## Create an Unscanable Letter

Some time ago I’ve heard about the EURion constellation. Never heard about it? Has nothing to do with stars or astrology. It’s the thing on your money! :)

Take a closer look at your bills and you’ll discover plenty of EURions, as shown in the picture on the right. Just a few inconspicuous dots. So what’s it all about? The EURion constellation is a pattern to be recognized by imaging software, so that it can recognize banknotes. It was invented to prevent people from copying money :)

But I don’t know of any law that prohibits using that EURion, so I’ve been playing around with it. Took me some trials to find the optimal size, but I was able to create a $LaTeX$ document that includes the EURion. That’s the essential tex code:

The whole $LaTeX$ environment can be found on GitHub, together with the EURion image and stuff. I also provide the resulting letter.

Of course I immediately asked some friends to try to scan the letter, but it turns out, that not all scanners/printers are aware of the EURion… So it’s a bit disappointing, but I learned another thing. Good good. And to be honest, I do not have a good use case. Why should I prevent someone from printing my letters? Maybe photographers can use the EURion in their images. Copyright bullshit or something…

## Monitoring of XOS devices

This week I developed some plugins for Nagios/Icinga to monitor network devices of the vendor Extreme Networks. All these plugins receive status information of, eg. switches, via SNMP.

## The Basic: Check Mem, CPU, and Fans

Checking for available memory, for the device’s temperature, for the power supplies, and for fan states is quite straight forward. You just ask the switch for the values of a few OIDs, evaluate the answer, and tell Nagios/Icinga what to do.

The Simple Network Management Protocol (SNMP) is actually a very easy to use protocol. There is an SNMP server, such as a router or a switch, which exposes management data through the SNMP protocol. To access these data you just send an object identify (OID) to an SNMP server and receive the corresponding value. So called management information bases (MIB) can tell you what a certain OID stands for.

On the command line, for example, you could use snmpwalk to iterate over an OID subtree to, e.g., obtain information about the memory on a device:

usr@srv $snmpwalk -v 2c -c publicCommunityString switch.address.com 1.3.6.1.4.1.1916.1.32.2.2.1 1.3.6.1.4.1.1916.1.32.2.2.1.1.1 = Gauge32: 1 1.3.6.1.4.1.1916.1.32.2.2.1.2.1 = STRING: "262144" 1.3.6.1.4.1.1916.1.32.2.2.1.3.1 = STRING: "116268" 1.3.6.1.4.1.1916.1.32.2.2.1.4.1 = STRING: "7504" 1.3.6.1.4.1.1916.1.32.2.2.1.5.1 = STRING: "138372"  The OID 1.3.6.1.4.1.1916.1.32.2.2.1 addresses the memory information table of the SNMP provider at switch.address.com. The value at *.2.1 shows how much memory is installed, *.3.1 shows how much memory is free, *.4.1 shows how much is consumed by the system, and *.5.1 shows how much is consumed by user processes. Basic calculations tell us there are 262144/1024 = 256KB in total and 100*116268/262144 = 44.35% is free. A bit more logic for a warning/critical switch and the plugin is done. ## The Feature: Monitoring of the FDB But I would probably not write about that basic stuff if there was not an extra feature! I implemented a script to also monitor the FDB. FDB is and abbreviation for forwarding databases: The switch maintains a forwarding database (FDB) of all MAC addresses received on all of its ports. It, for example, uses the information in this database to decide whether a frame should be forwarded or filtered. Each entry consists of • the MAC address of the device behind the port • the associated VLAN • the age of the entry – depending on the configuration the entries age out of the table • some flags – e.g. is the entry dynamic or static • the port The table may look like the following: > show fdb Mac Vlan Age Flags Port / Virtual Port List ------------------------------------------------------------------------------ 01:23:45:67:89:ab worknet(0060) 0056 n m 9 01:23:42:67:89:ab mobnet(0040) 0001 n m 21 Flags : d - Dynamic, s - Static, p - Permanent, n - NetLogin, m - MAC, i - IP, x - IPX, l - lockdown MAC, L - lockdown-timeout MAC, M- Mirror, B - Egress Blackhole, b - Ingress Blackhole, v - MAC-Based VLAN, P - Private VLAN, T - VLAN translation, D - drop packet, h - Hardware Aging, o - IEEE 802.1ah Backbone MAC, S - Software Controlled Deletion, r - MSRP  As soon as the switch gets a frame on one port it learns the corresponding MAC address, port number, etc. into this table. So if a frame for this MAC address arrives it know where to send it to. However, that content of a networking class. All we need to know is that a switch can tell you which device which MAC address is is connected to which port. And that’s the idea of check_extreme_fdb.pl! It compares the entries of the FDB with some expected entries in an CSV file. The CSV is supposed to contain three coloumns: mac,port,vlan  If a MAC address in the FDB matches the MAC address in the CSV file it checks the ports and vlans. If those do not match, it will raise an error. For the CSV: Feel free to leave port or vlan empty if you do not care about this detail. That means, if you just want to make sure that the device with the MAC 01:23:45:67:89:ab is in vlan worknet you add an entry such as: 01:23:45:67:89:ab,,worknet  Use -e <FILE> to pass the CSV file containing expected entry to the program and call it like beckham: perl -w check_extreme_fdb.pl -s <SWITCH> -C <COMMUNITY-STRING> -e <EXPECTED>  Here, SWITCH being the switch’s address and COMMUNITY-STRING beeing the SNMP “passphrase”. You may also want to add -w to raise a warning if one of the entries in the CSV file wasn’t found in the FDB. To create a sample CSV file that matches the current FDB you can call it with --print. To get the script have a look at the check_extreme_fdb.pl software page. ## More Extreme Stuff In addition there are some other scripts to monitor Extreme Networks devices: ## Do I have a CD-RW? You don’t know whether the CD drive on your machine is able to burn CDs? And too lazy to go off with your head under your table? Or you’re remote on the machine? Then that’s your command line: $ cat /proc/sys/dev/cdrom/info
CD-ROM information, Id: cdrom.c 3.20 2003/12/17

