A struct in C programming language is a structured type that aggregates a fixed set of labelled objects, possibly of different types, into a single object.A struct declaration consists of a list of fields, each of which can have any type...
(structure) in the C programming language. The ip struct is used as a template to form an IPv4 header
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
In computer networking, a raw socket is a socket that allows direct sending and receiving of network packets by applications, bypassing all encapsulation in the networking software of the operating system. Most socket application programming interfaces , especially those based on Berkeley sockets,...
. The structure can be found in the default include files of most Unix distributions. It is most commonly located in the header file
Header file
Some programming languages use header files. These files allow programmers to separate certain elements of a program's source code into reusable files. Header files commonly contain forward declarations of classes, subroutines, variables, and other identifiers...
.
Definition
struct ip {
unsigned int ip_hl:4; /* both fields are 4 bits */
unsigned int ip_v:4;
uint8_t ip_tos;
uint16_t ip_len;
uint16_t ip_id;
uint16_t ip_off;
uint8_t ip_ttl;
uint8_t ip_p;
uint16_t ip_sum;
struct in_addr ip_src;
struct in_addr ip_dst;
};
Fields
unsigned int ip_hl:4
IP header length expressed as a multiple of 32-bit octets or DWORDS (i.e. header length in bytes = value set in ip_hl x 4 [each # counts for 4 octets]). From the hex dump of an IP header this can be read off the value of an unsigned character at offset 0. Typically it will read 45 where 5 is a common default for ip_hl and 4 is ip_v.
Common Defaults: 5; sets header length to 20 bytes (header length without any routing options)
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
The type of service field in the IPv4 header has had various purposes over the years, and has been defined in different ways by five RFCs...
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).
Common Defaults: 0x00 (normal)
unsigned short int ip_len;
Total length must contain the total length of the IP datagram
Datagram
A datagram is a basic transfer unit associated with a packet-switched network in which the delivery, arrival time, and order are not guaranteed....
The Internet Control Message Protocol is one of the core protocols of the Internet Protocol Suite. It is chiefly used by the operating systems of networked computers to send error messages indicating, for example, that a requested service is not available or that a host or router could not be...
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
header and payload size in bytes.
unsigned short int ip_id;
The ID sequence number is mainly used for reassembly of fragmented IP datagrams.
Common Defaults: Single datagrams - arbitrary ID, Multiple datagrams - sequential ID.
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)
Common Defaults: 64, 255 (max)
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.
struct in_addr ip_src;
Source IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
- must be converted into binary format (suggested function is inet_pton)
struct in_addr ip_dst;
Destination IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
- must be converted into binary format (suggested function is inet_pton)
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