Class: PacketGen::Types::Fields Abstract

Inherits:
Object
  • Object
show all
Defined in:
lib/packetgen/types/fields.rb

Overview

This class is abstract.

Set of fields

This class is a base class to define headers or anything else with a binary format containing multiple fields.

Basics

A Fields subclass is generaly composed of multiple binary fields. These fields have each a given type. All types from PacketGen::Types module are supported, and all Fields subclasses may also be used as field type.

To define a new subclass, it has to inherit from Fields. And some class methods have to be used to declare attributes/fields:

class MyBinaryStructure < PacketGen::Types::Fields
  # define a first Int8 attribute, with default value: 1
  define_field :attr1, PacketGen::Types::Int8, default: 1
  #define a second attribute, of kind Int32
  define_field :attr2, PacketGen::Types::Int32
end

These defintions create 4 methods: #attr1, #attr1=, #attr2 and #attr2=. All these methods take and/or return Integers.

Fields may also be accessed through #[] ans #[]=. These methods give access to type object:

mybs = MyBinaryStructure.new
mybs.attr1     # => Integer
mybs[:attr1]   # => PacketGen::Types::Int8

#initialize accepts an option hash to populate attributes. Keys are attribute name symbols, and values are those expected by writer accessor.

#read is able to populate object from a binary string.

#to_s returns binary string from object.

Add Fields

Fields.define_field adds a field to Fields subclass. A lot of field types may be defined: integer types, string types (to handle a stream of bytes). More complex field types may be defined using others Fields subclasses:

# define a 16-bit little-endian integer field, named type
define_field :type, PacketGen::Types::Int16le
# define a string field
define_field :body, PacketGen::Types::String
# define afield using a complex type (Fields subclass)
define_field :mac_addr, PacketGen::Eth::MacAddr

This example creates six methods on our Fields subclass: #type, #type=, #body, #body=, #mac_addr and #mac_addr=.

Fields.define_field has many options (third optional Hash argument):

  • :default gives default field value. It may be a simple value (an Integer for an Int field, for example) or a lambda,

  • :builder to give a builder/constructor lambda to create field. The lambda takes one argument: Fields subclass object owning field,

  • :optional to define this field as optional. This option takes a lambda parameter used to say if this field is present or not,

  • :enum to define Hash enumeration for an Enum type.

For example:

# 32-bit integer field defaulting to 1
define_field :type, PacketGen::Types::Int32, default: 1
# 16-bit integer field, created with a random value. Each instance of this
# object will have a different value.
define_field :id, PacketGen::Types::Int16, default: ->{ rand(65535) }
# a size field
define_field :body_size, PacketGen::Types::Int16
# String field which length is taken from body_size field
define_field :body, PacketGen::Types::String, builder: ->(obj, type) { type.new('', length_from: obj[:body_size]) }
# 16-bit enumeration type. As :default not specified, default to first value of enum
define_field :type_class, PacketGen::Types::Int16Enum, enum: { 'class1' => 1, 'class2' => 2}
# optional field. Only present if another field has a certain value
define_field :opt1, PacketGen::Types::Int16, optional: ->(h) { h.type == 42 }

Fields.define_field_before and Fields.define_field_after are also defined to relatively create a field from anoher one (for example, when adding a field in a subclass).

Generating bit fields

Fields.define_bit_fields_on creates a bit field on a previuously declared integer field. For example, frag field in IP header:

define_field :frag, Types::Int16, default: 0
define_bit_fields_on :frag, :flag_rsv, :flag_df, :flag_mf, :fragment_offset, 13

This example generates methods:

  • #frag and #frag= to access frag field as a 16-bit integer,

  • #flag_rsv?, #flag_rsv=, #flag_df?, #flag_df=, #flag_mf? and #flag_mf= to access Boolean RSV, MF and DF flags from frag field,

  • #fragment_offset and #fragment_offset= to access 13-bit integer fragment offset subfield from frag field.

