Struct unpacking a Read Request in TFTP implementation

Question

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!

The strings are terminated by a 0 byte. Search for that byte and then extract the string up to it. — Barmar
– Barmar, Commented May 22, 2018 at 21:04
You can't use unpack() to extract the fields, except for the Opcode. — Barmar
– Barmar, Commented May 22, 2018 at 21:05
Are you sure the mode is binari? I'd expect it to be binary, since the designers of TFTP spoke English. — Barmar
– Barmar, Commented May 22, 2018 at 21:06
@Barmar actually there are three modes which are "netascii", "octet" (equivalent to binary), and "mail". Problem still the same.. different lenghts. How would you extract the string until it finds a 0? — xBurnsed
– xBurnsed, Commented May 22, 2018 at 21:14
If the packet is in p, p[2:p.find('\0')] is the filename — Barmar
– Barmar, Commented May 22, 2018 at 21:18

Barmar · Accepted Answer · 2018-05-22 21:23:30Z

1

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')

answered May 22, 2018 at 21:23

Barmar

789k57 gold badges555 silver badges669 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

xBurnsed Over a year ago

Thanks a lot! I really appreciate it! I knew how to use the packet[ : ] thing but I didn't know it could be combined with .find() method! I really appreciate your help!

Barmar Over a year ago

Why wouldn't you be able to combine it with find()? It's just numbers, it doesn't matter how you get the numbers.

Collectives™ on Stack Overflow

Struct unpacking a Read Request in TFTP implementation

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related