Regarding the RFC of TFTP (Trivial File Transfer Protocol), the RRQ/WRQ (Read\write requests) packet has this format:
2 bytes string 1 byte string 1 byte
------------------------------------------------
| Opcode | Filename | 0 | Mode | 0 |
------------------------------------------------
Mode can be either "netascii", "octet" (equivalent to binary) or "mail". The thing is netascii is 8 letters, octet has 5 and mail has 4.
I am creating my packet in my client like this:
paq = struct.pack('!H'+str(len(fileName))+'sB'+str(len(mode))+'sB', 02, fileName, 0, mode, 0)
And then I send the packet to the server so the server knows what to expect (A read in case of an RRQ or a write otherwise).
The thing is, I don't know how to unpack the packet if I don't know the string lengths on the server's side... Only the client knows the file length and the mode length, since he makes the packet.
Should I send the lengths to the server before the RRQ/WRQ packet so I know the format to use when unpacking? Is there another way?
Thank you!
If the received packet is in the byte array p, you can search for the 0 delimiters with find().
opcode = p[0:2].decode('ASCII')
nameEnd = p.find(b'\0', start=2)
filename = p[2:nameEnd].decode('ASCII')
modeEnd = p.find(b'\0', start=nameEnd+1)
mode = p[nameEnd+1:modeEnd].decode('ASCII')