YUM Configuration in Hindi - Excellent

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understanding Linux configuration files

Introduction

Every Linux program is an executable file holding the list of opcodes the CPU executes to accomplish specific operations. For instance, the ls command is provided by the file /bin/ls, which holds the list of machine instructions needed to display the list of files in the current directory onto the screen. The behaviour of almost every program can be customized to your preferences or needs by modifying its configuration files.

Is there a standard configuration file format in Linux?

In a word, no. Users who are new to Linux (rightly) feel frustrated that each configuration file looks like a new challenge to figure out. In Linux each programmer is free to choose the configuration file format he or she prefers. Format options range from the /etc/shells file, which contains a list of possible shells separated by a newline, to Apache's complex /etc/httpd.conf file.

What are system configuration files?

The kernel itself may be considered a "program." Why does the kernel need configuration files? The kernel needs to know the list of users and groups in the system, and manage file permissions (that is, determine if a file can be opened by a specific user, according to the permissions, UNIX_USERS). Note that these files are not specifically read by programs, but by a function provided by a system library, and used by the kernel. For instance, a program needing the (encrypted) password of a user should not open the /etc/passwd file. Instead, it should call the system library function getpw(). This kind of function is also known as a system call. It is up to the kernel (through the system library) to open the /etc/passwd file and after that, search for the password of the requested user.

Most of the configuration files in the Red Hat Linux system are in the /etc directory unless otherwise specified. The configuration files can be broadly classified into the following categories:

Access files

/etc/host.conf	Tells the network domain server how to look up hostnames. (Normally /etc/hosts, then name server; it can be changed through netconf.)
/etc/hosts	Contains a list of known hosts (in the local network). Can be used if the IP of the system is not dynamically generated. For simple hostname resolution (to dotted notation), /etc/hosts.conf normally tells the resolver to look here before asking the network nameserver, DNS or NIS.
/etc/hosts.allow	Man page same as hosts_access. Read by tcpd at least.
/etc/hosts.deny	Man page same as hosts_access. Read by tcpd at least.

Booting and login/logout

/etc/issue & /etc/issue.net	These files are read by mingetty (and similar programs) to display a "welcome" string to the user connecting from a terminal (issue) or through a telnet session (issue.net). They include a few lines stating the Red Hat release number, name, and Kernel ID. They are used by rc.local.
/etc/redhat-release	Includes one line stating the Red Hat release number and name. Used by rc.local.
/etc/rc.d/rc	Normally run for all run levels with level passed as argument. For example, to boot your machine in the Graphics mode (X-Server), run the following command from your command line: init 5. The runlevel 5 is starts the system in graphics mode.
/etc/rc.d/rc.local	Not official. May be called from rc, rc.sysinit, or /etc/inittab.
/etc/rc.d/rc.sysinit	Normally the first script run for all run levels.
/etc/rc.d/rc/rcX.d	Scripts run from rc (X stands for any number from 1 to 5). These directories are "run-level" specific directories. When a system starts up, it identifies the run-level to be initiated, and then it calls all the startup scripts present in the specific directory for that run-level. For example, the system usually starts up and the message "entering run-level 3" is shown after the boot messages; this means that all the init scripts in the directory /etc/rc.d/rc3.d/ will be called.

File system

The kernel provides an interface to display some of its data structures that can be useful for determining the system parameters like interrupts used, devices initialised, memory statistics, etc. This interface is provided as a separate but dummy filesystem known as the /proc filesystem. Many system utilities use the values present in this filesystemf or displaying the system statistics. For example, the file /proc/modules lists the currently loaded modules in the system. This information is read by the command lsmod, which then displays it in a human readable format. In the same way, the file mtab specified in the following table reads the /proc/mount file, which contains the currently mounted filesystems.

/etc/mtab	This changes continuously as the file /proc/mount changes. In other words, when filesystems are mounted and unmounted, the change is immediately reflected in this file.
/etc/fstab	Lists the filesystems currently "mountable" by the computer. This is important because when the computer boots, it runs the command mount -a, which takes care of mounting every file system marked with a "1" in the next-to-last column of fstab.
/etc/mtools.conf	Configuration for all the operations (mkdir, copy, format, etc.) on a DOS-type filesystem.

