Configure OpenSSH to automatically authenticate

Sometimes entering your pass-phrase to gain SSH access can get redundant, or you may want an automated script to be able to authenticate as you (as described at http://sial.org/howto/rsync/).

I've been trying to do this for some time now--apparently if you read the man pages carefully enough you can figure this out. I thought I'd need a more advanced SSH client, but OpenSSH already has everything you need--the ssh-keygen program is key. Following is a brief walk-through of how to configure connection from client to server for SSH. Replace the words client and server as appropriate.

client$ ssh-keygen -t rsa

It will ask where to save, you want the default since this is where ssh will look. It will ask for a pass-phrase, and since we're trying to get around having to enter a pass-phrase, we don't want to protect our authentication, so just hit enter twice. Then it will give you a fingerprint and put files in your ~/.ssh directory.

Copy the ~/.ssh/id_rsa.pub to the server machine taking care not to overwrite the same file there:

client$ scp ~/.ssh/id_rsa.pub user@server.address:.ssh/client.pub

Now SSH to the server and add the public key to the authorized keys file like so:

server$ cd ~/.ssh
server$ cat client.pub >> authorized_keys
server$ rm client.pub

The SSH server won't let you use the key unless the file is secured. That is, the keys should only be readable by you.

server$ ls -l authorized_keys
-rw-rw-r-- 1 user users authorized_keys
server$ chmod og-rw authorized_keys

Now we should be able to open an SSH session simply by:

client$ ssh user@server.address

where you can omit the 'user@' part if the user names are the same on the local and remote machines.

Finally, note that doing all this means if you leave your machine open, an intruder has more doors they can open! This is why you should always lock and/or time-out your screen. If the key has no passphrase, then if someone copies it, they can gain access to the machine you opened with this procedure. Therefore, you should restrict access to your machine by configuring /etc/hosts.allow and /etc/hosts.deny. If you're concerned, don't let the passphrase for the key be null (use a passphrase). Taking these steps can help protect your key.

Cost Review for Luxeon LED Headlight

To justify building on myself, I had to do a cost analysis.

Purchased:

4x D-cell Batteries ($5)
1x LED Dynamics BoostPuck ($35)
4x Lumileds Luxeon I Star/O ($25)

Subtotal: $65

Spare parts:
1x 270uF capacitor
1x 24-gauge (AWG) lead wire
1x Pentium3 Heat Sink
1x Mounting bracket
1x Rocker switch
2x 2 D-cell Battery Holder
1x 18-gauge (AWG) lead wire

Note that comparable systems are water-resistant and include a rechargeable battery system, but can cost from $120 up to $300.

Rebuild of Luxeon I Bicycle Headlight

This is a practical comparison between two light sources: an old 2-C cell halogen headlight and a single Luxeon I Star (with optics), both driven at the proper power. This is an image with no special adjustments and the sources about 5 feet away. It is easy to see that the Luxeon I has both more flux (total amount of light) and better throw (maximum intensity of beam). Note that halogen bulbs are known to be more efficient than traditional vacuum bulbs, see Lighting efficacy for more. According to my calculations, the power delivered to the halogen bulb is about 1.5 Watts, and the power to the Luxeon I (obviously) is 1 Watt. Note the spectrum of light output, and how this would make riding safer. This comparison allows the efficiency of the LED to really "shine".

Note that my power supply (limited to 1 amp) may not have been able to provide enough current to drive the Halogen bulb to the same power as the 2 C-cells, so the halogen light may get brighter with actual batteries. However, the power measurements quoted are absolutely correct and (together with the comparison picture) still illustrate the point of efficiency. Also note that only one Luxeon I (1-watt) is switched on in the comparison picture, whereas four are used in my headlight assembly.

The LED Dynamics BoostPuck 350mA current regulator came today. The model with lead wires attached was not available, so I had to solder on wires. It really is small as they say; even smaller than my rocker switch. I left the extra pins (reference voltage, and dim) not connected.


Using my power supply, I verified all the parts and connections were OK. The meter pictured is showing volts across the input power pins of the BoostPuck (out of frame). The system is was actually pulling down the voltage on my 1-amp-maximum power supply set at 6.0V by 200mV. You can see that the LED's are fully illuminated, which requires at least 12 Volts in their wiring configuration...


I soldered everything together as I mentioned I would in my last post (in series). I included a 270uF capacitor. The only problem with the BoostPuck is that it's a sealed unit and has no way to mount it to anything, unless you use the pins to anchor it (which I don't). It's not heavy, so I didn't think it's all that important. Using industrial hot glue, I fixed it to the rocker switch, and the capacitor to it as well. I expect the hot glue not to hold for long, especially if things heat up. I'll use some epoxy if all goes well--the hot glue comes off easily, epoxy won't.

The light output looks about right. It might be a little dimmer than the unregulated circuit, but it's worth the gains in battery efficiency/power delivery. The BoostPuck is producing over 12VDC across the four LED's in series, which is about what I expected. I haven't actually checked, but I'm assuming that the diode current is 350mA. I didn't make measurements of the current drawn from the batteries because of the AC components.


