compute CRC of struct in Python

Question

I have the following struct, from the NRPE daemon code in C:

typedef struct packet_struct {
  int16_t packet_version;
  int16_t packet_type;
  uint32_t crc32_value;
  int16_t result_code;
  char buffer[1024];
} packet;

I want to send this data format to the C daemon from Python. The CRC is calculated when crc32_value is 0, then it is put into the struct. My Python code to do this is as follows:

cmd = '_NRPE_CHECK'
pkt = struct.pack('hhIh1024s', 2, 1, 0, 0, cmd)
# pkt has length of 1034, as it should
checksum = zlib.crc32(pkt) & 0xFFFFFFFF
pkt = struct.pack('hhIh1024s', 2, 1, checksum, 0, cmd)
socket.send(....)

The daemon is receiving these values: version=2 type=1 crc=FE4BBC49 result=0

But it is calculating crc=3731C3FD

The actual C code to compute the CRC is:

https://github.com/KristianLyng/nrpe/blob/master/src/utils.c

and it is called via:

calculate_crc32((char *)packet, sizeof(packet));

When I ported those two functions to Python, I get the same as what zlib.crc32 returns.

Is my struct.pack call correct? Why is my CRC computation differing from the server's?

Answer 1

From the Python struct documentation:

To handle platform-independent data formats or omit implicit pad bytes, use standard size and alignment instead of native size and alignment: see Byte Order, Size, and Alignment for details.

Use '!' as the first format character to make the packed structure platform-independent. It forces big-endian, standard type sizes, and no pad bytes. Then the CRCs should be consistent.