# ExDhcp

**An instrumentable DHCP Packet GenServer for Elixir**

_Largely inspired by [one_dhcpd][1]_

<img src="https://api.travis-ci.com/RstorLabs/ex_dhcp.svg?branch=master"/>

## 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][2] 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:

```elixir

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](https://hexdocs.pm/ex_dhcp).

### Deployment

The [DHCP protocol][3] 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:

```bash
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:

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

## Installation

If available in [Hex](https://hex.pm/docs/publish), the package can be installed
by adding `ex_dhcp` to your list of dependencies in `mix.exs`:

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

<!-- References -->
[1]: https://github.com/fhunleth/one_dhcpd
[2]: https://en.wikipedia.org/wiki/Preboot_Execution_Environment
[3]: https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol