Understanding Linux Network Internals

Tuning via /proc Filesystem

The /proc filesystem was introduced in Chapter 3; it provides a simple interface for users to view and change kernel parameters and is the model for the newer sysfs directory. It contains a huge number of files (or rather, virtual data structures that look to the user just like files) that map to variables and functions inside the kernel and that can be used to tune the behavior of the networking component of the kernel as well.

The files used for IPv4 tuning are located mainly in two directories:

/proc/sys/net/ipv4/

Table 23-2 shows some of the files in this directory that are used by IPv4. The kernel variables associated with those files are declared in net/ipv4/sysctl_net_ipv4.c and are statically registered at boot time (see Chapter 3). Note that the directory contains many more files than the ones in Table 23-2. Most of the extra files are associated with L4 protocols, especially TCP.

/proc/sys/net/ipv4/conf/

This directory contains a subdirectory for each network device recognized by the kernel, plus other special directories (see Figure 36-4 in Chapter 36). Those subdirectories include configuration parameters that are device specific; among them are accept_redirects, send_redirects, accept_source_route, and forwarding. These will be covered in Chapter 36, with the exception of promote_secondaries, which is described in the section "Main Functions That Manipulate IP Addresses and Configuration."

Table 23-2. IPv4-related files in /proc/sys/net/ipv4 ...