System Admin Q & A – II

Ques 1: – What is Greylisting ?

Ans: – Greylisting (or graylisting) is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will “temporarily reject” any email from a sender it does not recognize. If the mail is legitimate the originating server will, after a delay, try again and, if sufficient time has elapsed, the email will be accepted. If the mail is from a spam sender,sending to many thousands of email addresses, it will probably not be retried.

Ques 2: – Can we have two apache servers having diff versions?

Ans: – Yes, you can have two different apache servers on one server, but they can’t listen to the same port at the same time.Normally apache listens to port 80 which is the default HTTP port. The second apache version should listen to another port with the Listen option in httpd.conf, for example to port 81.

For testing a new apache version before moving your sites from one version to another, this might be a good option.You just type in the browser window and you will be connected to the second apache instance.

Ques 3: – What is the load average of the server and What is an acceptable Server Load Average ?

Ans: – The load average is the sum of the run queue length and the number of jobs currently running on the CPUs. The three load-average values in the first line of top output are the 1-minute, 5-minute and 15-minute average. (These values also are displayed by other commands, such as uptime, not only top.)

There are a few factors involved to determine the server average load. If your server (s) use dual processors, the acceptable Server Load Average is 2.00. This load is considered “optimal”.

Ques 4: – What is Super Block in Linux/Unix ?

Ans: – Each file system is different and they have type like ext2, ext3 etc.Further eachfile system has size like 5 GB, 10 GB and status such as mount status. In short each file system has a superblock, which contains informationabout file system such as:

File system type
Information about other metadata structures
If this information lost, you are in trouble (data loss) so Linux maintains multiple redundant copies of thesuperblock in every file system. This is very important in many emergency situation, for example you can usebackup copies to restore damaged primary super block.

Following command displays primary and backup superblock location on /dev/sda3:

# dumpe2fs /dev/hda3 | grep -i superblock

Ques 5: – What is the difference between TCP & UDP ?

Ans: – TCP

TCP -Transmission control protocol
TCP is a connection oriented protocol.
Three way handshake happens between client and server.
TCP is a reliable data transfer
Slow transmission of data compare to UDP.
TCP is used to send file like database,where reliability play the first role


UDP -User Datagram protocol
UDP is connectionless protocol
There is no three way handshake between client and server
Data transfer is not reliable
Faster than TCP
It used to send data like video,audio

Ques 6: – What is the purpose of nsswitch.conf ?

– nsswitch.conf is the name service switch configuration file, generally located in the /etc directory. Its purpose is to tell the system in what order it should consult various name services to resolve information about users, name resolution, and netgroups. This information can be provided by files on the filesystem (e.g. /etc/hosts containing IP address/host name pairs or /etc/passwd containing user data) or by a network name service such as DNS for host names or NIS or LDAP for account data.

Ques 7: – What command is used to remove the password assigned to a group ?

#gpasswd -r

The gpasswd command is used to change the password assigned to a group. Use the -r option to remove the password from the group.

Ques 8: – What command would you type to use the cpio to create a backup called backup.cpio of all the users’ home directories ?

 # find /home | cpio -o > backup.cpio

The find command is used to create a list of the files and directories contained in home. This list is then piped to the cpio utility as a list of files to include and the output is saved to a file called backup.cpio

Ques 9: – What is Domain Key ?

This can be achieved by Limit Request Body directive.

Direct Domain Keys is an e-mail authentication system designed to verify the DNS domain of an e-mail sender and the message integrity. The DomainKeys specification has adopted aspects of Identified Internet Mail to create an enhanced protocol called DomainKeys Identified Mail (DKIM).

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