LINUX Administrator’s Quick Reference Card



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LINUX Administrator’s Quick Reference Card Jialong He 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 and 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. chfn, 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 / makebootdisk make a bootable floppy disk 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/rc.d/rc.netdevice /etc/modules.conf Put network card driver (e.g., e100) in “rc.netdevice” /sbin/modprobe e100 Or in “modules.conf” alias eth0 e100 /etc/HOSTNAME /etc/NETWORKING (Slackware) /etc/sysconfig/network (Redhat) hostname is set by “/bin/hostname” during boot and the name is read from these files. May change manually. etc/resolv.conf specify name server, DNS domain and search order. For Example: search nameserver /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 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) ntsysv menu driven SYSV service configuration (Redhat) chkconfig command line SYSV service configuration (Redhat) Redhat files in /etc/sysconfig Configuration Files keyboard keyboard map, e.g., KEYBOARD=”/usr/lib/kdb/keytables/” mouse Mouse type, e.g., MOUSETYPE=Microsoft XEMU3=yes network network settings, contains NETWORKING=yes 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 “” is the configuration file. “” is a macro file which can be used to generate “” by: m4 > aliases mail aliases, must run “newaliases” after change. use :include: to include external list in a file. .forward per user aliases, use \yourname to prevent further expand and keeps a copy in mailbox. access mail access control, FEATURE(access_db) should be set in For example, in /etc/mail/access REJECT RELAY DISCARD makemap hash /etc/mail/access < /etc/mail/access /etc/mail/relay- domains list all host/domain accepted for relaying. Commands newaliases rebuild the data base for the mail aliases file. makemap build access database, e.g, makemap hash access.dbcmd.txt save a man page as a text file and remove control characters. ntop –w 3000 Run ntop and listen on web port 3000. View traffic with browser to http://hostsname:3000 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”. To start web server with SSL support , use “httpd –DSSL” or “apachectl startssl”, otherwise, commented out in ssl.conf. (*) Trouble shoot SSl connection with command openssl s_client –connect 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 # log to remote host Note: separator between first column and second colume (log file name) must be TAB, not spaces. 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: Printing with CUPS Introduction Common Unix Printing System (CUPS) is the default printing system on many Linux distros and Mac OSX. The latest version can be downloaded from You have to download CUPS package and optionally ESP Ghostscript package if you don’t have a Postscript printer. You compile and install both packages with commands configure; (see configure –help) make; make install The printing daemon “/usr/sbin/cupsd” is controlled by a configuration file “/etc/cups/cupsd.conf”. The syntax of this file is similar to Apache’s httpd.conf. You can edit this file with a text editor, but normally the default settings work fine. After change any configuration, you restart “cupsd” to let it read new settings (for example: rc.cups start). Another important configuration file is “Printers.conf”. This file defines each local or network (socket or IPP) printer. You can edit this file with a text editor and then restart “cupsd” to have it read the new settings. Another way to change printer settings is to use command line program “lpadmin”. CUPS has a web-based administration tool. You point a web browser to http://localhost:631. Each Linux distribution also has its own GUI printer administration tool. lpadmin -p myprint -E -v parallel:/dev/lp0 –m laserjet.ppd lpadmin -p myprint -E -v socket:// -m myprint.ppd lpadmin -p myprint -E -v lpd:// -m myprint.ppd lpadmin -p myprint -E -v ipp:// -m myprint.ppd lpadmin -p myprint -E -v ipp://user:passwd@ -m myprint.ppd The about commands add a printer connected to (1) local parallel port, (2) JetDirect printer, and (3) LPD printer. –m option specifying a Postscript Printing Definition (PPD) files. CUPS has a few PPD files preinstalled. In order to use full features of your printer, you may need to find a proper PPD file and put it in “/usr/share/cups/model” directory. kcmshell printmgr KDE printer manager http://localhost:631 CUPS web administration interface lpadm –d myprint lpadm –x myprint -d option set default printer -x option delete a installed printer. enable/disable accept/reject Control printing queue lpadmin –p myprint –P another.PPD Change PPD file lpoptions –p myprint -l Display associate PPD lpinfo –v List supported printing protocols lp –d myprint filename lpr –P myprint filename Print a file cancel id lprm id remove a print job from queue /etc/cups/printers.conf /etc/cups/classes.conf /etc/cups/cupsd.conf CUPS related configuration files /etc/cups /usr/lib/cups /usr/share/cups CUPS related directories Samba File and Printer Sharing Introduction Samba provides file and printer sharing with MS Windows computers. It makes UNIX speaks SMB/ICFS file and printer sharing protocol. The latest version of samba can be downloaded from Samba is controlled by a configuration file “smb.conf”. On Redhat Linux, one can use “redhat-config-samba” to modify the configuration file. On other systems, SWAT is a web based GUI