Understanding Linux Network Internals

Data Structures Featured in This Part of the Book

The section "Main IPv4 Data Structures" in Chapter 19 gave a brief overview of the main data structures. This section has a detailed description of each data structure type. Figure 23-3 shows the file that defines each data structure.

iphdr Structure

The meaning of its fields has already been covered in the section "IP Header" in Chapter 18.

ip_options Structure

This structure represents the options for a packet that needs to be transmitted or forwarded. The options are stored in this structure because it is easier to read than the corresponding portion of the IP header itself.

Figure 23-3. Distribution of data structures in kernel files

Let's go field by field. They should be fairly simple to understand if you have read the section "IP Options" in Chapter 18. After this description, you will be able to understand more easily how the parsing is done and how its results are used by the IP layer subsystems, such as the code that processes incoming IP packets. Some of the bit fields are grouped together into an unsigned char; the declarations of these end with :1.

unsigned char optlen

Length of the set of options. As explained in Chapter 18, this is limited to a maximum of 40 bytes by the definition of the IP header.

unsigned char is_changed:1

Set if the IP header has been modified (such as an IP address or a timestamp). This is useful to know ...