drive name:             sr0
drive speed:            32
drive # of slots:       1
Can close tray:         1
Can open tray:          1
Can lock tray:          1
Can change speed:       1
Can select disk:        0
Reports media changed:  1
Can play audio:         1
Can write CD-R:         1
Can write CD-RW:        1
Can write DVD-R:        1
Can write DVD-RAM:      1
Can write MRW:          1
Can write RAM:          1


Works on Debian based systems :)

## Docker Jail for Skype

As I’m now permanently installed at our University (yeah!) I probably need to use skype more often than desired. However, I still try to avoid proprietary software, and skype is the worst of all. Skype is an

obfuscated malicious binary blob with network capabilities

as jvoisin beautifully put into words. I came in contact with skype multiple times and it was always a mess. Ok, but what are the options if I need skype? So far I’ve been using a virtual box if I needed to call somebody who insisted on using skype, but now that I’ll be using skype more often I need an alternative to running a second OS on my machine. My friend Tom meant to make a joke about using Docker and … TA-DAH! … Turns out it’s actually possible to jail a usable skype inside a Docker container! Guided by jvoisin’s article Running Skype in docker I created my own setup:

# The Dockerfile

The Dockerfile is available from the skype-on-docker project on GitHub. Just clone the project and change into the directory:

$git clone https://github.com/binfalse/skype-on-docker.git$ cd skype-on-docker
$ls -l total 12 -rw-r--r-- 1 martin martin 32 Jan 4 17:26 authorized_keys -rw-r--r-- 1 martin martin 1144 Jan 4 17:26 Dockerfile -rw-r--r-- 1 martin martin 729 Jan 4 17:26 README.md  The Docker image is based on a Debian:stable. It will install an OpenSSH server (it exposes 22) and download the skype binaries. It will also install the authorized_keys file in the home directories of root and the unprivileged user. Thus, to be able to connect to the container you need to copy your public SSH key into that file: $ cat ~/.ssh/id_rsa.pub >> authorized_keys


Good so far? Ok, then go for it! Build a docker image:

