kea-hook-runscript

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, on Kea 1.1 and above.

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. An example is included in examples/slash32_leases/;
• 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.

Examples

If you have more examples of usage, feel free to contribute your Kea config and your scripts!

Handing out IPv4 addresses in /32 subnets

This example allows to lease IPv4 addresses individually (/32 subnets), by inserting routes in the kernel each time a DHCP client connects, and sending custom routes to clients using DHCP option 121. This is mostly useful to hand out public IPv4 addresses to customers.

See the included README for more explanations and the source with the script and an example Kea configuration.

Debug script

To experiment, a simple debug 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

How to build

You first need to compile the hook. For this, you need Kea and Boost development headers installed: on Debian, the packages are kea-dev and libboost-dev.

To build, simply run:

$ make

Some notes on Kea versions:

• Kea 1.1 does not install all required headers (most notably dhcpsrv/), so you may need to build against Kea's source tree.
• Kea 1.2 is missing a header file by mistake, so depending on your distribution, you may need to manually copy option6_pdexclude.h from the Kea git repository to /usr/include/kea/dhcp/.
• Kea 1.3 should work out-of-the-box.

To build against a local Kea source tree, assumed to be in ~/kea:

• build Kea (cd ~/kea && make -j)
• install Kea to a local directory (cd ~/kea && make install DESTDIR=/tmp/kea)

Then build this hook with:

$ export KEA_MSG_COMPILER=~/kea/src/lib/log/compiler/kea-msg-compiler
$ export KEA_INCLUDE=~/kea/src/lib
$ export KEA_LIB=/tmp/kea/usr/local/lib
$ make

How to use this hook

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, documented below.

To debug, see the examples/debug.sh script described above.

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

Reference of variables passed to the external script

DHCPv4 variables

Here are all possible variables for DHCPv4, with their type, description and reference of the possible values. Booleans are simply expressed with 0 and 1.

Variable name	Type	Description	Reference
KEA_QUERY4_TYPE	string	Type of DHCP message	dhcp/dhcp4.h
KEA_QUERY4_INTERFACE	string	Interface on which query was received
KEA_QUERY4_RELAYED	bool	Whether query was relayed	dhcp/pkt4.h
KEA_QUERY4_HWADDR	string	Hardware address of the client (often MAC address)
KEA_QUERY4_HWADDR_TYPE	int	Type of hardware address	dhcp/dhcp4.h
KEA_QUERY4_HWADDR_SOURCE	int	Currently always set to 0	dhcp/hwaddr.h
KEA_RESPONSE4_TYPE	string	Type of DHCP message	dhcp/dhcp4.h
KEA_RESPONSE4_INTERFACE	string	Interface on which response is being sent
KEA_SUBNET4_PREFIX	IPv4	IP prefix of the subnet (without prefix length)
KEA_SUBNET4_PREFIXLEN	int	Prefix length of the subnet (0 to 32)
KEA_SUBNET4	string	KEA_SUBNET4_PREFIX/KEA_SUBNET4_PREFIXLEN
KEA_LEASE4_ADDRESS	IPv4	IPv4 address leased to client
KEA_REMOVE_LEASE	bool	Whether the lease should be removed from the lease database	DHCPv4 hook API
KEA_FAKE_ALLOCATION	bool	Whether the query is a DISCOVER or a REQUEST	DHCPv4 hook API

DHCPv4 hook points

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

pkt4_receive

• KEA_QUERY4_TYPE
• KEA_QUERY4_INTERFACE
• KEA_QUERY4_HWADDR
• KEA_QUERY4_HWADDR_SOURCE
• KEA_QUERY4_HWADDR_TYPE
• KEA_QUERY4_RELAYED

pkt4_send

• KEA_QUERY4_TYPE
• KEA_QUERY4_INTERFACE
• KEA_QUERY4_HWADDR
• KEA_QUERY4_HWADDR_SOURCE
• KEA_QUERY4_HWADDR_TYPE
• KEA_QUERY4_RELAYED
• KEA_RESPONSE4_TYPE
• KEA_RESPONSE4_INTERFACE

subnet4_select

• KEA_QUERY4_TYPE
• KEA_QUERY4_INTERFACE
• KEA_QUERY4_HWADDR
• KEA_QUERY4_HWADDR_SOURCE
• KEA_QUERY4_HWADDR_TYPE
• KEA_QUERY4_RELAYED
• KEA_SUBNET4_PREFIX
• KEA_SUBNET4_PREFIXLEN
• KEA_SUBNET4

lease4_select

• KEA_QUERY4_TYPE
• KEA_QUERY4_INTERFACE
• KEA_QUERY4_HWADDR
• KEA_QUERY4_HWADDR_SOURCE
• KEA_QUERY4_HWADDR_TYPE
• KEA_QUERY4_RELAYED
• KEA_SUBNET4_PREFIX
• KEA_SUBNET4_PREFIXLEN
• KEA_SUBNET4
• KEA_FAKE_ALLOCATION
• KEA_LEASE4_ADDRESS

