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Good evening everyone.

I'm having trouble creating a TFTP client for an assignment in C++ that uses #include

I'm sending the char buffer:

buffer={'0','2','f','i','l','e','n','a','m','e','0','o','c','t','e','t','0'};

Using the code:

sendto(sock,buffer,strlen(buffer), 0, (sockaddr *) &serverAddr, sizeof(sockaddr));

But when I look at this transfer in the WireShark it says "Opcode: Unknown (12338)" when I look at the Opcode portion of the packet. Even though it has selected the opcode from the string

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I'm absolutely stuck, any help would be appreciated. I'm pretty sure if I can send and receive a message I can handle the rest pretty easily.

Just throwing down here what I did but the marked answer was basically it Used the BYTE from winsock2 (don't know if I need to but better safe than sorry)

buffer={(BYTE)0,(BYTE)2,'f','i','l','e','n','a','m','e',(BYTE)0,'o','c','t','e','t',(BYTE)0};

and

sendto(sock,buffer,sizeof(buffer), 0, (sockaddr *) &serverAddr, sizeof(sockaddr));

Siemsen
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2 Answers2

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Not strlen(buffer) but sizeof(buffer)...

strlen stops couting characters at the first occurrence of '\0' - so it returns unspecified value for your buffer array since '\0' is not present in your buffer.

If you want to start your buffer with opcode 02 then pass it as integers not their digit representation: 

buffer={0,2,'f','i','l','e','n','a','m','e','0','o','c','t','e','t','0'};
//      ^ ^  simple numbers - not characters - not sure about rest of your buffer
PiotrNycz
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  • It is basically what you said but I convert the decimal using the BYTE cast from winsock2. So I'm marking your answer as correct. – Siemsen Oct 21 '12 at 18:35
  • Originally I thought you were suggesting \0 was in the buffer, but later realized that strlen() will return lengths much larger than the buffer. You should probably cast the first 2 entries in the buffer either to a (char) or (uint_8) – Brady Oct 21 '12 at 18:46
  • @Brady - not sure if this casting is needed - maybe with some pedantic compiler options. C++ allows here implicit casting. Anyway OP declared to add this casting in previous comment. – PiotrNycz Oct 21 '12 at 20:05
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12338 in decimal is 0x30, 0x32 in hex, which corresponds to '0' and '2' from your buffer.

Your using ascii chars, but you need to use a decimal/integer, by setting them to (char) 0x00 and (char) 0x02 in your buffer.

Brady
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  • Yeah, I'm really tired at this point. If it is any consolation the original version I had created looked gorgeous. – Siemsen Oct 21 '12 at 18:37