LINUX Admin Quick Reference

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User Management Files
/etc/group /etc/passwd /etc/shadow
User account information.
/etc/bashrc /etc/profile $HOME/.bashrc $HOME/.bash_profile
bash system wide and per user init files.
/etc/csh.cshrc /etc/csh.login $HOME/.cshrc $HOME/.tcshrc $HOME/.login
tcsh system wide and per user init files.
/etc/skel template files for new users. /etc/default default for certain commands.
/etc/redhat-release /etc/slackware-version

Redhat/Slackware version info (Linux kernel version with “uname –a”) Commands

adduser
script to create an new user interactively (slackware) or link to useradd (Redhat).
useradd, userdel, usermod
create, delete, modify an new user or update default new user information..
newusers update and create new users (batch mode). groupadd, groupdel, groupmod
add, delete or modify group.
chage. ch fn, chsh
modify account policy (password length, expire data etc.) or finger information (full name, phone number etc.) change default login shell.
linux init=/bin/sh rw

gain root access during boot prompt without password, can be used to fix some problems. mount –w -n –o remount /

Network Configuration Files
/etc/rc.d/rc.inet1 (Slackware) /etc/sysconfig/nework- scripts/ifcfg-eth0 (Redhat)
IP address, Network mask, Default gateway are in these files. May edit manually to modify network parameters.
/etc/HOSTNAME hostname is set by “/bin/hostname” during
boot /etc/NETWORKING

and the name is read from these files. (Slackware)

May change manually.
/etc/sysconfig/network (Redhat)
etc/resolv.conf

specify name server, DNS domain and search order. For Example: search la.asu.edu nameserver 129.219.17.200 /etc/hosts host name to IP mapping file.

/etc/host.conf
host name information look up order. Example: order hosts, bind multi on /etc/nsswitch.conf new way to specify information source. /etc/networks /etc/protocols /etc/services
TCP/IP services and ports mapping.
/etc/rpc RPC service name to their program numbers

mapping. Commands

netconfig menu driven Ethernet setup program. pppsetup setup PPP connection (Slackware).
ifconfig
setup Ethernet during boot, for example
/sbin/ifconfig eth0 ${IPADDR} broadcast ${BROADCAST} netmask ${NETMASK}
/sbin/route add -net ${NETWORK} netmask ${NETMASK} eth0

/sbin/route add default gw ${GATEWAY} netmask 0.0.0.0 metric 1

host lookup host name or IP (similar to nslookup). dnsdomainname show DNS domain name. arping; arp find out Ethernet address by first arping then arp. ipchains firewall and NAT (/etc/sysconfig/ipchains on Redhat) iptables firewall and NAT (/etc/sysconfig/iptables on Redhat)
Redhat files in /etc/sysconfig Configuration Files
keyboard
keyboard map, e.g., KEYBOARD=”/usr/lib/kdb/keytables/us.map”
mouse
Mouse type, e.g., MOUSETYPE=Microsoft XEMU3=yes
network
network settings, contains NETWORKING=yes
HOSTNAME=hostname.domain.com
NFS File Sharing Files
/etc/fstab file systems mounted during boot. /etc/exports NFS server export list. /etc/auto.master auto mount master file. Commands
mount mount a file system or all entries in fstab. exportfs export file system listed in exports
showmount –e hostname
show file systems exported
Printer Configuration Files
/etc/printcap /etc/printcap.local
Printer capabilities data base.
/etc/lpd.conf LPRng configuration file.
/etc/lpd.perms
permissions control file for the LPRng line printer spooler
/etc/hosts.lpd Access control (BSD lpd). /etc/hosts.equiv trusted hosts. PRINTER Environment variable of default printer. /dev/lp0 parallel port. Commands
lpc, lpq, lprm
line printer control program, print queue maintain
Sendmail Files
sendmail.cf sendmail.mc
“sendmail.cf” is the configuration file. “sendmail.mc” is a macro file which can be used to generate “sendmail.cf” by: m4 sendmail.mc > sendmail.cf
aliases
mail aliases, must run “newaliases” after change. use :include: to include external list in a file.
access
mail access control, FEATURE(access_db) should be set in sendmail.mc. For example, in /etc/mail/access cyberpromo.com REJECT mydomain.com RELAY spam@somewhere.com DISCARD

makemap hash /etc/mail/access < /etc/mail/access /etc/mail/relay- list all host/domain accepted for relaying.

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domains Commands
newaliases rebuild the data base for the mail aliases file.
makemap
build access database, e.g, makemap hash access.db<access
Useful Configuration Files Files
httpd.conf Apache web server configuration file. smb.conf Samba server (file and print for Windows).
lilo.conf LILO boot loder configuration file. syslog.conf System log daemon (syslogd) configuration. ssh_config sshd_config
SSH client and server configuration files.
ld.so.conf default dynamic library search path (run

ldconfig). mtools.conf mtool configuration file (access DOS file). named.conf DNS name server (BIND). sysctl.conf kernel parameters by sysctl (Redhat). ntp.conf net time server. inetd.conf Internet super server.

