Linux with Operating System Concepts

filename
address
.
The host program by default responds with the IP address of the given alias. The 
option –d (or –v) provides more detail (or a verbose response). Like dig, host will respond 
with question, answer, authority, and additional sections. Unlike dig, host, when sup-
plied with –d (or –v) responds with information from the SOA record. For instance, the 
; 
<<
>>
DiG 9.7.3-P3-RedHat-9.7.3-8.P3.el6_2.1 
<<
>>
-t NS www.nku.edu
;; global options: 
+
cmd
;; Got answer:
;; -
>>
HEADER
<<
- opcode: QUERY, status: NOERROR, id: 
57418
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, 
AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;www.nku.edu. 
IN 
NS
;; ANSWER SECTION:
www.nku.edu. 
3600 IN 
CNAME hhilwb6005.
hh.nku.edu.
;; Query time: 1 msec
;; SERVER: 172.28.102.11#53(172.28.102.11)
;; WHEN: Tue Aug 28 14:23:32 2012
;; MSG SIZE rcvd: 57
FIGURE 5.10 
dig Command Response

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instruction 
host –d www.nku.edu
will provide the following in the AUTHORITY 
SECTION:
;; AUTHORITY SECTION:
hh.nku.edu.
3600 IN 
SOA 
nkuserv1.hh.nku.edu. 
postmaster.exchange.nku.edu. 37368706 900 600 86400 3600
Notice here that we see the serial number, refresh rate, retry rate, expiration time, and 
minimum TTL. While this information is available via dig, it is not provided by default.
Figure 5.11 provides the results of 
host –d www.nku.edu
. The information in this 
response is similar to that from dig, but here, we go beyond the single machine www.nku.
edu to obtain information about the domain (nku.edu) as well.
Finally, with host, the –i option asks the DNS server to perform a 
reverse IP lookup
. This is 
used to translate an IP address into an IP alias. You might wonder why anyone would want 
to perform a reverse IP lookup when the whole point of the DNS is to allow humans to use 
IP aliases so that they do not need to remember IP addresses even though the addresses are 
essential for Internet communication. But there are many approaches to attacking a com-
puter by pretending to be an IP address that you are not. The reverse IP lookup can ensure 
that an incoming communication that claims to be from a particular IP address is correct.
5.8 CHAPTER REVIEW
Concepts and terms introduced in this chapter:
• ARPAnet—original form of the Internet, developed in the late 1960s through the 
early 1980s.
• Asymmetric encryption—a form of encryption that uses two separate keys, a public 
key to encrypt messages and a private key to decrypt messages. This form of encryp-
tion is primarily used to support E-commerce and secure communications between 
different users or sites on the Internet.
• FTP—file transfer protocol, for transferring files between computers over the Internet.
• HTTP/HTTPS—hypertext protocol transfer and the secure form, for transferring 
web pages from a web server to a web client.
• Insert mode—in vi, this mode inserts typed characters into the document rather 
than interpreting the characters as commands.
• Command mode—in vi, this mode interprets special-purpose keystrokes as com-
mands to move the cursor, cut/copy/paste characters, perform file operations, or 
change from command to insert/replace mode.
• Computer network—a collection of computers and computer resources connected 
together by some medium to support resource sharing, document sharing, and elec-
tronic communication.

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Linux with Operating System Concepts
Trying “www.nku.edu”
;; -
>>
HEADER
<<
- opcode: QUERY, status: NOERROR, id: 2258
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, 
ADDITIONAL: 0
;; QUESTION SECTION:
;www.nku.edu. 
IN 
A
;; ANSWER SECTION:
www.nku.edu. 
3600 IN 
CNAME hhilwb6005.hh.nku.edu.
hhilwb6005.hh.nku.edu. 3600 IN 
A 
172.28.119.82
Received 73 bytes from 172.28.102.11#53 in 2 ms
Trying “hhilwb6005.hh.nku.edu”
;; -
>>
HEADER
<<
- opcode: QUERY, status: NOERROR, id: 84
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, 
ADDITIONAL: 0
;; QUESTION SECTION:
;hhilwb6005.hh.nku.edu.
IN 
AAAA
;; AUTHORITY SECTION:
hh.nku.edu. 3600 IN 
SOA 
nkuserv1.hh.nku.edu. postmaster.
exchange.nku.edu. 37369675 900 600 86400 3600
Received 104 bytes from 172.28.102.11#53 in 5 ms
Trying “hhilwb6005.hh.nku.edu”
;; -
>>
HEADER
<<
- opcode: QUERY, status: NOERROR, id: 15328
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, 
ADDITIONAL: 0
;; QUESTION SECTION:
;hhilwb6005.hh.nku.edu.
IN 
MX
;; AUTHORITY SECTION:
hh.nku.edu.
3600 IN 
SOA 
nkuserv1.hh.nku.edu. 
postmaster.exchange.nku.edu. 37369675 900 600 86400 3600
Received 104 bytes from 172.28.102.11#53 in 3 ms
FIGURE 5.11 
host Command Response