The light beam itself is continuous (as opposed to strobed); waving hands in front of the beam doesn't reveal individual light pulses as with other charge-pumping lights I've encountered. Also, the intensity differences between the new and old LED's is gone as I expected. The newer LED's needed a higher voltage than the older LED's to produce the same amount of light, and fixing the current at 350mA equalized the light intensity.

For clarity, below is a crude circuit diagram including all the parts.

Luxeon LED headlight assembly

Have an old processor heat sink laying around? This Pentium 3 heatsink is the perfect size for four Luxeon I Star/O assemblies (LumiLEDs Product# LXHL-NWE8). Luxeon light sources need very good heat dissipation, and this might be over-doing it, but why not prolong the life of the light sources?

I wanted to get around paying up to $300 for a battery-powered bicycle headlamp, so I decided to meet or exceed the expectations with available parts.

I initially bought only two of these Luxeon Stars (with optics) over a year ago. I didn't have a good way to drive them other than my Elenco Precision (TM) power supply and alkaline batteries, both of which are voltage sources. I tried using two D cell alkaline batteries, this provided 3.0V which is less than the typical 3.4V, and not didn't force enough current to for the LED's to be all the way on. I also didn't have a good mounting bracket, or a good way to mount the batteries to the bicycle frame.


Recently, using the mounting bracket from and old mechanical speedometer and zip ties, I was able to make an excellent mount. This stops the lights from changing pitch (very important for this application) or breaking free. I connected them all in parallel to try to give them all 3.0V from the two D cells.

Lesson learned: Don't solder when the device is a metal core PCB and it's sitting on a heat sink with silicon paste in between! Leave these PCB's on an insulator while soldering.


I noted that they were only drawing around 800mA, nowhere near 1.4A (4x350mA). I decided to increase my power supply voltage potential, but this meant there needs to be another component in series with the light sources to allow the excess voltage to be dissipated. In the application notes from LumiLEDs, they recommend using a 6V power supply and a 10 Ohm resistor for a Luxeon I. I added another battery pack to the frame to get 6.3V, but I decided to use 0.9V rectifying diodes (3A max) rather than four resistors (one in series with each supply) which would require desoldering. This achieved 1.6A with one diode in series and 1.1A with two diodes in series. I stuck with the two diodes in series to stay on the safe side. The diodes get rather hot very quickly since they're dissipating so much power. This is a major problem, by my measurements -- 45% of the power was being dissipated in the diodes themselves, delivering only 55% of the power from the batteries to the light sources. Another reason that a passive approach (linear mode voltage regulation) is bad for alkaline battery applications is that the higher the power delivery, the more the batteries self heat and increase internal resistance. In other words, Alkaline's are less efficient when discharged quickly. It is a common trend that batteries are more efficient with intermittent loading for reasons related to battery theory. For much more, see the IEEE paper Battery­Driven System Design: A New Frontier in Low Power Design, by Kanishka Lahiri, Anand Raghunathan, Sujit Dey, and Debashis Panigrahi.

Lesson learned: The voltage drop across an LED is related to the energy released in each photon--relating to the wavelength of the light emitted. (Note that this is the P-N junction voltage drop, and differs from the external voltage drop and light spectrum is also highly dependent upon construction materials). The current through the LED corresponds to the number of photons emitted--relating to the intensity of light emitted. These brief rules of thumb really help in understanding how to apply high-power LED's.

Lesson learned: Luxeon LED's are constant-current devices, so to get them to the proper luminous intensity and power consumption, they need to be driven at the proper current amperage. For Luxeon I's this is 350mA, for Luxeon III's this is 700mA, etc. This point remained subtle to me because I figured I could always just find a voltage potential which would result in the proper current being drawn. The problem is that LumiLEDs has a tighter manufacturer tolerance of 350mA operating conditions than the forward voltage (Vf) which is loosely maintained. This means each light source will require a slightly different Vf to draw 350mA and use run at the full 1 Watt. The most robust solution is a constant current source which can be used to drive multiple light source in series. This also allows the supply voltage to fluctuate (eg: a draining battery) and not affect the luminous intensity as it would without a regulator.

Lesson learned: A common optimization for alkaline battery-powered applications is to use a charge-pumping power regulator. These are capable of producing voltages higher than their input voltage (12VDC from 6VDC for instance) by intermittently presenting a larger load and driving an oscillating circuit. The typical result is luminous intensity remaining mostly flat until the battery is mostly depleted. Just connecting a 12VDC battery supply would result in an exponentially decaying luminous intensity.

These conclusions lead me to replace my diode-dissipating circuit with a switch-mode power supply including a boost circuit. Because of the scarce availability of through-hole mounting switch-mode power supplies and the necessity for a PCB, I was very happy to find products from LED Dynamics including the BoostPuck. Their Application Figure 3 in their datasheet is my exact application. The regulators are unfortunately around $34, but it's worth the efficiency of a switch-mode power supply, the optimized battery utilization, and proper driving (constant current of 350mA) of the light sources to get the most light without risking burning out the LED.

