Blog Articles

Reminders with VoIP Phones, Asterisk & Crontab


Update: As it turns out, this solution causes people to get used to the ringing phone, causing not only the reminder to be ignored, but actual incoming calls as well. We have thus stopped using this solution.

It is considered best practice to regularly and often ventilate closed rooms due to SARS-CoV-2. But how can we make sure we remember to open the windows every now and then? We wanted to build some kind of visual or acoustic mechanism that would tell us when to open the windows.

I came up with the idea to abuse our already existing telephone system as a reminder system. This system consists of a central Asterisk server, and multiple SIP phones. The idea was to have Asterisk dial all the SIP phones, and set the reminder text as caller ID. The "secret" behind this is Asterisk's console calling feature. And here is how it's done:

First, we need to enable the chan_alsa module in Asterisk's /etc/asterisk/modules.conf:

load =>
; Comment out existing noload directives
;noload =>

Afterwards restart Asterisk.

In a default installation of Asterisk on Debian, this should be all that's required for console calling to work. In our case, Asterisk is running in a VM and the default audio device is a virtual sound card not used otherwise, so there was no need for additional configuration.

Next, we can configure the dialplan in /etc/asterisk/extensions.conf for our reminders:

exten => ventilate,1,NoOp()
same => n,Answer()
; Set the caller ID name to the reminder text
same => n,Set(CALLERID(name)=Lueften!)
; Dial all the phones
same => n,Dial(PJSIP/phone1&PJSIP/phone2)

To apply this change, we have to reload the dialplan:

# asterisk -rx 'dialplan reload'

We can then test our reminder by telling Asterisk to originate a call from its console context:

# asterisk -rx 'originate console/dsp extension ventilate@reminders'

Now your phones should start ringing, and display your reminder text as the caller ID.

Finally, in order to schedule the reminders, just put them into the crontab of the Asterisk server. In our case, I configured it to ring every full hour on Tuesdays between 8 and 11 PM:

0 20-23 * * 2 root /usr/sbin/asterisk -rx 'originate console/dsp extension ventilate@reminders'

Cisco 7900 series IP Phone Logo Converter


Last week we discovered a box full of old Cisco 7900 series IP phones hidden deep in a pile of boxes in our hackerspace. Of course we tried to get them up and running and figure out how to configure them.

After a bit of research and reading manuals, we learned that the phones can be configured via TFTP using a binary config file format. When we learned that the phones can display a custom 88x27 monochrome image instead of the default Cisco logo, we made it our top priority to get the image customization working.

What we learned was that the images had to be served alongside the configuration via TFTP in a proprietary format. The tool to convert images into this format comes bundled with every firmware release for the phone. However, we quickly encountered some limitations:

  • The converter, called bmp2logo.exe, was a proprietary, Windows only piece of software. Luckily, the tool ran in Wine without any problems (as long as you don't mess with the input file header).
  • The tool required the input file to be in a very specific format, namely a 88x27 pixel, monochrome, 1 bit per pixel BMP file.
  • While the image is drawn as dark pixels on a light background, the input file had to be white on black. Effectively, the colors were inverted.

We also learned that each image had to be tagged with a serial number. The phone uses this serial number to decide whether a new image has to be loaded from the TFTP server. If the serial number is the same as the number of the previous image, the new image is not loaded. So the serial number needs to be incremented by at least 1 for each new image.

To get around the limitations of the converter tool, I attempted to figure out the file format and write a free and cross-platform converter without these limitations.

Reverse Engineering the File Format

So let's have a look at a hexdump of one of those image files converted with bmp2logo.exe:

Hexdump of a converted image file

We know that the image consists of 88 x 27 = 2376 pixels. And since the input to bmp2logo.exe has to be a 1-bit-per-pixel uncompressed image, let's just assume that the same holds true for the output. This would give us a payload size of 297 bytes, so with a file size of 304 bytes, there should be a 11 byte header.

The first two bytes were always the same, no matter what image was converted. So it should be fairly safe to assume that they are a magic number. The next two bytes were always different for different serial numbers or different images. Also, if the serial number was changed by one, these two bytes would also change in a fairly consistent manner, so this appeared to be some some sort of checksum. Let's just ignore that for now and put it aside for later.

The next three bytes were all zeros, followed by a byte representing the serial number. Or so i thought at first, until I passed a serial number of -1 to bmp2logo.exe, and got ff ff ff ff as these four bytes, so this pretty clearly is a 32-bit representation of this number. The next two bytes were pretty obvious as well, they are the height and width of the image, 27 and 88 respectively.