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Linux Applications
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197
• Macro—a definition of keystrokes to be performed many times to save the user time, 
available in both vi and emacs.
• Private key—a key that can be used to encrypt and decrypt messages in symmetric 
(private key) encryption, or generate a public key and decrypt messages in asymmet-
ric key encryption.
• Public key—a key made available to the public to encrypt messages but not decrypt 
messages, used in asymmetric (public key) encryption.
• R-utility—a suite of Linux network programs that permits a user of one computer in 
a Linux network to access the other Linux computers without having to log in.
• Symmetric encryption—a form of encryption in which one key, the private key, is 
used to both encrypt and decrypt messages. Mostly used when there is only one user 
such as when encrypting and decrypting a file system.
• Text editor—a program to create and edit text files (as opposed to a word processor 
which also permits formatting of text).
• WYSIWYG—an acronym for “what you see is what you get,” a popular term express-
ing GUI-based software where you can see what the final product looks like before 
you print it. Early text-based word processors used embedded commands (see LaTeX 
in Section 5.4 for instance) so that the version of a file as seen during editing is not 
how the final product would look.
NOTE: See Chapters 12 and 15 for review terms on TCP/IP and DNS.
Linux commands introduced in this chapter:
• bibtex—generates the bibliography for a LaTeX document.
• dig—to query a DNS server about IP address information.
• emacs—a text-based text editor with many powerful features.
• ftp—a text-based program to transfer files between computers.
• host—like dig, to query a DNS server about IP address information.
• ifconfig—to obtain or reconfigure the IP address of a computer.
• ip—a network tool that supersedes many older network programs such as ifconfig 
(covered in detail in Chapter 12).
• latex—to process a .tex (LaTeX) file and create a neutral, device-independent (dvi) file.
• mail—also mailx, the built-in text-based Linux mail client program.
• nc—the netcat program offering a suite of network operations, including the ability 
to send HTTP requests to a web server without using a web browser.

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Linux with Operating System Concepts
• nslookup—like dig and host, but a more primitive DNS query tool.
• openssl—a collection of encryption algorithms that permit the user to generate pub-
lic and private keys, encrypt and decrypt messages, and create digital certificates and 
signatures.
• ping—tests the availability and access of a networked resource.
• rlogin—one of the r-utilities, to remotely log into another Linux computer of the 
same network.
• rsh—one of the r-utilities, to open a shell on another Linux computer of the same 
network.
• rwho—one of the r-utilities, to execute who on another Linux computer of the same 
network.
• rcp—one of the r-utilities, to copy a file from another Linux computer of the same 
network to this computer.
• ssh—a secure form of telnet (see below) that uses public key encryption.
• telnet—a program to log into another computer across the Internet. Telnet is no lon-
ger used because messages, including passwords, are passed in an unencrypted form.
• traceroute—tests the route taken in communications between two networked 
computers.
• vi—a text-based text editor that comes with all Linux implementations.
• wget—a command-line noninteractive program to retrieve files from a web server 
without having to operate through a web browser.
REVIEW PROBLEMS
1. In vi, what is the difference between the command ‘r’ and ‘i’?
2. In vi, what is the difference between the command ‘o’ and ‘O’?
3. In vi, what is the difference between the command ‘w’ and ‘W’?
4. In vi, what is the difference between the command ‘e’ and ‘E’?
5. In vi, you are editing a document and currently on line 21. How would you reach line 
1? Line 12? The last line of the document?
6. In vi, to copy the current line and paste it below the current line, you would do yy 
followed by p. How would you do this in emacs?
7. As a Linux user, what feature of Bash helps you learn how to control emacs?
8. What is a macro? Provide an example of why you might want to define one.

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Linux Applications
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199
9. Research LibreOffice and OpenOffice. Create a list of five features available in 
LibreOffice that are not available in OpenOffice.
10. Why might you want to use LaTeX rather than a WYSIWYG word processor?
11. Given the following LaTeX definition for a table, draw how it would appear in a 
document.
\begin{tabular}{│l││l│}
Item 1 & Item 2 \\
Item 3 & Item 4 \\
Item 5 \\
& Item 6 \\
\end{tabular}
12. What is Tex? How does it differ from LaTeX?
13. If I have a public key to your private key and you send me an encrypted message, can 
I decrypt it? If I have a private key to your public key and you send me an encrypted 
message, can I decrypt it?
14. Using openssl, what are the steps that you would need to generate a private key?
15. Using openssl, what are the steps that you would need to generate a public key?
16. Using openssl, what are the steps that you would need to generate a digital certificate?
17. Assume a digital certificate has the following information:
Issuer: C 
=
UK, L 
=
London, O 
=
Bobs Books, OU 
=
Sales,
CN 
=
tardis.bobsbooks.com.uk
What do each of C, L, O, OU, and CN stand for?
18. How do you control the valid dates when generating a digital certificate in openssl?
19. What is the difference between an email client and an email server?
20. How does the command 
mail
differ from the command 
mail