System administration

/etc/group	Contains the valid group names and the users included in the specified groups. A single user can be present in more than one group if he performs multiple tasks. For example, is a "user" is the administrator as well as a member of the project group "project 1", then his entry in the group file will look like: user: * : group-id : project1
/etc/nologin	If the file /etc/nologin exists, login(1) will allow access only to root. Other users will be shown the contents of this file and their logins refused.
etc/passwd	See "man passwd". Holds some user account info including passwords (when not "shadowed").
/etc/rpmrc	rpm command configuration. All the rpm command line options can be set together in this file so that all of the options apply globally when any rpm command is run on that system.
/etc/securetty	Contains the device names of tty lines (one per line, without leading /dev/) on which root is allowed to login.
/etc/usertty /etc/shadow	Contains the encrypted password information for users' accounts and optionally the password aging information. Included fields are: Login name Encrypted password Days since Jan 1, 1970 that password was last changed Days before password may be changed Days after which password must be changed Days before password is to expire that user is warned Days after password expires that account is disabled Days since Jan 1, 1970 that account is disabled
/etc/shells	Holds the list of possible "shells" available to the system.
/etc/motd	Message Of The Day; used if an administrator wants to convey some message to all the users of a Linux server.

Networking

/etc/gated.conf	Configuration for gated. Used only by the gated daemon.
/etc/gated.version	Contains the version number of the gated daemon.
/etc/gateway	Optionally used by the routed daemon.
/etc/networks	Lists names and addresses of networks accessible from the network to which the machine is connected. Used by route command. Allows use of name for network.
/etc/protocols	Lists the currently available protocols. See the NAG (Network Administrators Guide) and man page. C interface is getprotoent. Should never change.
/etc/resolv.conf	Tells the kernel which name server should be queried when a program asks to "resolve" an IP Address.
/etc/rpc	Contains instructions/rules for RPC, which can be used in NFS calls, remote file system mounting, etc.
/etc/exports	The file system to be exported (NFS) and permissions for it.
/etc/services	Translates network service names to port number/protocol. Read by inetd, telnet, tcpdump, and some other programs. There are C access routines.
/etc/inetd.conf	Config file for inetd. See the inetd man page. Holds an entry for each network service for which inetd must control daemons or other servicers. Note that services will be running, but comment them out in /etc/services so they will not be available even if running. Format:
/etc/sendmail.cf	The Mail program sendmail's configuration file. Cryptic to understand.
/etc/sysconfig/network	Indicates NETWORKING=yes or no. Read by rc.sysinit at least.
/etc/sysconfig/network-scripts/if*	Red Hat network configuration scripts.

System commands

System commands are meant exclusively to control the system, and make everything work properly. All the programs like login (performing the authentication phase of a user on the console) or bash (providing the interaction between a user and the computer) are system commands. The files associated with them are therefore particularly important. This category has the following files of interest to users and administrators.

/etc/lilo.conf	Contains the system's default boot command line parameters and also the different images to boot with. You can see this list by pressing Tab at the LILO prompt.
/etc/logrotate.conf	Maintains the log files present in the /var/log directory.
/etc/identd.conf	Identd is a server that implements the TCP/IP proposed standard IDENT user identification protocol as specified in the RFC 1413 document. identd operates by looking up specific TCP/IP connections and returning the user name of the process owning the connection. It can optionally return other information instead of a user name. See the identd man page.
/etc/ld.so.conf	Configuration for the Dynamic Linker.
/etc/inittab	This is chronologically the first configuration file in UNIX. The first program launched after a UNIX machine is switched on is init, which knows what to launch, thanks to inittab. It is read by init at run level changes, and controls the startup of the main process.
/etc/termcap	A database containing all of the possible terminal types and their capabilities.

Daemons

A daemon is a program running in non-interactive mode. Typically, daemon tasks are related to the networking area: they wait for connections, so that they can provide services through them. Many daemons are available for Linux, ranging from Web servers to ftp servers.

/etc/syslogd.conf	The configuration file for the syslogd daemon. syslogd is the daemon that takes care of logging (writing to disk) messages coming from other programs to the system. This service, in particular, is used by daemons that would not otherwise have any means of signaling the presence of possible problems or sending messages to users.
/etc/httpd.conf	The configuration file for Apache, the Web server. This file is typically not in /etc. It may be in /usr/local/httpd/conf/ or /etc/httpd/conf/, but to make sure, you need to check the particular Apache installation.
/etc/conf.modules or /etc/modules.conf	The configuration file for kerneld. Ironically, it is not the kernel "as a daemon". It is rather a daemon that takes care of loading additional kernel modules "on the fly" when needed.

User programs

In Linux (and UNIX in general), there are countless "user" programs. A most common user program config file is /etc/lynx.cfg. This is the configuration file for lynx, the well-known textual browser. Through this file you can define the proxy server, the character set to use, and so on. The following code sample shows a part of the lynx.cfg file that can be modified to change the proxy settings of the Linux system. These settings apply (by default) to all the users running lynx in their respective shells, unless a user overrides the default config file by specifying --cfg = "mylynx.cfg.

