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dhclient-pd

dhclient-pd is a script that takes an incoming IPv6 prefix delegation from dhclient and assigns subnet prefix addresses to one or more additional interfaces on the system. It is also a standalone tool that may be called from any source, not just dhclient. Only the hook script is dhclient specific.

Why two scripts?

dhclient actually sources the hook scripts, which means that the hook script must be valid shell script code. However, working with IP addresses in shell scripting is both error prone and complex, so I implemented that logic in python instead. Python was chosen because it is nearly always available on any modern linux system.

Requirements

  • a modern version of python3

Installation

sudo cp ./src/prefix-delegation /etc/dhcp/dhclient-exit-hooks.d/prefix-delegation
sudo chmod 644 /etc/dhcp/dhclient-exit-hooks.d/prefix-delegation
sudo cp ./src/dhclient-pd /usr/local/bin/dhclient-pd
sudo chmod 755 ./src/dhclient-pd /usr/local/bin/dhclient-pd
sudo mkdir /etc/dhclient-pd

Configuration

Create /etc/dhclient-pd/interfaces.json. Example syntax:

[
    {
        "name": "enp4s0f0.3",
        "prefix_id": "3"
    },
    {
        "name": "enp4s0f1.4",
        "prefix_id": "4"
    },
    {
        "name": "enp4s0f1.5",
        "prefix_id": "af1"
    },
    {
        "name": "enp4s0f1.6",
        "prefix_id": "6"
    },
    {
        "interface_ids": [
            "::1",
            "::3"
        ],
        "name": "lo",
        "prefix_id": "0"
        "prefix_length: 128
    }
]

name and prefix_id are mandatory. Note that prefix_id is a string containing the hex representation of the id, so the value needs to be quoted to avoid being interpreted as an integer if there are no letters in the id. interface_ids is set to ["::1"] by default unless overriden. You may define multiple interface_ids within the same prefix, which is normally useful only for loopback addresses (/128). If you want to have multiple different prefixes on the same interface, just add another configuration block for that interface with an unique prefix_id.

Manual execution

You are not bound to dhclient. The hook script manages all the dhclient specifics. dhclient-pd is a standalone tool that may be called from any source. Typically you would call it manually with:

dhclient-pd config -p $prefix [-o $old_prefix]

You must replace $prefix with your actual delegated prefix.

If you run config without any prefix options nothing will happen as dhclient-pd then doesn't know which prefixes to manage.

To quickly show the configuration:

dhclient-pd show