadmin Lesson 135: Do not configure your firewall over SSH.

Wanted to make sure my server was secure, so I was updating IPTables config. I was also curious what would happen if I configured my firewall over SSH. I was correct: my SSH connection was instantly dropped. Time for a walk to the other side of campus to hit the reset button.

UPDATE: So I managed to clear out everything and reconfigure IPTables. Seems like a program that would be cool to learn more about (re: setting up my own NAT router with a linux computer and two NICs). Overall, I’ve simply allowed all outgoing and only allowed incoming on ports that I’m actually using (just in case some random thing is running that I’m not aware of).

Of course I’m still vulnerable to the reverse SSH tunnel…

God Bless the HP 8924C: A Tale and Tutorial of the Service Monitor

God bless the HP 8924C: without you poor college students would not be able to possess RF test equipment way above their pay grade. My model is the 60 W continuous version, US with green CRT. I was able to secure this wonder for $869 shipped from Washington to Boston, MA on eBay. It also seems like my purchase raised the price of these units on eBay by $200-$500. Sorry. Mine is in “Almost New” condition despite being over 17 years old. It originally retailed for $30,000. Pretty cool stuff. It cost more than cars back then.

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In the picture, you can see it dominating my dorm room desk (room mate is thrilled, obviously). It’s huge and heavy and says “TWO PERSON LIFT!” on the side. I have also officially named it “Bruce.” Continue reading God Bless the HP 8924C: A Tale and Tutorial of the Service Monitor

A Tutorial on the Motorola i50r Basic Telephone Interconnect Repeater Controller

The Motorola i50r is a rather archaic device produced by Motorola in the 1990s for use in controlling simple repeaters while allowing telephone interconnect functionality, remote setup and knockdown of the repeater, and a few other simple functions. It is, in essence, a repeater controller that can be setup in minutes with a few dip switches on the front and runs reliably. Its most important feature is that it provides telephone interconnect or autopatch in a pretty slick and efficient way. Its one big shortcoming is that it lacks an automatic identification feature, so your repeater will be going stealth. (Please refer to FCC or applicable regional rules/regulations regarding the legal operation of your repeater. The enterprise is not responsible for your illegal actions while using this device.)


Continue reading A Tutorial on the Motorola i50r Basic Telephone Interconnect Repeater Controller

Updated: Repeater on the Cheap: The Chinese SQ-450N Duplexer Test Results

This was purchased from jaco3378 on 1/12 with tuning service. Shipped the 18th and received the 29th to Boston, MA.

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I ordered a Motorola “duo-repeater” at the end of 2012. It consisted of two SM120 radios and came with two antennas, something I later learned was not a fantastic way to make a repeater. The high power of the transmitting antenna would “de-sense” the receiver of the repeater (I assume some early gain stage is overloaded by the high level of the nearby signal and some gain control effectively reduces the level of your intended signal into the receiver). Continue reading Updated: Repeater on the Cheap: The Chinese SQ-450N Duplexer Test Results

No More Wall Warts: The DC-DC Buck Step-Down Converter

A few months ago I saw this little gem on eBay and snagged it for something like $3.50. After a month (gotta love Chinese envelopes) it showed up at my door.


My mom asked, appropriately, “What the hell is that?”

To her, probably nothing. To me, however, a very cool gadget.

If you’ve experimented with electronics, you’re probably aware of the really annoying fact that all these devices require different voltages and currents and practically no two are alike. If you’re exceedingly lucky, your device will take a standard 12V 1A center positive wall-wart, available in droves. Or maybe a 5V 1A supply (those are also common).

But a lot of the time, you end up with equipment that requires odd voltages. That’s where this device comes in handy. Plugged into the input, I have a 19V laptop transformer that I got for $6 on Amazon Prime for a friend (I thought I lost her adapter, I later found it, ended up with it). Clipping off the annoying connector revealed two beautiful red and white wires which I stripped and tinned. Many of these adapters have coaxial arrangements of the cables (this one has one of those little plastic cylindrical enclosures. Maybe that’s where it changes from coaxial to double strand?).

Basically this converter consists of a simple switching circuit with a built in inductor. Two filter capacitors on the input and output stabilize the voltage. By switching very quickly between charging and discharging the inductor it’s able to maintain a steady voltage across the outputs which is stepped down from it’s input. The max voltage it can produce is 2V less than the input or something like 26V, whichever comes first. It can supply something like 2.5A.

Basically, for $10 I have a 2.5A fully adjustable power supply. A tiny screw allows fluid adjustment along a very wide range. When combined with a 19V supply at 3A, it’s capable of supplying most every gadget I have in my collection with the necessary voltage. Is it stable as the dickens?


It tends to jump a little high after I dramatically reduce the current draw (releasing the PTT of a radio). But all in all, not bad for $10. And you can always attach a gigantic capacitor across the outputs for stability (check out the MIT Flea Market if you’re interested in gigantic cheap capacitors for arc welding or other uses. be careful though they’ll burn you into a fried chicken if you’re not careful).

Here it’s powering my Turnigy Accucell 6 universal battery charger to give my radio batteries a boost. I have a 25 Amp supply but for charging at 1A current, that’s a little wasteful (not to mention huge).

Bottom line is that I would highly recommend perusing eBay for one of these things. The screw sets the output voltage which doesn’t change as input voltage varies, and these are supposedly 95%+ efficient.

The Andre seal of approval is bestowed upon this Chinese gizmo.