Question

I have defined the following struct to represent an IPv4 header (up until the options field):

struct IPv4Header
{
    // First row in diagram
    u_int32 Version:4;
    u_int32 InternetHeaderLength:4;     // Header length is expressed in units of 32 bits.
    u_int32 TypeOfService:8;
    u_int32 TotalLength:16;

    // Second row in diagram
    u_int32 Identification:16;
    u_int32 Flags:3;
    u_int32 FragmentOffset:13;

    // Third row in diagram
    u_int32 TTL:8;
    u_int32 Protocol:8;
    u_int32 HeaderChecksum:16;

    // Fourth row in diagram
    u_int32 SourceAddress:32;

    // Fifth row in diagram
    u_int32 DestinationAddress:32;
};

I now also captured an IP frame with Wireshark. As an array literal it looks like this:

// Captured with Wireshark
const u_int8 cIPHeaderSample[] = {
    0x45, 0x00, 0x05, 0x17,
    0xA7, 0xE0, 0x40, 0x00,
    0x2E, 0x06, 0x1B, 0xEA,
    0x51, 0x58, 0x25, 0x02,
    0x0A, 0x04, 0x03, 0xB9
};

My question is: How can I create a IPv4Header object using the array data?

This doesn’t work because of incompatible endianness:

IPv4Header header = *((IPv4Header*)cIPHeaderSample);

I’m aware of the functions like ntohs and ntohl, but it can’t figure out how to use them correctly:

u_int8 version = ntohs(cIPHeaderSample[0]);
printf("version: %x \n", version);

// Output is:
// version: 0

Can anyone help?

Answer 1

The most portable way to do it is one field at a time, using memcpy() for types longer than a byte. You don’t need to worry about endianness for byte-length fields:

uint16_t temp_u16;
uint32_t temp_u32;
struct IPv4Header header;

header.Version = cIPHeaderSample[0] >> 4;

header.InternetHeaderLength = cIPHeaderSample[0] & 0x0f;

header.TypeOfServer = cIPHeaderSample[1];

memcpy(&temp_u16, &cIPHeaderSample[2], 2);
header.TotalLength = ntohs(temp_u16);

memcpy(&temp_u16, &cIPHeaderSample[4], 2);
header.Identification = ntohs(temp_u16);

header.Flags = cIPHeaderSample[6] >> 5;

memcpy(&temp_u16, &cIPHeaderSample[6], 2);
header.FragmentOffset = ntohs(temp_u16) & 0x1fff;

header.TTL = cIPHeaderSample[8];

header.Protocol = cIPHeaderSample[9];

memcpy(&temp_u16, &cIPHeaderSample[10], 2);
header.HeaderChecksum = ntohs(temp_u16);

memcpy(&temp_u32, &cIPHeaderSample[12], 4);
header.SourceAddress = ntohl(temp_u32);

memcpy(&temp_u32, &cIPHeaderSample[16], 4);
header.DestinationAddress = ntohl(temp_u32);