TFTP Packet Factory

Straightforward, but fun.

class PacketFactory # TFTP = RFC 1350, 2347, 2348 (not needed here)
#                                                                             #
#       2 bytes  string  1b   string  1b  string   1b  string   1b ...        #
#      +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+-->       #
#      |   01  |filename| 0 |  mode  | 0 |  opt1  | 0 | value1 | 0 | <        #
#      +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+-->       #
#        string  1b   string  1b                                              #
#       >-------+---+---~~---+---+                                            #
#      <  optN  | 0 | valueN | 0 |                                            #
#       >-------+---+---~~---+---+                                            #
#                                                                             #
#                             RRQ packet                                      #
#                                                                             #
#  The mode field contains the string "netascii", "octet", or "mail"          #
#                                                                             #
   def buildRRQ( filename, mode, *opts )
      packet = "\0\1" + filename + "\0" + mode + "\0"
      opts.each { |o| packet += o + "\0" }
      return packet
   end

#                                                                             #
#       2 bytes  string  1b   string  1b  string   1b  string   1b ...        #
#      +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+-->       #
#      |   02  |filename| 0 |  mode  | 0 |  opt1  | 0 | value1 | 0 | <        #
#      +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+-->       #
#        string  1b   string  1b                                              #
#       >-------+---+---~~---+---+                                            #
#      <  optN  | 0 | valueN | 0 |                                            #
#       >-------+---+---~~---+---+                                            #
#                                                                             #
#                             WRQ packet                                      #
#                                                                             #
   def buildWRQ( filename, mode, blocksize, octets )
      packet = "\0\2" + filename + "\0"
                      + mode + "\0"
                      + blocksize + "\0"
                      + octets + "\0"
      return packet
   end
#                                                                             #
#                   2 bytes     2 bytes      n bytes                          #
#                   ----------------------------------                        #
#                  |   03   |   Block #  |   Data     |                       #
#                   ----------------------------------                        #
#                                                                             #
#                              DATA packet                                    #
#                                                                             #
   def buildDAT( blocknum, data )
      return "\0\3" + [blocknum, data].flatten.pack("n*")
   end
#                                                                             #
#                         2 bytes     2 bytes                                 #
#                         ---------------------                               #
#                        |   04   |   Block #  |                              #
#                         ---------------------                               #
#                                                                             #
#                              ACK packet                                     #
#                                                                             #
   def buildACK( blocknum ) return "\0\4" + [blocknum].pack("n") end

   def buildACKfromDAT( packet )
      packet = packet[0..3]
      packet[1] = "\04"
      return packet
   end
#                                                                             #
#       2 bytes  string  1b   string  1b...string  1b   string  1b            #
#      +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+          #
#      |   06  |  opt1  | 0 | value1 | 0 |  optN  | 0 | valueN | 0 |          #
#      +-------+---~~---+---+---~~---+---+---~~---+---+---~~---+---+          #
#                                                                             #
#                           OACK packet [RFC 2347]                            #
#                                                                             #
   def buildOACK( opt_hash )
      packet = "\0\6"
      opt_hash.each_pair { |k,v| packet += k + "\0" + v + "\0" }
      return packet
   end

#                                                                             #
#               2 bytes     2 bytes      string    1 byte                     #
#               -----------------------------------------                     #
#              |   05   |  ErrorCode |   ErrMsg   |   0  |                    #
#               -----------------------------------------                     #
#                                                                             #
#                             ERROR packet                                    #
#                                                                             #
#   Error Codes                                                               #
#                                                                             #
#      Value     Meaning                                                      #
#                                                                             #
#      0         Not defined, see error message (if any).                     #
#      1         File not found.                                              #
#      2         Access violation.                                            #
#      3         Disk full or allocation exceeded.                            #
#      4         Illegal TFTP operation.                                      #
#      5         Unknown transfer ID.                                         #
#      6         File already exists.                                         #
#      7         No such user.                                                #
#      8         Invalid option value requested.  [RFC 2347]                  #
#                                                                             #
   def buildERR( errcode, errmsg )
      packet = "\0\5" + [errcode, errmsg].pack("na*x")
      return packet
   end
end