Linux with Operating System Concepts

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All in all, the design of IPv6 is well thought out, having been engineered over a period of 
years starting in 1998. This is not the case with IPv4 that was designed for an incarnation of 
the Internet preceding its popularity and in fact preceding the popularity of personal com-
puters. But until all networks on the Internet are compliant with IPv6, we will continue 
to see a large number of IPv4 users. According to www.worldipv6launch.org as of October 
2013, only about 12% of Internet communication with popular websites use IPv6 while 
other estimates state that only about 16% of all Internet networks support IPv6.
12.2.5 Domains, the DNS, and IP Aliases
Now that we have covered some details of TCP/IP, let us examine how the Internet itself 
works. You want to send a message to another computer. This message might be an HTTP 
request generated by your web browser in response to clicking on a link, it might be an email 
message sent from your email program, it might be a ping request to see if a given network 
resource is responding, or it might be an ssh command so that you can remotely log into 
another computer. Most likely, all you know of the remote computer (the destination) is its 
IP alias. This alias must first be translated into an IP address through address resolution.
Your local site probably has a DNS server. The DNS is set up to perform alias to address 
translation for you. If the destination is not part of your local domain, it is likely that your 
DNS server does not have that information locally. If it does, it can look up the IP address 
of the requested resource and respond with that address. Otherwise, it kicks the request 
up to another DNS server located elsewhere on the Internet. Eventually, the request will 
be kicked up to a DNS server that knows the domain of the destination computer. Your 
computer will either receive an IP address of the domain or a message that the alias could 
not be resolved. Given the address, your computer can now add this to your message and 
send the message out onto the Internet. If no DNS server knows of the domain specified, 
you will be given an error.
What do we mean by a domain? The Internet is divided into top-level domains such 
as .com, .edu, .gov, and .net. Within each of these general domains, specific domains are 
established such as amazon.com or nku.edu.

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DNS name servers generally come in two forms, 
authorities
and 
caches
. An authority 
is responsible for its own domain. This means that for each domain, there will be at least 
one authoritative DNS server that contains information about that domain. Other DNS 
servers can cache information about domains outside of their own. Thus, your local 
DNS server may know how to find a domain such as nku.edu. If not, then it will know 
of a DNS server to communicate with one that will either know of the domain or know 
of another DNS server to hand the request onto. Typically, a specific resource’s address 
will only be stored in an authoritative DNS server’s table. But the domain information 
itself should be known elsewhere.
As an example, you want to reach www.nku.edu. The domain is nku.edu. While 
your local DNS server may not know nku.edu, it knows of a DNS server that does. Your 
request goes to that DNS server that responds with the IP address for the DNS server 
for nku.edu. Now, your request for www.nku.edu goes to that DNS server that knows 
about the specific machine www. In its DNS table is a mapping of www to the IP address. 
This address is sent back to your computer and finally, you have the IP address needed 
to communicate with the computer www.nku.edu. While this seems complicated, the 
actual amount of time that this operation might take will typically be a few seconds or 
less, probably less than a second. Further, this entire interaction happens transparently 
for the user.
To establish proper communication, you will have to set up a table that indicates the 
location of your DNS server(s). In Linux, these IP addresses are placed in the file 
/etc/
resolv.conf
. We will look at setting up a DNS server in Linux using the bind program 
in Chapter 15 (available at http://www.crcpress.com/product/isbn/9781482235890).
If there are machines whose IP addresses are static and which you communicate with 
often, you may wish to bypass the entire DNS address-translation process. In Linux, you 
can set up your own mapping information in the file 
/etc/hosts
. Before any name reso-
lution is attempted, Linux first examines the hosts file to see if there any entries that match 
the request. Entries in this file are denoted as ip address followed by host name(s).
Let us consider a local server with a static IP address called 
ourserver.internal-
net.com
. The organization has aliased this machine to the name 
internalserver.
com
. As we might contact this server frequently and since it has an IP address, this server 
seems a useful target for inclusion in the /etc/hosts file to reduce the amount of traffic for 
our DNS servers. If the IP address is 10.11.12.13, we could add this entry to /etc/hosts.
10.11.12.13 ourserver.internalnet.com internalserver.com
What else do you have to do to establish how your computer communicates on a net-
work? First, your computer needs an IP address. This is discussed in Section 12.4. Second, 
you need for your computer to respond to incoming messages. The network service needs 
to be running, and this is discussed in Section 12.3. You may also wish to protect your 
computer so that incoming messages are scrutinized first. We use a firewall for this. The 
Linux firewall is discussed in Section 12.6. Third, you need to establish your interface(s) to 
the network. We also discuss this in Section 12.3.

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Linux with Operating System Concepts
12.3 NETWORK SERVICES AND FILES
The primary network services are all controlled by script files stored under /etc/init.d as 
are most of the important services in Linux (refer back to Chapter 11). The services them-
selves are typically found under /usr/sbin. Many of the network services can be thought of 
as umbrella services in that other services call upon them to help fulfill their tasks. Other 
supporting scripts for network services are found under /etc/sysconfig and /etc/sysconfig/
network-scripts.
12.3.1 The Network Service
The network service is used to start or stop network communication. If this service is not 
running, then you are unable to communicate over the network. Even lo (the loopback 
device over 127.0.0.1) is unreachable. Unless you wish your computer to serve as a stand-
alone machine, you will want your network service running in all situations except when 
you are working on the network (changing interface devices or altering some network con-
figuration). All other services related to the network rely on network running. Thus, if you 
stop network, services such as netconsole, httpd, iptables, autofs, ntpd, and dnsmasq are 
useless even if they are running.
When you start the network service through the controlling script (from /etc/
init.d), it first executes the script 
/etc/init.d/functions
which contains numer-
ous script functions that support other scripts. It then executes the script 
/etc/sys-
config/network
to establish two environment variables: 
NETWORKING
=
yes
and 
HOSTNAME
=