ExDhcp

An instrumentable DHCP Packet GenServer for Elixir

Largely inspired by one_dhcpd

General Description

ExDhcp is an instrumentable DHCP GenServer, with an opinionated interface that takes after the GenStage design. We couldn't use GPL licenced material in-house, so this project was derived from one_dhcpcd.

At the moment, unlike one_dhcpcd, it does not implement a full DHCP server, but you could use ExDhcp to implement that functionality. ExDhcp is ideal for using DHCP functionality for some other purpose, such as PXE booting.

If you would like to easily implement distributed DHCP with custom code hooks for custom functionality, ExDhcp might be for you.

Usage Notes

A minimal ExDhcp server implements the following three methods:

• handle_discover
• handle_request
• handle_decline

It might look something like this:

defmodule MyDhcpServer do
  use ExDhcp
  alias ExDhcp.Packet

  def start_link(init_state) do
    ExDhcp.start_link(__MODULE__, init_state)
  end

  @impl true
  def init(init_state), do: {:ok, init_state}

  @impl true
  def handle_discover(request, xid, mac, state) do

    # insert code here. Should assign the unimplemented values 
    # for the response below:

    response = Packet.respond(request, :offer,
      yiaddr: issued_your_address,
      siaddr: server_ip_address,
      subnet_mask: subnet_mask,
      routers: [router],
      lease_time: lease_time,
      server: server_ip_address,
      domain_name_servers: [dns_server]))

    {:respond, response, new_state}
  end

  @impl true
  def handle_request(request, xid, mac, state) do
    
    # insert code here

    response = Packet.respond(request, :ack,
      yiaddr: issued_your_address ...)

    {:respond, response, state}
  end

  @impl true
  def handle_decline(request, xid, mac, state) do
    
    # insert code here

    response = Packet.respond(request, :offer,
      yiaddr: new_issued_address ...)

    {:respond, response, state}
  end

end

For more details, see the documentation.

Deployment

The DHCP protocol listens in on port 67, which is below the privileged port limit (1024) for most, e.g. Linux distributions.

ExDhcp doesn't presume that it will be running as root or have access to that port, and by default listens in to port 6767. If you expect to have access to privileged ports, you can set the port number in the module start_link options.

Alternatively, on most linux distributions you can use iptables to forward broadcast UDP from port 67 to port 6767 and vice versa. The following incantations will achieve this:

iptables -t nat -I PREROUTING -p udp --src 0.0.0.0 --dport 67 -j DNAT --to 0.0.0.0:6767
iptables -t nat -A POSTROUTING -p udp --sport 6767 -j SNAT --to <server ip address>:67

NB: If you're using a port besides 6767, be sure to replace it with your chosen port.

Interface Binding

There may be situations where you would like to bind DHCP activity to a specific ethernet interface; this is settable in the module start_link options.

In order to successfully bind to the interface on Linux machines, do the following as superuser:

setcap cap_net_raw=ep /path/to/beam.smp

Installation

If available in Hex, the package can be installed by adding ex_dhcp to your list of dependencies in mix.exs:

def deps do
  [
    {:ex_dhcp, "~> 0.1.1"}
  ]
end