Understanding Linux Network Internals

General Packet Handling

The rest of this chapter covers some general considerations that the kernel has to take into account when handling ingress IP packets, such as checksumming and options. Subsequent chapters go into detail about how they are forwarded, transmitted, and fragmented/defragmented.

Protocol Initialization

The IPv4 protocol is initialized by ip_init, defined in net/ipv4/ip_output.c. Because IPv4 support cannot be removed from the kernel (i.e., it cannot be compiled as a module), there is no ip_uninit function.

Here are the main tasks accomplished by ip_init:

• Register the handler for IP packets with the dev_add_pack function (see Chapter 13). This handler is a function named ip_rcv.

• Initialize the routing subsystem, including the protocol-independent cache (see Chapter 32).

• Initialize the infrastructure used to manage IP peers (see the section "Long-Living IP Peer Information" in Chapter 23.

ip_init is invoked at boot time by inet_init, which takes care of the initialization of all the subsystems related to IPv4, including the L4 protocols.

Interaction with Netfilter

We will not examine the Netfilter firewalling subsystem in this book, but we can examine its main working principles now, particularly its relationship to the aspects of the IPv4 implementation we discuss in this part of the book.

Firewalling, essentially, hooks into certain places in the network stack code that packets always pass through when the packets or the kernel meet certain conditions; at those points, ...