ExPcap

A PCAP library written in Elixir. This does not wrap a C or Erlang PCAP library, rather it attempts to be an idiomatic Elixir library.

This library parses pcap files, however it does not yet support most protocols that can be contained within a pcap file. The only supported protocols at the moment are:

• Ethernet
• IPv4
• UDP
• DNS

Documentation

Documentation can be found at http://cobenian.github.io/expcap

To Build Documentation

You must have run mix deps.get and mix deps.compile first.

mix docs

To Build

You must have Elixir 1.0.0+ installed along with mix.

mix deps.get
mix deps.compile
mix compile
mix escript.build

To Test

You must have run mix deps.get and mix deps.compile first.

mix test

To Run

Once the project has been built, the following escript can be run:

./expcap -f <path-file-pcap-file>

A sample DNS cap file can be found in test/data/dns.cap. This file is provided by Wireshark as a sample capture.

./expcap -f test/data/dns.cap

Windows

Escript does not run on Windows so the expcap escript will not work. However, the code in this library should work on Windows if used as an Elixir library. This has not been tested that we are aware of.

Adding Support For Additional Protocols

Adding support for additional protocols is not difficult. Any protocol that may contain your protocol in its body should be updated to indicate that your protocol is supported.

• Update encapsulating protocols to be aware of the new protocol

For example, if we are adding the UDP protocol and it can be encapsulated in IPv4 packets, we need to modify the IPv4 PayloadType protocol. In this case we would add the following line:

    <<17>> -> Protocol.Udp

Note that each protocol is different in this regard. For example, in IPv4 the header contains a 'protocol' field that indicates the content type of the body. The new IPv4 PayloadType implementation would look like:

    defimpl PayloadType, for: Protocol.Ipv4 do
      @doc """
      Returns the parser that will parse the body of this IPv4 packet.
      """
      @spec payload_parser(binary) :: PayloadParser.t
      def payload_parser(data) do
        case data.header.protocol do
          <<06>> -> Protocol.Tcp
          <<17>> -> Protocol.Udp
        end
      end
    end

Bare Bones:

    defimpl PayloadType, for: Protocol.Ipv4 do
      def payload_parser(data) do
        case data.header.protocol do
          <<06>> -> Protocol.Tcp
          <<17>> -> Protocol.Udp
        end
      end
    end

• Create a module and struct for your protocol

For many protocols this means having header and data sections only. You may want to include a "parsed data" element as well. Here is an example:

    defmodule Protocol.Udp do
      @moduledoc """
      A parsed UDP packet
      """

      @bytes_in_header 8

      defstruct header: %Protocol.Udp.Header{},
                data: <<>>

      @type t :: %Protocol.Udp{
        header: Protocol.Udp.Header.t,
        data: binary
      }
    end

Bare Bones:

    defmodule Protocol.Udp do
      defstruct header: %Protocol.Udp.Header{},
                data: <<>>
    end

• Create a struct for your protocol's header (optional)

This is not strictly required, but is generally a good practice.

Here is an example:

    defmodule Protocol.Udp.Header do
      @moduledoc """
      A parsed UDP packet header
      """
      defstruct srcport:     <<>>,
                destport:    <<>>,
                length:      <<>>,
                checksum:    <<>>

      @type t :: %Protocol.Udp.Header{
        srcport:  non_neg_integer,
        destport: non_neg_integer,
        length:   binary,
        checksum: binary
      }
    end

Bare Bones:

    defmodule Protocol.Udp.Header do
      defstruct srcport:     <<>>,
                destport:    <<>>,
                length:      <<>>,
                checksum:    <<>>
    end

• Implement the PayloadType protocol

Example:

    defimpl PayloadType, for: Protocol.Udp do
      @doc """
      Returns the parser that will parse the body of the UDP packet
      """
      @spec payload_parser(binary) :: Protocol.Udp.t
      def payload_parser(_data) do
        Protocol.Dns
      end
    end

