Understanding Linux Configuration Files

Introduced

Each Linux program is an executable file that contains a list of opcode that the CPU will execute to accomplish certain operations. For example, the LS command is provided by the/bin/ls file, which contains a list of machine instructions that need to be used when displaying a list of files in the current directory on the screen. Almost every program's behavior can be customized according to your preferences or needs by modifying its profile.

Is there a standard configuration file format for Linux?

In a word, no. Users who are unfamiliar with Linux will feel frustrated because each profile looks like a new challenge to meet. In Linux, each programmer is free to choose the profile format he or she likes. You can choose from a lot of formats, from the/etc/shells file (which contains a list of shells separated by a newline character), to the complex/etc/httpd.conf file of Apache.

What is a system configuration file?

The kernel itself can also be seen as a "program." Why does the kernel need configuration files? The kernel needs to understand the list of users and groups in the system, thus managing file permissions (that is, determining whether a particular user (Unix_users) can open a file based on permissions). Note that these files are not explicitly read by the program, but are read by a function provided by the system library and used by the kernel. For example, a program that requires a user's (encrypted) password should not open the/etc/passwd file. Instead, the program should call the GETPW () function of the system library. This function is also known as a system call. Opening the/etc/passwd file and then finding the password for the requested user is determined by the kernel (through the system library).

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

accessing files

/etc/host.conf	Tell the network name server how to find the host name. (usually/etc/hosts, then a name server, which can be changed by netconf)
/etc/hosts	Contains a list of known hosts (on the local network). If the system's IP is not dynamically generated, it can be used. For simple host name resolution (dot notation), before requesting a DNS or NIS Network name server,/etc/hosts.conf usually tells the resolver to check here first.
/etc/hosts.allow	See the online help page for hosts_access. Read at least by TCPD.
/etc/hosts.deny	See the online help page for hosts_access. Read at least by TCPD.

Boot and Logon/logoff

/etc/issue &/etc/issue.net	These files are read by Mingetty (and similar programs) to display a "welcome" string to a user who is connected from a terminal (issue) or through a Telnet session (Issue.net). They include several lines of information stating the Red Hat version number, name, and kernel ID. They are used by rc.local.
/etc/redhat-release	Includes a line of information that declares the Red Hat version number and name. Used by rc.local.
/etc/rc.d/rc	Typically run at all run levels, and levels are passed as parameters. For example, to boot a machine in graphical (graphics) mode (x-server), run the following command at the command line: init 5 . Run Level 5 indicates that the system is booted in graphical mode.
/etc/rc.d/rc.local	Informal. Can be called from RC, Rc.sysinit, or/etc/inittab.
/etc/rc.d/rc.sysinit	is usually the first script of all runlevel.
/etc/rc.d/rc/rcx.d	A script that runs from RC ( X represents any number between 1 and 5). These directories are specific "runlevel" directories. When the system starts, it recognizes the runlevel to start, and then calls all startup scripts that exist in the specific directory for that RunLevel. For example, a "entering Run-level 3" message is typically displayed after the boot message when the system starts, which means that all initialization scripts in the/etc/rc.d/rc3.d/directory are called.

File system

The kernel provides an interface to display some of its data structures, which may be useful for system parameters such as interrupts used, initialized devices, and memory statistics. This interface is provided as a standalone but virtual file system, called the/proc file system. Many system utilities use the values present in this file system to display system statistics. For example, the/proc/modules file enumerates the modules currently loaded in the system. The Lsmod command reads this information and then displays it in a format that people can understand. The mTAB file specified in the following table reads the/proc/mount file containing the currently installed file system in the same way.

/etc/mtab	This will change as the/proc/mount file changes. In other words, when the file system is installed and uninstalled, changes are immediately reflected in this file.
/etc/fstab	Lists the file systems that the computer is currently "ready to install". This is important because the command is run when the computer Boots mount -a , which installs every file system with the "1" tag in the penultimate column of Fstab.
/etc/mtools.conf	The configuration of all operations on a DOS type file system (create directory, copy, format, and so on).

