Kernel

  1. Kernel Modules - lsmod, insmod rmmod and modprobe commands
  2. Proc Filesystem
  3. Changing kernel parameters using sysctl

Kernel Modules

Many components of the kernel can be compiled as dynamically loadable modules. This allows for increased kernel functionality without increasing the size of the kernel image loaded at the boot time.

Good candidates for modularization are

  • Components that need not be resident in the kernel for all configurations and hardware
  • peripheral device drivers
  • supplementary filesystems

The kernel modules reside in /lib/modules/<kernel_version> directory

Controlling modules

  • lsmod lists the modules currently resident in the kernel
  • insmod is used to load a particular module
  • rmmod unloads a module if it is idle

Controlling modules intelligently

Many modules depends on other module to be present.

  • A database of module dependencies can be generated using depmod command
  • modinfo shows the information about a linux kernel module
  • modprobe can be used to load kernel modules

The advandage of using modprobe over insmod are:

  • modprobe will load any underlying modules required by a given module
  • modprobe will consult /etc/modprobe.conf (/etc/modules.conf in old systems) for default module configurations
    modprobe -v module_name

Module configuration
Many modules accept parameters that can be specified at the load time. Default values for various parameters can be specifield in /etc/modprobe.conf (/etc/modules.conf in old systems). When modprobe loads a module, it will consult this file for appropriate defaults.

Examples: 

# lsmod
Module                  Size  Used by
md5                     4161  1 
ipv6                  268865  14 
sunrpc                168453  1 
dm_mod                 58613  0 
uhci_hcd               35281  0 
i2c_piix4               8913  0 
i2c_core               21825  1 i2c_piix4
......
......

# modinfo md5
filename:       /lib/modules/2.6.12-1.1447_FC4/kernel/crypto/md5.ko
license:        GPL
description:    MD5 Message Digest Algorithm
vermagic:       2.6.12-1.1447_FC4 686 REGPARM 4KSTACKS gcc-4.0
depends:        
srcversion:     70F89E71D506DA03F4BDCA3


# cat /etc/modprobe.conf
alias eth0 e100
alias eth1 3c59x
alias snd-card-0 snd-cs4236
options snd-card-0 index=0
options snd-cs4236 index=0

Tips: If a kernel module is not loading (test it using lsmod), run depmod command first. It will create all the dependents for all the modules. Then load the module using modprobe -v <module_name> command.

The /proc filesystem

  • /proc is a virtual filesystem containing information about the running kernel
  • It is a map to the running kernel
  • Contens of “files” under /proc can be viewed using ‘cat’ commnad
  • Provides information on system hardware, networking settings and activity, memory usage and more.
  • /proc has number of subdirectories
  • The /proc/sys subdirectory allows administrators to modify certain parameters of a running kernel

Some of the key files in top-level directory include:

/proc/interrupts, /proc/dma, /proc/ioports - IRQ, DMA and I/O settings
/proc/cpuinfo - Information about the system CPUs
/proc/meminfo - Information on availabel, free memory, swap, cached memory
/proc/version - Information on Kernel version, build host, build date, etc.

Some important subdirectories include:

/proc/scsi, /proc/ide -- information about SCSI and IDE devices
/proc/net -- Network activity and configuration
/proc/sys -- Kernel configuration parameters
/proc/<PID> -- information about process ID

The /proc/sys subdirectory allows administrators to modify certain parameters of a running kernel.

echo “1” > /proc/sys/net/ipv4/ip_forward # Turn on IP forwarding.

Changing Kernel Parameters (/proc/sys configuration) using sysctl :

  • /proc/sys modifications are temporary and not saved at system shutdown
  • The sysctl command manages such settings in a static and centralized fashion using /etc/sysctl.conf file
  • sysctl is called at boot time by rc.sysinit and uses settings in /etc/sysctl.conf

With the sysctl, you specify a path to the variable, with /proc/sys being the base.

For example, to view /proc/sys/net/ipv4/ip_forward, use the following command: 

  # sysctl net.ipv4.ip_forward
  net.ipv4.ip_forward = 1

To then update this variable temporarily, use the -w (write) option: 

  # sysctl -w net.ipv4.ip_forward="0"
  net.ipv4.ip_forward = 0

The same thing can be done by modifying /proc/sys/net/ipv4/ip_forward file 

  # echo "0" > /proc/sys/net/ipv4/ip_forward

To make it permanent, edit /etc/sysctl.conf file. Add or modify the "net.ipv4.ip_forward" line 

   net.ipv4.ip_forward=0

To re-read (make the changes to take effect) the /etc/sysctl.conf file, use the -p option 

 # sysctl -p 
 net.ipv4.ip_forward = 0
 net.ipv4.conf.default.rp_filter = 1