Tuesday, June 14, 2011
Boot Process Linux Sort
In sort boot process of linux OS ....
power on --> BIOS load into RAM --> BIOS moto is to load OS --> for that he perform POST inventry and calls CMOS to know boot order
---> after getting boot order BIOS load it whatever it finds in first sector of hdd into RAM this is what we called MBR
M ----------------------------- B ----------------------------------- R
<----------------------------- 512 bytes --------------------------->
+--------------BS------------+ +--------PT--------+ +----MN----+
446 64 2
M ------------------------------ B ----------------------------------- R
--> Exaine PT of MBR to know where is active partition (AP) -> Then BIOS load whatever it finds on BS of AP
--> BIOS then load 1SBL(1st satge boot loader) which were put there by anocanda during install time.
--> 1SBL gives CHS # of the /boot/boot.map to BIOS to load 2SBL(2st satge boot loader)
--> 2SBL examine the binary format of grub.conf and follows all instructions given there.
--> with the help of BIOS 2SBL loads Linux kernel vmlinuz-2.4.20-8 into the RAM
--> 2SBL uncompress kernal vmlinuz and filled it into ram waht we see in /proc dirr...
--> Now Linux Kernel is into RAM which is much powerfull than 2SBL and takes total control from it.
--> With the help of BIOS Kernal uncompresses the /boot/initrd-2.4.20-8.img to find filesystem driver.
-> Now HDd driver available and Kernel is into RAM
-> Now Kernel Load /sbin/init program into RAM.
-> /sbin/init* examines /etc/inittab and
1> run level (0-6 what ever is is defined) and run an script name
2> /etc/rc.d/rc.sysinit in a subshell of inittab shell
which calls /etc/sysconfig/network to load networking
and start various functions defined in /etc/init.d/functions
and Mounts all local filesystems from /etc/fstab and updates /etc/mtab
and now function of ec.sysinit ends
Short summary of what rc.sysinit does
===================================== in sort waht done by rc.sysinit
hostname
system logs
date and time
keyboard
system font
swap partitions
usb
file system checks
mount the filesystems read/write
=====================================
3> /etc/rc.d/rc { as a subshell }
/etc/rc.d/rc3.d/K*
/etc/rc.d/rc3.d/S* --> start & stop of services
/etc/rc.d/rc.local -->
--> shell inittab is active and by defaults there are 6 mingettys out of which all entered into sleep state
--- except 1
---> this mingetty then executes the /bin/login process and is then it put into sleep state by init
---> and we get promped for username & password
--> * /bin/login execs /usr/bin/passwd which challenges for passwd from
us which then does authentication and authorization using PAM
---> after Right username and password
---> mingetty then goes into zombie state and is killed by init
---> defined shell loads and we get our shell based command prompt:#$
power on --> BIOS load into RAM --> BIOS moto is to load OS --> for that he perform POST inventry and calls CMOS to know boot order
---> after getting boot order BIOS load it whatever it finds in first sector of hdd into RAM this is what we called MBR
M ----------------------------- B ----------------------------------- R
<----------------------------- 512 bytes --------------------------->
+--------------BS------------+ +--------PT--------+ +----MN----+
446 64 2
M ------------------------------ B ----------------------------------- R
--> Exaine PT of MBR to know where is active partition (AP) -> Then BIOS load whatever it finds on BS of AP
--> BIOS then load 1SBL(1st satge boot loader) which were put there by anocanda during install time.
--> 1SBL gives CHS # of the /boot/boot.map to BIOS to load 2SBL(2st satge boot loader)
--> 2SBL examine the binary format of grub.conf and follows all instructions given there.
--> with the help of BIOS 2SBL loads Linux kernel vmlinuz-2.4.20-8 into the RAM
--> 2SBL uncompress kernal vmlinuz and filled it into ram waht we see in /proc dirr...
--> Now Linux Kernel is into RAM which is much powerfull than 2SBL and takes total control from it.
--> With the help of BIOS Kernal uncompresses the /boot/initrd-2.4.20-8.img to find filesystem driver.
-> Now HDd driver available and Kernel is into RAM
-> Now Kernel Load /sbin/init program into RAM.
