Linux

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introduction to linux+redhat+centos

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Linux

  1. 1. In the name of god LINUX red hat(centos) Abolfazl Hashemi
  2. 2. Title • • • • • • • • • • • What’s Linux? working with directories, files Using text editors like vi, vim Control on mount & un-mounting process File permissions & ownerships Linux installation & package management (yum, rpm, wget ) process & threads Configure disk partitions Manage disk quota & create quota report Symbolic links, FHS Writing shell script • • • • • • • • • • • Working with archive files System resource management Working with debuggers in Linux gpg command Introduction to network Security in linux Do automate tasks in Linux, cron job Configure & working with OpenSSH Working with tcpdump Configure web server Network configuration
  3. 3. Summary of Linux • • • • Linux clone of unix os Linux can run on small computer Open source os Distributed of Linux – – – – – – – Debian GNU/Linux Fedora Linux Gentoo Linux Libranet GNU/Linux Red Hat Linux Yellow Dog Linux bash ……. tsh zsh x86_64 i386 User Shell Kernel Graphic User Interface(GUI)
  4. 4. Installation Requirements Customize installation X configuration GUI: graphical user interface Install What is shell, terminal, virtual terminal, super terminal, konsole, tty. • man, --help • • • • • •
  5. 5. working with directories, files, … • Command line for directories pwd: current working directory-> /root cd: change working directory -> cd /etc/init.d ls: list of directory, files, ... . • ls –a: list of all directory,… . • ls –l : list of all directory,... With details mkdir: make directories -> mkdir test
  6. 6. working with directories, files, …(cont’d) rm: remove file, directory, … .  rm –r: remove recursive ->rm –r file mv: move file, directory, … .  mv /etc/test /var cp: copy file, directory, … .  cp /etc/test /var
  7. 7. working with directories, files, …(cont’d)  echo: display a line of text  date: print or set the system date and time  date –s: set time and date  more: filter and show text  less: opposite of more  head: output the first part of file  Head –n 5 /etc/init.d/netfs  tail: output the last part of file  tail–n 5 /etc/init.d/netfs  alias: summary of commands that you now it  alias ll=“ls –l”  : -> arg out to arg in
  8. 8. working with directories, files, …(cont’d) Important files:        / /etc /var /sbin /bin /dev /usr  /home  /root  /proc Exercise1  What is . ?  What is .. ?
  9. 9. Using text editors like vi, vim vi & vim -> vi test.txt vim: vi improved command for vi:     i -> insert :w -> save :q ->quit :! ->force  :wq! ->save and quit with force