Proxy settings in /etc/lynx.cfg 

.h1 proxy .h2 HTTP_PROXY .h2 HTTPS_PROXY .h2 FTP_PROXY .h2 GOPHER_PROXY .h2 NEWS_PROXY .h2 NNTP_PROXY # Lynx version 2.2 and beyond supports the use of proxy servers that can act as # firewall gateways and caching servers. They are preferable to the older # gateway servers. Each protocol used by Lynx can be mapped separately using # PROTOCOL_proxy environment variables (see Lynx Users Guide). If you have # not set them externally, you can set them at run time via this configuration file. # They will not override external settings. The no_proxy variable can be used # to inhibit proxying to selected regions of the Web (see below). Note that on # VMS these proxy variables are set as process logicals rather than symbols, to # preserve lowercasing, and will outlive the Lynx image. # .ex 15 http_proxy:http://proxy3.in.ibm.com:80/ ftp_proxy:http://proxy3.in.ibm.com:80/ #http_proxy:http://penguin.in.ibm.com:8080 #ftp_proxy:http://penguin.in.ibm.com:8080/ .h2 NO_PROXY # The no_proxy variable can be a comma-separated list of strings defining # no-proxy zones in the DNS domain name space. If a tail substring of the # domain-path for a host matches one of these strings, transactions with that # node will not be proxied. .ex no_proxy:demiurge.in.ibm.com, demiurge

Changing configuration files

When changing a configuration file, make sure that the program using that configuration is restarted if it's not controlled by the system administrator or the kernel. A normal user doesn't usually have privileges to start or stop system programs and/or daemons.

The kernel

Changing configuration files in the kernel immediately affects the system. For example, changing the passwd file to add a user immediately enables that user. Also there are some kernel tunable parameters in the /proc/sys directory on any Linux system. The write-access to all these files is given only to the super-user; other users have only readonly access. The files in this directory are classified in the same manner as the Linux kernel source. Every file in this directory represents a kernel data structure that can be dynamically modified to change the system performance.

Note: Before changing any value in any of these files, make sure you know everything about the file to avoid irreparable damage to the system.
Files in the /proc/sys/kernel/ directory

File name	Description
threads-max	The maximum number of tasks the kernel can run.
ctrl-alt-del	If 1, then pressing this key sequence cleanly reboots the system.
sysrq	If 1, then Alt-SysRq is active.
osrelease	Displays the release of the operating system.
ostype	Displays the type of the operating system.
hostname	The host name of the system.
domainname	Network domain of which the system is a part.
modprobe	Specifies whether modprobe should be automatically run at startup, and load the necessary modules.

Daemons and system programs

A daemon is a program that is always running in background, quietly carrying out its task. Common ones are in.ftpd (ftp server daemon), in.telnetd (telnet server daemon), and syslogd (system logging daemon). Some daemons, while running, keep a close watch on the configuration file and reload it automatically when it changes. But most of the daemons do not reload automatically. We need to "tell" them somehow that the configuration file has changed and that it should be reloaded. This can be achieved (on Red Hat Linux systems) by restarting the services using the service command.

For example, if we have changed the network configuration, we need to issue:
service network restart.

Note: The services are most commonly the scripts present in the /etc/rc.d/init.d/* directory and are started by the init when the system is booted. So, to restart the service you can also do the following:
/etc/rc.d/init.d/ start | stop | status
start, stop, and status are the values that these scripts take as input to perform the action.

User programs

A user or system program reads its configuration file every time it is launched. Remember, though, that some system programs are spawned when the computer is turned on, and their behaviour depends on what they read in the configuration files in /etc/. So, the first time a user program is started, the default configuration is read from the files present in the /etc/ directory. Later, the user can customise the programs by using rc and . (dot) files as explained in the next section.

User configuration files: . (dot) files and rc files

We have seen how programs can be easily configured. But what if someone does not like the way a program has been configured in /etc/? A "normal" user cannot simply go into /etc and change the configuration files; they are owned -- from the filesystem's point of view -- by root! This is why most user programs define two configuration files: the first one at a "system" level, located in /etc/; and the other one, "private" to the user, that can be found in his or her home directory.

For example, in my system I have installed the very useful wget utility. In /etc/ there is an /etc/wgetrc file. In my home directory, there is a file named .wgetrc, which describes my customised configuration (which will be loaded only when I, the user run the wget command). Other users may also have the .wgetrc file in their home directory (/home/other); this file will be read, of course, only when the user runs the wget command. In other words, the /etc/wgetrc file provides "default" values for wget, while the /home/xxx/.wgetrc file lists the "customisations" for a certain user. It is important to understand that this is the "general rule," and is not necessarily true for all cases. A program like pine, for instance, does not have any files in /etc/, but only the custom configuration in the users' home directory, in a file named .pinerc. Other programs may only have a default configuration file in /etc/, and may not let users "customize" them (it's the case with only a few of the config. files in the /etc dir.).