-> /sbin/init* examines /etc/inittab and
1> run level (0-6 what ever is is defined) and run an script name
2> /etc/rc.d/rc.sysinit in a subshell of inittab shell
which calls /etc/sysconfig/network to load networking
and start various functions defined in /etc/init.d/functions
and Mounts all local filesystems from /etc/fstab and updates /etc/mtab
and now function of ec.sysinit ends
Short summary of what rc.sysinit does
===================================== in sort waht done by rc.sysinit
hostname
system logs
date and time
keyboard
system font
swap partitions
usb
file system checks
mount the filesystems read/write
=====================================
3> /etc/rc.d/rc { as a subshell }
/etc/rc.d/rc3.d/K*
/etc/rc.d/rc3.d/S* --> start & stop of services
/etc/rc.d/rc.local -->
--> shell inittab is active and by defaults there are 6 mingettys out of which all entered into sleep state
--- except 1
---> this mingetty then executes the /bin/login process and is then it put into sleep state by init
---> and we get promped for username & password
--> * /bin/login execs /usr/bin/passwd which challenges for passwd from
us which then does authentication and authorization using PAM
---> after Right username and password
---> mingetty then goes into zombie state and is killed by init
---> defined shell loads and we get our shell based command prompt:#$
Linux Interview Questions
1> Diff between RHEL 4 & 5 ?
2> About boot process in sort?
3> Types of DNS?
4> how dns works?
5> what is squid?
6> port number of data ftp, ftp, telnet, smtp, sql
7> what is sendmail hows it's works
9> How to block downloading in squid
10> How you handle Load on server?
11> Fields in crontab?
12> Any backup tools you know?
13> what is amanda why to use?
14> How to upadte your server?
15> rpm commands to update, erase, install etc...
16> Use squid to do authentication of users on browser?
17> USe of NFS and samba?
18> Diff NfS & Samba?
19> What is SeLinux?
20> DEfault cache size in Squid?
21> I want to assign NISdomainname via DHCP is it possible how?
22> How to setup networking IP doaminname etc...
23> What is & Difference between find and grep
24> About Raid tool used & level diff
25> I want to do remote login how?
26> Use of NIS?
27> Use of Log files?
28> I sent a mail but lost how to find it where is ..?
Some other
- How to find which user surfed which site? squid not log file
- which command you use for maintaing system info to submit to your manager?
- How you reduce load on your server
- tools used to check system load and performance
- Do you know anything about DHCP on windows server
- Application that you much like can confident about that
- In sort what you know in Linux
- Can you compile C program in linux how?
- about Linux shell scripting any other scripting language you know
Another that some asked to me?
1> why i hire you?
2> what is mean and responsibilities of a sys admin
3> why you chose linux as career?
4> About your current job and responsibilities
5> After doing job in diff field near about 2 years why you want to move your path
6> Why Linux not Windows? :)
7> What is your strenght?
8> Do you have any question?
9> Anything that you want to ask?
10> What is yor expection?
11> expected salary?
12> notice period how soon you can join?
Our HR department will inform you -- Why this?? :(
Some exact questions i don't remember so i just written in short!!!
1> size of MBR, fields in MBR, PT size in MBR
2> Dir /abcd have 744 perm to user but file /abcd/file1 has 777 permission can a normal
user edit file1, if no why?
4> what called #! /bin/bash
5> There a parameter in vsftpd conf file without which vsftpd server will not work
6> Diff GRUP & LILO techinical DIFF ...
7> Can we authenticate only users login on vsftpd by LDAP how
8> Port number SAMBA, DHCPD, SQUID, DNS, POP, POP, NTP on secure level, IMAP on secure level,
sendmail on secure level, Postfix..
9> Diff TCP/UDP
10> What are 7 Layers TCP/IP eg of each
11> Fields in DNS
12> USe of ip_farwording
13> where is config file of sshd service.
14> what is tunnel
15> What is mutt
16> fields in top command meanings
1>load average 2> %id 3> Mem: 4> SWAP
17> What is the use of swap partition
18> can we encrease swap partition size how
19> syntax of for loop in bash
14> awk stand for
15> can we change vsftpd default directory location
16> Can we change Default Directory in apache
16> Can we use index.php insted of index.html as default page in apache how .