Author:

  • Sylvain Daubert

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(options = {}) ⇒ Fields

Create a new header object

Parameters:



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# File 'lib/packetgen/types/fields.rb', line 291

def initialize(options={})
  @fields = {}
  @optional_fields = {}

  self.class.class_eval { @field_defs }.each do |field, ary|
    type, default, builder, optional, enum, field_options = ary
    default = default.call if default.is_a?(Proc)
    @fields[field] = if builder
                       builder.call(self, type)
                     elsif enum
                       type.new(enum)
                     elsif !field_options.empty?
                       type.new(field_options)
                     else
                       type.new
                     end

    value = options[field] || default
    if value.class <= type
      @fields[field] = value
    elsif type < Types::Enum
      case value
      when ::String
        @fields[field].value = value
      else
        @fields[field].read(value)
      end
    elsif type < Types::Int
      @fields[field].read(value)
    elsif type <= Types::String
      @fields[field].read(value)
    elsif @fields[field].respond_to? :from_human
      @fields[field].from_human(value)
    end

    @optional_fields[field] = optional if optional
  end
  self.class.class_eval { @bit_fields }.each do |bit_field|
    self.send "#{bit_field}=", options[bit_field] if options[bit_field]
  end
end

Class Method Details

.define_bit_fields_on(attr, *args) ⇒ void

This method returns an undefined value.

Define a bitfield on given attribute

class MyHeader < PacketGen::Types::Fields
  define_field :flags, Types::Int16
  # define a bit field on :flag attribute:
  # flag1, flag2 and flag3 are 1-bit fields
  # type and stype are 3-bit fields. reserved is a 6-bit field
  define_bit_fields_on :flags, :flag1, :flag2, :flag3, :type, 3, :stype, 3, :reserved, 7
end

A bitfield of size 1 bit defines 2 methods:

  • #field? which returns a Boolean,

  • #field= which takes and returns a Boolean.

A bitfield of more bits defines 2 methods:

  • #field which returns an Integer,

  • #field= which takes and returns an Integer.

Parameters:

  • attr (Symbol) —

    attribute name (attribute should be a Int subclass)

  • args (Array) —

    list of bitfield names. Name may be followed by bitfield size. If no size is given, 1 bit is assumed.

Raises:

  • (ArgumentError) 


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# File 'lib/packetgen/types/fields.rb', line 233

def self.define_bit_fields_on(attr, *args)
  attr_def = @field_defs[attr]
  raise ArgumentError, "unknown #{attr} field" if attr_def.nil?
  type = attr_def.first
  unless type < Types::Int
    raise TypeError, "#{attr} is not a PacketGen::Types::Int"
  end
  total_size = type.new.width * 8
  idx = total_size - 1

  field = args.shift
  while field
    next unless field.is_a? Symbol
    size = if args.first.is_a? Integer
             args.shift
           else
             1
           end
    unless field == :_
      shift = idx - (size - 1)
      field_mask = (2**size - 1) << shift
      clear_mask = (2**total_size - 1) & (~field_mask & (2**total_size - 1))

      if size == 1
        class_eval "        def \#{field}?\n          val = (self[:\#{attr}].to_i & \#{field_mask}) >> \#{shift}\n          val != 0\n        end\n        def \#{field}=(v)\n          val = v ? 1 : 0\n          self[:\#{attr}].value = self[:\#{attr}].to_i & \#{clear_mask}\n          self[:\#{attr}].value |= val << \#{shift}\n        end\n        METHODS\n      else\n        class_eval <<-METHODS\n        def \#{field}\n          (self[:\#{attr}].to_i & \#{field_mask}) >> \#{shift}\n        end\n        def \#{field}=(v)\n          self[:\#{attr}].value = self[:\#{attr}].to_i & \#{clear_mask}\n          self[:\#{attr}].value |= (v & \#{2**size - 1}) << \#{shift}\n        end\n        METHODS\n      end\n\n      @bit_fields << field\n    end\n\n    idx -= size\n    field = args.shift\n  end\nend\n"

.define_field(name, type, options = {}) ⇒ void

This method returns an undefined value.

Define a field in class

class BinaryStruct < PacketGen::Types::Fields
  # 8-bit value
  define_field :value1, Types::Int8
  # 16-bit value
  define_field :value2, Types::Int16
  # specific class, may use a specific constructor
  define_field :value3, MyClass, builder: ->(obj, type) { type.new(obj) }
end

bs = BinaryStruct.new
bs[value1]   # => Types::Int8
bs.value1    # => Integer

Parameters:

  • name (Symbol) —

    field name

  • type (Object) —

    class or instance

  • options (Hash) (defaults to: {}) —

    Unrecognized options are passed to object builder if :builder option is not set.