Xinetd.conf, Xinet.d directory
Extended inetd configuration.
proftpd.conf proftpd FTP server. amanda.conf network backup server.
/etc/pine.conf /etc/pine.conf.fixed
PINE mail client system wide settings.
Rebuild Kernel Configure Kernel Parameters
make config make menuconfig make xconfig
Configuring the kernel with interactive, menu or X window interface.
Compile Kernel Source
make dep make zImage make zdisk make zlilo make bzImage
Building and installing a new kernel.
Compile Modules
make modules make modules_install
Building and installing modules.
Manage Modules
insmod, lsmod, modinfo, modprobe, rmmod, depmod
Manage loadable modules.
Miscellaneous Files
/etc/shells allowed login shells /etc/ftpusers user names NOT allowed to use ftp.
/etc/host.allow /etc/host.deny
TCP wrapper host control files.
/etc/sysconfig (redhat)
contains system configuration files.
/dev/fd0 floppy drive A
/etc/inittab /etc/init.d
system run level control file.
Commands
fromdos, todos (Slackware) dos2unix, unix2dos (Redhat)
convert text file from/to linux format.
pwck, grpck verify integrity of password and group files. pwconv, pwunconv, grpconv, grpuncov
convert to and from shadow passwords and groups.
shadowconfig toggle shadow passwords on and off.
quota, edquota, quotacheck, quotaon, quotaoff, repquota,
Manage disk quota.
lilo -D dos set LILO default OS (default=dos in lilo.conf) ldd find out shared library dependencies. lsof list opened files. fuser filename show processes that using the file.
ifdown ifup
bring up/down a network interface (Redhat)
sysctl configure kernel parameters (Redhat). socklist list opened socked. shutdown [–r|h] now reboot / halt computer
nmap scan a host for opened ports.
crontab show or edit cron jobs. sys-unconfig unconfigure system chkconfig --list list services started at different run level. kudzu probe for new hardware (Redhat).
rpm
rpm -i INSTALL a package rpm -e UNINSTALL a package rpm -q QUERY a package rpm -U UPDATE a package
man cmd | col –b >cmd.txt
save a man page as a text file and remove control characters.
Configure Apache 2.0 with SSL mod_ssl
(1) when compile apache, specify –enable-ssl for configure script.
By default, ssl is not enabled. After compiling, use “httpd –l” to list the modules. “mod_ssl” should be in them. (2) generate private key with command: openssl genrsa -out server.key 1024
(3) generate certificate request
openssl req -new -key server.key -out server.csr
(4) generate self-signed certificate openssl x509 -req -days 60 -in server.csr -signkey server.key -out server.crt
(5) modify “ssl.conf” which is included in “httpd.conf”. Note, specify “httpd –DSSL”, otherwise, commented out <IfDefine SSL> in ssl.conf.
Syslog.conf
Each line consists of a selector and an action. A selector has two parts: facilities and priorites, separated by a period (.),You may precede every priority with an equation sign (``='') to specify only this single priority and not any of the above. You may also (both is valid, too) precede the priority with an exclamation mark (``!'') to ignore all that priorities, either exact this one or this and any higher priority.
Example: mail.notice /var/log/mail # log to a file *.emerg @myhost.mydomain.org # log to remote host
facilities
auth, auth-priv, cron, daemon, kern, lpr, mail, mark, news, syslog, user, uucp, local0 – local7.
priorities debug, info, notice, warning, err, crit, alert, emerg.
action
Regular File: File with full pathname beginning with “/”.
Terminal and Console: Specify a tty, same with /dev/console. Remote Machine: @myhost.mydomain.org

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IPtables (Netfilter) Command Syntax

iptables [-t <table >] <command> <chain > <parameters> Save and Restore rules /sbin/iptables-save > /etc/sysconfig/iptables /sbin/iptables-restore < /etc/sysconfig/iptables

Firewall script sample http://tiger.la.asu.edu/iptables_examples.htm Build-in Table
filter This is the default table for handling network packets. Build-
in chains are:
1. INPUT — This chain applies to packets received

via a network interface. 2. OUTPUT — This chain applies to packets sent

out via the same network interface which received the packets. 3. FORWARD — This chain applies to packets

received on one network interface and sent out on another. nat This table used to alter packets that create a new connection.