lease4_renew

• KEA_QUERY4_TYPE
• KEA_QUERY4_INTERFACE
• KEA_QUERY4_HWADDR
• KEA_QUERY4_HWADDR_SOURCE
• KEA_QUERY4_HWADDR_TYPE
• KEA_QUERY4_RELAYED
• KEA_SUBNET4_PREFIX
• KEA_SUBNET4_PREFIXLEN
• KEA_SUBNET4
• KEA_LEASE4_ADDRESS

lease4_release

• KEA_QUERY4_TYPE
• KEA_QUERY4_INTERFACE
• KEA_QUERY4_HWADDR
• KEA_QUERY4_HWADDR_SOURCE
• KEA_QUERY4_HWADDR_TYPE
• KEA_QUERY4_RELAYED
• KEA_LEASE4_ADDRESS

lease4_decline

• KEA_QUERY4_TYPE
• KEA_QUERY4_INTERFACE
• KEA_QUERY4_HWADDR
• KEA_QUERY4_HWADDR_SOURCE
• KEA_QUERY4_HWADDR_TYPE
• KEA_QUERY4_RELAYED
• KEA_LEASE4_ADDRESS

lease4_expire

• KEA_LEASE4_ADDRESS
• KEA_REMOVE_LEASE

lease4_recover

• KEA_LEASE4_ADDRESS

DHCPv6 variables

Here are all possible variables for DHCPv6, with their type, description and reference of the possible values. Booleans are simply expressed with 0 and 1.

Variable name	Type	Description	Reference
KEA_QUERY6_TYPE	string	Type of DHCPv6 message	dhcp/dhcp6.h
KEA_QUERY6_INTERFACE	string	Interface on which query was received
KEA_QUERY6_DUID	string	TODO
KEA_QUERY6_HWADDR	string	TODO
KEA_RESPONSE6_TYPE	string	Type of DHCPv6 message	dhcp/dhcp6.h
KEA_RESPONSE6_INTERFACE	string	Interface on which response is being sent
KEA_RESPONSE6_ADDRESS	string	TODO
KEA_RESPONSE6_PREFERRED_LIFETIME	int	TODO
KEA_RESPONSE6_VALID_LIFETIME	int	TODO
KEA_SUBNET6_PREFIX	IPv6	IP prefix of the subnet (without prefix length)
KEA_SUBNET6_PREFIXLEN	int	Prefix length of the subnet (0 to 128)
KEA_SUBNET6	string	KEA_SUBNET6_PREFIX/KEA_SUBNET6_PREFIXLEN
KEA_LEASE6_ADDRESS	IPv6	IPv6 address leased to client
KEA_REMOVE_LEASE	bool	Whether the lease should be removed from the lease database	DHCPv6 hook API
KEA_FAKE_ALLOCATION	bool	Whether the query is a SOLICIT or a REQUEST	DHCPv6 hook API

DHCPv6 hook points

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

pkt6_receive

• KEA_QUERY6_TYPE
• KEA_QUERY6_INTERFACE
• KEA_QUERY6_DUID
• KEA_QUERY6_HWADDR

pkt6_send

• KEA_QUERY6_TYPE
• KEA_QUERY6_INTERFACE
• KEA_QUERY6_DUID
• KEA_QUERY6_HWADDR
• KEA_RESPONSE6_TYPE
• KEA_RESPONSE6_INTERFACE
• KEA_RESPONSE6_ADDRESS
• KEA_RESPONSE6_PREFERRED_LIFETIME
• KEA_RESPONSE6_VALID_LIFETIME

subnet6_select

• KEA_QUERY6_TYPE
• KEA_QUERY6_INTERFACE
• KEA_QUERY6_DUID
• KEA_QUERY6_HWADDR
• KEA_SUBNET6_PREFIX
• KEA_SUBNET6_PREFIXLEN
• KEA_SUBNET6

lease6_select

• KEA_QUERY6_TYPE
• KEA_QUERY6_INTERFACE
• KEA_QUERY6_DUID
• KEA_QUERY6_HWADDR
• KEA_SUBNET6_PREFIX
• KEA_SUBNET6_PREFIXLEN
• KEA_SUBNET6
• KEA_FAKE_ALLOCATION
• KEA_LEASE6_ADDRESS

lease6_renew

• KEA_QUERY6_TYPE
• KEA_QUERY6_INTERFACE
• KEA_QUERY6_DUID
• KEA_QUERY6_HWADDR
• KEA_LEASE6_ADDRESS

lease6_rebind

• KEA_QUERY6_TYPE
• KEA_QUERY6_INTERFACE
• KEA_QUERY6_DUID
• KEA_QUERY6_HWADDR
• KEA_LEASE6_ADDRESS

lease6_decline

• KEA_QUERY6_TYPE
• KEA_QUERY6_INTERFACE
• KEA_QUERY6_DUID
• KEA_QUERY6_HWADDR
• KEA_LEASE6_ADDRESS

lease6_release

• KEA_QUERY6_TYPE
• KEA_QUERY6_INTERFACE
• KEA_QUERY6_DUID
• KEA_QUERY6_HWADDR
• KEA_LEASE6_ADDRESS

lease6_expire

• KEA_LEASE6_ADDRESS
• KEA_REMOVE_LEASE

lease6_recover

• KEA_LEASE6_ADDRESS

TODO

• take stdout/stderr of script and turn it into proper Kea logs
• handle DHCPv6 hook points better
• also call the script at load/unload
• 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...).