Commonly used rc and . (dot) files

Filename	Description
~/.bash_login	Look at "man bash". Treated by bash like ~/.bash_profile if that doesn't exist.
~/.bash_logout	Look at "man bash".Sourced by bash login shells at exit.
~/.bash_profile	Sourced by bash login shells after /etc/profile.
~/.bash_history	The list of commands executed previously.
~/.bashrcÂ	Look at "man bash". Sourced by bash non-login interactive shells (no other files are). Non-interactive shells source nothing unless BASH_ENV or ENV are set.
~/.emacs	Read by emacs at startup.
~/.forward	If this contains an e-mail address, then all mail to owner of ~ will be forwarded to that e-mail address.
~/.fvwmrc ~/.fvwm2rc	Config files for fvwm and fvwm2 (the basic X Window manager).
~/.hushlogin	Look at "man login". Causes a "quiet" login (no mail notice, last login info, or MOD).
~/.mail.rc	User init file for mail program.
~/.ncftp/	Directory for ncftp program; contains bookmarks, log, macros, preferences, trace. See man ncftp. The purpose of ncftp is to provide a powerful and flexible interface to the Internet standard File Transfer Protocol. It is intended to replace the stock ftp program that comes with the system.
~/.profile	Look at "man bash". Treated by bash like ~/.bash_profile if that and ~/.bash_login don't exist, and used by other Bourn-heritage shells too.
~/.pinerc	Pine configuration
~/.muttrc	Mutt configuration
~/.exrc	Configuration of vi can be controlled by this file. Example: set ai sm ruler Writing the above line in this file makes vi set the auto-indentation, matching brackets and displaying line number and rows-columns options.
~/.vimrc	Default "Vim" configuration file. Same as .exrc.
~/.gtkrc	GNOME Toolkit.
~/.kderc	KDE configuration.
~/.netrc	Default login names and passwords for ftp.
~/.rhosts	Used by the r-tools: rsh, rlogin, etc. Very weak security since host impersonation is easy. Must be owned by user (owner of ~/) or superuser. Lists hosts from which users may access this account. Ignored if it is a symbolic link.
~/.rpmrc	See "man rpm". Read by rpm if /etc/rpmrc is not present.
~/.signature	Message text that will be appended automatically to the mail sent from this account.
~/.twmrc	Config file for twm (The Window Manager).
~/.xinitrc	Read by X at startup (not by xinit script). Mostly starts some progs. Example: exec /usr/sbin/startkde If the above line is present in this file, then the KDE Window Manager is started in when the startx command is issued from this account.
~/.xmodmaprc	This file is passed to the xmodmap program, and could be named anything (~/.Xmodmap and ~/.keymap.km, for example).
~/.xserverrc	Run by xinit as the X server if it can find X to execute.
~/News/Sent-Message-IDs	Default mail history file for gnus.
~/.Xauthority	Read and written by xdm program to handle authorization. See the X, xdm, and xauth man pages.
~/.Xdefaults, ~/.Xdefaults-hostname	Read by X applications during startup on hostname. If the -hostname file can't be found, .Xdefaults is looked for.
~/.Xmodmap	Points to .xmodmaprc; Red Hat had (has) .xinitrc using this name.
~/.Xresources	Usually the name for the file passed to xrdb to load the X resources database, to avoid the need for applications to read a long .Xdefaults file. (~/.Xres has been used by some.)
~/mbox	User's old mail.

Understanding Linux Kernel Parameters

 Documentation for /proc/sys/kernel/* kernel version 2.2.10
  (c) 1998, 1999,  Rik van Riel 
  (c) 2009,        Shen Feng
 
 For general info and legal blurb, please look in README.
 
 ==============================================================
 
 This file contains documentation for the sysctl files in
 /proc/sys/kernel/ and is valid for Linux kernel version 2.2.
 
 The files in this directory can be used to tune and monitor
 miscellaneous and general things in the operation of the Linux
 kernel. Since some of the files _can_ be used to screw up your
 system, it is advisable to read both documentation and source
 before actually making adjustments.
 
