heipei’s weblog

You often hear the phrase “the swiss knife of …”. Well, for networking, OpenSSH clearly deserves that title in my opinion. Besides the obvious feature of providing secure logins with different options for mutual authentication, it can also be used to transfer files. It is also a core component of software like git, which depend on ssh to securely push changes instead of reinventing their own server and dealing with the whole authentication/authorization stuff once again.
I’ve blogged about using SSH to access hosts behind a firewall/router more conveniently, and I really suggest you sit down and read the manpage front-to-back, it will probably be rewarding for your current workflow.

Reusing a connection
Anyway, today I wanted to present another advanced feature, one I only stumbled upon while learning for an upcoming exam. The slides mentioned the possibility to share connections on one SSH-channel (to the same destination, obviously). Now, why do you need more than one connection in the first place? Well, sometimes you just need the space, and using screen just doesn’t cut it. Or you use git or scp to the same host you’re logged in already interactively. In that case, you just have to establish one connection and any further connection will use the first one. The benefit? Login is way faster, almost instantly, because the whole handshake and authentication doesn’t have to take place again. The caveat: You’ll have to close your master-connection last, otherwise the other connections will drop too. For me that’s no problem, since the host I use most of the time is also the host I’ve got a session open with 24/7 (IRC).

I’m just gonna show the fully automated version here, though you can use it manually as well, just when you need it. A better description can be found here. To automate it, put this in your ~/.ssh/config (you don’t have a config yet? shocking!):
ControlMaster auto
ControlPath /tmp/%r@%h:%pAs usual, you can put it at the very top to apply for all hosts or you can activate it for specific hosts by putting below a Host declaration. Now all you have to do is use ssh host as usual. You’ll see it’s working when you terminate the connection:
Shared connection to host.de closed.

Reverse tunnel
Yes, you can tunnel with OpenSSH. You can even have it act as a SOCKS proxy, which is really neat when combined with Firefox-plugins like FoxyProxy. But you can also reverse-tunnel your way out of a closed network (or NATed network for that matter) when you don’t have access to any intermediate host (like the router/firewall). It’s easy:
ssh -NR 12345:localhost:22 home.deThis assumes you’re issuing the command from the machine that you want to access (from home) later on. It will bind the port 12345 on home.de so that it is forwarded to port 22 (SSH) on the local host. Yeah, it requires some reverse thinking too to get it right ;) The -N-switch prevents a login, by the way. Obviously it is of little use if your home.de has a flaky dialup connection, so you might want to reverse-tunnel to a stable endpoint (or use something like autossh).

Wow, the last few weeks I shot more photos (and learned more) than in a whole year, at least that’s how it feels.
First and foremost I got a new lens for my birthday. The Canon EF-S 18-200mm 3.5-5.6 IS. This was meant as a replacement for the rather basic 18-55mm that came with my 400D body. So far the 18-200 hasn’t disappointed me. I’m really using the big span of the focal length and since it is meant as my alround-lens I didn’t care to much about the 3.5 open aperture (I’ll mostly use it for trips or well-lit situations, my EF 50mm 1.4USM is still the basement-party weapon). Check out a few of my photos on flickr to get an idea of the lens. The IS is really nice, the handling feels rich. The only downsides of this lens are it’s weight and the autofocus which sometimes has a hard time in low light conditions.

A few weeks ago I decided to finally buy a flash. I didn’t hesitate long and bought the Canon 430EX II. Before that (and while waiting for DHL) I read a lot of strobist.com, so that I hit the ground running when the flash did arrive. The nice thing about buying flashes is that you don’t have to worry so much about capabilities/size, because you can buy a bigger one which can act as master at some point in the future should you feel the need to. Along with the flash I ordered some small utilities, like the Phottix Tetra radio-triggers for off-camera flash, the Roscoe Strobist Gel collection (gel from “gelatine”. It’s called “Effektfolien” in german) and a small diffuser (just for fun). The Phottix Tetra have been great as well, especially considering the price-tag of about 30€, which is next to nil compared to other wireless triggers which quickly add up to ten times that much. However the Phottix triggers don’t support TTL, which is a measuring-system so the camera knows how to adjust for the flash. Well, what better time to really get acquainted with the M mode ;)

Aachen Open 2010, which just ended today, was the only real event I looked forward to, photography-wise, and it turned out really nice. Not only was the event itself a smashing success (no other way to put it, the organisation was flawless, the attendance was record-breaking and the venue perfect for our needs), but also meant many diverse personalities to photograph. You can see my complete photos of the event here: photos.heipei.net/Aachen_Open_2010/ and photos by other people will be collected here: http://cube.hackvalue.de/ao10/media-de.

Tonight I salvaged an old (and broken) Umbrella and simply covered the inside with white xerox paper. I sawed of the round part of the handle and now it fits into the grip of my tripod. The Phottix has a mount option for standard tripods. The results are amazing, and as usual I now feel much better when I buy an umbrella (I’m not quite sure whether I want reflective or shoot-through or both ;)

A polfilter (pol for polarizing) can give you all kinds of nifty improvements in your outdoor pictures. Here is a little preview. I expect to achieve much more drastic differences come summer:

Left: No filter (or at least filter turned towards “normal”). Right: Filter set to the right angle. Make sure you check out the full-sized version of the photo and pay attention not only to the sky but also to the trees and the windows of the building. Image was sharpened with Gimp btw.

