Red Hat LINUX 7.2 Reference Manual page 95

Section 4.3:Directories in /proc
• ip_forward — Permits interfaces on the system to forward packets to one other. By default,
this file is set to 0 to disable forwarding, but setting this file to 1 will enable forwarding.
• ip_local_port_range — Specifies the range of ports to be used by TCP or UDP when
a local port is needed. The first number is the lowest port to be used, and the second number
specifies the highest port. Any systems that expect to require more ports than the default 1024 to
4999 should use the 32768 to 61000 range in this file.
• tcp_syn_retries — Provides a limit on the number of times your system will re-transmit a
SYN packet when attempting to make a connection.
• tcp_retries1 — Sets the number of permitted re-transmissions attempting to answer an in-
coming connection. Default of 3.
• tcp_retries2 — Sets the number of permitted re-transmissions of TCP packets. Default of
15.
For a complete list of files and options available, see /usr/src/linux-2.4/Documenta-
tion/networking/ip-sysctl.txt.
A number of other directories exist within the /proc/sys/net/ipv4 directory cover specific
topics. The conf directory allows each of the systems interfaces to be configured in different ways,
including the use of a default settings for unconfigured devices (in the default subdirectory) and
settings that override all special configurations (in the all subdirectory).
In order to control connections between direct neighbors, meaning any other system directly connected
to your system, the neigh directory allows special configurations for each interface. This would
allow you to treat systems differently that you might trust more due to their relatively proximity or
relation to your system. At the same time, it also makes it easy to put strict rules in place for systems
several hops away.
Routing over IPV4 also has its own directory, route. Unlike conf and neigh, the route directory
contains specifications that apply to routing with any interfaces on the system. Many of these settings,
such as max_size, max_delay, and min_delay, relate to controlling the size of the routing
cache. To clear the routing cache, simply write any value to the flush file.
Additional information about these directories and the possible values for their configuration files can
be found in /usr/src/linux-2.4/Documentation/filesystems/proc.txt.
/proc/sys/vm
This directory facilitates the configuration of the Linux kernel's virtual memory (VM) subsystem. The
kernel makes extensive and intelligent use of virtual memory, which is commonly called swap space.
The following files are commonly found in the /proc/sys/vm directory:
• bdflush — Sets various values related to the bdflush kernel daemon.
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