17> visudo command will open file?
18> Diff RHEL & Centos
19> Diff LILO GRUB
20> Advantages of GRUB
21> What is iptables, Firewall, netfilter ..?
22> write iptable to block use of sshd service via port 22 from outside of range 192.168.8.10-192.168.8.50
23> Tables in iptables
24> what is cluster & virtualizaton just basic, what you understand by term..
25> what is domain and hostname concept.
26> can a system have multiple hostname/domainname how?
27> how many virtual ips you can assign to a single interface.
Power of vi editor Linux
# vim /home/suresh/myscripts/scratcher.sh <-- colorfull look
# vi /home/suresh/myscripts/scratcher.sh <-- simple
Moving through the text is usually possible with the arrow keys. If not, try:
h -> move Left
l -> move Right
k -> move up
j -> move down
shift+H : top of file (or :1 similarly you can go to any line by line number(n) :n)
shift+G : end of file (or :$)
dd --> delete current line
5+dd -> delete 5 line from current line (similarly n line)
5+dw -> delete 5 words from current word
x -> delete single character from current position
cc -->cut all current line content & leave the editor in insert mode
C --> delete all words from current position to end of line.
s -->cur the current character and leave te curser in insert mode
yy -->copy line
n+yy --> copy n lines
pp -->paste copied line etc..
n+pp -->paste n times
Insert Mode:
1> i or I (cpas i)
2> Insert button
3> a (+move curser to 1 position right)
4> A (+ move the curser at end of line)
5> o (+ a blank line below current)
6> O (+ a blank line above current)
Command mode: ESC button
:w -> Save
ESC+z+z --> Save & exit
:wq -> Save & Exit
:q -> Exit
:q! -> Exit without Save
:w file2 will save the text to file2.
:wq! overrides read-only permission
/find -->Search string "find" use "n" to search new location
? find --> same as above but search in opposite dir
:1, $s/shirish/shukla/g -->replace shirish by shukla throught file
undo & redo:
esc+u --> undo
ctl+r --> redo
esc . --> repeate the previous change
Bookmark
step 1> Go to the required word (type ESC+ma) you can use any lowercase a - z, A - Z, [ ect,.. note that a & A store diff bookmarks etc..
step 2>To go to bookmarked location (type ESC+ `+a ) note ` is above TAB button, similarly you can give various bookmarkes
* You can go to yu last edited line by --> ESC+`+ .
:marks --> Will show your bookmarks in current file.
Quick Summary of Vim Bookmark Commands
* ma –> Creates a bookmark called a
* `a –> Jump to the exact location (line and column) of the bookmark a
* ‘a –> Jump to the beginning of the line of the bookmark a
* :marks –> Display all the bookmarks
* :marks a –> Display the details of the bookmark with name a
`. –> Jump to the exact location (line and column) where the last change was performed
* ‘. –> Jump to the beginning of the line where the last change was performed
Advanced:
K --> Go to the man page of the word currently under the curser.
# vi /home/suresh/myscripts/scratcher.sh <-- simple
Moving through the text is usually possible with the arrow keys. If not, try:
h -> move Left
l -> move Right
k -> move up
j -> move down
shift+H : top of file (or :1 similarly you can go to any line by line number(n) :n)
shift+G : end of file (or :$)
dd --> delete current line
5+dd -> delete 5 line from current line (similarly n line)
5+dw -> delete 5 words from current word
x -> delete single character from current position
cc -->cut all current line content & leave the editor in insert mode
C --> delete all words from current position to end of line.
s -->cur the current character and leave te curser in insert mode
yy -->copy line
n+yy --> copy n lines
pp -->paste copied line etc..
n+pp -->paste n times
Insert Mode:
1> i or I (cpas i)
2> Insert button
3> a (+move curser to 1 position right)
4> A (+ move the curser at end of line)
5> o (+ a blank line below current)
6> O (+ a blank line above current)
Command mode: ESC button
:w -> Save
ESC+z+z --> Save & exit
:wq -> Save & Exit
:q -> Exit
:q! -> Exit without Save
:w file2 will save the text to file2.
