There are two flags available to aid in debugging configuration scripts. Since configuration loading issues will manifest during boot, the flags are passed as kernel boot parameters.

ISO image build

When having trouble compiling your own ISO image or debugging Jenkins issues you can follow the steps at ISO Build Issues.

System Startup

The system startup can be debugged (like loading in the configuration file from /config/config.boot. This can be achieve by extending the Kernel command-line in the bootloader.


  • vyos-debug - Adding the parameter to the linux boot line will produce timing results for the execution of scripts during commit. If one is seeing an unexpected delay during manual or boot commit, this may be useful in identifying bottlenecks. The internal flag is VYOS_DEBUG, and is found in vyatta-cfg. Output is directed to /var/log/vyatta/cfg-stdout.log.

  • vyos-config-debug - During development, coding errors can lead to a commit failure on boot, possibly resulting in a failed initialization of the CLI. In this circumstance, the kernel boot parameter vyos-config-debug will ensure access to the system as user vyos, and will log a Python stack trace to the file /tmp/boot-config-trace. File boot-config-trace will generate only if config loaded with a failure status.

Live System

A number of flags can be set up to change the behaviour of VyOS at runtime. These flags can be toggled using either environment variables or creating files.

For each feature, a file called vyos.feature.debug can be created to toggle the feature on. If a parameter is required it can be placed inside the file as its first line.

The file can be placed in /tmp for one time debugging (as the file will be removed on reboot) or placed in ‘/config’ to stay permanently.

For example, /tmp/vyos.ifconfig.debug can be created to enable interface debugging.

It is also possible to set up the debugging using environment variables. In that case, the name will be (in uppercase) VYOS_FEATURE_DEBUG.

For example running, export VYOS_IFCONFIG_DEBUG="" on your vbash, will have the same effect as touch /tmp/vyos.ifconfig.debug.

  • ifconfig - Once set, all commands used, and their responses received from the OS, will be presented on the screen for inspection.

  • command - Once set, all commands used, and their responses received from the OS, will be presented on the screen for inspection.

  • developer - Should a command fail, instead of printing a message to the user explaining how to report issues, the python interpreter will start a PBD post-mortem session to allow the developer to debug the issue. As the debugger will wait from input from the developer, it has the capacity to prevent a router to boot and therefore should only be permanently set up on production if you are ready to see the OS fail to boot.

  • log - In some rare cases, it may be useful to see what the OS is doing, including during boot. This option sends all commands used by VyOS to a file. The default file is /tmp/full-log but it can be changed.


In order to retrieve the debug output on the command-line you need to disable vyos-configd in addition. This can be run either one-time by calling sudo systemctl stop vyos-configd or make this reboot-safe by calling sudo systemctl disable vyos-configd.


Recent versions use the vyos.frr framework. The Python class is located inside our vyos-1x:python/vyos/ It comes with an embedded debugging/ (print style) debugger as vyos.ifconfig does.

To enable debugging just run: $ touch /tmp/vyos.frr.debug

Debugging Python Code with PDB

Sometimes it might be useful to debug Python code interactively on the live system rather than a IDE. This can be achieved using pdb.

Let us assume you want to debug a Python script that is called by an op-mode command. After you found the script by looking up the op-mode-defitions you can edit the script in the live system using e.g. vi: vi /usr/libexec/vyos/op_mode/

Insert the following statement right before the section where you want to investigate a problem (e.g. a statement you see in a backtrace): import pdb; pdb.set_trace() Optionally you can surrounded this statement by an if which only triggers under the condition you are interested in.

Once you run show xyz and your condition is triggered you should be dropped into the python debugger:

> /usr/libexec/vyos/op_mode/
-> rule_type = rule.get('type', '')

You can type help to get an overview of the available commands, and help command to get more information on each command.

Useful commands are:

  • examine variables using pp(var)

  • continue execution using cont

  • get a backtrace using bt

Config Migration Scripts

When writing a new configuration migrator it may happen that you see an error when you try to invoke it manually on a development system. This error will look like:

vyos@vyos:~$ /opt/vyatta/etc/config-migrate/migrate/ssh/0-to-1 /tmp/config.boot
Traceback (most recent call last):
  File "/opt/vyatta/etc/config-migrate/migrate/ssh/0-to-1", line 31, in <module>
    config = ConfigTree(config_file)
  File "/usr/lib/python3/dist-packages/vyos/", line 134, in __init__
    raise ValueError("Failed to parse config: {0}".format(msg))
ValueError: Failed to parse config: Syntax error on line 240, character 1: Invalid syntax.

The reason is that the configuration migration backend is rewritten and uses a new form of “magic string” which is applied on demand when real config migration is run on boot. When running individual migrators for testing, you need to convert the “magic string” on your own by:

vyos@vyos:~$ /usr/libexec/vyos/ --virtual --set-vintage vyos /tmp/config.boot

Configuration Error on System Boot

Being brave and running the latest rolling releases will sometimes trigger bugs due to corner cases we missed in our design. Those bugs should be filed via Phabricator but you can help us to narrow down the issue. Login to your VyOS system and change into configuration mode by typing configure. Now re-load your boot configuration by simply typing load followed by return.

You should now see a Python backtrace which will help us to handle the issue, please attach it to the Phabricator task.

Boot Timing

During the migration and extensive rewrite of functionality from Perl into Python a significant increase in the overall system boottime was noticed. The system boot time can be analysed and a graph can be generated in the end which shows in detail who called whom during the system startup phase.

This is done by utilizing the systemd-bootchart package which is now installed by default on the VyOS 1.3 (equuleus) branch. The configuration is also versioned so we get comparable results. systemd-bootchart is configured using this file: bootchart.conf

To enable boot time graphing change the Kernel commandline and add the following string: init=/usr/lib/systemd/systemd-bootchart

This can also be done permanently by changing /boot/grub/grub.cfg.


VyOS CLI is all about priorities. Every CLI node has a corresponding node.def file and possibly an attached script that is executed when the node is present. Nodes can have a priority, and on system bootup - or any other commit to the config all scripts are executed from lowest to highest priority. This is good as this gives a deterministic behavior.

To debug issues in priorities or to see what’s going on in the background you can use the /opt/vyatta/sbin/ script which lists to you the execution order of the scripts.