System Management

/etc/group	Contains a valid group name and the users that are included in the specified group. A single user can exist in multiple groups if they perform multiple tasks. For example, if a user is a member of the Project 1 project group and is also an administrator, his entry looks like this in the group file:user: * : group-id : project1
/etc/nologin	If a/etc/nologin file exists, login (1) will only allow access by the root user. It will display the contents of this file to other users and refuse to sign in.
etc/passwd	Please refer to "Man passwd". It contains some user account information, including the password (if it has not been encrypted by the shadow program).
/etc/rpmrc	RPM command configuration. All RPM command-line options can be set together in this file, so that when any RPM command runs on the system, all options are applied globally.
/etc/securetty	Contains the device name, consisting of a TTY line (one name per line, excluding the previous/dev/), where the root user is allowed to log in.
/etc/usertty /etc/shadow	Contains encrypted user account password information, and can also include password aging information. The fields included are: Login Name Password after encryption From January 1, 1970 to the number of days the password was last changed The number of days before the password can be changed Number of days before password must be changed Number of days the user was warned before the password expires Number of days the password expires payback disabled Number of days from January 1, 1970 to account disabled
/etc/shells	Contains a list of possible "shells" that are available to the system.
/etc/motd	Daily messages, used when an administrator wants to communicate a message to all users of a Linux server.

Networking

/etc/gated.conf	The configuration of the gated. Can only be used by the gated daemon.
/etc/gated.version	Contains the version number of the gated daemon.
/etc/gateway	Used by the routed daemon, optionally.
/etc/networks	Lists the network names and network addresses that can be accessed from the network to which the machine is connected. Used by routing commands. Allows the use of network names.
/etc/protocols	Lists the currently available protocols. Please refer to the NAG (network Administrator's Guide) and the online help page for Administrators. The C interface is getprotoent. can never be changed.
/etc/resolv.conf	Tells the kernel which name server should be queried when the program requests "resolve" an IP address.
/etc/rpc	Contains RPC directives/rules that can be used in NFS calls, remote file system installations, and so on.
/etc/exports	The file system (NFS) to export and the permissions to it.
/etc/services	Converts the network service name to a port number/protocol. Read by inetd, Telnet, tcpdump, and some other programs. There are some C access routines.
/etc/inetd.conf	The inetd configuration file. See the inetd online help page. Contains entries for each network service, inetd must control daemons or other services for these network services. Note that the services will run, but they will be commented out in/etc/services, so that they will not be available even if the services are running. Format:<service_name> <sock_type> <proto> <flags> <user> <server_path> <args>
/etc/sendmail.cf	The SendMail configuration file for the mail program. Rather obscure, it is difficult to understand.
/etc/sysconfig/network	Indicate Networking=yes or No. Read at least by Rc.sysinit.
/etc/sysconfig/network-scripts/if*	Red Hat Network configuration script.

System commands

System commands want to control the system exclusively and let everything work properly. All programs such as login (completing the console user authentication phase) or bash (providing interaction between users and computers) are system commands. Therefore, the documents relating to them are also of particular importance. The following files are of interest to users and administrators in this category.

/etc/ lilo.conf	contains the default boot command line parameters for the system, as well as the different images used at startup. You can see this list by pressing the Tab key at the LILO boot prompt.
/etc/logrotate.conf	maintain log files in the/var/log directory.
/etc/identd.conf	Identd is a server that implements T in the manner specified in the RFC 1413 documentation CP/IP proposed standard IDENT user identification protocol. Identd works by locating a specific TCP/IP connection and returning the user name of the process that owns the connection. As a choice, it can also return other information, rather than the user name. See the Identd online help page.
/etc/ld.so.conf	Dynamic Linker (Linker) configuration.
/etc/inittab	Chronologically speaking, this is the first configuration file in UNIX. The first program that starts after a UNIX machine is opened is init, and it knows what to start, which is due to the existence of the inittab. When the runlevel changes, Init reads Inittab and then controls the start of the main process.
/etc/termcap	A database that contains all possible terminal types and the performance of those terminals.

Daemon process

A daemon is a program that runs in non-interactive mode. In general, daemon tasks are related to networking zones: They wait to connect to provide services over a connection. Linux can use many daemons from a WEB server to an FTP server.

/etc/syslogd.conf	The configuration file for the syslogd daemon. SYSLOGD is a daemon that is responsible for recording (writing to disk) messages sent from other programs to the system. This service is especially used by certain daemons, which do not have another way to emit signals that may be problematic or to send messages to users.
/etc/httpd.conf	The WEB server Apache configuration file. This file is generally not in/etc. It may be in/usr/local/httpd/conf/or/etc/httpd/conf/, but to determine its location, you will also need to check for specific Apache installation information.
/etc/conf.modules or/etc/modules.conf	The Kerneld configuration file. Interestingly, Kerneld is not a "daemon" kernel. It is actually a daemon that is responsible for "fast" loading additional kernel modules when needed.