Options Hash (options):

  • :default (Object) —

    default value

  • :builder (Lambda) —

    lambda to construct this field. Parameters to this lambda is the caller object and the field type class.

  • :optional (Lambda) —

    define this field as optional. Given lambda is used to known if this field is present or not. Parameter to this lambda is the being defined Field object.

  • :enum (Hash) —

    mandatory option for an Enum type. Define enumeration: hash’s keys are String, and values are Integer.



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# File 'lib/packetgen/types/fields.rb', line 144

def self.define_field(name, type, options={})
  define = []
  if type < Types::Enum
    define << "def #{name}; self[:#{name}].to_i; end"
    define << "def #{name}=(val) self[:#{name}].value = val; end"
  elsif type < Types::Int
    define << "def #{name}; self[:#{name}].to_i; end"
    define << "def #{name}=(val) self[:#{name}].read val; end"
  elsif type.instance_methods.include?(:to_human) &&
        type.instance_methods.include?(:from_human)
    define << "def #{name}; self[:#{name}].to_human; end"
    define << "def #{name}=(val) self[:#{name}].from_human val; end"
  else
    define << "def #{name}; self[:#{name}]; end\n"
    define << "def #{name}=(val) self[:#{name}].read val; end"
  end

  define.delete(1) if type.instance_methods.include? "#{name}=".to_sym
  define.delete(0) if type.instance_methods.include? name
  class_eval define.join("\n")
  @field_defs[name] = [type, options.delete(:default),
                       options.delete(:builder),
                       options.delete(:optional),
                       options.delete(:enum),
                       options]
  @ordered_fields << name
end

.define_field_after(other, name, type, options = {}) ⇒ void

This method returns an undefined value.

Define a field, after another one

Parameters:

  • other (Symbol, nil) —

    field name to create a new one after. If nil, new field is appended.

  • name (Symbol) —

    field name to create

  • type (Object) —

    class or instance

  • options (Hash) (defaults to: {}) —

    See define_field.

Raises:

  • (ArgumentError)

See Also:



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# File 'lib/packetgen/types/fields.rb', line 197

def self.define_field_after(other, name, type, options={})
  define_field name, type, options
  return if other.nil?
  @ordered_fields.delete name
  idx = @ordered_fields.index(other)
  raise ArgumentError, "unknown #{other} field" if idx.nil?
  @ordered_fields[idx + 1, 0] = name
end

.define_field_before(other, name, type, options = {}) ⇒ void

This method returns an undefined value.

Define a field, before another one

Parameters:

  • other (Symbol, nil) —

    field name to create a new one before. If nil, new field is appended.

  • name (Symbol) —

    field name to create

  • type (Object) —

    class or instance

  • options (Hash) (defaults to: {}) —

    See define_field.

Raises:

  • (ArgumentError)

See Also:



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# File 'lib/packetgen/types/fields.rb', line 180

def self.define_field_before(other, name, type, options={})
  define_field name, type, options
  return if other.nil?
  @ordered_fields.delete name
  idx = @ordered_fields.index(other)
  raise ArgumentError, "unknown #{other} field" if idx.nil?
  @ordered_fields[idx, 0] = name
end

.delete_field(name) ⇒ void

This method returns an undefined value.

Delete a previously defined field

Parameters:

  • name (Symbol) 


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# File 'lib/packetgen/types/fields.rb', line 209

def self.delete_field(name)
  @ordered_fields.delete name
  @field_defs.delete name
end

.inherited(klass) ⇒ void

This method returns an undefined value.

On inheritage, create @field_defs class variable

Parameters:

  • klass (Class) 


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# File 'lib/packetgen/types/fields.rb', line 107

def self.inherited(klass)
  ordered = @ordered_fields.clone
  field_defs = @field_defs.clone
  bf = @bit_fields.clone
  klass.class_eval do
    @ordered_fields = ordered
    @field_defs = field_defs
    @bit_fields = bf
  end
end

Instance Method Details

#[](field) ⇒ Object

Get field object

Parameters:

  • field (Symbol)

Returns:

  • (Object) 


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# File 'lib/packetgen/types/fields.rb', line 336

def [](field)
  @fields[field]
end

#[]=(field, obj) ⇒ Object

Set field object

Parameters:

  • field (Symbol)
  • obj (Object)

Returns:

  • (Object) 


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# File 'lib/packetgen/types/fields.rb', line 344

def []=(field, obj)
  @fields[field] = obj
end

#body=(value) ⇒ void

This method returns an undefined value.

