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PacketFu::TCPPacket

TCPPacket is used to construct TCP packets. They contain an EthHeader, an IPHeader, and a TCPHeader.

Example

tcp_pkt = PacketFu::TCPPacket.new
tcp_pkt.tcp_flags.syn=1
tcp_pkt.tcp_dst=80
tcp_pkt.tcp_win=5840
tcp_pkt.tcp_options="mss:1460,sack.ok,ts:#{rand(0xffffffff)};0,nop,ws:7"

tcp_pkt.ip_saddr=[rand(0xff),rand(0xff),rand(0xff),rand(0xff)].join('.')
tcp_pkt.ip_daddr=[rand(0xff),rand(0xff),rand(0xff),rand(0xff)].join('.')

tcp_pkt.recalc
tcp_pkt.to_f('/tmp/tcp.pcap')

Parameters

:eth
  A pre-generated EthHeader object.
:ip
  A pre-generated IPHeader object.
:flavor
  TODO: Sets the "flavor" of the TCP packet. This will include TCP options and the initial window
  size, per stack. There is a lot of variety here, and it's one of the most useful methods to
  remotely fingerprint devices. :flavor will span both ip and tcp for consistency.
 :type
  TODO: Set up particular types of packets (syn, psh_ack, rst, etc). This can change the initial flavor.
:config
 A hash of return address details, often the output of Utils.whoami?

Attributes

eth_header[RW]
ip_header[RW]
tcp_header[RW]

Public Class Methods

can_parse?(str) click to toggle source
# File lib/packetfu/protos/tcp.rb, line 47
def self.can_parse?(str)
  return false unless str.size >= 54
  return false unless EthPacket.can_parse? str
  return false unless IPPacket.can_parse? str
  return false unless str[23,1] == "\x06"
  return true
end
new(args={}) click to toggle source
# File lib/packetfu/protos/tcp.rb, line 68
def initialize(args={})
  @eth_header =     (args[:eth] || EthHeader.new)
  @ip_header        =     (args[:ip] || IPHeader.new)
  @tcp_header =     (args[:tcp] || TCPHeader.new)
  @tcp_header.flavor = args[:flavor].to_s.downcase

  @ip_header.body = @tcp_header
  @eth_header.body = @ip_header
  @headers = [@eth_header, @ip_header, @tcp_header]

  @ip_header.ip_proto=0x06
  super
  if args[:flavor]
    tcp_calc_flavor(@tcp_header.flavor)
  else
    tcp_calc_sum
  end
end

Public Instance Methods

peek_format() click to toggle source

TCP packets are denoted by a "T ", followed by size, source and dest information, packet flags, sequence number, and IPID.

# File lib/packetfu/protos/tcp.rb, line 182
def peek_format
  peek_data = ["T  "]
  peek_data << "%-5d" % self.to_s.size
  peek_data << "%-21s" % "#{self.ip_saddr}:#{self.tcp_src}"
  peek_data << "->"
  peek_data << "%21s" % "#{self.ip_daddr}:#{self.tcp_dst}"
  flags = ' ['
  flags << self.tcp_flags_dotmap
  flags << '] '
  peek_data << flags
  peek_data << "S:"
  peek_data << "%08x" % self.tcp_seq
  peek_data << "|I:"
  peek_data << "%04x" % self.ip_id
  peek_data.join
end
read(str=nil, args={}) click to toggle source
# File lib/packetfu/protos/tcp.rb, line 55
def read(str=nil, args={})
  raise "Cannot parse `#{str}'" unless self.class.can_parse?(str)
  @eth_header.read(str)

  # Strip off any extra data, if we are asked to do so.
  if args[:strip]
    tcp_body_len = self.ip_len - self.ip_hlen - (self.tcp_hlen * 4)
    @tcp_header.body.read(@tcp_header.body.to_s[0,tcp_body_len])
  end
  super(args)
  self
end
tcp_calc_flavor(str) click to toggle source