  10. 10. ‫)‪Using text editors like vi, vim(cont’d‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫میسيد.‬ ‫‪ J‬باال بشدن خطَای پاییه محل مًسد وظش.‬ ‫‪ H‬بٍ ايلیه خط میسيد ‪ L‬بٍ آخشیه خط میسيد.‬ ‫‪ R‬بٍ مذ جایگزاسی میسيد.‬ ‫‪ :s/search-text/replace-text/g‬فقط دس‬ ‫خط مًسد وظش جایگزاسی میکىذ. ‪ /gci‬میپشسذ‬ ‫کٍ میخًاَی تغییش دَی یا خیش.‪ :%s‬بشای کل‬ ‫خطَا اوجام میشًد. ‪:g/start/s/text/rep‬‬ ‫َشجایی کٍ ‪َtext‬ست‪ rep‬قشاس میدَذ بٍ ششط‬ ‫ششيع خط با ‪:5,10s/search- .start‬‬ ‫‪ text/replace-text/g‬جایگزاصی میکىذ اص‬ ‫خط 5 تا 21.‬ ‫‪ :3,9d‬اص خط 3 تا خط 9 سا پاک میکىذ.‬ ‫#: سفته بٍ خط #‬ ‫‪ :5,10w filename‬اص خط 5 تا 01 سا دس فایل‬ ‫مًسد وظش کپی میکىذ.‬ ‫‪ :v‬سفته بٍ محیط يیژيیال‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬ ‫‪ u‬بٍ حالت قبل بش میگشدد معادل ‪Ctrl+z‬‬ ‫‪ Ctrl+r‬بٍ حالت بعذ میسيد معادل ‪Ctrl+y‬‬ ‫‪ :r‬تمام مته سا کپی ي بعذ بٍ مکان دلخًاٌ اضافٍ‬ ‫میکىذ.‬ ‫‪x‬پاک کشدن کاساکتش.‬ ‫‪dw‬پاک کشدن کلمٍ.‬ ‫‪ZZ‬رخیشٌ کشدن ي خشيج.‬ ‫‪:e‬باصکشدن فایل جذیذ.‬ ‫‪:n‬سفته بٍ فایل بعذی کٍ دس حال تغییش است..‬ ‫‪ Ctrl+u‬باال بشدن مته ي ‪Ctrl+e‬باال بشدن‬ ‫صفحٍ.‬ ‫‪ Ctrl+d‬پاییه بشدن مته ي ‪Ctrl+y‬پاییه بشدن‬ ‫صفحٍ.‬ ‫‪ :e filename‬يیشایش فایل مًسد وظش.‬ ‫‪ Ctrl+g‬اطالعاتی دس مًسد فایلی کٍ داخل آن‬ ‫َستیذ میدَذ.‬ ‫}+‪ Ctrl‬ي {+‪ Ctrl‬بٍ ابتذا ي اوتُای پاساگشاف‬
  11. 11. Using text editors like vi, vim(cont’d) • • • • • • • • • • • :set remap Accept macros within macros :set report Indicates largest size of changes reported on status line :set ro Changes file type to "read only" :set scroll=n set n lines for CTRL-d and z :set sh=shell_path set shell escape (default is /bin/sh) to shell_path :set showmode Indicates input or replace mode at bottom :set slow Postpone display updates during inserts :set sm Show matching { or ( as ) or } is typed :set sw=n Sets shift width to n characters :set tags=x Path for files checked for tags (current directory included in default) :set term Prints terminal type • • • • • • • • • • :set terse Shorten messages with terse :set timeout Eliminates one-second time limit for macros :set tl=n Sets significance of tags beyond n characters (0 means all) :set ts=n Sets tab stops to n for text input :set wa Inhibits normal checks before write commands :set warn warn :set window=n Sets number of lines in a text window to n :set wm=n Sets automatic wraparound n spaces from right margin. :set ws Sets automatic wraparound n spaces from right margin.
  12. 12. Using text editors like vi, vim(cont’d) • • • • • • • • • • • :set ai Turns on auto indentation :set all Prints all options to the screen :set ap Prints line after d c J m :s t u commands :set aw Automatic write on :n ! e# ^^ :rew ^} :tag :set bf Discards control characters from input :set dir=tmp Sets tmp to directory or buffer file :set eb Precedes error messages with a bell :set ed Precedes error messages with a bell :set ht= Sets terminal hardware tabs :set ic Ignores case when searching :set lisp Modifies brackets for Lisp • • • • • • • • • compatibility. :set list Shows tabs (^l) and end of line ($) :set magic Allows pattern matching with special characters :set mesg Allows others to send messages :set nooption Turns off option :set nu Shows line numbers :set opt Speeds output; eliminates automatic RETURN :set para= macro names that start paragraphs for { and } operators :set prompt Prompts for command input with : :set re Simulates smart terminal on dumb terminal
  13. 13. Using text editors like vi, vim(cont’d) Exercise2  How to search word in text?  How to save output command in text?  What are grep and find command?