User Program

In Linux (and General UNIX), there are countless "user" programs. The most common type of user program configuration file is/etc/lynx.cfg. This is the configuration file for the famous text browser lynx. With this file, you can define a proxy server, the character set to use, and so on. The following code sample shows part of the Lynx.cfg file, and modifying this part of the code can change the proxy server settings for the Linux system. By default, these settings apply to all users who run lynx in their own shell, unless a user resets the default profile by specifying it --cfg = "mylynx.cfg" .

Proxy server 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 Beyo ND supports the use of proxy servers that can act as# firewall gateways and caching servers. They is preferable to the older# gateway servers. Each protocol used by Lynx can is mapped separately using# protocol_proxy environment variables (see Lynx Users Guide). If you have the # not set them externally, you can set the them at run time via the This configuration file.# they would not override E xternal 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 is set as process logicals rather than symbols, to# preserve lowercasing, and wil L OUTLIVE the Lynx image.#.ex 15http_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 the these strings, transactions with that# node won't be Proxied. Exno_proxy:demiurge.in.ibm.com, Demiurge

Change the configuration file

If the program is not controlled by the system administrator or the kernel when the configuration file is changed, make sure that the program that uses the configuration is restarted. Normal users typically do not have permission to start or stop system programs and/or daemons.

Kernel

Changing the configuration file in the kernel immediately affects the system. For example, changing the passwd file to increase the user will immediately make the user available. And there are some kernel tunable parameters in the/proc/sys directory of any Linux system. Only Superuser can get write access to all of these files, while other users have read-only access rights. The classification of files in this directory is the same as the way the Linux kernel source code is categorized. Each file in this directory represents a kernel data structure that can be modified dynamically to alter system performance.

Note: before you change any of the values in any of these files, you should make sure that you fully understand the file to avoid irreparable damage to the system.
Files in the/proc/sys/kernel/directory

Filename	Describe
Threads-max	The maximum number of tasks that the kernel can run.
Ctrl-alt-del	If the value is 1, then pressing these keys in order will "completely" reboot the system.
SysRq	If the value is 1,ALT-SYSRQ, the activation state is active.
Osrelease	Displays the release version number of the operating system
OSType	Displays the type of operating system.
Hostname	The host name of the system.
DomainName	Network domain, the system is part of the network domain.
Modprobe	Specifies whether modprobe should automatically run and load the required modules at startup.

Daemons and System programs

The daemon is a program that runs in the background forever, and it silently executes its own task. Common daemons are IN.FTPD (FTP server daemon), in.telnetd (Telnet Server daemon), and syslogd (System logging daemon). Some daemons monitor the configuration file closely at run time and reload it automatically when the configuration file changes. However, most daemons do not automatically reload the configuration file. We need to "tell" in some way that these daemon profiles have been changed and should be reloaded. You can do this by restarting the service using the Service command (on the Red Hat Linux system).

For example, if we change the network configuration, we need to issue:
service network restart。

Note: The most common of these services is the script that exists in the/etc/rc.d/init.d/* directory, which is initiated by Init when the system is booted. Therefore, you can also restart the service by doing the following:
/etc/rc.d/init.d/<script-for-the-service> start | stop | status
Start, stop, and status are the input values that these scripts accept to perform operations.

User Program

The user or System program reads its configuration file each time it is started. However, keep in mind that some system programs do not behave the same when the computer is opened, and their behavior depends on what is read in the configuration file in/etc/. Therefore, the default configuration is read from a file that exists in the/etc/directory the first time the user program starts. The user can then customize the program by using the RC and. (dot) files, as shown in the following section.

User profiles:. (dot) files and RC files

We've seen how easy it is to configure the program. But what if someone doesn't like how to configure the program in/etc/? "Normal" users can not simply enter/etc and then change the configuration file, from the point of view of the file system, the owner of the configuration file is the root user! This is why most user programs define two profiles: The first is the "system" level, located in/etc/, and the other belongs to the user "private", which can be found in his or her home directory.