:wq! overrides read-only permission
/find -->Search string "find" use "n" to search new location
? find --> same as above but search in opposite dir
:1, $s/shirish/shukla/g -->replace shirish by shukla throught file
undo & redo:
esc+u --> undo
ctl+r --> redo
esc . --> repeate the previous change
Bookmark
step 1> Go to the required word (type ESC+ma) you can use any lowercase a - z, A - Z, [ ect,.. note that a & A store diff bookmarks etc..
step 2>To go to bookmarked location (type ESC+ `+a ) note ` is above TAB button, similarly you can give various bookmarkes
* You can go to yu last edited line by --> ESC+`+ .
:marks --> Will show your bookmarks in current file.
Quick Summary of Vim Bookmark Commands
* ma –> Creates a bookmark called a
* `a –> Jump to the exact location (line and column) of the bookmark a
* ‘a –> Jump to the beginning of the line of the bookmark a
* :marks –> Display all the bookmarks
* :marks a –> Display the details of the bookmark with name a
`. –> Jump to the exact location (line and column) where the last change was performed
* ‘. –> Jump to the beginning of the line where the last change was performed
Advanced:
K --> Go to the man page of the word currently under the curser.
tar,zip,bzip,bzip2 linux commands
================== Compress Archiever ===========================
==> tar
tar -cvf file.tar ----> tared
tar -tvf file.tar ----> tabular view content of file
tar -xvf file.tar ----> untar
==> compress & uncompress >>> RH-9 absent in RHEL versions
tar -Zcvf file.tar.Z ----> compress
tar -Zxvf file.tar.Z ----> uncompress
compress file.tar
uncompress file.tar.Z
==> zip & unzip
gip file.tar ----> ziped
gunzip file.tar.zip ----> unziped
==> gzip & gunzip
tar -zcvf file.tar.gz ----> gziped
tar -zxvf file.tar.gz
gzip file.tar
gunzip file.tar.gz
==> bzip2 & bunzip2
tar -jcvf file.tar.bz2 ----> bziped
tar -jxvf file.tar.bz2 ----> bunziped
bzip2 file.tar.bzip2
bunzip2 file.tar.bzip2
===========>> Test file contents of compressed file
tar -tvf
tar -Ztvf
tar -ztvf
tar -jtvf
zcat.gz
bcat.bz2 ---> absent in RHEL
cpio -idmv.cpio
zcat.cpio.gz | cpio -idmv
=================== Advanced =======================
1 line gungip and untar
# zcat backup.gz | tar -xvf -
==> tar
tar -cvf file.tar
tar -tvf file.tar ----> tabular view content of file
tar -xvf file.tar ----> untar
==> compress & uncompress >>> RH-9 absent in RHEL versions
tar -Zcvf file.tar.Z
tar -Zxvf file.tar.Z ----> uncompress
compress file.tar
uncompress file.tar.Z
==> zip & unzip
gip file.tar
gunzip file.tar.zip ----> unziped
==> gzip & gunzip
tar -zcvf file.tar.gz
tar -zxvf file.tar.gz
gzip file.tar
gunzip file.tar.gz
==> bzip2 & bunzip2
tar -jcvf file.tar.bz2
tar -jxvf file.tar.bz2 ----> bunziped
bzip2 file.tar.bzip2
bunzip2 file.tar.bzip2
===========>> Test file contents of compressed file
tar -tvf
tar -Ztvf
tar -ztvf
tar -jtvf
zcat
bcat
cpio -idmv
zcat
=================== Advanced =======================
1 line gungip and untar
# zcat backup.gz | tar -xvf -
Chaneg Date and Time Linux
============ Treating with date command =================
1> date 081811002007 [mmddhhmmyyyy]--To set the Linux clock to 11th-Aug-2007-11-AM
2> timeconfig --Set your TIMEZONE
3> hwclock --hc to sys--->-->To set the Linux[system]clock from the HW clock
4> hwclock --utc sys to hc -->To set the HWclock from the Linux[system] clock
touch -t yyyymmddhhmm filename
echo "DATE:" `date --date "today" +%d-%m-%Y` <--DATE:20-12-2010
echo "DATE:" `date --date "yesterday" +%d-%m-%Y`
echo "DATE:" `date --date "+5 minutes ago" +%d-%m-%Y-%M` <--present -5 min