  14. 14. Using text editors like vi, vim(cont’d)           od: octal and other format export: show variables env: environments variable wc: show word, char, … . sort: sort text uniq: delete repeated lines pr: print text touch: change file timestamps cpio: copy files to and from archives dd: convert and copy a file
  15. 15. Control on mount & un-mounting process • mount: verify hardware(device) to Linux • umount: unmount • command for mount:  mount -> show devices mounted  mount /dev/cdrom /mnt->mount cd rom  mount /dev/sda1 /mnt ->mount sda(storage device type a number1  mount –l –t <type> -> list and type of devices  /etc/udev/rules.d -> verify devices
  16. 16. Control on mount & un-mounting process(cont’d) Important file:  /etc/fstab  /etc/mtab  /proc/mounts Exercise3  What is mount point?  What is journaling?
  17. 17. File permissions & ownerships d r-- r-- r-- = d 4 4 4 -> directory & permission is 444 - --- --- --- rwx rwx rwx -> read, write, execute ll -> - --- --- --types are directory, link, … .
  18. 18. File permissions & ownerships(cont’d)  - Normal data file; may be text, an executable program, graphics, compressed data, or just about any other type of data.  d Directory; disk directories are files just like any others, but they contain filenames and pointers to disk in odes. Controlling Access to Files 195  l Symbolic link; the file contains the name of another file or directory. When Linux accesses the symbolic link, it tries to read the linked-to file.  p Named pipe; a pipe enables two running Linux programs to communicate with each other. One opens the pipe for reading, and the other opens it for writing, enabling data to be transferred between the programs.  s Socket; a socket is similar to a named pipe, but it permits network and bidirectional links.  b Block device; a file that corresponds to a hardware device to and from which data is transferred in blocks of more than one byte. Disk devices (hard disks, floppies, CDROMs, and so on) are common block devices.  c Character device; a file that corresponds to a hardware device to and from which data is transferred in units of one byte. Examples include parallel port, RS-232 serial port, and audio devices.
  19. 19. File permissions & ownerships(cont’d) command for permission  chmod: change file mode -> chmod 777 filename  chown: change file owner and group-> chown user1 filename -> chown –R  Set User ID (SUID)  Set Group ID (SGID)
  20. 20. File permissions & ownerships(cont’d)
  21. 21. File permissions & ownerships(cont’d)
  22. 22. File permissions & ownerships(cont’d)
  23. 23. File permissions & ownerships(cont’d) Command line:  groupadd: create new group -> groupadd test -p test  groupadd –r: create system group  groupdel: delete group  groupmems: add members to group ->groupmems –g test [-l] [-a] [-d]  groupmod: groupmod test –g 777
  24. 24. File permissions & ownerships(cont’d) find / [-group name] [-user name] newgrp: login to new group useradd: create new user Userdell: delete user passwd: change password
  25. 25. File permissions & ownerships(cont’d)  Important directory:      /etc/shadow /etc/group /etc/gshadow /etc/login.defs /etc/passwd  Exercise4  What is umask ?  What is file attributes?  What is sudoedit ?
  26. 26. symbol • > Creates a new file containing standard output. If the specified file exists, it’s overwritten. • >> Appends standard output to the existing file. If the specified file doesn’t exist, it’s created. • 2> Creates a new file containing standard error. If the specified file exists, it’s overwritten. • 2>> Appends standard error to the existing file. If the specified file doesn’t exist, it’s created. • &> Creates a new file containing both standard output and standard error. If the specified file exists, it’s overwritten. • < Sends the contents of the specified file to be used as standard input. • << Accepts text on the following lines as standard input. • <> Causes the specified file to be used for both standard input and standard output.
  27. 27. Linux installation & package management  Install: some time you should install packages(program) for example MySQL packages.  Type of packages are URL, .rpm, name, … .  yum install package1 [package2] …  Upgrade: some time you should upgrade program for example change MySQL 4 to 5.  yum upgrade [package1] [package2]  Update: some time you should update program for example MySQL 5 to 5.1.  yum update [package1] [package2] …  Uninstall: some time you should uninstall program for example erase MySQL.  remove | erase [package1] [package2] ….
