Chapter 2. The

proc

File System

49

  threads max

  Sets the maximum number of threads to be used by the kernel, with a default

value of

2048

.

  version

  Displays the date and time the kernel was last compiled. The first field in this file, such

as

#3

, relates to the number of times a kernel was built from the source base.

The

random

directory stores a number of values related to generating random numbers for the kernel.

2.3.9.4.

/proc/sys/net/

This directory contains assorted directories concerning various networking topics. Various configura 

tions at the time of kernel compilation make available different directories here, such as

appletalk

,

ethernet

,

ipv4

,

ipx

, and

ipv6

. Within these directories, you can adjust the assorted networking

values for that configuration on a running system.

Given the wide variety of possible networking options available with Linux and the great amount of

space required to discuss them, only the most common

/proc/sys/net/

directories will be dis 

cussed.

The

core

directory contains a variety of settings that control the interaction between the kernel and

networking layers. The most important files there are:

  message_burst

  The amount of time in tenths of a second required to write a new warning

message. This is used to prevent Denial of Service (DoS) attacks. The default setting is

50

.

  message_cost

  Also used to prevent DoS attacks by placing a cost on every warning message.

The higher the value of this file (default of

5

), the more likely the warning message will be ignored.

The idea of a DoS attack is to bombard your system with requests that generate errors and fill up

disk partitions with log files or require all of your system's resources to handle the error logging.

The settings in

message_burst

and

message_cost

are designed to be modified based on your

system's acceptable risk versus the need for comprehensive logging.

  netdev_max_backlog

  Sets the maximum number of packets allowed to queue when a partic 

ular interface receives packets faster than the kernel can process them. The default value for this

file is

300

.

  optmem_max

  Configures the maximum ancillary buffer size allowed per socket.

  rmem_default

  Sets the receive socket buffer's default size in bytes.

  rmem_max

  Sets the receive socket buffer's maximum size in bytes.

  wmem_default

  Sets the send socket buffer's default size in bytes.

  wmem_max

  Sets the send socket buffer's maximum size in bytes.

The

/ipv4

directory contains additional networking settings. Many of these settings, used in con 

junction with one another, are very useful in preventing attacks on the system or using the system to

act as a router.

Caution

An erroneous change to these files may affect your remote connectivity to the system.

Here are some of the most important files in the

/proc/sys/net/ipv4/

directory:

  icmp_destunreach_rate

,

icmp_echoreply_rate

,

icmp_paramprob_rate

and

icmp_timeexeed_rate

  Set the maximum ICMP send packet rate, in hundredths of a second,

to hosts under certain conditions. A setting of

0

removes any delay and is not a good idea.