Create Custom Protocol

PacketGen allows you adding your own header classes.

Quick start

To add a new/custom header, you first have to define the new header class. For example:

module MyModule
 class MyHeader < PacketGen::Header::Base
   define_field :field1, PacketGen::Types::Int32
   define_field :field2, PacketGen::Types::Int32
 end
end

Then, class must be declared to PacketGen:

PacketGen::Header.add_class MyModule::MyHeader

Finally, bindings must be declared:

# bind MyHeader as IP protocol number 254 (needed by Packet#parse and Packet#add)
PacketGen::Header::IP.bind_header MyModule::MyHeader, protocol: 254

And use it:

pkt = Packet.gen('IP').add('MyHeader', field1: 0x12345678)
pkt.myheader.field2.read 0x01

Add a new header type, in more detail

Define a header class

A new header class should inherit from PacketGen::Header::Base class (or from PacketGen::Header::ASN1Base, see below). This base class implements minimal API (see below) to parse a header from binary string, or to generate binary string from a header class.

PacketGen::Header::Base inherits from PacketGen::Types::Fields, which is a class to define headers or anything else with a binary format containing multiple fields.

Here, magical method is define_field. This method define a field and its name. You may set as fields as you need. A type may be a predefined type from PacketGen::Types, or any other PacketGen::Types::Fields subclass:

class CustomField < PacketGen::Types::Fields
  # add a 16-bit integer field
  define_field :one, PacketGen::Types::Int16
  # add another one
  define_field :two, PacketGen::Types::Int16
end

class ExampleHeader < PacketGen::Header::Base
  # define a first 32-bit integer field
  define_field :first, PacketGen::Types::Int32
  # add a custom field
  define_field :custom, CustomField
  # add a 8-bit integer field
  define_field :int8, PacketGen::Types::Int8

  # then bit fields on int8 field
  #  ack and error are boolean flags, as no size is specified (default to 1)
  #  rsv is a 4-bit field
  #  type is a 2-bit field
  define_bit_fields_on :int8, :ack, :error, :rsv, 4, :type, 2
end

Here, some example to access these fields:

example = ExampleHeader.new

example.first        #=> Integer
example.first = 0x12345678

example.custom       #=> CustomField
example.custom.one   #=> Integer

example.ack?         #=> Boolean
example.ack = true

example.type         #=> Integer
example.type = 1

Builtin Types

All builtin types are listed here.

Add some methods to a header class

By default, all fields will have accessors with the good type. By example, a Types::Int32 field may be accessed as an Integer, a Types::String may be accessed as a String, etc.

But, for some reason, you may need to add another accessor, or a method to compute some protocol data.

To do that, you have to understand Fields model. A field may be accessed through its accessor, as already seen. But it may also be accessed through Fields' hash.

Fields class defines #[] method to access to field object by its name. By example, with our previously defined ExampleHeader:

example.first     #=> Integer
example[:first]   #=> PacketGen::Types::Int32

Sometimes, you will need to access real field object. All field objects have common methods:

#read reads a binary string to set object,
#to_s gives binary string from object,
#sz gives binary size.

As an example of method in a header class, we will define one to calculate first field, which value should be size of custom field:

class ExampleHeader
  # compute first field
  def calc_first
    self[:first].read self[:custom].sz
  end
end

Add a new ASN.1 header

Some header may be defined using ASN.1 notation, like SNMP.

To define such a header, use PacketGen::Header::ASN1Base as base class:

class SNMP < PacketGen::Header::ASN1Base
  sequence :message,
            content: [enumerated(:version, value: 'v2c',
                                 enum: { 'v1' => 0, 'v2c' => 1, 'v2' => 2, 'v3' =>}),
                      octet_string(:community, value: 'public'),
                      model(:data, PDUClass)]

  define_attributes :version, :community
end

This definition uses lots of stuffs from rasn1 gem:

sequence defines a ASN.1 Sequence named message. This sequence contains:
- an Enumerated named version,
- an Octet String named community,
- and a PDUClass (subclass of RASN1::Model) named data and not defined here.

define_attributes is a helper method to declare attributes from some ASN.1 fields. This helps to mimic standard header behavior.

Packet magics

PacketGen::Packet defines some magics using specific header methods.

If your header class has a checksum field and/or a length field, Packet provides magic for them. You have to define a #calc_checksum and/or a calc_length which appropriately set checksum and/or length fields respectively. Then Packet#calc_checksum and Packet#calc_length will calculate all checksum and length fields in all headers, including yours.

If your header class is not an application layer one, you should define a body field of type PacketGen::Types::String. This will allow Packet#parse to automagically parse headers embedded in yours. Same magic will happen for Packet#to_s, Packet#encapsulate, Packet#decapsulate and Packet#add.

Header minimal API

PacketGen::Header::Base and PacketGen::Header::ASN1Base are provided to simplify writing of new headers. But they may not be so useful for some protocol types.

So, here is minimal API needed by PacketGen to handle a header class.

A header MUST have accessors:

#packet : get/set packet to which header belongs.

A header MUST respond to:

#protocol_name: get protocol name, usually class name as a String, without
module path,
#method_name: get method name, usually same as protocol name but downcase.
This name is used as accessor from packet to access header object,
#read: method to parse binary string and decode header,
#to_s: method to generate binary data of header to send on wire,
#parse?: return true if decoded header is correct. Used when guessing if
header may be decoded from binary string. An example of use if checking first
4-bit field for IP version in a IP/IPv6 header.

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Last updated 7 years ago