  28. 28. Linux installation & package management(cont’d)  Another command for package management: rpm just manage .rpm packages rpm –i -> install rpm –U -> upgrade rpm –v -> print verbose information rpm –h -> print 50 hash marks as the package archive is unpacked. Use with –v  rpm –e -> erase or uninstall rpm –Uvh package.rpm     
  29. 29. Linux installation & package management(cont’d)  Another command for package management:  wget: download from network and support http, https and ftp.  wget –c -> continue  wget –d ->debug For install all packages you need make file that and compile and copy configure. For example ./configure -> make -> make install
  30. 30. Linux installation & package management(cont’d) Important file:  /etc/yum  /etc/yum.conf  /etc/yum.repos.d Exercise5  install mc packages from local use yum  go to /usr/share/doc and find mc files  Work by mc command
  31. 31. Configure disk partitions LVM(Logical Volume Manager) partition Standard partition Extended : same as primary but don’t have file system and create logical partition on it and their have fs LVM active sda1 Primary[4] sda2 Extended[5->] Sda(n)
  32. 32. Configure disk partitions(cont’d)
  33. 33. Configure disk partitions(cont’d)  A partition can be primary, extended and active  Just 1 active, 4 primary  Name of hard: sda or hda  Name of partition: sda1, sda2, … .  Name of device: cdrw, cdrom, sdb, … .  fdisk: partition table in linux  fdisk –l : list of all partition->fdisk –l name: details for name  fdisk [name of disk] -> fdisk /dev/sda : manage sda  Interactive area  m for help
  34. 34. Configure disk partitions(cont’d)      a : bootable flag n : new partition q : exit with out save w : write and exit d : delete a partition parted : same as fdisk partx : show number of partition for sda
  35. 35. Configure disk partitions(cont’d) Create file system on partition     mke2fs : create ext2,ext3,ext4 fs resize2fs : resize ext2,ext3,ext4 fs mkfs: build a linux fs mkswap : set up a linux swap area
  36. 36. Configure disk partitions(cont’d) Logical Volume Manager, or LVM, is a storage management solution that allows administrators to divide hard drive space into physical volumes (PV), which can then be combined into logical volume groups (VG), which are then divided into logical volumes (LV) on which the file system and mount point are created.
  37. 37. Configure disk partitions(cont’d)
  38. 38. Configure disk partitions(cont’d)  Example: lvm>  pvcreate pv1  vgcreate vg1  lvcraete –L 10g -n lv1 vg1  Mkfs.ext4 /dev/vg1/lv1  Important files:  /dev/*  /sbin/vg*  /etc/fstab  Exercise6  Create lvm disk and vg and pv  Create ext4 file system on lvm
  39. 39. Manage disk quota & create quota report  File system base disk quota allocation  User or group based disk quota allocation  Hard limit – For example, if you specify 2GB as hard limit, user will not be able to create new files after 2GB  Soft limit – For example, if you specify 1GB as soft limit, user will get a warning message “disk quota exceeded”, once they reach 1GB limit. But, they’ll still be able to create new files until they reach the hard limit  Grace Period – For example, if you specify 10 days as a grace period, after user reach their hard limit, they would be allowed additional 10 days to create new files. In that time period, they should try to get back to the quota limit.
  40. 40. Manage disk quota & create quota report(cont’d)  Create user and group for disk quota  Enable disk quota check : ->go fstab  LABEL=/home /home ext2 defaults,usrquota,grpquota 1 2  mount -n -o remount /  Reboot server  Show disk quota:      quotacheck –avug a: Check all quota-enabled file system v: Verbose mode u: Check for user disk quota g: Check for group disk quota  Assigned disk quota:  edquota username
  41. 41. Manage disk quota & create quota report(cont’d)  Report disk quota:  Repquota /home  Add quota check daily:  Create cron -> next session  Example: go fastab  /dev/VolGroup1/LogVol1 /home ext3 defaults,usrquota 1 2
  42. 42. Symbolic links, FHS Link : connection between 2 files. For example /var/spool/mail -> /var/mail Soft link (symbolic link) : point to file Hard link : pointer of file in directory so if delete file link is exist ln : make links between files  ln –s : create soft link  ln –p : hard link to soft link FHS(Filesystem Hierarchy System)
  43. 43. Writing shell script What is shell script : Shell scripts are plaintext files, so you create them in text editors. A shell script begins with a line that identifies the shell that’s used to run it. The execute text file begin with #!/bin and if you want use bash commands you insert #!/bin/bash ->sharp bang For execute that you should set execute permission and enter ./filename.
