Trivial File Transfer Protocol Server (TFTP) implementation in C

TFTP is a (trivial) File Transfer Protocol that ensures reliable data transfer over UDP. It does this by using a subset of mechanics that make TCP reliable.
The full RFC can be found below, with the packet structure highlighted in this read me.

You can find the full specification here: https://datatracker.ietf.org/doc/html/rfc1350

Usage

Lauch server:

$ clang -pedantic main.c tftp_server.c && ./a.out

Read file via tftp

$ tftp -e 127.0.0.1
get payload.txt

Use the following tcpdump command to view network traffic:

tcpdump -i lo0 -n udp port 69

Packet structure

The TFTP header of a packet contains one of the following opcodes to indicate the intended operation, these are:

opcode  operation
  1     Read request (RRQ)
  2     Write request (WRQ)
  3     Data (DATA)
  4     Acknowledgment (ACK)
  5     Error (ERROR)

Initial Read Request

ReadReq acts as the initial read request packet (RRQ) informing the server which file it would like to read

2 bytes     string    1 byte     string   1 byte
------------------------------------------------
| Opcode |  Filename  |   0  |    Mode    |   0  |
------------------------------------------------

Data Packet

The data packet contains an opcode, the block number (a monotonically incrementing number used by the server to ensure reliable transfer by waiting for the client to ack the packet by sending the block number back to the server)

2 bytes     2 bytes      n bytes
----------------------------------
| Opcode |   Block #  |   Data     |
----------------------------------

ACK Packet

Ack responds to the server with a block number to inform the server which packet it just received

2 bytes     2 bytes
---------------------
| Opcode |   Block #  |
---------------------