$docker build -t binfalse/skype .  This might take a while. Docker will execute the commands given in the Dockerfile and create a new Docker image with the name binfalse/skype. Feel free to choose a different name.. As soon as that’s finished you can instantiate and run a new container using: $ docker run -d -p 127.0.0.1:55757:22 --name skype_container binfalse/skype


This will start the container as a daemon (-d) with the name skype_container (--name skype_container) and the host’s port 55757 mapped to the container’s port 22 (-p 127.0.0.1:55757:22). Give it a millisecond to come up and then you should be able to connect to that container via ssh. From that shell you should be able to start an configure skype:

$ssh -X -p 55555 docker@127.0.0.1 The programs included with the Debian GNU/Linux system are free software; the exact distribution terms for each program are described in the individual files in /usr/share/doc/*/copyright. Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent permitted by applicable law. Last login: Mon Jan 4 23:07:37 2016 from 172.17.42.1$ skype


You can immediately go and do your chats and stuff, but you can also just configure skype. Do setup everything just like you want to find it when starting skype, for example tick the auto-login button to get rid of the login screen etc. As soon as that’s done, commit the changes to build a new image reflecting your preferences:

$docker commit skype_container binfalse/deb-skype  Now you’ll have an image called binfalse/deb-skype that contains a fully configured skype installation. Just kill the other container: $ docker stop skype_container
\$ docker rm skype_container


And now your typical workflow might look like:

docker run -d -p 127.0.0.1:55757:22 --name skype__ binfalse/deb-skype
sleep 1
ssh -X -p 55757 docker@127.0.0.1 skype && docker rm -f skype__


Feel free to cast it in a mould just as I did. The script is also available from my apt repo, it’s name is bf-skype-on-docker:

echo "deb http://apt.binfalse.de binfalse main" > /etc/apt/sources.list.d/binfalse.list
apt-get update && apt-get install bf-skype-on-docker


## Getting into a new group

You know, … you just got this new floppy disk with very important material but you cannot access it because you’re not in the system’s floppy group and, thus, you’re not allowed to access the floppy device. Solution is easy: add your current user to the floppy group! Sounds easy, doesn’t it? The annoying thing is that those changes won’t take affect in the current session. You need to log out and log in again – quite annoying, especially if you’re into something with lots of windows and stuff. Just happened to me with docker again..

However, there are two methods to get into the new groups without the need to kill the current session:

• su yourself: let’s say your username is myname you just need to su myname to get a prompt with the new group memberships.
• ssh localhost: that also gives you a new session with updated affiliations.

That way, you do not need to start a new session. However, you still need to start all applications/tools from that terminal - might be odd to those who are used to the gnome/kde menues.. :)

## Supplemental material

Display group membership:

 groups USERNAME


 groupadd GROUPNAME


Add a user to a group:

 useradd -G GROUPNAME USERNAME


## CyanogenMod Updates and the firewall

I’m running CyanogenMod on my phone and I have the firewall AFWall+ installed.

To update the list of available updates the application CM Updater need to be able to connect to the internet. It will contact a website from within the google empire and ask for available images.

To be remembered.

• The first sentence of each doc comment should be a summary sentence
• Write the description to be implementation-independent, but specifying such dependencies where necessary. (what is required, what is allowed)
• wrap keywords in <code>...</code>

## typical meta annotations

• @author is not critical, because it is not included when generating the API specification
• @version SCCS string "%I%, %G%", which converts to something like “ 1.39, 02/28/97” (mm/dd/yy) when the file is checked out of SCCS
• @since specify the product version when the Java name was added to the API specification (if different from the implementation)

## typical method definition

• @param parameter-name description
• followed by the name (not data type) of the parameter, followed by a description of the parameter
• the first noun in the description is the data type
• @return description
• omit for methods that return void and for constructors
• include it for all other methods, even if its content is entirely redundant with the method description
• @throws class-name description should be included for any checked exceptions (previously, it was @exception)

• {@value package.class#field}
• when {@value} is used (without any argument) in the doc comment of a static field, it displays the value of that constant: The value of this constant is {@value}.
• otherwise it displays the value of the specified constant: Evaluates the script starting with {@value #SCRIPT_START}.
• {@code text}
• Equivalent to <code>{@literal}</code>.