  44. 44. Writing shell script(cont’d)
  45. 45. Writing shell script(cont’d)  Commands for shell script:  set: for see system variables             BASH= Our shell name BASH_VERSION= Our shell version name COLUMNS= No. of columns for our screen HOME= Our home directory LINES= No. of columns for our screen LOGNAME= Our logging name OSTYPE= Our OS type PATH= Our path settings PS1= Our prompt settings PWD= Our current working directory SHELL= Our shell name USERNAME= User name who is currently login to this PC
  46. 46. Writing shell script(cont’d)  export: definition variable -> export var=10  echo: show quantity of variable -> echo $var  -n  -e        a b c n r t Do not output the trailing new line. Enable interpretation of the following backslash escaped characters in the strings: alert (bell) backspace suppress trailing new line new line carriage return horizontal tab backslash  if [ -n "$var" ]; then echo "not empty" else echo "empty"
  47. 47. Writing shell script(cont’d) How to use and create function:  What is function?  Example: function today { echo “today is” date +”%A,%B,%D,%Y” } Run today type: show type of command
  48. 48. Writing shell script(cont’d)  Conditions and loops: what is condition and loop?  if: what is .bashrc->directory of store information of user bash and variable and etc. if [ condition]; then commands elif commands else commands fi  while: for like while while [condition ] do commands done
  49. 49. Writing shell script(cont’d) • Example: #!/bin/bash x=“welcome” y=“welcome to linux” z=15 if [ $z –gt 10]; then echo “z=“$z; else if [ $x –eq $y ]; then echo “x=y”; fi echo “var=n” $x ”n” $y “n” $z fi
  50. 50. Writing shell script(cont’d)  for: for { variable name } in { list }  Condition:  Exercise7  Input 2 number and compare their. so calculate a*b, a+b, a/b, a10.
  51. 51. Working with archive files tar: extract, archive, … .  tar –cvf filename.tar filetoarchive  tar –xzf filename –C directory->extract & zip gzip: compress a file -> gunzip: expand a file zip & unzip: similar to gzip
  52. 52. System resource management  Run level: 0 – 6 -> /etc/grub.conf  0: turn on  6: turn off  pstree: Shows the Parent-Child Relation Between Processes  gstack: print a stack trace of a running process -> gstack [PID]  more /proc/cpuinfo  top: display Linux task  s -> change delay  z -> change color  h -> help  b -> bold  q -> exit
  53. 53. System resource management(cont’d)  ps: report a snapshot of current process -> ps aux  USER: The name of the user who started the process.  PID: The PID of the process. The command ps aux sorts the processes by their PID.  %CPU: The percentage of CPU time the process has used since startup.  %MEM: The percentage of memory the process is currently using.  VSZ: The virtual memory size, which is the total amount of memory claimed by this process.  RSS: The resident memory size, which is the amount of memory the process currently has in use.  TTY: The terminal (TTY) from which the process was started. A question mark indicates a daemon process that is not associated to any TTY.
  54. 54. System resource management(cont’d)  STAT: The current status of the process.  START: The time at which the process was started.  TIME: The total amount of system time this process has been using since it started.  COMMAND: The command that was used to start this process. If the name of this command is between square brackets (you can see quite a few examples of this in (Listing 9-5), the process is not started with a command at the command line, but is a kernel thread.