Just like the post on the Streulichtblende (sorry, german only) I thought I’d build my own Softbox before actually buying one (and the flash that goes with it ;). What’s a Softbox? Well, put simple it’s a device to distribute light/the flash evenly over a big surface, so that the resulting light and especially the shadows are “soft”. You can buy those for external flashes, or use the similar umbrellas, but I opted to try it with my builtin flash first :P

Construction
This is not a detailed guide how to build it (you should figure that out yourself, depending on the materials available to you), but just a summary of my results.
As you can see I used the cheapest materials I could find: an empty amazon.de-box, using the lashes to create an inner curvature, then covering everything inside the box with tin foil. Finally the diffusor is made from two Din A4 sheet of white paper, where you’ll have to be careful with the thickness and the color. I can’t say it’s comfortable to use or durable, but the time spent making it is minimal, so who cares.

Results
When are the effects of a softbox most obvious? Probably when there is absolutely no other light-source around (see my Black/White collage). It becomes even more obvious if you photograph something in front a background where it casts shadows. One thing I had to do was to increase my flash exposure compensation (i.e. make it brighter), and in a few cases I had to manually set the white balance, though that’s something I do very often anyway. And using the P-mode seems like the only usable way to incorporate the flash easily. I guess I’ll have to read up on the difference between “Green Square” and “P” again. If you can I’d try to build a bigger box and of course the flash should be close to the center (which can’t be done with the internal flash).

Bottom line
After quite a few shots (photographing people is probably the most rewarding) with my selfbuilt softbox and I know now that the money for a professional one will be well spent ;) If anyone knows of good brands for a) flashes (for Canon) b) softboxes c) remote flash triggers (wireless), let me know!

This is just a reminder that Aachen Open, a German speedcubing competition, will take place again next year. More precisely on January 16/17, 2010. What’s even better is the huge number of registered participants and the new venue for the competition. Since the last place became quite crowded (and began to smell after about 2 hours) the management secured a larger spot in RWTHs Computer-Science building (Aula II, Ahornstraße 55). The downside for visitors is the distance to downtown Aachen, but on the bright side we’ll be equipped with internet throughout the competition.

Rest assured, I’ll be there, but probably less cubing and more photographing. So be sure to come by, even if just to get a glimpse of hardcore-nerds in a different area than your own profession. In case you missed the last Aachen Open, be sure to check out last years website for impressions.

Manchmal muss man, trotz eduroam, noch das alte Cisco-VPN benutzen um sich ins Uni-Netz einzuwählen. Z.B. wenn man von Zuhause auf Dinge wie Springerlink oder andere Online-Bibliotheken zugreifen will, oder um auf bestimmte Hosts per SSH zu kommen die (sinnvollerweise) von aussen nicht erreichbar sind.
In Mac OS 10.6 Snow Leopard kann man Cisco-Verbindungen glücklicherweise direkt mit dem Betriebssystem machen und ist endlich frei von dem nervigen Cisco-Client. Allerdings hat das ganze noch einen kleinen Bug, nämlich dass man sein Passwort bei jedem Verbindungsversuch neu eingeben muss. Wie man das vermeidet steht hier: 10.6: Save Cisco IPSec password in the keychain

When you start using a small and slow laptop over an unreliable GPRS connection (say while travelling in an ICE at 300km/h) you start noticing certain things that are missing from your current shell-setup. One of these problems manifests itself when you want to send the mail you just typed in mutt. Fortunately there is an easy and elegant fix (if you use mutt with msmtp): msmtpqueue. This gives you three little scripts to enqueue, list and send messages which would otherwise have gone directly into msmtp. Just put them somewhere and then add this to your .muttrc:set sendmail="$HOME/local/bin/msmtp-enqueue -i"
macro index \Cy "!$HOME/local/bin/msmtp-runqueue<enter>"Now “sending” mail from mutt happens instantly and you can manually push the batch of mails out to your smtp once the train has stopped in a station and your reception is stable ;). msmtp-listqueue shows you the queued mails while msmtp-runqueue mails them (using msmtp of course!). For the downstream-direction of offline mutt-usage there’s offlineimap, but I feel like this deserves an extra post ;)

When using X on my Acer Aspire One (110L) and playing videos with mplayer, X would occasionally blank the screen after a few minutes without any way out but to suspend/resume to get your X back. This didn’t only happen with mplayer, and it didn’t have anything to do with xscreensaver.Anyway, after some digging I found that adding Option "FramebufferCompression" "off"to the Device-section of my xorg.conf resolved the problem.
Another cool thing I found out while going through ssh manpages is that you can silence the motd on login simply by touching ~/.hushlogin.

I just found this wallpaper I seem to have made in ‘06 (around August) and posted to the Gentoo Forums. It never went anywhere, but it would be a shame to let it go to waste, so here you go. Don’t ask for any higher resolutions though ;)

Download it in 800×600, 1024×768 or 1280×1024.

Without anything spectacular to blog about I fall back on my trusty photostream to keep you guys interested ;) You can find photos from my weekend-trip to Munich (including a short visit to the TU in Garching) as well as impressions of my holiday in France. We’re planning to continue the hiking season next weekend with a visit to the Eifelsteig, so I’m hoping to snap some nice fall-foliage. Enjoy!