Below are a few good interview questions for networking or Linux related jobs. These will give a relatively good assessment if the candidate has good knowledge of some fundamentals. Alternatively, this makes a great study guide if you are applying for any networking or Linux related jobs. Wikipedia is the best guide and much better than any books out there on fundamentals. Reading up on articles on TCP/IP, SMTP, HTTP status codes and other topics on Wikipedia to better your skills is a great exercise to do.
Networking specific questions
What is the difference between TCP and UDP?
What are some example uses of UDP?
What is ICMP? What are some example ICMP responses and what do they mean?
What is the difference between a switch and a hub?
What is a scenario in which you would need to use a hub and why?
What is ARP? How does it work? What is an arp table, and where does this exist?
What is a router? How does a router work?
What is a NAT router?
What is DNS? What ports does DNS operate on?
What are some example DNS records, and what do they do? What software is used commonly for running a DNS server?
What is SMTP and how does it work?
What are common ports that SMTP runs on? Why would one use different ports for SMTP?
What is SMTP-TLS? When is this used and why?
What is HTTP and HTTPS? What are common ports for these protocols?
What are examples of common web servers used on any platform?
What are a few example HTTP status codes and what do they mean?
What are a few example HTTP headers?
What is SSL?
What are example ciphers used for SSL connections?
Who decides what cipher to use (client or server) to establish an SSL connection?
What is a firewall? How does it work? What is a state table?
What are the differences in closed, open, and filtered ports on a firewall?
What is an RST packet?
What is an ACK packet?
What is a SYN packet?
Linux specific questions
What is the difference between a thread and a process?
What is the difference between a hard link and a symbolic or soft link?
What is an inode? What is one piece of information that is not contained in the inode?
How do change a lost root password? Consider no sudo privileges are available either.
One log in /var/log is growing very large in size, and not rotating. How do you check/fix this?
What command lists open files actively being accessed by a process?
What is the command to make a dump of a mysql database?
What is the administrative group name on Red Hat or CentOS?
Who should own the /var/www/ directory (user as well as group) and why?
What is the difference in locate and find?
What is the difference between these:
drwxr-xr-x 2 jsmith jsmith 4096 2012-03-22 18:51 stuff
-rwxr-xr-x 2 jsmith jsmith 4096 2012-03-22 18:51 stuff
What does this command do on Red Hat or CentOS:
$ rpm -qa | grep php
What does the following command show:
$ sudo blkid
If you cannot ping localhost, what do you check to see what is going wrong?
$ ping localhost
$ ping: unknown host localhost
What are the differences in these commands:
$ mv [abc]* myfolder/
$ mv [!abc]* myfolder/