22. PreOS

22.1. Description

With cdist you can install and configure new machines. You can use cdist to create PreOS, minimal OS whose purpose is to boot a new machine. After PreOS is booted, the machine is ready for installing the desired OS and afterwards it is ready for configuration.

22.2. PreOS creation

With cdist you can create PreOS. Currently supported PreOS-es include:

  • debian
  • ubuntu
  • devuan

PreOS is created using the cdist preos command. This command has subcommands that determine the desired PreOS.

For example, to create an ubuntu PreOS:

$ cdist preos ubuntu /preos/preos-ubuntu -B -C \
    -k ~/.ssh/id_rsa.pub -p /preos/pxe-ubuntu \
    -t "/usr/bin/curl 192.168.111.5:3000/cdist/install/"

For more info about the available options see the cdist manual page.

This will bootstrap (-B) ubuntu PreOS in /preos/preos-ubuntu directory, it will be configured (-C) using default built-in initial manifest and with specified ssh authorized key (-k) and with specified trigger command (-t). After bootstrapping and configuration PXE boot directory will be created (-p) in /preos/pxe-ubuntu.

After PreOS is created, new machines can be booted using the created PXE (after proper dhcp and tftp settings).

Since PreOS is configured with ssh authorized key it can be accessed through ssh, i.e. it can be further installed and configured with cdist.

When installing and configuring new machines using cdist's PreOS concept cdist can use triggering for host installation/configuration, which is described in the previous chapter.

When new machine is booted with PreOS then trigger command is executed. Machine will connect to cdist trigger server. If the request is, for example, for installation then cdist trigger server will start install command for the client host using parameters specified at trigger server startup.

22.3. Implementing new PreOS sub-command

preos command is implemented as a plugin system. This plugin system scans for preos subcommands in the cdist/preos/ distribution directory and also in ~/.cdist/preos/ directory if it exists.

preos subcommand is a module or a class that satisfies the following:

  • it has the attribute _cdist_preos set to True
  • it defines a function/method commandline.

For a module-based preos subcommand, the commandline function accepts a module object as its first argument and the list of command line arguments (sys.argv[2:]).

For a class-based preos subcommand commandline method should be static-method and must accept a class as its first argument and the list of command line arguments (sys.argv[2:]).

If preos scanning finds a module/class that has _cdist_preos set to True and a function/method commandline then this module/class is registered to preos subcommands. The name of the command is set to _preos_name attribute if defined in the module/class, defaulting to the module/class name in lowercase. When a registered preos subcommand is specified, commandline will be called with the first argument set to module/class and the second argument set to sys.argv[2:].

22.3.1. Example of writing new dummy preos sub-command

22.3.1.1. Module-based preos:

  1. Create directory ~/.cdist/preos/ if it does not exist
  2. Create ~/.cdist/preos/netbsd.py with the following contents:
_preos_name = 'netbsd'
_cdist_preos = True

def commandline(cls, args):
    print("NetBSD PreOS: {}".format(args))

When you try to run this new preos you will get:

$ cdist preos -L
Available PreOS-es:
    - debian
    - devuan
    - netbsd
    - ubuntu
$ cdist preos netbsd
NetBSD PreOS: []

22.3.1.2. Class based preos:

  1. Create directory ~/.cdist/preos/ if it does not exist
  2. Create ~/.cdist/preos/freebsd.py with the following contents:
class FreeBSD(object):
    _cdist_preos = True

    @classmethod
    def commandline(cls, args):
        print("FreeBSD dummy preos: {}".format(args))

When you try to run this new preos you will get:

$ cdist preos -h
Available PreOS-es:
    - debian
    - devuan
    - freebsd
    - ubuntu
$ cdist preos freebsd
FreeBSD dummy preos: []

In the commandline function/method you have all the freedom to actually create a PreOS.

22.4. Simple tipical use case for using PreOS and trigger

Tipical use case for using PreOS and trigger command include the following steps.

  1. Create PreOS PXE with ssh key and trigger command for installation.

    $ cdist preos ubuntu /preos/ubuntu -b -C \
    -k ~/.ssh/id_rsa.pub -p /preos/pxe \
    -t "/usr/bin/curl 192.168.111.5:3000/cdist/install/"

  2. Configure dhcp server and tftp server.

  3. On cdist host (192.168.111.5 from above) start trigger command (it will use default init manifest for installation).

    $ cdist trigger -b -v

  4. After all is set up start new machines (PXE boot).

  5. New machine boots and executes trigger command, i.e. triggers installation.

  6. Cdist trigger server starts installing host that has triggered it.

  7. After cdist install is finished new host is installed.