  55. 55. System resource management(cont’d)  Use limited system resource use ulimit command:  Core File Limits The -c option limits the size of core dumps, which are fi les created for  debugging purposes in certain types of program crashes.  File Limits The -f option limits the size of fi les that may be created by the shell, and –n limits the number of open fi le descriptors. (Most systems don’t honor the -n limits, though.)  Process Limits The -u option limits the number of processes a user may run, and -t limits the total CPU time in seconds.  Memory Limits The -v option sets the total amount of virtual memory available to the shell, -s sets the maximum stack size, -m sets the maximum resident set size, -d limits programs’ data set size, and -l sets the maximum size that may be locked into memory.
  56. 56. System resource management(cont’d)  Hard and Soft Limits The -H and -S options modify other options, causing them to be set as hard or soft limits, respectively. Hard limits may not be subsequently increased, but soft limits may be. If neither option is provided, ulimit sets both the hard and soft limits for the feature specified.  Current Settings Passing -a causes ulimit to report its current settings.  Important directories:  /proc/*  /boot/System.map –  Exercise8  Read command : pgrep, pkill, free, kill,
  57. 57. Working with debuggers in Linux After you write shell script you need debug it and some time you need debug software for this problem we use gdb command. gdb: GNU debugger  In shell script use (gdb) commands……(gdb)  For software use gdb (option) [ pid or name of software]  Some option of gdb
  58. 58. Working with debuggers in Linux(cont’d)  h: for help  q: quit  run: run [program name]  bt: print call stack = where     up: move up one stack frame down: != up frame: frame n ->go to frame n info: info frame ->information current frame  Exercise9  If you need Debugger command see Debugging with gdb Tenth Edition, for gdb version 7.6.1.2013, Richard Stallman, Roland Pesch, Stan Shebs, 658paper
  59. 59. gpg command Checksum: when we transfer or storage data between computers maybe information destroyed and we don’t understand or understand not solve their so we use a method to solve it problem. For example CRC checksum. You Consider, checksum different with encrypt. In data encryption goal is encrypt and in checksum goal is solve and understand mistake. For best data transfer we encrypt and checksum methods.
  60. 60. gpg command(cont’d) MD5: is a hashing method. First get checksum of data then get hash their.
  61. 61. gpg command(cont’d)  Encryption (gpg GNU Privacy Guard) : 2 way for encryption  signature file: just who make file can access it  gpg --output file.sig --detach-sign file  Primary and public key: just who Possessing primary key can access it and other just encrypt file with public key.      gpg --gen-key->generate private and public key gpg --list-key->list of key gpg --edit-key keyID->edit key gpg --output [fileforsendkey.gpg] --export->create export key gpg --armor --output [fileforsendkey.gpg] --export-> Creates ASCII armored output
  62. 62. Security in linux  Physical Access Problems  Set bios password  Set grub password  [grub-crypt --sha-256] or [grub-crypt --md5]  Copy to /etc/grub.conf -> password –md5 ………………….  Stolen Passwords  Local Program Bugs  Set SUID and SGID bit for program  find / -perm +6000 -type f  Server Bugs  Denial-of-Service Attacks  Encryption Issues
  63. 63. Security in linux(cont’d)  Creating Firewall Rules: iptable  /etc/services: see the services run on linux and more details about them.  netstat -> netstat –ap : see active internet connection
  64. 64. Security in linux(cont’d)
  65. 65. Security in linux(cont’d)  The iptables program is the utility that manages firewall.  First you should know about packet and packet filtering.  iptables -L -t filter       Chain target Chain target Chain target INPUT prot opt FORWARD prot opt OUTPUT prot opt (policy ACCEPT) source destination (policy ACCEPT) source destination (policy ACCEPT) source destination
  66. 66. Security in linux(cont’d)
