simple_dns_server.ex

simple_dns_server.ex

  defmodule SimpleDNSServer do
    @moduledoc """
    A simple DNS server that responds to TXT queries with fixed sample values.
    """

    # Use 5353 for testing to avoid requiring root privileges for port 53
    @dns_port 9999
    @txt_records ["v=spf1 include:_spf.example.com ~all", "key1=value1", "key2=value2"]

    def start do
      {:ok, socket} = :gen_udp.open(@dns_port, [:binary, active: true, ip: {0, 0, 0, 0}])
      IO.puts("DNS server running on port #{@dns_port}")
      loop(socket)
    end

    defp loop(socket) do
      receive do
        {:udp, _socket, client_ip, client_port, query} ->
          IO.inspect({client_ip, client_port, query}, label: "Received DNS query")
          response = build_response(query)
          :gen_udp.send(socket, client_ip, client_port, response)
          loop(socket)
      end
    end

    defp build_response(query) do
      # Extract the transaction ID (first 2 bytes of the query)
      <<transaction_id::16, flags::16, qdcount::16, _rest::binary>> = query

      # Build the DNS response header
      header = <<
        # Transaction ID
        transaction_id::16,
        # Flags: Standard query response, no error
        0b10000100_00000000::16,
        # Questions: Echo the number of questions
        qdcount::16,
        # Answer RRs: 1
        1::16,
        # Authority RRs: 0
        0::16,
        # Additional RRs: 0
        0::16
      >>

      # Extract the question section (everything after the header)
      <<_header::binary-size(12), question::binary>> = query

      # Build the answer section for a single TXT record
      txt_value = "v=spf1 include:_spf.example.com ~all"
      txt_data = encode_txt_data(txt_value)

      answer = <<
        # Name: Pointer to the question
        0xC00C::16,
        # Type: TXT
        16::16,
        # Class: IN (Internet)
        1::16,
        # TTL: 60 seconds
        60::32,
        # Data length
        byte_size(txt_data)::16,
        # TXT data
        txt_data::binary
      >>

      # Combine the header, question, and answer sections
      header <> question <> answer
    end

    defp encode_txt_data(txt_value) do
      # Encode the TXT value as a length-prefixed string
      <<byte_size(txt_value)::8, txt_value::binary>>
    end
  end