In some network environments, where for example administration lans or some private lans are deployed, it might still be necessary to access a specific port of a machine inside that lan from the outside. Commonly, you would have to access a jump host and from there you would be able to reach the respective machine.

In our case, we had to reach the management port of a switch in a private lan. For example:

• the private has the IP address range 192.168.10.0/24
• the switch is configured with 192.168.10.254 and its management port is 80
• the jump host with access to both networks has the external address 10.10.10.1

To access the switch directly at address 10.10.10.1 with port 81, you can configure xinetd on the jump host with the following configuration:

# cat /etc/xinetd.d/http-switch
service http-switch
{
 disable = no
 type = UNLISTED
 socket_type = stream
 protocol = tcp
 wait = no
 redirect = 192.168.10.254 80
 bind = 10.10.10.1
 port = 81
 user = nobody
}

After reloading (or starting if not yet done so) xinetd, you can reach the switch by pointing your browser to http://10.10.10.1:81:

chkconfig xinetd on
rcxinetd restart

The same principle can also be used when forwarding e.g. ssh ports of machines.

as you might know, ownCloud uses the Open Build Service (short OBS) to produce and distribute the binary packages for the various distributions we want to support.

The OBS is a great system as it does not only build for various Linuxes but also lets the users download the binary packages utilizing a huge mirroring infrastructure. All for you, all for the good of free software.

And there is even more: OBS offers a nice download page which can either be embedded into a projects webpage or be linked from it. Depending on the settings of the OBS project, for example the number of Linuxes it builds for, the page is adopted automatically. So you never again have to deal with lots of download links to the projects repository downloads. See the ownCloud Client Package download page as an living example.

Now the page had a shortcoming for apt based distributions. I became aware of it as the problem as well as the solution to it was nicely pointed out by this blog: Apt wants to have the key before it trusts the download source. OBS generates the key but its not obvious how to import.

This morning I told one of the brave OBS developers to the blog post and already now the solution is online. From now on, the download page also shows the exact command line to import the key. Isn’t that great how quickly things become better if a couple of people do a little bit work together?

Thanks a lot, guys, really cool…

By a continuing process of inflation, government can confiscate, secretly and unobserved, an important part of the wealth of their citizens.

-- John Maynard Keynes, The Economic Consequences of the Peace

Akademy 2012 isn’t far any more and I just registered for a BOF at this years Akademy in Tallin. It will take place in Room 419 on monday, July 2nd, 2012 at 10:30.

Subject of the BOF will be the integration between KDE and ownCloud. As onwCloud originated from the KDE project we should meet and think together again how we can provide useful integration for the users combining the power of both technologies.

After a short overview of where the ownCloud project is going and what the recent achievements are (if participants are interested) I think we should concentrate on the technical level.

There are a couple of rough ideas already how we can integrate on a technical level, such as

• Integration of the ownCloud data storage in the KDE user experience.
• KDE features for the ownCloud Desktop Client like KWallet integration or KDE SSL certificate management.
• Synchronisation of PIM data like contacts, addresses, bookmarks or RSS feeds
• a syncing API for application data to be synced in KDE applications.
• Syncing of KDE application configurations.
• Plasma integration of ownCloud/syncing client.

I am sure there will be more we can come up with. Let me know upfront if you want me to put it on the list or even better show up at the BOF in person.

Ferrocement is a building technique that allows for very flexible forms to be made with concrete. You can make vaults, walls, round tanks or differently-shaped ones, cisterns...

For a normal concrete pour, you need some kind of formwork that is difficult to build and awkward to remove later. Ferrocement generally lets you avoid most or all of the formwork, as what is built can support itself even as the concrete is curing.

I am not an expert in ferrocement, but I've had had enough things in my house made with this technique that I can give a little introduction.

Building a bridge - a little example

For this example, we'll build a small bridge to cross a ditch in the ground - a Permaculture-style swale for water catchment. The bridge is not designed to sustain big loads, or for a great span; it is just a little example.

You form a rebar mesh into the shape you want, overlay it with hexagonal wire mesh, and tie it all together with wire. The more taut the hexagonal mesh, the better. You can make the wire ties with pliers, or with an L-shaped tool that builders use to twist wire elegantly.

Pour a little concrete base on each end of the bridge, on which to seat the wire mesh. The basic recipe is:

• Four 20-liter buckets of gravel.
• Four 20-liter buckets of sand.
• One 50 Kg bag of Portland cement.

Put the mesh over the concrete bases, seating it reasonably well.

Then you start pouring concrete over the bridge's mesh. Do the lower ends first, and climb up to the center. You have to spread the concrete mixture with a trowel, but without tamping it down - that would cause it to spill through the mesh. Something will spill, but it is generally only the most liquid part of the mixture, and not the majority of the material that clings to the gravel.

Continue up. The mixture at the bottom holds what goes at the top, and you go like that until you reach the center.

The next day, the bridge is strong enough to walk on. The concrete is not fully cured yet, but it is strong enough. Then you can coat it and smooth it with a flat trowel to refine the surface.

Ferrocement vaults

Some time ago, Christopher Alexander's book A Pattern Language inspired my wife and myself to expand our house, and to build concrete vaults instead of flat roofs. Vaults are easy to build, don't require expensive and wasteful formwork, drain automatically, give you tall ceilings, and make rooms quite special.

It was then that I found Flying Concrete, a fantastic web page of ferrocement construction, especially vaulted structures. The idea is to build catenary vaults. A catenary is the curve that results from hanging a chain from two points; when you turn it upside down, you get the most efficient vault possible. The church of the Sagrada Familia, in Barcelona, by Antoni Gaudí, is a very tall structure built entirely out of catenary vaults and arches.

Ignore the light brown strips under the structure; originally I wanted to use flexible wooden strips instead of rebar, but got MDF by accident - and only later discovered that it got unusably soft when wet. Other than that, the structure of the vault is the same as for the little bridge: rebar and tied wire mesh.

First you can lay on a thin layer of concrete, let it harden a bit, and then lay a thicker layer on top of that. This prevents the mesh from sagging too much from the initial weight.

In the picture above you can see a larger vault alredy poured, and a similar vault in the process of being formed. The base of each of those is a square of about 4x4 meters, and the vault itself is under 1 meter tall. This requires no formwork at all; the rebar/mesh and concrete support themselves.

Free-form structures

Finally, let me show you two staircases. The vaulted arches on the first one, and the funny curve on the second one, are a thin ferrocement shell. Then the steps are poured on top of the hardened shell, each one with little wood to hold the concrete.

References

Flying Concrete is Steve Kornher's no-bullshit resource about practical construction with ferrocement. He builds vaults, staircases, and chimneys reminiscent of Antoni Gaudí's capricious forms. Last time I checked, he was experimenting with building an entire house with no "ferro" - no rebar at all, just a compression structure based on catenary curves. Based on his advice, I was able to build concrete vaults at my house.