Wednesday, July 31, 2013

Linux system administrator interview questions and answers for experienced

1. Ping  uses which protocol ?

ICMP, The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet Protocol Suite. It is used by the operating systems of networked computers to send error messages indicating, for example, that a requested service is not available or that a host or router could not be reached. ICMP can also be used to relay query messages.)

2. What is Syslogd?

Syslogd provides a kind of logging that many modern programs use. Every logged message contains at least a time and a hostname field, normally a program name field, too, but that depends on how trusty the logging program is.

main configuration file for syslogd is /etc/syslog.conf

3. What is Cname?

A CNAME(Canonical Name record) Record is a type of resource record in the Domain Name System (DNS) that specifies that the domain name is an alias of another, canonical domain name. Here "canonical" usually means: a more generally accepted or standard name.

4. Dns port number & protocol ?

DNS primarily uses User Datagram Protocol (UDP) on port number 53 to serve requests. DNS queries consist of a single UDP request from the client followed by a single UDP reply from the server.
The Transmission Control Protocol (TCP) is used when the response data size exceeds 512 bytes, or for tasks such as zone transfers. Some resolver implementations use TCP for all queries.

5. what is the Run levels configuration file & how to shift run levels?

A run level is a state of init and the whole system that defines what system services are operating. 

The following table defines how most Linux Distributions define the different run levels. However, run-levels 2 through 5 can be modified to suit your own tastes.

Run level numbers

0 Halt the system.
1 Single-user mode (for special administration).
2 Local Multiuser with Networking but without network service (like NFS)
3 Full Multiuser with Networking
4 Not Used
5 Full Multiuser with Networking and X Windows(GUI)
6 Reboot.

Configuration file: /etc/inittab 

to shift to different run levels use: init 1 (to shift to run level 1)

6. Dmesg command purpose?

dmesg obtains its data by reading the kernel ring buffer. A buffer is a portion of a computer's memory that is set aside as a temporary holding place for data that is being sent to or received from an external device, such as a hard disk drive (HDD). A ring buffer is a buffer of fixed size for which any new data added to it overwrites the oldest data in it.

dmesg can be very useful when troubleshooting or just trying to obtain information about the hardware on a system. Its basic syntax is

dmesg [options]

The output of dmesg is maintained in the log file /var/log/dmesg, and it can thus also be easily viewed by reading that file with a text editor, such as vi or gedit, or with a command such as cat, e.g.,

less /var/log/dmesg 

7. How to find our User login details ?

On a Linux (on Solaris/FreeBSD or any other UNIX) machine, many users will be sharing the same server.

Users will uses putty or ssh (secure and highly recommended) to login remotely.

use who comand . you can see loggedin users list.
command: who  or w

if u want to check for a particular user : who | grep <user id>

8. Purpose of nsswith.conf file ?

nsswitch.conf - System Databases and Name Service Switch configuration file


Various functions in the C Library need to be configured to work correctly in the local environment. Traditionally, this was done by using files (e.g., `/etc/passwd'), but other nameservices (like the Network Information Service (NIS) and the Domain Name Service (DNS)) became popular, and were hacked into the C library, usually with a fixed search order.

The Linux libc5 with NYS support and the GNU C Library 2.x ( contain a cleaner solution of this problem. It is designed after a method used by Sun Micro systems in the C library of Solaris 2. We follow their name and call this scheme "Name Service Switch" (NSS). The sources for the "databases" and their lookup order are specified in the /etc/nsswitch.conf file.

9. How to check errors in Ifconfig –a command and what information will you see in ifconfig –a ?

The following ifconfig command with -a argument will display information of all active or inactive network interfaces on server. It displays the results for eth0, lo, sit0 and tun0.

[root@testmachine ~]# ifconfig -a

eth0      Link encap:Ethernet  HWaddr 00:0B:CD:1C:18:2A
          inet addr:  Bcast:  Mask:
          inet6 addr: fe80::20b:cdff:fe1c:185a/64 Scope:Link
          RX packets:2344927 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2220777 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:293839516 (280.2 MiB)  TX bytes:1043722206 (995.3 MiB)
          Interrupt:185 Memory:f7fe0000-f7ff0000

lo        Link encap:Local Loopback
          inet addr:  Mask:
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:5022927 errors:0 dropped:0 overruns:0 frame:0
          TX packets:5022927 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:2175739488 (2.0 GiB)  TX bytes:2175739488 (2.0 GiB)

sit0      Link encap:IPv6-in-IPv4
          NOARP  MTU:1480  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)

tun0      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
          inet addr:  P-t-P:  Mask:
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100
          RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)

10. How To check the disk usage fot tmp directory?

du -sh /tmp

11. What is the diff b/w soft link and hard link? 

A hard link will creates another file with a link to the same underlying inode.

When you delete a file it removes one link to the underlying inode. The inode is only deleted (or deletable/over-writable) when all links to the inode have been deleted.

A symbolic link is a link to another name in the file system.

Once a hard link has been made the link is to the inode. deleting renaming or moving the original file will not affect the hard link as it links to the underlying inode. Any changes to the data on the inode is reflected in all files that refer to that inode.

Note: Hard links are only valid within the same File System. Symbolic links can span file systems as they are simply the name of another file.

12. How to change the Path in environmental variables  for /temp dir is make it temporary and permanent

Ans: will Post soon.

13. Diff b/w AD and LDAP

14. Diff b/w Terminal Service and VPN? 

15. Do you have knowledge on load balancer?

16. How you are fit in pxe and xen virtualisation?

17. How to resolve  grub reload issue to get data recovery?

18. Raid concepts what is raid 5, 1,0,1+0,0+1?

19. What is rds in vmware esxi?

20. How to monitoring running processes and how to kill particular heavy load process?

21. Diff b/w ssh and telnet? 

22. Similarity and Diff b/w AD and Ldap ?

23. What is scheduling service in windows like cron in linux?

24. What is Fault tolerance?

25. Kernal panic errors and resolving methods?

26. Type of Grub issues and fix methods ?

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