Linux Admin Interview Questions

Once a Linux admin starts looking out for a job, there are n number of things one has to refresh, get concepts clarified and brush up things worked upon long ago. Skill level requirements vary from company to company and one may not be sure about what all topics to touch upon before taking up the interview.

Here are some of the common linux admin questions asked in an interview

• Explain Linux boot process

• What does /sbin/init process do?

• Difference between SIGKILL(9) and SIGTERM(15).

• What is a zombie?

• Difference between zombie and orphan process.

• Does zombie process utilize system resource?

• What is a socket?

• How traceroute works?

• What does * * * mean in traceroute result?

• RAID Concepts - RAID 0,RAID 1,RAID 5,RAID 10.

• Which RAID is faster in terms of Read and Write requests?

• LVM - configuration,resizing

• What is journalling - Advantages, different types of journalling.

• Where journalling filesystem need not be used?

• Difference between ext4 and ext3 filesystems.

• Explain Forward Proxy and Reverse Proxy.

• Setting up L7 loadbalancing by using Reverse Proxy concept. Using Nginx as loadbalancer for Apache. 

• Difference between TCP wrappers and iptables.

• Difference between SNAT and MASQURADE in iptables.

• Flow of packet in a iptables chain.

• Why NFS needs portmap service?

• Will NFS service start or linux boot if /etc/exports file is empty?

• Difference between soft link and hard link.

• Why hard link cannot be created for a directory?

• Why sometimes du and df commands give different results?

• Why secure sites(https) need to have dedicated IP address?

• Difference between process and threads?

• Difference between Paging and Swapping.

• What is virtual memory?

• Explain the path of an email or how email works basically.

• How to check for Open relay mail servers?

• How does a browser fetch a webpage? Explain the entire process with reference to 7 layer OSI model.

• What is a packet and frame?

• When does DNS(BIND) use UDP and TCP?

• Explain various DNS Records - A,PTR,CNAME,MX 

• What is a shared memory?

• What is context switching? What does high amount of context switchings imply?

• What is an interrupt?

• What is unicast,multicast and broadcast packets?

• What is a multi-homed network?

• What is DMZ?

• If there are two swap partitions, which swap partition will  the system use?

• How to increase swap space on the fly?

• How to install a package from source? Explain what each does - configure,make and make install.

• Display rpm package architechture(i386 or x86_64). 

• How to find system architechture - 32bit or 64bit?

• HTTP status codes, mainly common error codes.

• About modules in Linux - lsmod,modprobe,depmod

• How to avoid using initrd in grub file?

• There are two root entries in grub configuration file for each kernel. What do they stand for?

• What is superblock in linux?

• How to install a package in hundreds of servers?

• How to look for a string pattern in a growing log file? What happens if the log file rotates over?

• How to identify and restart a process if only the port number of the process is known?

• How many instances of Apache can be run in a server?

• How many IPs can be configured in a network card(NIC)?

• Managing remote servers using IPMI.

• Which type of hard disk is best - SCSI,ATA,SAS,SATA?

• How to debug a process - strace, gcore?

• Telnet - What "Connection refused" error means?

• How to check for open ports in a node?

• Where can iptables DNAT be used?

• What is MTU and what is the max size possible?

• Scenario involving arp cache - Node A can talk to node B,node B can talk to node C, but node A cannot talk to node C although they all are in same subnet.

• Nagios - Difference between active and passive checks.

• FTP - Difference between active and passive ftp.

• Port numbers of standard services.

• Difference between POP and IMAP protocols. Which is better?

• TCP handshake process.

• How to install/upgrade/compile a kernel?

• How to pass arguments to a shell script

• What is the purpose of execute flag bit for a directory?

• What is Real User ID(UID) and Effective User ID? 

• Difference between SAN and NAS. 

• How to check File system block size in Linux? What if block sizes are too large or too small? 

• Unable to create new files - Why? How to find Inode usage? Can Inode limit be increased? 

• How to increase The Maximum Number Of Open Files / File Descriptors (FD)? How to deal with "Too many open files" error?

• How does MySQL replication work?

•  MySQL backup howto? Dump or Raw Backup?

• What does init process do?

• What is tcp half open?

• What is tcp slowstart?

• What happens if a log file is renamed or moved in perl?

• Difference between @INC and %INC in perl.

• Explain the difference between my and local variable scope declarations in perl.

• Difference between use and require in perl.

• How to install a perl module?