The last byte of the header seems to represent the number of bytes that comprise a single row. I'm not entirely sure about this, but it made the most sense, especially if we assume that there are other phones out there which may support grayscale images, and need a greater color depth and consequently more bits per row.

Now, on to the last part: The checksum. I was a bit lost here, so I just tried various ways of compressing data into 16 bits: Adding them, xor'ing them, swapping bytes around before adding them, always wrapping the results to 16 bits. In the end it was a typo that brought me to the solution: Instead of sum = (sum + swapped) & 0xffff, I wrote sum = (sum + swapped) % 0xffff. So instead of performing addition mod 65536, i was performing addition mod 65535 by accident. The result of this turned out to be just the binary inverse of the intended checksum, so let's add a final sum ^= 0xffff before returning the result and call it a day.

File Format

With all that information, and verifying that the payload did indeed match the uncompressed monochrome image, we finally got our file format:

| 10h 60h |  CHKSUM |       SERIAL      |
| H  | W  | RW |                        |
+----+----+----+                        |
|                                       |
:               PAYLOAD                 :
:                                       :
|                                       |
|                                       |

To summarize, the file consists of an 11 byte header, followed by the payload. The header consists of:

  1. The magic number 0x10 0x60
  2. A 16 bit checksum
  3. The 32 bit serial number in big endian
  4. H, the height of the image in pixels, 1 byte. Since the image is 27px high, this is 27, or 0x1b
  5. W, the width of the image in pixels, 1 byte. Since the image is 88px wide, this is 88, or 0x58
  6. RW, the number of bytes in a single row, 1 byte. Since the image is 88px wide and each byte holds 8 pixels at once, this is 11, or 0x0b

Following the header is the image payload, with each pixel expressed as a single bit, in row-major order starting in the top left corner.

A Python implementation of the checksum algorithm could like this:

sum = 0
# Iterate the file starting at byte 4 (directly after the checksum)
# Iterate two bytes at a time (a and b), stick them together in
# reverse order, and add to the total sum mod 0xffff.
for a, b in zip(data[4::2], data[5:2]):
    sum = (sum + ((b << 8) | a)) % 0xffff
# Flip all bits in the result
sum ^= 0xffff


Now that we know how such an image file is composed, we can write a tool to generate these files. And here's the result:

Cisco 7900 phone screen with a cat instead of the Cisco

You can find the converter script on Gitlab. It can be used in exactly the same way as the original bmp2logo.exe converter:

  1. ./ <serial> <infile.something> <outfile.dat>
  2. Copy <outfile.dat> to your TFTP server
  3. Update the phone configuration to point to the new file
  4. Recompile the configuration file and copy it to your TFTP server
  5. Reboot your phone

Remember to increment the serial number every time you change the image, or the phone won't pick up the new file.

Scaling and Slicing PDF Documents with pdfjam and mutool


I recently encountered the following challenge: I had a PDF document consisting of multiple A4 pages, each of which I needed to print scaled up to A2. However, I only had an A4 printer available, and the printer driver was not able to perform the scaling and/or slicing on its own.

After a long search, and lots of disappointments (since neither of Libre Office, Inkscape and Evince could do what I wanted, or I simply couldn't find the feature), I came up with the following solution:

$ pdfjam -o scaled.pdf --a2paper input.pdf
$ mutool poster -x 2 -y 2 scaled.pdf sliced.pdf

The first command takes the input file, scales it up to A2 size, and writes it to an intermediate file. The second command slices each page from the intermediate file into 2 slices both vertically and horizontally, totaling in 4 slices per page, which again results in A4-sized slices the printer can print.

Automated Debian package building with Gitlab CI and Reprepro


I previously deployed some ad-hoc services using ugly and difficult to maintain solutions, such as binaries manually extracted from container images and copied into /usr/local/bin. Of course, this is a lot of manual work for each upgrade of the service, and if you upgrade often, a lot of repetitive manual work. Or, in other words, the perfect kind of work to be automated.

So I decided to package the software in question for Debian, which is the Linux distribution I run these services on, and to automate the process of building the packages and publishing them to a repository. There's three different "categories" of software I wanted to have in my repository:

  • Binaries published as build artifacts in releases of their upstream repository. An example for this is Gitea. The binaries can be automatically downloaded and put into a package.