Bare Bones:

    defimpl PayloadType, for: Protocol.Udp do
      def payload_parser(_data) do
        Protocol.Dns
      end
    end

• Implement the PayloadParser protocol

Example:

    defimpl PayloadParser, for: Protocol.Udp do
      @doc """
      Returns the parsed body of the UDP packet
      """
      @spec from_data(binary) :: any
      def from_data(data) do
        Protocol.Udp.from_data data
      end
    end

Bare Bones:

    defimpl PayloadParser, for: Protocol.Udp do
      def from_data(data) do
        Protocol.Udp.from_data data
      end
    end

• Add support for parsing your header (optional if you do not have a header)

Example:

    @doc """
    Parses the header of a UDP packet
    """
    @spec header(binary) :: Protocol.Udp.Header.t
    def header(data) do
      <<
      srcport       :: unsigned-integer-size(16),
      destport      :: unsigned-integer-size(16),
      length        :: bytes-size(2),
      checksum      :: bytes-size(2),
      _payload       :: binary
      >> = data
      %Protocol.Udp.Header{
        srcport: srcport,
        destport: destport,
        length: length,
        checksum: checksum
      }
    end

Bare Bones:

    def header(data) do
      <<
        srcport       :: unsigned-integer-size(16),
        destport      :: unsigned-integer-size(16),
        length        :: bytes-size(2),
        checksum      :: bytes-size(2),
        _payload       :: binary
      >> = data
      %Protocol.Udp.Header{
        srcport: srcport,
        destport: destport,
        length: length,
        checksum: checksum
      }
    end

• Add support for parsing your protocol

Example:

    @doc """
    Returns a parsed UDP packet
    """
    @spec from_data(binary) :: Protocol.Udp.t
    def from_data(data) do
      << _header :: bytes-size(@bytes_in_header), payload :: binary >> = data
      %Protocol.Udp{
        header: header(data),
        data: payload
      }
    end

Bare Bones:

    def from_data(data) do
      << _header :: bytes-size(@bytes_in_header), payload :: binary >> = data
      %Protocol.Udp{
        header: header(data),
        data: payload
      }
    end

• Add support for printing your header to string (optional if you do not have a header)

Example:

    defimpl String.Chars, for: Protocol.Udp.Header do
      @doc """
      Prints a UDP packet header to a human readable string
      """
      @spec to_string(Protocol.Udp.t) :: String.t
      def to_string(udp) do
        String.strip("""
        srcport:          #{udp.srcport}
        srcport:          #{udp.destport}
        length:           #{ExPcap.Binaries.to_uint16(udp.length)}
        checksum:         #{ExPcap.Binaries.to_hex(udp.checksum)}
        """)
      end
    end

Bare Bones:

    defimpl String.Chars, for: Protocol.Udp.Header do
      def to_string(udp) do
        String.strip("""
        srcport:          #{udp.srcport}
        srcport:          #{udp.destport}
        length:           #{ExPcap.Binaries.to_uint16(udp.length)}
        checksum:         #{ExPcap.Binaries.to_hex(udp.checksum)}
        """)
      end
    end

• Add support for printing your protocol to string

Example:

    defimpl String.Chars, for: Protocol.Udp do
      @doc """
      Prints a UDP packet to a human readable string
      """
      @spec to_string(Protocol.Udp.t) :: String.t
      def to_string(udp) do
        String.strip("""
        Udp:
        #{udp.header}
        Length:           #{byte_size(udp.data)}
        Raw:              #{ExPcap.Binaries.to_raw(udp.data)}
        """)
      end
    end

Bare Bones:

    defimpl String.Chars, for: Protocol.Udp do
      def to_string(udp) do
        String.strip("""
        Udp:
        #{udp.header}
        Length:           #{byte_size(udp.data)}
        Raw:              #{ExPcap.Binaries.to_raw(udp.data)}
        """)
      end
    end

Limitations

• Very few protocols are supported at this time.
• Well formed pcap files can be parsed properly, however corrupted pcap files may not have helpful error messages.
• Escript will not run on Windows, but the code should.