Traversal of the Netfilter Code

The following ascii image describes the way a packet takes when traversing the netfilter firewalling code inside the linux kernels 2.4 and 2.6. It is followed by a description of the visualized paths.

Stateful Filtering with Netfilter

When implementing security policies with a packet filter you will usually have to specify rules for each and every packet that you wish to handle. This will result in a myriad of rules that are hard to maintain. Still, you might be able to prolong choas by distributing rule definitions among several files and cascading chains of similar rules. Though, there is an easier way.

Remote Commands

SSH is most commonly known for its ability to login to remote hosts. But it also allows the execution of commands on the remote host without a prior login although it still requires successful authentication (otherwise, there would be no sense in using SSH, right?!):


A series of measurements $x_i = x_1, \dots, x_n$ is a one dimensional list or array which is by nature very space inefficient to store. A histogram is a two dimensional data structure that can be configured to a custom trade off between space and accuracy. The values are sorted into bucket according to their size.


When comparing two or more series of measurements, it is useful have a relative measure like the distribution of values.


A mean value is only valid for the exact series of measurements that it was calculated from. When adding one or more measurements to the series, the mean value needs to be calculated again and probably result in a similar but not the same mean value. The reason for this is the fact that the final mean value can only be calculated from an infinite number of measurements but this is apparently impossible to do.

Statistical Terms and their German Translation

The following table provides a list of well-known german term and their english translation:

Private Perl Instance

Imagine you intend to give threaded programming in perl a try. You do not want to update the system-wide perl installation because many perl modules are not thread-safe. A private perl installation is the only viable solution. After unpacking the perl distribution and changing into the newly created directory executethe following commands:


Subversion supports some keywords which it is able to substitute in a versioned file though there are two things you need to do:

Multi-Line svn:ignore

The property svn:ignore may contain new-line separated patterns which describe filenames to ignore for repository actions. Specifying a single pattern is rather straight forward.