echo "DATE:" `date --date "-5minutes ago" +%d-%m-%Y-%M` <---Present +5 min
echo "DATE:" `date --date "1 month ago -5 minutes ago" +%d-%m-%Y-%M`
<--present -1 month +5 min
Some importnat journal use paramiters
%a --> Mon
%b --> Jan
%d --> date 01..31 %e --> 1..31
%H --> hour 0..23) %k --> 1..23
%M --> minute (00..59)
%S --> Seconds(00..60)
%I --> hour 00..12) %m --> 1-12
%Y --> year 2010 %y --> year 10(last 2 digit)
%p --> AM-PM %p --> am-pm
%T --> %H:%M:%S
=================================== More n more ==================
%%--->a literal %
%a--->localeâs abbreviated weekday name (e.g., Sun)
%A--->localeâs full weekday name (e.g., Sunday)
%b--->localeâs abbreviated month name (e.g., Jan)
%B--->localeâs full month name (e.g., January)
%c--->localeâs date and time (e.g., Thu Mar 3 23:05:25 2005)
%C--->century; like %Y, except omit last two digits (e.g., 21)
%d--->day of month (e.g, 01)
%D--->date; same as %m/%d/%y
%e--->day of month, space padded; same as %_d
%F--->full date; same as %Y-%m-%d
%g--->last two digits of year of ISO week number (see %G)
%G--->year of ISO week number (see %V); normally useful only with %V
%h--->same as %b
%H--->hour (00..23)
%I--->hour (01..12)
%j--->day of year (001..366)
%k--->hour ( 0..23)
%l--->hour ( 1..12)
%m--->month (01..12)
%M--->minute (00..59)
%n--->a newline
%N--->nanoseconds (000000000..999999999)
%p--->localeâs equivalent of either AM or PM; blank if not known
%P--->like %p, but lower case
%r--->localeâs 12-hour clock time (e.g., 11:11:04 PM)
%R--->24-hour hour and minute; same as %H:%M
%s--->seconds since 1970-01-01 00:00:00 UTC
%S--->second (00..60)
%t--->a tab
%T--->time; same as %H:%M:%S
%u--->day of week (1..7); 1 is Monday
%U--->week number of year, with Sunday as first day of week (00..53)
%V--->ISO week number, with Monday as first day of week (01..53)
%w--->day of week (0..6); 0 is Sunday
%W--->week number of year, with Monday as first day of week (00..53)
%x--->localeâs date representation (e.g., 12/31/99)
%X--->localeâs time representation (e.g., 23:13:48)
%y--->last two digits of year (00..99)
%Y--->year
%z--->+hhmm numeric timezone (e.g., -0400)
%:z -> +hh:mm numeric timezone (e.g., -04:00)
%::z -> +hh:mm:ss numeric time zone (e.g., -04:00:00)
%:::z -> numeric time zone with : to necessary precision (e.g., -04, +05:30)
%Z -> alphabetic time zone abbreviation (e.g., EDT)
1> date 081811002007 [mmddhhmmyyyy]--To set the Linux clock to 11th-Aug-2007-11-AM
2> timeconfig --Set your TIMEZONE
3> hwclock --hc to sys--->-->To set the Linux[system]clock from the HW clock
4> hwclock --utc sys to hc -->To set the HWclock from the Linux[system] clock
touch -t yyyymmddhhmm filename
echo "DATE:" `date --date "today" +%d-%m-%Y` <--DATE:20-12-2010
echo "DATE:" `date --date "yesterday" +%d-%m-%Y`
echo "DATE:" `date --date "+5 minutes ago" +%d-%m-%Y-%M` <--present -5 min
echo "DATE:" `date --date "-5minutes ago" +%d-%m-%Y-%M` <---Present +5 min
echo "DATE:" `date --date "1 month ago -5 minutes ago" +%d-%m-%Y-%M`
<--present -1 month +5 min
Some importnat journal use paramiters
%a --> Mon
%b --> Jan
%d --> date 01..31 %e --> 1..31
%H --> hour 0..23) %k --> 1..23
%M --> minute (00..59)
%S --> Seconds(00..60)
%I --> hour 00..12) %m --> 1-12
%Y --> year 2010 %y --> year 10(last 2 digit)
%p --> AM-PM %p --> am-pm
%T --> %H:%M:%S
=================================== More n more ==================
%%--->a literal %
%a--->localeâs abbreviated weekday name (e.g., Sun)
%A--->localeâs full weekday name (e.g., Sunday)
