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Using AllStarLink/app_rpt with Cloud Kernels

Many virtual hosts or VPS providers leverage the Debian "cloud kernel" version of the Debian Linux OS. These providers include Amazon AWS, Microsoft Azure, Linode, Vultr, DreamHost, and DigitalOcean. Using their stock Debian 12 installation image comes with the "cloud kernel" installed. While this kernel is usually beneficial due to its smaller footprint than the full kernel through the removal of hardware elements, the Asterisk app_rpt module requires the support of these elements.

Diagnosing a DAHDI Kernel Issue

Problems with Asterisk/app_rpt related to the DAHDI module being unavailable can be see by the following log messages at boot:

[2024-12-10 15:09:19.780] ERROR[930155] loader.c: res_timing_dahdi declined to load.
[2024-12-10 15:09:19.818] NOTICE[930201] app_rpt.c: Normal Repeater Init  63001
[2024-12-10 15:09:19.818] WARNING[930220][C-00000001] chan_dahdi.c: Unable to open '/dev/dahdi/pseudo': No such file or directory
[2024-12-10 15:09:19.818] ERROR[930220][C-00000001] chan_dahdi.c: Unable to dup channel: No such file or directory
[2024-12-10 15:09:19.818] ERROR[930220] app_rpt/rpt_bridging.c: Failed to request dahdi/pseudo
[2024-12-10 15:09:19.818] WARNING[930220] app_rpt.c: Disabled broken repeater 63001

This appears in /var/log/asterisk/messages.log not long after Asterisk begins to start.

Converting from Cloud to Normal Kernel

After installing a Debian Linux 12 image that is using a cloud kernel, the following procedure can be used to change the OS to use the normal kernel that will allow the building of the DAHDI module needed for app_rpt.

  • Become root by doing sudo -s.

  • Check the running kernel of the system to see if it is the reduced-footprint "cloud" version using uname -a:

    # uname -a
Linux localhost 6.1.0-28-cloud-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.119-1 (2024-11-22) x86_64 GNU/Linux

    If the name of the kernel includes the phrase "cloud", this is the reduced-sized cloud kernel that removes all of the hardware drivers not relevant to a cloud VM. Unfortunately, use of Asterisk's DAHDI module requires the hardware-drivers-enabled kernel.

  • Install the full kernel on the host:

    apt install linux-image-amd64

  • Remove the cloud kernels:

    apt remove -y ` dpkg -l | grep -E '^ii\s+linux' | grep cloud | awk '{print $2}'`

    A box will appear warning that the running kernel is being removed. The default prompt is to abandon the process with "yes". Select "no" to remove the kernel.

  • Reboot the system

  • After reboot, check the running kernel with the command uname -a. It should now be running the "non cloud" version of the kernel:

    # uname -a
Linux localhost 6.1.0-28-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.119-1 (2024-11-22) x86_64 GNU/Linux

  • Note: depending on your platform type you may need to create a MOK for kernel modules as documented at UEFI SecureBoot. Test the boot type as follows:

    [ -d /sys/firmware/efi ] && echo UEFI || echo BIOS

    It will print "UEFI" for SecureBoot/UEFI systems that need MOKs or "BIOS" for everything else and does not need an MOK.

  • Now ensure that the development headers are installed:

    apt install -y linux-headers-$(uname -r)

  • (Re)install the dahdi-dkms kernel so it will build the modules:

    apt install --reinstall -y dahdi-dkms

    The installation should include something that looks like this:

    Setting up dahdi-dkms (1:3.4.0-4+asl) ...
Loading new dahdi-3.4.0 DKMS files...
Building for 6.1.0-28-amd64
Building initial module for 6.1.0-28-amd64
Done.

dahdi_dummy.ko:
Running module version sanity check.
- Original module
- No original module exists within this kernel
- Installation
- Installing to /lib/modules/6.1.0-28-amd64/updates/dkms/

dahdi.ko:
Running module version sanity check.
- Original module
- No original module exists within this kernel
- Installation
- Installing to /lib/modules/6.1.0-28-amd64/updates/dkms/

dahdi_transcode.ko:
Running module version sanity check.
- Original module
- No original module exists within this kernel
- Installation
- Installing to /lib/modules/6.1.0-28-amd64/updates/dkms/
depmod...

  • Load the dahdi module:

    modprobe dahdi

  • Check that the module is loaded with lsmod | grep dahdi:

    # lsmod | grep dahdi
 dahdi                 258048  0

Now setup of ASL3/app_rpt should be able to continue.