For example, I have a very useful wget utility installed in my system. There is a/etc/wgetrc file in the/etc/. In my home directory, there is a file named. WGETRC, which describes my custom configuration (this configuration file will only be loaded when I, the user runs the wget command). Other users can also have. wgetrc files in their own home directory (/home/other); Of course, only those users will be able to read this file when they run the wget command. In other words, the/etc/wgetrc file provides a "default" value for Wget, and the/home/xxx/.wgetrc file lists the custom items for a user. It is important that this is only a "general rule", not all cases. For example, a program like pine, which does not have any files in/etc/, has a custom configuration file in the user's home directory, named. Pinerc. Other programs may have only the default profile in/etc/and may not allow users to "customize" these profiles (only a few config files in the/etc directory.) File is the case).

RC and. (point) files commonly used

Filename	Describe
~/.bash_login	Please refer to "Man bash". If ~/.bash_profile does not exist, Bash takes ~/.bash_login as ~/.bash_profile processing.
~/.bash_logout	Please refer to "Man bash". It is referenced by the bash login shell when exiting.
~/.bash_profile	Referenced by the bash login shell after referencing/etc/profile.
~/.bash_history	A list of previously executed commands.
~/.bashrc	Please refer to "Man bash". By bash non-logon interactive shell references (no other files). Non-interactive shells do not reference any files unless bash_env or ENV is set.
~/.emacs	Read at startup by Emac.
~/.forward	If this contains an email address, all mail to the owner will be forwarded to this email address.
~/.FVWMRC ~/.FVWM2RC	Configuration files for fvwm and fvwm2 (basic X Window manager).
~/.hushlogin	Please refer to "Man login". Causes a "silent" login (no email notification, last login information, or MOD information).
~/.mail.rc	The user initialization file for the mail program.
~/.ncftp/	A directory of NCFTP programs that contain bookmarks, logs, macros, preferences, and tracking information. See Man NcFTP. The purpose of the NCFTP is to provide a powerful and flexible interface for the Internet standard File Transfer Protocol, the Internet standards files Transfer Protocol. It is intended to replace the standard FTP program used by the system.
~/.profile	Please refer to "Man bash". If the ~/.bash_profile and ~/.bash_login files do not exist, Bash takes ~/.profile as ~/.bash_profile and is used by other shells that inherit Bourn.
~/.pinerc	Pine Configuration
~/.muttrc	Mutt Configuration
~/.exrc	This file can control the configuration of VI. Example: Set AI sm ruler Writing the above line in this file causes VI to set the options for automatic indentation, matching parentheses, displaying line numbers, and row-column.
~/.vimrc	The default "Vim" configuration file. Same as the. Exrc.
~/.gtkrc	GNOME Toolkit (Gnome Toolkit).
~/.kderc	KDE configuration.
~/.netrc	FTP default login name and password.
~/.rhosts	Used by R-tools such as rsh, Rlogin, and so on. Because impersonating a host is easy, security is very low. Must be owned by the user (owner of the ~/) or Superuser. Lists some hosts from which users can access the account. If it is a symbolic link, it is ignored.
~/.rpmrc	Please refer to "Man rpm". If/ETC/RPMRC does not exist, it is read by RPM.
~/.signature	The message text is automatically appended to the end of the message sent from this account.
~/.twmrc	TWM ( THe Window Manager) configuration file.
~/.xinitrc	Read at startup by X (not read by Xinit script). Some programs are usually started. Example: Exec/usr/sbin/startkde If the above line is present in the file, this line launches the KDE Window Manager (KDE window manager) when issuing the STARTX command from this account.
~/.xmodmaprc	This file is transferred to the XMODMAP program and can be named any file (for example, ~/. Xmodmap and ~/.keymap.km).
~/.xserverrc	If Xinit can find the X,xinit to execute, it will run the file as an X server.
~/news/sent-message-ids	The default message history file for Gnus.
~/. Xauthority	Read and write by the XDM program to handle permissions. See the X, XDm, and Xauth online help pages.
~/. Xdefaults, ~/. Xdefaults-hostname	It is read by the X application during the startup of the host hostname. If the-hostname file is not found, find it. Xdefaults file.
~/. Xmodmap	Point to . XMODMAPRC; Red Hat has a. xinitrc file that uses this name.
~/. Xresources	The name of the file that is typically transferred to Xrdb to load the X resource database is designed to avoid the application needing to read a very long. Xdefaults files in this case. (In some cases, ~/has been used.) Xres. ）
~/mbox	The user's old message.

Reprinted from: http://www.ibm.com/developerworks/cn/linux/management/configuration/index.html

Understanding Linux Configuration Files