Understanding Linux configuration files


Summary: This article explains configuration files on a Linux system that control user permissions, system applications, daemons, services, and other administrative tasks in a multi-user, multi-tasking environment. These tasks include managing user accounts, allocating disk quotas, managing e-mails and newsgroups, and configuring kernel parameters. This article also classifies the config files present on a Red Hat Linux system based on their usage and the services they affect.


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: <service_name> <sock_type> <proto> <flags> <user> <server_path> <args>
/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/<script-for-the-service> 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.
~/.bashrc 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 ~/.fvwm2rcConfig files for fvwm and fvwm2 (the basic X Window manager).~/.hushloginLook at “man login”. Causes a “quiet” login (no mail notice, last login info,
or MOD).~/.mail.rcUser 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.~/.profileLook 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.~/.pinercPine configuration~/.muttrcMutt configuration~/.exrcConfiguration 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.~/.vimrcDefault “Vim” configuration file. Same as .exrc.~/.gtkrcGNOME Toolkit.~/.kdercKDE configuration.~/.netrcDefault login names and passwords for ftp.~/.rhostsUsed by the r-tools: rsh, rlogin, etc. Very
weak security since host impersonation is easy.

  1. Must be owned by user (owner of ~/) or superuser.
  2. Lists hosts from which users may access this account.
  3. Ignored if it is a symbolic link.

~/.rpmrcSee “man rpm”. Read by rpm if /etc/rpmrc is not present.~/.signatureMessage text that will be appended automatically to the mail sent
from this account.~/.twmrcConfig file for twm (The Window Manager).~/.xinitrcRead 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.~/.xmodmaprcThis file is passed to the xmodmap program, and could be named anything (~/.Xmodmap and ~/.keymap.km, for example).~/.xserverrcRun by xinit as the X server if it can find X to execute.~/News/Sent-Message-IDsDefault mail history file for gnus.~/.XauthorityRead and written by xdm program to handle authorization. See the X, xdm, and xauth man pages.~/.Xdefaults,

~/.Xdefaults-hostnameRead by X applications during startup on hostname. If the -hostname
file can’t be found, .Xdefaults is looked for.~/.XmodmapPoints to .xmodmaprc; Red Hat had (has) .xinitrc using this name.~/.XresourcesUsually 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.)~/mboxUser’s old mail.

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Posted on October 8, 2012, in Linux Introduction, Rhel 6 Covers & Notes, Servers, Softwares and tagged , , , . Bookmark the permalink. 2 Comments.

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