ip is a struct (structure) in the C programming language. The ip struct is used as a template to form an IPv4 header in a raw socket. The structure can be found in the default include files of most Unix distributions. It is most commonly located in the
unsigned int ip_hl:4IP header length in 32-bit octets (i.e. value set in ip_hl * 4 = header length in bytes)
unsigned int ip_v:4Internet Protocol version
unsigned char ip_tos;Type of Service controls the priority of the packet. The first 3 bits stand for routing priority, the next 4 bits for the type of service (delay, throughput, reliability and cost).
unsigned short int ip_len;Total length must contain the total length of the IP datagram. This includes IP, ICMP, TCP or UDP header and payload size in bytes.
unsigned short int ip_id;The ID sequence number is mainly used for reassembly of fragmented IP datagrams.
unsigned short int ip_off;The fragment offset is used for reassembly of fragmented datagrams. The first 3 bits are the fragment flags, the first one always 0, the second the do-not-fragment bit (set by ip_off |= 0x4000) and the third the more-flag or more-fragments-following bit (ip_off |= 0x2000). The following 13 bits is the fragment offset, containing the number of 8-byte big packets already sent.
unsigned char ip_ttl;Time to live is the amount of hops (routers to pass) before the packet is discarded, and an ICMP error message is returned. Can sometimes be used to reverse engineer the client distance from server (e.g. if ttl = 250 at server, client is probably 5 hops away)
unsigned char ip_p;The transport layer protocol. Can be tcp (6), udp(17), icmp(1), or whatever protocol follows the IP header. Look in /etc/protocols for more.
unsigned short int ip_sum;The header checksum. Every time anything in the header changes, it needs to be recalculated, or the packet will be discarded by the next router.
unsigned int ip_src;Source IP address - must be converted into binary format (suggested function is inet_pton())
unsigned int ip_dst;Destination IP address - must be converted into binary format (suggested function is inet_pton())