CODEWITHSUNDEEP
cstructraw-sockets
K. Shores
K. Shores

Reputation: 1005

Layout of an ip header and network programming

I am taking a class on computer and network security. We are writing a packet spoofer. I could just download one from the internet and use it, but I prefer writing the stuff myself. Below is the struct that I use to represent the ip header which I am basing off of the wikipedia article. I am attempting to send an icmp ping packet. I have done it successfully, but only after assigning the value of the ip header length to the version field, and vice versa. Somehow I have setup my struct wrong, or I am assigning the values wrong, and I am not sure what I am doing incorrectly.

struct ip_header
{
    uint8_t version : 4 // version
        , ihl : 4; // ip header length
    uint8_t dscp : 6 // differentiated services code point
        , ecn : 2; // explicit congestion notification
    uint16_t total_length; // entire packet size in bytes
    uint16_t identification; // a unique identifier
    uint16_t flags : 3 // control and identify fragments
        , frag_offset : 13; // offset of fragment relative to the original
    uint8_t ttl; // how many hops the packet is allowd to travel
    uint8_t protocol; // what protocol is in use
    uint16_t checksum; // value used to determine bad packets
    uint32_t src_ip; // where the packet is form
    uint32_t dest_ip; // where the packet is going
};

If I assign the version and ihl, like below, wireshark reports an error with the header, "Bogus IPV4 version (0, must be 4)".

char buffer[1024];
struct ip_header* ip = (struct ip_header*) buffer;
ip->version = 4;
ip->ihl = 5;

However, after changing to the following listing, the ICMP request goes through just fine.

char buffer[1024];
struct ip_header* ip = (struct ip_header*) buffer;
ip->version = 5;
ip->ihl = 4;

I have tried placing htons around the numbers, but that doesn't seem to do anything useful. What am I missing here?

Upvotes: 0

Views: 1048

Answers (1)

Sky Dynamics
Sky Dynamics

Reputation: 91

You simply need to correct your structure's endianness. Look at the IP header structure defined in the <netinet/ip.h> file:

  struct iphdr
  {
#if __BYTE_ORDER == __LITTLE_ENDIAN
    unsigned int ihl:4;
    unsigned int version:4;
#elif __BYTE_ORDER == __BIG_ENDIAN
    unsigned int version:4;
    unsigned int ihl:4;
#else
# error "Please fix <bits/endian.h>"
#endif
    uint8_t tos;
    uint16_t tot_len;
    uint16_t id;
    uint16_t frag_off;
    uint8_t ttl;
    uint8_t protocol;
    uint16_t check;
    uint32_t saddr;
    uint32_t daddr;
    /*The options start here. */
  };

Upvotes: 3

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