 Currently, these files might (depending on your configuration)
 show up in /proc/sys/kernel:
 
 - acct
 - acpi_video_flags
 - auto_msgmni
 - bootloader_type      [ X86 only ]
 - bootloader_version      [ X86 only ]
 - callhome       [ S390 only ]
 - cap_last_cap
 - core_pattern
 - core_pipe_limit
 - core_uses_pid
 - ctrl-alt-del
 - dmesg_restrict
 - domainname
 - hostname
 - hotplug
 - kptr_restrict
 - kstack_depth_to_print       [ X86 only ]
 - l2cr                        [ PPC only ]
 - modprobe                    ==> Documentation/debugging-modules.txt
 - modules_disabled
 - msgmax
 - msgmnb
 - msgmni
 - nmi_watchdog
 - osrelease
 - ostype
 - overflowgid
 - overflowuid
 - panic
 - panic_on_oops
 - panic_on_unrecovered_nmi
 - panic_on_stackoverflow
 - pid_max
 - powersave-nap               [ PPC only ]
 - printk
 - printk_delay
 - printk_ratelimit
 - printk_ratelimit_burst
 - randomize_va_space
 - real-root-dev               ==> Documentation/initrd.txt
 - reboot-cmd                  [ SPARC only ]
 - rtsig-max
 - rtsig-nr
 - sem
 - sg-big-buff                 [ generic SCSI device (sg) ]
 - shm_rmid_forced
 - shmall
 - shmmax                      [ sysv ipc ]
 - shmmni
 - softlockup_thresh
 - stop-a                      [ SPARC only ]
 - sysrq                       ==> Documentation/sysrq.txt
 - tainted
 - threads-max
 - unknown_nmi_panic
 - version
 
 ==============================================================
 
 acct:
 
 highwater lowwater frequency
 
 If BSD-style process accounting is enabled these values control
 its behaviour. If free space on filesystem where the log lives
 goes below % accounting suspends. If free space gets
 above % accounting resumes.  determines
 how often do we check the amount of free space (value is in
 seconds). Default:
 4 2 30
 That is, suspend accounting if there left <= 2% free; resume it
 if we got >=4%; consider information about amount of free space
 valid for 30 seconds.
 
 ==============================================================
 
 acpi_video_flags:
 
 flags
 
 See Doc*/kernel/power/video.txt, it allows mode of video boot to be
 set during run time.
 
 ==============================================================
 
 auto_msgmni:
 
 Enables/Disables automatic recomputing of msgmni upon memory add/remove
 or upon ipc namespace creation/removal (see the msgmni description
 above). Echoing "1" into this file enables msgmni automatic recomputing.
 Echoing "0" turns it off. auto_msgmni default value is 1.
 
 
 ==============================================================
 
 bootloader_type:
 
 x86 bootloader identification
 
 This gives the bootloader type number as indicated by the bootloader,
 shifted left by 4, and OR'd with the low four bits of the bootloader
 version.  The reason for this encoding is that this used to match the
 type_of_loader field in the kernel header; the encoding is kept for
 backwards compatibility.  That is, if the full bootloader type number
 is 0x15 and the full version number is 0x234, this file will contain
 the value 340 = 0x154.
 
 See the type_of_loader and ext_loader_type fields in
 Documentation/x86/boot.txt for additional information.
 
 ==============================================================
 
 bootloader_version:
 
 x86 bootloader version
 
 The complete bootloader version number.  In the example above, this
 file will contain the value 564 = 0x234.
 
 See the type_of_loader and ext_loader_ver fields in
 Documentation/x86/boot.txt for additional information.
 
 ==============================================================
 
 callhome:
 
 Controls the kernel's callhome behavior in case of a kernel panic.
 
 The s390 hardware allows an operating system to send a notification
 to a service organization (callhome) in case of an operating system panic.
 
 When the value in this file is 0 (which is the default behavior)
 nothing happens in case of a kernel panic. If this value is set to "1"
 the complete kernel oops message is send to the IBM customer service
 organization in case the mainframe the Linux operating system is running
 on has a service contract with IBM.
 
 ==============================================================
 
 cap_last_cap
 
 Highest valid capability of the running kernel.  Exports
 CAP_LAST_CAP from the kernel.
 
 ==============================================================
 
 core_pattern:
 
 core_pattern is used to specify a core dumpfile pattern name.
 . max length 128 characters; default value is "core"
 . core_pattern is used as a pattern template for the output filename;
   certain string patterns (beginning with '%') are substituted with
   their actual values.
 . backward compatibility with core_uses_pid:
  If core_pattern does not include "%p" (default does not)
  and core_uses_pid is set, then .PID will be appended to
  the filename.
 . corename format specifiers:
  % '%' is dropped
  %% output one '%'
  %p pid
  %u uid
  %g gid
  %s signal number
  %t UNIX time of dump
  %h hostname
  %e executable filename (may be shortened)
  %E executable path
  % both are dropped
 . If the first character of the pattern is a '|', the kernel will treat
   the rest of the pattern as a command to run.  The core dump will be
   written to the standard input of that program instead of to a file.
 