%b--->localeâs abbreviated month name (e.g., Jan)
%B--->localeâs full month name (e.g., January)
%c--->localeâs date and time (e.g., Thu Mar 3 23:05:25 2005)
%C--->century; like %Y, except omit last two digits (e.g., 21)
%d--->day of month (e.g, 01)
%D--->date; same as %m/%d/%y
%e--->day of month, space padded; same as %_d
%F--->full date; same as %Y-%m-%d
%g--->last two digits of year of ISO week number (see %G)
%G--->year of ISO week number (see %V); normally useful only with %V
%h--->same as %b
%H--->hour (00..23)
%I--->hour (01..12)
%j--->day of year (001..366)
%k--->hour ( 0..23)
%l--->hour ( 1..12)
%m--->month (01..12)
%M--->minute (00..59)
%n--->a newline
%N--->nanoseconds (000000000..999999999)
%p--->localeâs equivalent of either AM or PM; blank if not known
%P--->like %p, but lower case
%r--->localeâs 12-hour clock time (e.g., 11:11:04 PM)
%R--->24-hour hour and minute; same as %H:%M
%s--->seconds since 1970-01-01 00:00:00 UTC
%S--->second (00..60)
%t--->a tab
%T--->time; same as %H:%M:%S
%u--->day of week (1..7); 1 is Monday
%U--->week number of year, with Sunday as first day of week (00..53)
%V--->ISO week number, with Monday as first day of week (01..53)
%w--->day of week (0..6); 0 is Sunday
%W--->week number of year, with Monday as first day of week (00..53)
%x--->localeâs date representation (e.g., 12/31/99)
%X--->localeâs time representation (e.g., 23:13:48)
%y--->last two digits of year (00..99)
%Y--->year
%z--->+hhmm numeric timezone (e.g., -0400)
%:z -> +hh:mm numeric timezone (e.g., -04:00)
%::z -> +hh:mm:ss numeric time zone (e.g., -04:00:00)
%:::z -> numeric time zone with : to necessary precision (e.g., -04, +05:30)
%Z -> alphabetic time zone abbreviation (e.g., EDT)
Telnet configuration in RHEL5
First check whether telnet-server rpm in installed,
# rpm -q telnet-server
telnet-server-0.17-38.el5
If it is installed, then edit
# vi /etc/xinet.d/telnet
change the line disable = yes to
disable = no
Then # service xinetd restart
Now We can able to telnet connection of linux system.
Note : It works for the system which is not protected with IPtables and Selinux
If you have iptables enabled. then please follow as below,
Make a rule to allow telnet from external clients
# iptables -I INPUT -p tcp --dport 23 -j ACCEPT
# service iptables save
# service iptables restart
Just give following command and make sure that rule is saved.
# iptables -L
This rule is to allow request coming to port number 23 i.e. telnet connections from any client.
============
My conclusion
1> off firewall
2> chkconfig krb5-telnet on
3> please note do not on krb5-telnet
4> service xinetd restart
# rpm -q telnet-server
telnet-server-0.17-38.el5
If it is installed, then edit
# vi /etc/xinet.d/telnet
change the line disable = yes to
disable = no
Then # service xinetd restart
Now We can able to telnet connection of linux system.
Note : It works for the system which is not protected with IPtables and Selinux
If you have iptables enabled. then please follow as below,
Make a rule to allow telnet from external clients
# iptables -I INPUT -p tcp --dport 23 -j ACCEPT
# service iptables save
# service iptables restart
Just give following command and make sure that rule is saved.
# iptables -L
This rule is to allow request coming to port number 23 i.e. telnet connections from any client.
============
My conclusion
1> off firewall
2> chkconfig krb5-telnet on
3> please note do not on krb5-telnet
4> service xinetd restart
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