  • Software released as container images only. An example for this is the Drone CI Server. This is a bit more tricky, as the binaries must be extracted from the image before it can be put into a package.

  • Software already released as Debian packages, but not available through a repository. An example for this is a project of my own, the iCalendar Timeseries Server, where the CI pipeline automatically builds Debian packages and adds them as build artifacts to releases. Here, the already-built package just needs to be fetched and added to the repository.

For creating and maintaining the repository, I'm using Reprepro, as it is extremely simple to use, and generally is rather uncomplex and lightweight. It does however come with some limitations; I especially encountered the problem that Reprepro will only keep the latest version of a package in the repo index, so older versions won't be available to clients.

For automating the packaging and repository build process, I'm using Gitlab CI. The pipeline is running each night, building the latest stable version for each package, adding the packages to the in-container Reprepro repository, and then synchronize the repository to a web server of mine.

The source repository with the package build scripts and pipeline configuration is available on Gitlab. Though, before using this as a template, be advised that the packages built by this pipeline are not exceptionally high-quality. They are also usually built to fit my personal needs, so don't expect ready-to-use packages.

Recording IPTV Using ffmpeg


I don't usually watch TV. But from time to time there is something interesting on the programme, such as debates on local politics. Unfortunately, those usually run at a time of day where I'm not able (or more likely not willing) to tune in an pay attention to an hour of political discourse. So I want to record them and watch later instead.

My ISP provides its IPTV programme as MPEG-TS streams via multicast UDP. And they even link a M3U playlist of all stations on their website, so you can basically watch TV with any client whatsover, as long as it speaks IGMP and understands MPEG-TS video streams. This makes recording very easy, as this is supported by a lot of multimedia processing software, including ffmpeg.

The playlist consists of a list of TV stations, each of which is represented by its own multicast group and an UDP port. So let's just take the first station and see what ffmpeg finds in there:

Input #0, mpegts, from 'udp://':
  Duration: N/A, start: 41892.675600, bitrate: N/A
  Program 9038 
      service_name    : SRF 1 HD
      service_provider: Schweizer Radio und Fernsehen
    Stream #0:0[0x50]: Video: h264 (High) ([27][0][0][0] / 0x001B), yuv420p(tv, bt709, progressive), 1280x720 [SAR 1:1 DAR 16:9], 50 fps, 50 tbr, 90k tbn, 100 tbc
    Stream #0:1[0x51](deu): Audio: mp2 ([3][0][0][0] / 0x0003), 48000 Hz, stereo, fltp, 192 kb/s (clean effects)
    Stream #0:2[0x52](eng): Audio: mp2 ([3][0][0][0] / 0x0003), 48000 Hz, stereo, fltp, 192 kb/s (clean effects)
    Stream #0:3[0x5b](deu): Audio: ac3 ([6][0][0][0] / 0x0006), 48000 Hz, 5.1(side), fltp, 448 kb/s (clean effects)
    Stream #0:4[0x6e](deu,deu): Subtitle: dvb_teletext ([6][0][0][0] / 0x0006)
    Stream #0:5[0x70]: Unknown: none ([5][0][0][0] / 0x0005)
    Stream #0:6[0x72]: Unknown: none ([12][0][0][0] / 0x000C)

We can see that the MPEG-TS stream contains multiple indiviual streams, which are listed in the output above. Now, I don't know what's up with 0:5 and 0:6, or why ffmpeg doesn't understand them. Anyway, I only need the video and one audio channel. Let's just pick the first two, and record an one hour TV show:

ffmpeg -f mpegts -i udp:// -map 0:0 -map 0:1 -c copy -t 3600 recording.mkv

To break it down:

  • -f mpegts tells ffmpeg that the input is an MPEG transport stream.
  • -i udp:// tells ffmpeg to join the specified multicast group and receive the MPEG-TS stream on UDP port 5000.
  • -map 0:0 -map 0:1 only extracts the streams 0:0 (the H.264 video stream) and 0:1 (the German audio channel in MP2)
  • -c copy causes the input stream to be demuxed only, and the selected streams to be written to the output without CPU-intensive decoding and reencoding.
  • -t 3600 terminates the stream after one hour, when the show is over.
  • recording.mkv is the output filename. The container format (here MKV) is deduced from the filename.

So the whole ffmpeg command takes the original stream as input, demultiplexes it to get the individual media streams, then throws out all but one video and one audio stream, multiplexes them into a Matroska container, which is then written to disk.