Build-in chains:
1. PREROUTING — This chain alters packets

received via a network interface when they arrive. 2. OUTPUT — This chain alters locally -generated

packets before they are routed via a network interface. 3. POSTROUTING — This chain alters packets

before they are sent out via a network interface. ## Masquerade everything out ppp0. iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE

## Change source addresses to 1.2.3.4. iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to 1.2.3.4 mangle This table is used for specific types of packet alteration.
Build-in chains:
1. PREROUTING — This chain alters packets
received via a network interface before they are routed. 2. OUTPUT — This chain alters locally-generated
packets before they are routed via a network interface.
Commands
--flush | -F Flush (delete) rules in the selected chain. --policy | -P Set default policy for a particular chain. --list | -L List all rules in filter table, use [–t tablename] to
specify other tables.
--append | -A A appends a rule to the end of the specified chain.
-insert | -I Inserts a rule in a chain at a particular point. Other commands: (1) --new | -N (2) --delete | -D (3) --replace | -D (4) --zero | -Z (5) –check | -C (6) delete-chain | -X (7) rename-chain | -E
Parameters
--proto | -p [!] name protocol: by number or name, including tcp,

udp, icmp or all. --source | -s [!] addr/mask source IP address.

--destination | -d addr/mask destination IP address.
--in-interface | -i incoming interface name, e.g. eth0 or ppp0. --out-interface | -o outgoing interface name. --jump | -j jump to a particular target when matching a

rule. Standard options: ACCEPT, DROP, QUEUE, RETURN, REJECT. May jump to a user defined chain. --fragment | -f match second or further fragments only.

Options for TCP and UDP protocol
--sport | --source-port --dport | destination-port
source and/or destination port. Can specify a range like 0:65535, use exclamation character (!) to NOT match ports.
Options for TCP only
--syn Match SYN packets. --tcp-flags Match TCP packets with specific bits set. For example, -p tcp –tcp-flags ACK,FIN,SYN SYN will only match TCP packets that have the SYN flag set and the ACK and FIN flags unset.
Options for ICMP only
--icmp-type [!] type Match specified ICMP type. Valid ICMP type can be
list by iptables –p icmp -h
Option for state module (-m state --state)
ESTABLISHED The matching packet is associated with other

packets in an established connection. RELATED The matching packet is starting a new connection

related in some way to an existing connection. NEW The matching packet is either creating a new

connection or is part of a two-way connection not previously seen.
INVALID The matching packet cannot be tied to a known
connection.
X Window (XFree86) Files
To set screen resolution, in “Screen” section and Subsection “Display”, specify a mode. For example: Modes “1024x768”
To specify screen refresh rate, in “Monitor” section, specify vertical rate. For example: VertRefresh 70-120
/etc/X11/xinit/xinitrc $HOME/.xinitrc
clients to run after X server started
/etc/X11/fs/config configure X11 font path (font server). Commands
startx start X window system. Xconfigurator (Redhat) xfree86setup (Slackware) xf86config
setup X server and generate XF86config.
XFree86 -configure
XFreee86 auto configuration (Plug-n-Play), generate a template named “XF86Config.new” Ctrl+Alt+Del stop X server (on some system Ctrl+Alt+ESC). Ctrl+Alt+F1 Ctrl+Alt+F7
F1 temporary switch to text mode, F7 switch back to graphic mode.
SuperProbe detect graphic hardware. xvidtune adjust X server origin and size. xmodmap modifying key map and mouse button map. xhost server access control program for X.
xsetroot root window parameter setting utility for X. xlsfonts server font list displayer for X. xset ser preference utility for X.
XF86Config

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XFree86 uses a configuration file called XF86Config for its initial setup. This file is normally located in “/etc/X11” or “/etc” directory. The XF86Config file is composed of a number of sections which may be present in any order. Each section has the form:
Section "SectionName"
SectionEntry ... EndSection
The graphics boards are described in the Device sections, and the monitors are described in the Monitor sections. They are bound toget her by a Screen section. Keyboard and Mouse are described in InputDevice sections, although Keyboard and Pointer are still recognized. ServerLayout section is at the highest level and bind together the InputDevice and Screen sections.
A special keyword called Option may be used to provide free-form data to various components of the server. The Option keyword takes either one or two string arguments. The first is the option name, and the optional second argument is the option value. All Option values must be enclosed in quotes. File Section
FontPath "path" Font path elements may be either absolute directory paths, or a font server identifier
RGBPath "path" Sets the path name for the RGB color database.
ModulePath "path" Allows you to set up multiple directories to use for storing modules loaded by the XFree86 server.
EXAMPLE
Section "Files"
RgbPath "/usr/X11R6/lib/X11/rgb" FontPath "unix/:7100" EndSection Serverflags Section
Option "DontZap" "boolean" Disable use Ctrl+Alt+Backspace to termin ate X server.
Option "DontZoom" "boolean" Disable use ‘Ctrl+Alt +Keypad +’ and ‘Ctrl+Alt +Keypad -’ to switch video mode.
Option "BlankTime" "time" Sets the inactivity timeout for the blanking phase of the screensaver in minutes. Default 10 min.
Option "StandbyTime" "time" Sets the inactivity timeout for the "standby" phase of DPMS mode in minutes. Default 20 min.
Option "SuspendTime" "time" Sets the inactivity timeout for the "suspend" phase of DPMS mode, default 30 min.
Option "OffTime" "time" Sets the inactivity timeout for the "off" phase of DPMS mode, default 40 min.
Option "DefaultServerLayout" "layout_id" Specify the default ServerLayout section to use. Default is the first ServerLayout section.
EXAMPLE
Section "ServerFlags"
Option "BlankTime" "99999" Option "StandbyTime" "99999" Option "SuspendTime" "99999" Option "OffTime" "99999" EndSection
Module Section
Load "modulename" Load a module. The module name given should be the module's standard name, not the module file name.
EXAMPLE
Section "Module"