 ==============================================================
 
 core_pipe_limit:
 
 This sysctl is only applicable when core_pattern is configured to pipe
 core files to a user space helper (when the first character of
 core_pattern is a '|', see above).  When collecting cores via a pipe
 to an application, it is occasionally useful for the collecting
 application to gather data about the crashing process from its
 /proc/pid directory.  In order to do this safely, the kernel must wait
 for the collecting process to exit, so as not to remove the crashing
 processes proc files prematurely.  This in turn creates the
 possibility that a misbehaving userspace collecting process can block
 the reaping of a crashed process simply by never exiting.  This sysctl
 defends against that.  It defines how many concurrent crashing
 processes may be piped to user space applications in parallel.  If
 this value is exceeded, then those crashing processes above that value
 are noted via the kernel log and their cores are skipped.  0 is a
 special value, indicating that unlimited processes may be captured in
 parallel, but that no waiting will take place (i.e. the collecting
 process is not guaranteed access to /proc//).  This
 value defaults to 0.
 
 ==============================================================
 
 core_uses_pid:
 
 The default coredump filename is "core".  By setting
 core_uses_pid to 1, the coredump filename becomes core.PID.
 If core_pattern does not include "%p" (default does not)
 and core_uses_pid is set, then .PID will be appended to
 the filename.
 
 ==============================================================
 
 ctrl-alt-del:
 
 When the value in this file is 0, ctrl-alt-del is trapped and
 sent to the init(1) program to handle a graceful restart.
 When, however, the value is > 0, Linux's reaction to a Vulcan
 Nerve Pinch (tm) will be an immediate reboot, without even
 syncing its dirty buffers.
 
 Note: when a program (like dosemu) has the keyboard in 'raw'
 mode, the ctrl-alt-del is intercepted by the program before it
 ever reaches the kernel tty layer, and it's up to the program
 to decide what to do with it.
 
 ==============================================================
 
 dmesg_restrict:
 
 This toggle indicates whether unprivileged users are prevented
 from using dmesg(8) to view messages from the kernel's log buffer.
 When dmesg_restrict is set to (0) there are no restrictions. When
 dmesg_restrict is set set to (1), users must have CAP_SYSLOG to use
 dmesg(8).
 
 The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the
 default value of dmesg_restrict.
 
 ==============================================================
 
 domainname & hostname:
 
 These files can be used to set the NIS/YP domainname and the
 hostname of your box in exactly the same way as the commands
 domainname and hostname, i.e.:
 # echo "darkstar" > /proc/sys/kernel/hostname
 # echo "mydomain" > /proc/sys/kernel/domainname
 has the same effect as
 # hostname "darkstar"
 # domainname "mydomain"
 
 Note, however, that the classic darkstar.frop.org has the
 hostname "darkstar" and DNS (Internet Domain Name Server)
 domainname "frop.org", not to be confused with the NIS (Network
 Information Service) or YP (Yellow Pages) domainname. These two
 domain names are in general different. For a detailed discussion
 see the hostname(1) man page.
 
 ==============================================================
 
 hotplug:
 
 Path for the hotplug policy agent.
 Default value is "/sbin/hotplug".
 
 ==============================================================
 
 kptr_restrict:
 
 This toggle indicates whether restrictions are placed on
 exposing kernel addresses via /proc and other interfaces.  When
 kptr_restrict is set to (0), there are no restrictions.  When
 kptr_restrict is set to (1), the default, kernel pointers
 printed using the %pK format specifier will be replaced with 0's
 unless the user has CAP_SYSLOG.  When kptr_restrict is set to
 (2), kernel pointers printed using %pK will be replaced with 0's
 regardless of privileges.
 
 ==============================================================
 
 kstack_depth_to_print: (X86 only)
 
 Controls the number of words to print when dumping the raw
 kernel stack.
 
 ==============================================================
 
 l2cr: (PPC only)
 
 This flag controls the L2 cache of G3 processor boards. If
 0, the cache is disabled. Enabled if nonzero.
 
 ==============================================================
 
 modules_disabled:
 
 A toggle value indicating if modules are allowed to be loaded
 in an otherwise modular kernel.  This toggle defaults to off
 (0), but can be set true (1).  Once true, modules can be
 neither loaded nor unloaded, and the toggle cannot be set back
 to false.
 
 ==============================================================
 
 nmi_watchdog:
 
 Enables/Disables the NMI watchdog on x86 systems. When the value is
 non-zero the NMI watchdog is enabled and will continuously test all
 online cpus to determine whether or not they are still functioning
 properly. Currently, passing "nmi_watchdog=" parameter at boot time is
 required for this function to work.
 