Sets the correct flavor for TCP Packets. Recognized flavors are:

windows, linux, freebsd
# File lib/packetfu/protos/tcp.rb, line 89
def tcp_calc_flavor(str)
  ts_val = Time.now.to_i + rand(0x4fffffff)
  ts_sec = rand(0xffffff)
  case @tcp_header.flavor = str.to_s.downcase
  when "windows" # WinXP's default syn
    @tcp_header.tcp_win = 0x4000
    @tcp_header.tcp_options="MSS:1460,NOP,NOP,SACKOK"
    @tcp_header.tcp_src = rand(5000 - 1026) + 1026
    @ip_header.ip_ttl = 64
  when "linux" # Ubuntu Linux 2.6.24-19-generic default syn
    @tcp_header.tcp_win = 5840
    @tcp_header.tcp_options="MSS:1460,SACKOK,TS:#{ts_val};0,NOP,WS:7"
    @tcp_header.tcp_src = rand(61_000 - 32_000) + 32_000
    @ip_header.ip_ttl = 64
  when "freebsd" # Freebsd
    @tcp_header.tcp_win = 0xffff
    @tcp_header.tcp_options="MSS:1460,NOP,WS:3,NOP,NOP,TS:#{ts_val};#{ts_sec},SACKOK,EOL,EOL"
    @ip_header.ip_ttl = 64
  else
    @tcp_header.tcp_options="MSS:1460,NOP,NOP,SACKOK"
  end
  tcp_calc_sum
end
tcp_calc_sum() click to toggle source

tcp_calc_sum() computes the TCP checksum, and is called upon intialization. It usually should be called just prior to dropping packets to a file or on the wire.

# File lib/packetfu/protos/tcp.rb, line 119
def tcp_calc_sum
  checksum =  (ip_src.to_i >> 16)
  checksum += (ip_src.to_i & 0xffff)
  checksum += (ip_dst.to_i >> 16)
  checksum += (ip_dst.to_i & 0xffff)
  checksum += 0x06 # TCP Protocol.
  checksum +=       (ip_len.to_i - ((ip_hl.to_i) * 4))
  checksum += tcp_src
  checksum += tcp_dst
  checksum += (tcp_seq.to_i >> 16)
  checksum += (tcp_seq.to_i & 0xffff)
  checksum += (tcp_ack.to_i >> 16)
  checksum += (tcp_ack.to_i & 0xffff)
  checksum += ((tcp_hlen << 12) + 
               (tcp_reserved << 9) + 
               (tcp_ecn.to_i << 6) + 
               tcp_flags.to_i
              )
  checksum += tcp_win
  checksum += tcp_urg

  chk_tcp_opts = (tcp_opts.to_s.size % 2 == 0 ? tcp_opts.to_s : tcp_opts.to_s + "\x00") 
  chk_tcp_opts.unpack("n*").each {|x| checksum = checksum + x }
  if (ip_len - ((ip_hl + tcp_hlen) * 4)) >= 0
    real_tcp_payload = payload[0,( ip_len - ((ip_hl + tcp_hlen) * 4) )] # Can't forget those pesky FCSes!
  else
    real_tcp_payload = payload # Something's amiss here so don't bother figuring out where the real payload is.
  end
  chk_payload = (real_tcp_payload.size % 2 == 0 ? real_tcp_payload : real_tcp_payload + "\x00") # Null pad if it's odd.
  chk_payload.unpack("n*").each {|x| checksum = checksum+x }
  checksum = checksum % 0xffff
  checksum = 0xffff - checksum
  checksum == 0 ? 0xffff : checksum
  @tcp_header.tcp_sum = checksum
end
tcp_recalc(arg=:all) click to toggle source

Recalculates various fields of the TCP packet.

Parameters

:all
  Recomputes all calculated fields.
:tcp_sum
  Recomputes the TCP checksum.
:tcp_hlen
  Recomputes the TCP header length. Useful after options are added.
# File lib/packetfu/protos/tcp.rb, line 165
def tcp_recalc(arg=:all)
  case arg
  when :tcp_sum
    tcp_calc_sum
  when :tcp_hlen
    @tcp_header.tcp_recalc :tcp_hlen
  when :all
    @tcp_header.tcp_recalc :all
    tcp_calc_sum
  else
    raise ArgumentError, "No such field `#{arg}'"
  end
end

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