Used to set body as value of body object.

Parameters:

Raises:

  • (BodyError) —

    no body on given object

  • (ArgumentError) —

    cannot cram body in :body field



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# File 'lib/packetgen/types/fields.rb', line 436

def body=(value)
  raise BodyError, 'no body field' unless @fields.key? :body
  case body
  when ::String
    self[:body].read value
  when Int, Fields
    self[:body] = value
  when NilClass
    self[:body] = Types::String.new.read('')
  else
    raise ArgumentError, "Can't cram a #{body.class} in a :body field"
  end
end

#fields ⇒ Array<Symbol>

Get all field names

Returns:



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# File 'lib/packetgen/types/fields.rb', line 350

def fields
  @ordered_fields ||= self.class.class_eval { @ordered_fields }
end

#force_binary(str) ⇒ String

Force str to binary encoding

Parameters:

Returns:



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# File 'lib/packetgen/types/fields.rb', line 453

def force_binary(str)
  PacketGen.force_binary(str)
end

#inspect ⇒ String

Common inspect method for headers

Returns:



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# File 'lib/packetgen/types/fields.rb', line 402

def inspect
  str = Inspect.dashed_line(self.class, 2)
  fields.each do |attr|
    next if attr == :body
    next unless is_present?(attr)
    str << Inspect.inspect_attribute(attr, self[attr], 2)
  end
  str
end

#is_optional?(field) ⇒ Boolean

Say if this field is optional

Returns:

  • (Boolean) 


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# File 'lib/packetgen/types/fields.rb', line 361

def is_optional?(field)
  @optional_fields.key? field
end

#is_present?(field) ⇒ Boolean

Say if an optional field is present

Returns:

  • (Boolean) 


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# File 'lib/packetgen/types/fields.rb', line 367

def is_present?(field)
  return true unless is_optional?(field)
  @optional_fields[field].call(self)
end

#offset_of(field) ⇒ Integer

Get offset of given field in PacketGen::Types::Fields structure.

Parameters:

  • field (Symbol)

Returns:

  • (Integer)

Raises:

  • (ArgumentError) —

    unknown field



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# File 'lib/packetgen/types/fields.rb', line 461

def offset_of(field)
  raise ArgumentError, "#{field} is an unknown field of #{self.class}" unless @fields.include?(field)
  offset = 0
  fields.each do |f|
    break offset if f == field
    next unless is_present?(f)
    offset += self[f].sz
  end
end

#optional_fields ⇒ Object

Get all optional field name



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# File 'lib/packetgen/types/fields.rb', line 355

def optional_fields
  @optional_fields.keys
end

#read(str) ⇒ Fields

Populate object from a binary string

Parameters:

Returns:



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# File 'lib/packetgen/types/fields.rb', line 375

def read(str)
  return self if str.nil?
  force_binary str
  start = 0
  fields.each do |field|
    next unless is_present?(field)
    obj = nil
    if self[field].respond_to? :width
      width = self[field].width
      obj = self[field].read str[start, width]
      start += width
    elsif self[field].respond_to? :sz
      obj = self[field].read str[start..-1]
      size = self[field].sz
      start += size
    else
      obj = self[field].read str[start..-1]
      start = str.size
    end
    self[field] = obj unless obj == self[field]
  end

  self
end

#sz ⇒ nteger

Size of object as binary string

Returns:

  • (nteger) 


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# File 'lib/packetgen/types/fields.rb', line 421

def sz
  to_s.size
end

#to_h ⇒ Hash

Return object as a hash

Returns:

  • (Hash) —

    keys: attributes, values: attribute values



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# File 'lib/packetgen/types/fields.rb', line 427

def to_h
  Hash[fields.map { |f| [f, @fields[f].to_human] }]
end

#to_s ⇒ String

Return object as a binary string

Returns:



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# File 'lib/packetgen/types/fields.rb', line 414

def to_s
  fields.select { |f| is_present?(f) }
        .map! { |f| force_binary @fields[f].to_s }.join
end