 If LAPIC NMI watchdog method is in use (nmi_watchdog=2 kernel
 parameter), the NMI watchdog shares registers with oprofile. By
 disabling the NMI watchdog, oprofile may have more registers to
 utilize.
 
 ==============================================================
 
 osrelease, ostype & version:
 
 # cat osrelease
 2.1.88
 # cat ostype
 Linux
 # cat version
 #5 Wed Feb 25 21:49:24 MET 1998
 
 The files osrelease and ostype should be clear enough. Version
 needs a little more clarification however. The '#5' means that
 this is the fifth kernel built from this source base and the
 date behind it indicates the time the kernel was built.
 The only way to tune these values is to rebuild the kernel :-)
 
 ==============================================================
 
 overflowgid & overflowuid:
 
 if your architecture did not always support 32-bit UIDs (i.e. arm,
 i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to
 applications that use the old 16-bit UID/GID system calls, if the
 actual UID or GID would exceed 65535.
 
 These sysctls allow you to change the value of the fixed UID and GID.
 The default is 65534.
 
 ==============================================================
 
 panic:
 
 The value in this file represents the number of seconds the kernel
 waits before rebooting on a panic. When you use the software watchdog,
 the recommended setting is 60.
 
 ==============================================================
 
 panic_on_unrecovered_nmi:
 
 The default Linux behaviour on an NMI of either memory or unknown is
 to continue operation. For many environments such as scientific
 computing it is preferable that the box is taken out and the error
 dealt with than an uncorrected parity/ECC error get propagated.
 
 A small number of systems do generate NMI's for bizarre random reasons
 such as power management so the default is off. That sysctl works like
 the existing panic controls already in that directory.
 
 ==============================================================
 
 panic_on_oops:
 
 Controls the kernel's behaviour when an oops or BUG is encountered.
 
 0: try to continue operation
 
 1: panic immediately.  If the `panic' sysctl is also non-zero then the
    machine will be rebooted.
 
 ==============================================================
 
 panic_on_stackoverflow:
 
 Controls the kernel's behavior when detecting the overflows of
 kernel, IRQ and exception stacks except a user stack.
 This file shows up if CONFIG_DEBUG_STACKOVERFLOW is enabled.
 
 0: try to continue operation.
 
 1: panic immediately.
 
 ==============================================================
 
 
 pid_max:
 
 PID allocation wrap value.  When the kernel's next PID value
 reaches this value, it wraps back to a minimum PID value.
 PIDs of value pid_max or larger are not allocated.
 
 ==============================================================
 
 ns_last_pid:
 
 The last pid allocated in the current (the one task using this sysctl
 lives in) pid namespace. When selecting a pid for a next task on fork
 kernel tries to allocate a number starting from this one.
 
 ==============================================================
 
 powersave-nap: (PPC only)
 
 If set, Linux-PPC will use the 'nap' mode of powersaving,
 otherwise the 'doze' mode will be used.
 
 ==============================================================
 
 printk:
 
 The four values in printk denote: console_loglevel,
 default_message_loglevel, minimum_console_loglevel and
 default_console_loglevel respectively.
 
 These values influence printk() behavior when printing or
 logging error messages. See 'man 2 syslog' for more info on
 the different loglevels.
 
 - console_loglevel: messages with a higher priority than
   this will be printed to the console
 - default_message_loglevel: messages without an explicit priority
   will be printed with this priority
 - minimum_console_loglevel: minimum (highest) value to which
   console_loglevel can be set
 - default_console_loglevel: default value for console_loglevel
 
 ==============================================================
 
 printk_delay:
 
 Delay each printk message in printk_delay milliseconds
 
 Value from 0 - 10000 is allowed.
 
 ==============================================================
 
 printk_ratelimit:
 
 Some warning messages are rate limited. printk_ratelimit specifies
 the minimum length of time between these messages (in jiffies), by
 default we allow one every 5 seconds.
 
 A value of 0 will disable rate limiting.
 
 ==============================================================
 
 printk_ratelimit_burst:
 
 While long term we enforce one message per printk_ratelimit
 seconds, we do allow a burst of messages to pass through.
 printk_ratelimit_burst specifies the number of messages we can
 send before ratelimiting kicks in.
 
 ==============================================================
 
 randomize_va_space:
 
 This option can be used to select the type of process address
 space randomization that is used in the system, for architectures
 that support this feature.
 
 0 - Turn the process address space randomization off.  This is the
     default for architectures that do not support this feature anyways,
     and kernels that are booted with the "norandmaps" parameter.
 
