kea-hook-runscript/README.md

About kea-hook-runscript

This a hook for the Kea DHCP server that allows to run an external script at various points in the processing of DHCP requests and responses.

The goal is to simplify integration with Kea: for many simple use-cases, it is overkill to have to write a full-blown Kea hook, where a simple shell script can do the job.

What it can and can't do

Integration is mostly done one-way: thanks to this hook, Kea passes information to the external script, but the script cannot easily modify Kea's behaviour.

The external script can be any kind of executable program, but often it will be a simple script (shell, Perl, Python...). Information about what Kea is doing is provided to the external script through environment variables: MAC address of the requesting DHCP client, IP address being handed out, etc.

Each time Kea encounters a hook point, it will call the script synchronously. That is, Kea will do absolutely nothing else while the script is running. Thus, it is a good idea to perform only lightweight processing in the script, and absolutely avoid blocking operations. Also, scripting languages that need to initialise a huge interpreter (such as Python or Ruby) will cause a large amount of CPU usage and a massive slowdown of Kea, because the script is run multiple times for each DHCP transaction.

This hook works for both DHCPv4 and DHCPv6.

In the future, the hook will possibly feed the return code of the external script back into Kea. This would allow the external script to cancel part of Kea's normal processing (for instance, it could be possible to easily implement a flexible host blacklist this way).

Use-cases

Given the limitations exposed above, here are some example use-cases for which this hook is well-suited:

• add/remove routing entries when DHCP clients arrive or leave. This can be useful when handing out IPv4 addressing in /32 subnets, or IPv6 Prefix Delegation with DHCPv6-PD;
• update firewall rules to allow/refuse access to new DHCP clients;
• log information about successful leases.

For more complex use-cases, including non-trivial changes to Kea's behaviour, it may be easier to just write a Kea hook yourself.

How to use this hook

You first need to compile the hook. For this, you need Kea development headers installed. It has been developed with Kea 1.2 but should work with later version.

To build, simply run:

$ make

Kea 1.2 is missing a header file, so depending on your distribution, you may need to manually copy option6_pdexclude.h from the Kea git repository to /usr/include/kea/dhcp/.

If all goes well, you should obtain a kea-hook-runscript.so file. Then, here is how to tell Kea to use this hook, for DHCPv4:

{
"Dhcp4":
{
  "hooks-libraries": [
    {
      "library": "/path/to/hea-hook-runscript/kea-hook-runscript.so",
      "parameters": {
        "script": "/path/to/myscript.sh"
      }
    }
  ],
  ...
}
}

You can use the same script for both DHCPv4 and DHCPv6, or use two different scripts.

The script will receive the name of the hook point as first argument, and all relevant information available at the current hook point will be passed as environment variables.

Refer to the Kea documentation for more information about each hook point:

• DHCPv4 hooks reference: https://jenkins.isc.org/job/Kea_doc/doxygen/de/df3/dhcpv4Hooks.html
• DHCPv6 hooks reference: https://jenkins.isc.org/job/Kea_doc/doxygen/d1/d02/dhcpv6Hooks.html

Debug script

To experiment, an example script is provided: examples/debug.sh. It simply prints the name of the hook point and all environment variables passed to it.

The output of the script is at /tmp/kea-hook-runscript-debug.log. A nice way to debug is to continously display the content of this file:

tail -F /tmp/kea-hook-runscript-debug.log

Reference of variables passed to the external script

DHCPv4 variables

Here are all possible variables, with their type, description and reference of the possible values. Booleans are simply expressed with 0 and 1.

Variable name	Type	Description	Reference
QUERY4_TYPE	string	Type of DHCP message	dhcp/dhcp4.h
QUERY4_INTERFACE	string	Interface on which query was received
QUERY4_RELAYED	bool		dhcp/pkt4.h

DHCPv4 hook points

For each Kea hook point, here are all variables usable in the external script.

pkt4_receive

• QUERY4_TYPE
• QUERY4_INTERFACE
• QUERY4_HWADDR
• QUERY4_HWADDR_SOURCE
• QUERY4_HWADDR_TYPE
• QUERY4_RELAYED

TODO

• handle DHCPv6 hook points better
• also call the script at load/unload
• document environment variables
• allow to configure which hook points will trigger the script
• take into account the return code of the script to set the status of the callout (this should be configurable to avoid surprises...).