Error in packet parsing - c

Question

i trying to parse a packet. till the ip header everything is fine(i'm able to retrieve all the values correctly). but for the udp header( checked if the protocol is 17) , the values are coming out to be wrong( all the 4 fields).
I'm trying to do this:
struct udp_header{
uint16_t sport;
uint16_t dport;
uint16_t len;
uint16_t chksum;
};
struct udp_header* udp= (struct udp_header*)(packet + 14 + ip_hdr->ip_hl*4);
Packet is the pointer pointing to the beginning of the packet. 14 is for ethernet header.The header length ip when checked is giving out the correct value. But after performing this operation i'm getting all the fields wrongly. when tried with uint8_t as data type( i know its wrong! ) the destintion port somehow is coming out correct.

Answer 1

You have run into endianness. IP packets have all fields in network byte order (aka "big-endian"), and your host system probably runs little-endian. Look into ntohs() and friends for one approach.
The proper approach is to not copy the structure as-is from the network data, but instead extract each field manually and byte-swap it if necessary. This also works around any issues with padding and alignment, there's no guarantee that your struct is mapped into your computer's memory in exactly the same way as the packet is serialized.
So you would do e.g.:
udp_header.sport = ntohs(*(unsigned short*) (packet + 14 + 4 * ip_hdr->ip_hl));
This is also a bit iffy, since it assumes the resulting address can validly be cast into a pointer to unsigned short. On x86 that will work, but it's not epic.
Even better, in my opinion, is to drop the use of pointers and instead write a function called e.g. unsigned short read_u16(void *packet, size_t offset) that extracts the value byte-by-byte and returns it. Then you'd just do:
udp_header.sport = read_u16(packet, 14 + 4 * ip_hdr->ip_hl);

Answer 2

I always use this struct for IP header:
struct sniff_ip {
u_char ip_vhl; /* version << 4 | header length >> 2 */
u_char ip_tos; /* type of service */
u_short ip_len; /* total length */
u_short ip_id; /* identification */
u_short ip_off; /* fragment offset field */
#define IP_RF 0x8000 /* reserved fragment flag */
#define IP_DF 0x4000 /* dont fragment flag */
#define IP_MF 0x2000 /* more fragments flag */
#define IP_OFFMASK 0x1fff /* mask for fragmenting bits */
u_char ip_ttl; /* time to live */
u_char ip_p; /* protocol */
u_short ip_sum; /* checksum */
struct in_addr ip_src,ip_dst; /* source and dest address */
};
#define IP_HL(ip) (((ip)->ip_vhl) & 0x0f)
#define IP_V(ip) (((ip)->ip_vhl) >> 4)
And to get the UDP struct pointer:
udp = (struct sniff_udp*)(packet + SIZE_ETHERNET + (IP_HL(ip)*4));

Answer 3

As another answer remarked, you have to deal with endianness of your data.
The other thing you need to deal with is byte alignment. For speed, when you define a structure in C like this:
struct udp_header{
uint16_t sport;
uint16_t dport;
uint16_t len;
uint16_t chksum;
};
The C compiler may leave padding bytes between these fields so that member accesses can be done with faster single-instruction memory access assembly instructions. You can check if your c compiler is doing this by printf("struct size is: %u\n", sizeof(struct udp_header));
Assuming you are using GCC, you must disable padding bytes by adding #pragma pack(1) before the structure definition. To re-enable padding for speed you should then use #pragma pack().