• @see reference
• Adds a "See Also" heading
• reference is a string: Adds a text entry for string. No link is generated. The string is a book or other reference to information not available by URL.
• reference is an <a href="URL#value">label</a>: Adds a link as defined by URL#value. The URL#value is a relative or absolute URL.
• reference is an package.class#member label: Adds a link (with optional visible text label) that points to the documentation for the specified name in the Java Language that is referenced.
• {@link package.class#member label}
• in-line link with visible text label that points to the documentation for the specified package, class or member name of a referenced class
• not necessary to add links for all API names in a doc comment
• @serial (or @serialField or @serialData) for interoperability with alternative implementations of a Serializable class and to document class evolution

## deprecation

• @deprecated deprecated-text
• the first sentence should at least tell the user when the API was deprecated and what to use as a replacement
• a {@link} tag should be included that points to the replacement method

# References

## Block URLs on phone using AdAway

Using AdAway on your Android phones it is very easy to block a set of URLs.

AdAway (available from F-Droid, source from GitHub) is an ad-blocker. The application basically maintains a list of resources containing lists to ad-servers, see figure for example, e.g.:

# dest ip   url


This entry will redirect all traffic meant for some.ad.server.com to the server 127.0.0.1, which is your localhost. Entries from different resources are merged. Thus, all requests to typical ad-servers will fail → annoying advertisements won’t be delivered. It’s quite powerful and gives impressive results. However, I didn’t want to talk about ad-stuff. The cool thing is

## AdAway allows for extensions with own hosts files!

You can simply add another link to a file containing further host entries. I created my own AdAway file, uploaded it to https://stuff.lesscomplex.org/adaway.txt and added it to AdAway, as you can see in the figure. The current version contains a few entries for:

• facebook/yahoo/etc: I’m not using facebook or stuff, all these request shouldn’t leave my phone
• captive portal detection: To detect captive portals the phone downloads a few bytes from a google server and checks if it is able to access the real internet or just a captive portal
• swift key: I’m using swift key, but do not want it to communicate to the internet. Thus, I’m blocking a few URLs that swiftkey wants to talk to. As I’m already using a firewall this just adds another layer of privacy.

Feel free to use my “extension”, but I expect it to change over time. :)

## Shortcomings

This method only works if applications load contents from URLs. As soon as the IPs are hard-coded the hosts file

## Disable shutter sound on Cyanogenmod 12

Everyone knows that annoying shutter sound of the camera app on Android phones. It’s against the law to sell android phones which do not make sounds when taking pictures. And in general, it is a good feature as it improves other people privacies.

However, I still want to get rid of the sound. It’s a bit tricky, but having a rooted phone (with e.g. CyanogenMod) it’s very easy: Just delete the sound file /system/media/audio/ui/camera_click.ogg! :)

For the lazy: Get a root shell (eg. ssh or adb) and execute the following:

Problem solved.

## Kile menu bar hides entries

Kile, KDE’s Integrated LaTeX Environment, has a weird bug: Every time I update something in the UI it recreates its config file and I loose some menu options, such as Settings -> Configure Kile where you used to configure your preferences..

In Figure 1 you can see the menu as expected. There are some entries to Configure Kile, t0 Configure Toolbars, to Configure Shortcuts and to switch to Full Screen Mode, etc. However, as soon as I update certain things in the user interface (UI), eg. if I add a new action icon to the toolbar to quickly get the \textbf{} environment for bold fonts, these entries get lost. In those cases Kile won’t be configureable anymore. The resulting toolbar is shown in Figure 2. You see, the number of entries significantly decreased..

However, I just discovered the reason: Kile in these cases rewrites its config in an unexpected manner… If you have a look at the configuration stored in ~/.kde/share/apps/kile/kileui.rc you’ll find an XML subtree such as:

No idea where it comes from, but the 1 in noMerge prevents the default menu entries to be merged into the minimal set of entries defined in that snippet. However, now that we know what’s wrong we can easily fix it! Just replace the 1 with a 0 and restart Kile. You might want to do the same for all other menus to discover that you’ve also been missing some entries in the Help menu ;-)