  67. 67. Security in linux(cont’d) #!/bin/bash iptables -F INPUT iptables -F FORWARD iptables -F OUTPUT iptables -P INPUT DROP iptables -P FORWARD DROP iptables -P OUTPUT DROP # Let traffic on the loopback interface pass iptables -A OUTPUT -d 127.0.0.1 -o lo -j ACCEPT iptables -A INPUT -s 127.0.0.1 -i lo -j ACCEPT # Let DNS traffic pass iptables -A OUTPUT -p udp --dport 53 -j ACCEPT iptables -A INPUT -p udp --sport 53 -j ACCEPT # Let clients' TCP traffic pass iptables -A OUTPUT -p tcp --sport 1024:65535 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -p tcp --dport 1024:65535 -m state --state ESTABLISHED,RELATED -j ACCEPT # Let local connections to local SSH server pass iptables -A OUTPUT -p tcp --sport 22 -d 172.24.1.0/24 -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -p tcp --dport 22 -s 172.24.1.0/24 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
  68. 68. Do automate tasks in Linux, cron job  What is automate task ?  When use automate task?  Some system maintenance tasks should be performed at regular intervals and are highly automated  Automate task in linux?  at  cron: cron is a daemon  at & cron: at just run once and cron run ongoing  Command for cron job:
  69. 69. Do automate tasks in Linux, cron job  crontab: execute program to configuration cron  crontab –l : see cron job  crontab [-u user] [-l | -e | -r] [file]  /etc/cron.d: configuration directory -> sysstat  /var/spool/cron: cron job  Create cron job:  02 4 * * * root run-parts /etc/cron.daily  This line begins with five fields that specify the time. The fields are, in order, the minute  (0–59), the hour (0–23), the day of the month (1–31), the month (1–12), and the day of the  week (0–7; both 0 and 7 correspond to Sunday)
  70. 70. Do automate tasks in Linux, cron job  In all cases, you can specify multiple values in several ways:  An asterisk (*) matches all possible values.  A list separated by commas (such as 0,6,12,18) matches any of the specified values.  Two values separated by a dash (-) indicate a range, inclusive of the end points. For instance, 9-17 in the hour field specifies a time of from 9:00 a.m. to 5:00 p.m.  A slash, when used in conjunction with some other multi value option, specifies stepped values /a range in which some members are skipped. For instance, */10 in the minute field indicates a job that’s run every 10 minutes  at: at -f commands.txt noon
  71. 71. Working with tcpdump  Capture packets from a particular Ethernet interface using tcpdump -i  Capture only N number of packets using tcpdump -c  Display Captured Packets in ASCII using tcpdump -A
  72. 72. Working with tcpdump  Display Captured Packets in HEX and ASCII using tcpdump -XX  Capture the packets and write into a file using tcpdump -w  Reading the packets from a saved file using tcpdump -r  Capture packets with IP address using tcpdump -n  Capture packets with proper readable timestamp using tcpdump -tttt  Read packets longer than N bytes  tcpdump -w g_1024.pcap greater 1024  Receive only the packets of a specific protocol type  tcpdump -i eth0 arp
  73. 73. Working with tcpdump  Receive packets flows on a particular port using tcpdump port  tcpdump -i eth0 port 22  Capture packets for particular destination IP and Port  tcpdump -w comm.pcap -i eth0 dst 16.181.170.246 and port 22  tcpdump Filter Packets – Capture all the packets other than arp and rarp  tcpdump -i eth0 not arp and not rarp
  74. 74. Introduction to network  tcp/ip and osi model:        physical layer data link layer network layer transport layer session layer presentation layer application layer  Topology of network  Ring  Star  bus  protocol
  75. 75. Introduction to network (cont’d)  IP  DNS  DHCP  Port  telnet and ssh  ftp  http and https  Arp and rarp  icmp
  76. 76. Network configuration  Ifconfig  Setup  DNS  Forward  Primary  Secondary  Stub  Reverse  /etc/resolv.conf  /etc/hosts
  77. 77. Network configuration(cont’d)  DHCP  How to work dhcp ?  /etc/dhcp/dhcpd.conf  Telnet & SSH  how to work telnet or ssh?  ssh 192.168.1.100  telnet 192.168.1.100
  78. 78. Network configuration(cont’d) icmp  Ping  tracepath ftp  How to work ftp?  scp

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