When the unit arrives, I'll removed the hot glue (to keep the parts clean) and connect the light sources in series and connect the BoostPuck to the ligth sources after the rocker switch. As per the application notes, I'll need to include a capacitor since my batteries are a few feet away. More as soon as the part comes in.

1979 Free Spirit Ten Speed

Once I started riding this old Free Spirit ten-speed, I was hooked. Most of my riding is on a paved recreational trail, so the thin tires aren't a hazard and the bike is a lot more efficient than a Mountain Bike on those conditions.


The bicycle came from Sears & Roebuck circa 1979. I decided that even if I buy a new road bike, that I wanted to keep the Free Spirit around since it was in such good condition compared to a few much younger bikes I have seen. The parts are old (Shimano) parts, but most of the following procedures still apply to most bicycles.

The first problem was that the wheels needed to be "trued" very badly, the crank was loose, it had lots of grime around the chain hardware, the brakes needed to be tensioned, and it had some out of order accessories. The accessories it had were a luggage wrack in the back which had a broken support (which is laying on the ground behind the bike in the picture), and a mechanical speedometer which had a loose cable and just made chirping noises. It also had "soft" plastic handle grips on the end of the dropped handle bars which were petrified and provided no cushioning.


I started by disassembling the whole thing! I cleaned all the parts with CLOROX(tm) disinfecting wipes because they have a cleaning agent which does no harm and are great for absorbing grime. I put the luggage wrack aside because it just got in the way of mounting/dismounting (and was broken). As for the speedometer, modern bicycle computers (speedometer, odometer, tripometer, etc.) are now inexpensive, under $12 at your local "superstore", and allow you to program your wheel size (unlike the mechanical one), so I replaced the huge speedometer with the little computer.


I took the wheels to the local bike shop and had them trued for $5/wheel. They still have a little jitter, but it's nothing compared to the thumping and wobbling that they had before. They had both lateral (off rotation axis) wobble and radial (tending to not form a circle) wobble. I can ride over 20 mph now and not leave the ground.


While the wheels were being worked on, I was able to remove the plastic grips a lot easier than I expected (these things have tendency to become attached). Rubber grips can be a real pain, but I figured the plastic grips would have been fused to the handlebars. I just inserted a flat-head screw driver (on the bottom side to avoid any visible scratching) in between the grip and the handle bar. I didn't pry with the screwdriver, rather just inserted and removed it several times, spraying WD-40 down the length of the screwdriver each time. After about three times, I was able to break each grip free by trying to twist the grip around the handlebar. Once it came free, I took out the screwdriver and worked it off. I cleaned the lubricant off the bars. I bought some handlebar gel tape from Bartrager, and installed them according to these directions since there were none in the box. I guess I have long handlebars because I didn't have to cut the length of the tape, I just made a cut to taper the distal end of the tape so it was flush with the handlebar cap.





The bicycle has a disk brake in the back which needed to be manually tightened because it didn't lock the wheel. The brakes in the front were also too loose. Tip: If brake pads don't lock the wheel, they need more tension. Take the wheel off of the frame, and ask an assistant to hold the pads together. If your caliper makes it easy to disengage the recoiling springs, you won't need an assistant. Taking the wheel off will provide enough slack to make the adjustment--I found it was impossible otherwise. Adjust where the steel cable is fastened on the caliper side about a centimeter and tighten everything back up.


The bike was back in order but the crank still felt bad because it moved inside the frame. It's a one-piece crank and is a lot simpler than I thought before. Take the chain off first. It has a big locking nut which you'll need a big wrench for. Note that the threads are in the opposite direction from most threads, so you take the nut off by turning it in a clockwise direction. Under that, there were some washers which act as dust covers. There is a big nut that the ball bearings ride on which needs to be finger-tightened, it was loose and this is where the crank wobble came from. I de-greased the ball bearings and the journals inside the frame. It was essentially dry inside just a lot of black deposits on everything. There was some fatigue on the right-hand (gear) side where the ball bearings either crashed down or the bearing retainer was hitting. With one-piece cranks, look inside where the crank fits and look for the obstructions from the other tubes. Find a place to fit the corner of the crank and angle the crank through as to take the gear away from the frame. This will give you more space and will let the crank rest. After everything was clean (I had to use an abrasive brush in some places), I greased everything up as much as I could and put everything back together. I used a screwdriver to make the final torque on the hand-tightened nut, I stopped as soon as I felt no wobbling of the crank. When you put everything back together, you should be able to spin the crank and it should keep spinning for a while. If it doesn't make it more than a few turns after you let go, it's too tight.


After a few other adjustments, like the kickstand position, everything was ready to go. The bike feels a lot tighter like a new bike would. All the rattling, chirping, and wobbling is gone.

The bike will need new tires soon (the thin tires don't last very long), I have about 300 miles on mine and the tread in the back is disappearing. When I replace these, I'll try to disassemble the bearings on the wheels and try to revive them like I did the crank bearings. I'll also replace the chain at that time.