Load "extmod" Load "type1" EndSection

InputDevice Section
There are normally at least two InputDevice sections, one for Keyboard and one for Mouse.
Identifier Specify an unique name for this input device.
Drive r Specify the name of the driver to use for this input device..
Option "CorePointer" This input device is installed as the primary pointer device.
Option "CoreKeyboard" This input device is the primary Keyboard.
EXAMPLE
Section "InputDevice"
Identifier "Generic Keyboard" Driver "keyboard" Option "AutoRepeat" "500 30" Option "CoreKeyboard" EndSection
Section "InputDevice"
Identifier "PS2 Mouse" Driver "mouse" Option "CorePointer" Option "Device" "/dev/mouse" Option "Protocol" "PS/2" Option "Emulate3Buttons" "true" EndSection
Device Section
Specifies information about the video card used by the system. You must have at least one Device section in your configuration file. The active device is in ServerLayout->Screen.
Identifier Specify an unique name for this graphics card.
Driver Specify the name of the driver to use for this graphics card.
EXAMPLE
Section "Device"

Identifier "ATI Mach64" VendorName "ATI MACH64" VideoRam 2048 EndSection

Monitor Section
Monitor section describes a monitor. There must be at least one monitor section and the active one is used in ServerLayout ->Screen.
Identifier Specify an unique name for this monitor.
HorizSync horizsync-range Gives the range(s) of horizontal sync frequencies of this monitor in kHz.
VertRefresh vertrefresh-range Gives the range(s) of vertical sync frequencies of this monitor in Hz.
EXAMPLE
Section "Monitor"
Identifier "Generic Monitor " VendorName "Monitor Vendor" ModelName "Monitor Model" HorizSync 31.5-56.6 VertRefresh 40-70 EndSection

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Screen Section
Screen Section binds Device and Monitor sections. There must be at least one Screen Section. The active one is in ServerLayout section.
Identifier Specify an unique name for this Screen Section.
Device "device-id" This specifies the Identifier of Device section to be used for this screen.
Monitor "monitor-id" This specifies the Identifier of Monitor section to be used for this screen.
DefaultDepth depth Default color depth, like 8, 16 or 24.
Option "Accel" Enables XAA (X Acceleration Architecture), default is ON.
DISPLAY SUBSECTION Each Screen section must have at least one Display Subsection which matches the depth values in DefaultDepth. Depth depth This entry specifies what color depth of this Display Subsection. Virtual xdim ydim Specifies the virtual screen resolution to be used. ViewPort x0 y0 Sets the upper left corner of the initial display. Modes "mode-name" ... Secifies the list of video modes to use. Each mode-name specified must be in double quotes. They must correspond to those specified in the appropriate Monitor section (including implicitly referenced built -in ESA standard modes). mode can be switched with Ctrl+Alt+Keypad-Plus or Ctrl+Alt+Keypad-Minus.
EXAMPLE Section "Screen"
Identifier "My Screen” Device " ATI Mach64" Monitor " Generic Monitor" DefaultDepth 16 SubSection "Display"

Depth 16 Modes "1024x768" "800x600" "640x480" EndSubSection SubSection "Display"

Depth 24 Modes "1024x768" "800x600" "640x480" EndSubSection EndSection
ServerLayout Section
ServerLayout section binds a Screen section and one or more InputSection to form a complete configuration. The active ServerLayout section is specified in ServerFlags. If not, the first ServerLayout section is active. If no ServerLayout sections are present, the single active screen and two active (core) input devices are selected as described in the relevant sections.
Identifier An unique name for this ServerLayout Section.
Screen screen-num "screen-id" position-information The screen-id field is mandatory, and specifies the Screen section being referenced.
InputDevice "idev-id" "option" ... Normally at least two are required, one for the core pointer and the other for the primary keyboard devices.
EXAMPLE
Section "ServerLayout"
Identifier "Default Layout" Screen "My Screen" InputDevice "Generic Keyboard" InputDevice "PS/2 Mouse" EndSection