Understanding Linux Network Internals

ioctl

At the top of Figure 3-4, you can see how an ioctl call is issued. Let's see an example involving ifconfig.

We said earlier that the ifconfig command uses ioctl to communicate with the kernel. For example, when the system administrator types a command like ifconfig eth0 mtu 1250 to change the MTU of the interface eth0, ifconfig opens a socket, initializes a local data structure with the information received from the system administrator (data in the example), and passes it to the kernel with an ioctl call. SIOCSIFMTU is the command identifier.

    struct ifreq data;
    fd = socket(PF_INET, SOCK_DGRAM, 0);
    < ... initialize "data" ...>
    err = ioctl(fd, SIOCSIFMTU, &data);

ioctl commands are processed by the kernel in different places. Figure 3-4 shows how the most common ioctl commands used by the networking code are dispatched by sock_ioctl and routed to the right function handler. We will not see how sock_ioctl is invoked or how transport protocols like UDP and TCP register their handlers. If you desire to dig into this part of the code, you can use the figure as a starting point. For the routines that we cover in this book, the figure provides a reference to the right chapter.

Figure 3-3. Creation of the core directories in /proc/sys/net

The name of the ioctl commands in the figure is parsed (split into components) for your convenience. For example, the command used to add a route to a ...