System Admin Q & A – VI

Ques 1: – What is MTU ?

Ans: – The MTU is the “Maximum Transmission Unit” used by the TCP protocol. TCP stands for Transmission Control Prototcol. The MTU determines the size of packets used by TCP for each transmission of data. Too large of an MTU size may mean retransmissions if the packet encounters a router along its route that can’t handle that large a packet. Too small of an MTU size means relatively more overhead and more acknowledgements that have to be sent and handled. The MTU is rated in “octets” or groups of 8 bits. The so-called “official” internet standard MTU is 576, but the standard rating for ethernet is an MTU of 1500.

Ques 2: – What does nslookup do? Explain its two modes.

Ans: – Nslookup is used to find details related to a Domain name server. Details like IP addresses of a machine, MX records, servers etc. It sends a domain name query packet to the corresponding DNS.

Nslookup has two modes. Interactive and non interactive. Interactive mode allows the user to interact by querying information about different hosts and domains.

Non interactive mode is used to fetch information about the specified host or domain.
Interactive mode:

Nslookup [options] [server]

Ques 3: – Explain RPM (Red Hat Package Manager) features

Ans: – RPM is a package managing system (collection of tools to manage software packages).
Features:

RPM can verify software packages.
RPM can be served as a powerful search engine to search for software’s.
Components, software’s etc can be upgraded using RPM without having to reinstall them
Installing, reinstalling can be done with ease using RPM
During updates RPM handles configuration files carefully, so that the customization is not lost.

Ques 4: – daemon is used for scheduling of the commands?

Ans: – The crontab command is used for scheduling of the commands to run at a later time. SYNTAX

crontab [ -u user ] file
crontab [ -u user ] { -l | -r | -e }

Options
-l List – display the current crontab entries.
-r Remove the current crontab.
-e Edit the current crontab using the editor specified by the VISUAL or EDITOR environment variables.

When user exits from the editor, the modified crontab will be installed automatically. Each user can have their own crontab, and though these are files in /var, they are not intended to be edited directly. If the –u option is given than the crontab gives the name of the user whose crontab is to be tweaked. If it is given without this then it will display the crontab of the user who is executing the command.

Ques 5: – What is LD_LIBRARY_PATH?

Ans: – LD_LIBRARY_PATH is an environment variable. It is used for debugging a new library or a non standard library. It is also used for which directories to search. Path to search for directories needs to given. The variable can be set by using setenv—LD_LIBRARY_PATH–$PATH.

Ques 6: – What is LMHOSTS file?

Ans: – It’s a file stored on a host machine that is used to resolve NetBIOS to specific IP addresses.

Ques 7: – Describe a TCP connection sequence?

Ans: – The TCP three-way handshake describes the mechanism of message exchange that allows a pair of TCP devices to move from a closed state to a ready-to-use, established connection. Connection establishment is about more than just passing messages between devices to establish communication. The TCP layers on the devices must also exchange information about the sequence numbers each device wants to use for its first data transmission, as well as parameters that will control how the connection operates.

The former of these two data exchange functions is usually called sequence number synchronization, and is such an important part of connection establishment that the messages that each device sends to start the connection are called SYN (synchronization) messages.
You may recall from the TCP fundamentals section that TCP refers to each byte of data individually, and uses sequence numbers to keep track of which bytes have been sent and received. Since each byte has a sequence number, we can acknowledge each byte, or more efficiently, use a single number to acknowledge a range of bytes received.

Ques 8: – How to delete the files older than 7 days in linux/unix ?

 find /your_directory -mtime +7 -exec rm -f {} \;

Ques 9: – What the Purpose of VLAN?

Ans: – VLANs can enhance network security. VLANs increase the number of broadcast domains while decreasing their size. Network adds, moves, and changes are achieved by configuring a port into the appropriate VLAN.

A group of users needing high security can be put into a VLAN so that no users outside of the VLAN can communicate with them. As a logical grouping of users by function, VLANs can be considered independent from their physical or geographic locations.

Ques 10: – How to enable timestamp with history command in Linux ?

Ans: – History is a common command for shell to list out all the executed commands. It is very useful when it comes to investigation on what commands was executed that tear down the server. With the help of last command, you be able to track the login time of particular user as well as the the duration of the time he/she stays login

HISTTIMEFORMAT takes format string of strftime. Check out the strftime manual to choose and construct the timestamp that suit your taste

# export HISTTIMEFORMAT="%F %T"

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