 1 - Make the addresses of mmap base, stack and VDSO page randomized.
     This, among other things, implies that shared libraries will be
     loaded to random addresses.  Also for PIE-linked binaries, the
     location of code start is randomized.  This is the default if the
     CONFIG_COMPAT_BRK option is enabled.
 
 2 - Additionally enable heap randomization.  This is the default if
     CONFIG_COMPAT_BRK is disabled.
 
     There are a few legacy applications out there (such as some ancient
     versions of libc.so.5 from 1996) that assume that brk area starts
     just after the end of the code+bss.  These applications break when
     start of the brk area is randomized.  There are however no known
     non-legacy applications that would be broken this way, so for most
     systems it is safe to choose full randomization.
 
     Systems with ancient and/or broken binaries should be configured
     with CONFIG_COMPAT_BRK enabled, which excludes the heap from process
     address space randomization.
 
 ==============================================================
 
 reboot-cmd: (Sparc only)
 
 ??? This seems to be a way to give an argument to the Sparc
 ROM/Flash boot loader. Maybe to tell it what to do after
 rebooting. ???
 
 ==============================================================
 
 rtsig-max & rtsig-nr:
 
 The file rtsig-max can be used to tune the maximum number
 of POSIX realtime (queued) signals that can be outstanding
 in the system.
 
 rtsig-nr shows the number of RT signals currently queued.
 
 ==============================================================
 
 sg-big-buff:
 
 This file shows the size of the generic SCSI (sg) buffer.
 You can't tune it just yet, but you could change it on
 compile time by editing include/scsi/sg.h and changing
 the value of SG_BIG_BUFF.
 
 There shouldn't be any reason to change this value. If
 you can come up with one, you probably know what you
 are doing anyway :)
 
 ==============================================================
 
 shmmax:
 
 This value can be used to query and set the run time limit
 on the maximum shared memory segment size that can be created.
 Shared memory segments up to 1Gb are now supported in the
 kernel.  This value defaults to SHMMAX.
 
 ==============================================================
 
 shm_rmid_forced:
 
 Linux lets you set resource limits, including how much memory one
 process can consume, via setrlimit(2).  Unfortunately, shared memory
 segments are allowed to exist without association with any process, and
 thus might not be counted against any resource limits.  If enabled,
 shared memory segments are automatically destroyed when their attach
 count becomes zero after a detach or a process termination.  It will
 also destroy segments that were created, but never attached to, on exit
 from the process.  The only use left for IPC_RMID is to immediately
 destroy an unattached segment.  Of course, this breaks the way things are
 defined, so some applications might stop working.  Note that this
 feature will do you no good unless you also configure your resource
 limits (in particular, RLIMIT_AS and RLIMIT_NPROC).  Most systems don't
 need this.
 
 Note that if you change this from 0 to 1, already created segments
 without users and with a dead originative process will be destroyed.
 
 ==============================================================
 
 softlockup_thresh:
 
 This value can be used to lower the softlockup tolerance threshold.  The
 default threshold is 60 seconds.  If a cpu is locked up for 60 seconds,
 the kernel complains.  Valid values are 1-60 seconds.  Setting this
 tunable to zero will disable the softlockup detection altogether.
 
 ==============================================================
 
 tainted:
 
 Non-zero if the kernel has been tainted.  Numeric values, which
 can be ORed together:
 
    1 - A module with a non-GPL license has been loaded, this
        includes modules with no license.
        Set by modutils >= 2.4.9 and module-init-tools.
    2 - A module was force loaded by insmod -f.
        Set by modutils >= 2.4.9 and module-init-tools.
    4 - Unsafe SMP processors: SMP with CPUs not designed for SMP.
    8 - A module was forcibly unloaded from the system by rmmod -f.
   16 - A hardware machine check error occurred on the system.
   32 - A bad page was discovered on the system.
   64 - The user has asked that the system be marked "tainted".  This
        could be because they are running software that directly modifies
        the hardware, or for other reasons.
  128 - The system has died.
  256 - The ACPI DSDT has been overridden with one supplied by the user
         instead of using the one provided by the hardware.
  512 - A kernel warning has occurred.
 1024 - A module from drivers/staging was loaded.
 2048 - The system is working around a severe firmware bug.
 4096 - An out-of-tree module has been loaded.
 
 ==============================================================
 
 unknown_nmi_panic:
 
 The value in this file affects behavior of handling NMI. When the
 value is non-zero, unknown NMI is trapped and then panic occurs. At
 that time, kernel debugging information is displayed on console.
 
 NMI switch that most IA32 servers have fires unknown NMI up, for
 example.  If a system hangs up, try pressing the NMI switch.