interface. SWAT is run from “inetd” and listen to port 901. You just need point your browser to http://localhost:901 after starting swat. Commands To test if the syntax of “smb.conf” is correct, use testparm smb.conf List shares on a Samba or Windows server smbclient –L machinename -U username Connect to a Samba or Windows server and get/put files using FTP like commands: smbclient //machinename/sharename -U username Security Mode in “smb.conf” security = user In this (default) security mode, samba maintain its own user login database which is usually in /etc/samba/smbpasswd. This file is created with command /usr/sbin/smbpasswd. Note, the user login file and command have the same name but in different directories. Following settings are used: encrypt passwords = yes smb passwd file = /etc/samba/smbpasswd security = domain In this security mode, samba server must join to an NT domain (using net command) and authenticate users by a domain controller. A user must have both valid UNIX and NT account in order to access files. security = server Use another computer (NT or W2k) to authenticate users. No need to join a domain. Need to specify a login server: password server = security = share Give each share a password, no user name needed. IPtables (Netfilter) Command Syntax iptables [-t ] Save and Restore rules /sbin/iptables-save > /etc/sysconfig/iptables /sbin/iptables-restore < /etc/sysconfig/iptables Firewall script sample 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 iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to 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 $HOME/.xinitrc /etc/X11/xinit/xinitrc /etc/X11/xinit/xinitrc.d $HOME/.Xclients /etc/X11/xinit/Xclients scripts run after X server started /etc/sysconfig/desktop decide which desktop (GNORM, KDE) to start (Redhat). (by /etc/X11/prefdm) /etc/X11/fs/config configuration of 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 “” 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 (xorg.conf) 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 together 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 terminate 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. Driver 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 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 Boot Sequences Redhat Usually the Linux kernel file is /boot/vmlinuz and is loaded by a boot loder (e.g., LILO). The first process created by the kernel is /sbin/init. It uses a configuration file /etc/inittab. init process runs /etc/rc.d/rc.sysinit script first, then runs all scripts in /etc/rc.d/rcN.d, where N is the default run level defined in inittab. The actual scripts are stored in /etc/rc.d/init.d and proper links are created in run level directoris to point to corresponding scripts in init.d directory. The last script to run is /etc/rc.d/rc.local. Run level 1: Single user mode Run level 3: Multiuser mode Run level 5: Multiuser model with X11 Slackware In Slackware, Linux kernel is /boot/vmlinuz and the first process started by the kernel is /sbin/init. Its configuration file is /etc/inittab. init first runs script /etc/rc.d/rc.S, then runs /etc/rc.K for single user mode or /etc/rc.M for multiuser mode. The last script to run is /etc/rc.d/rc.local. rc.S calls scripts (rc.modules, rc.pcmcia, rc.serial and rc.sysvinit). rc.M calls scripts (rc.inet1, rc.inet2, rc.httpd, rc.samba) and start some network server (lpd, httpd etc.) rc.inet1 sets IP address, Mask, and default Gateway. Run level 1: Single user mode Run level 3: Multiuser mode Run level 4: Multiuser model with X11 Fix Slackware boot after Ghost • boot with slackware installation CD • mount root partition (mount /dev/hda2 /mnt) • change root (chroot /mnt) • re-run LILO (cd /mnt/etc; lilo) GRUB boot loader Introduction More Linux distributions are using GRUB as the boot loader instead of LILO. GRUB can be downloaded from The newer version of GRUB (called GRUB2) is totally rewritten and uses somewhat different syntax than Ver (called GRUB Legacy). After unpacking GRUN, compile and install it with following commands: ./configure make make install First test GRUB by creating a bootable floppy disk with command grub-install /dev/fd0 Suppose you have Windows installed on the first hard disk and Linux on the second disk, boot with just created floppy, when grub> prompt appears, type following commands === Boot Linux === grub> root (hd1,0) grub> kernel /boot/vmlinuz (or linux /boot/vmlinuz if using GRUB2) grub> boot === Boot Windows === grub>rootnoverify(hd0, 0) chainloader +1 boot Other Useful GRUB commands ls Display disks and partitions (GRUB2) geometry(hd0) Show hard disk geometry, test if HD exist configfile (hd1,0)/boot/grub/grub.conf Display boot menu cat (hd1, 0)/etc/fstab Display a file content, can figure out which partition was used as root partition in an unbootable system, then pass root parameter in “kernel” command help List available commands Sample GRUB configuration file #=================== # GRUB ver #=================== default=0 timeout=10 splashimage=(hd1,2)/grub/splash.xpm.gz title Linux root (hd1,0) kernel /boot/vmlinuz root=/dev/sdb1 title Windows XP rootnoverify (hd0,0) chainloader +1 #=================== # GRUB2 #=================== set timeout=10 set default=0 # Entry 0 - Load Linux kernel menuentry "Linux" { set root=(hd1,0) linux /boot/vmlinuz root=/dev/hda2 initrd /initrd } # Entry 1 - Chainload another bootloader menuentry "Windows" { set root=(hd0,0) chainloader +1 }



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