The Debian GNU/Linux install program lets you specify a network
configuration that's used when your system is first booted. If your
network configuration changes, you can re-install Linux. However, you
can spare yourself much inconvenience by learning how Linux stores its
network configuration. As you'll see, by using a text editor to revise
some files, you can alter your system's network configuration without
going through the pain of re-installing Linux.
If you replace your network adapter card with a different model
card, you must run the
modconf program, which lets
you specify the driver that operates your card. To do so, simply login
as
root and type the command:
modconf
You're already familiar with the
modconf
program. It's the same program you used to specify drivers when you
originally installed Linux. If you have difficulty using
modconf, refer to the section titled Section 3.1.2.13, "Configuring device driver modules" in Chapter 3,
Installing Linux. You must reboot your system before
changes made by
modconf take effect.
When you installed Linux, you specified a hostname for your
system. If you want to change the hostname associated with your
system, you can edit the file
/etc/hostname by
using
ae or another editor of your
choosing. Because the file - like most configuration files - has
restrictive permissions, you must login as
root in
order to modify it.
The format of the
/etc/hostname file is
simple. The file contains a single line, which contains the hostname
of your system; for example,
debian. If you
change the hostname, be sure to specify only the hostname itself; do
not specify a fully qualified hostname that includes the domain name
(for example,
debian.ora.com).
When you installed Linux, you may have specified one or more
nameservers. Your system accesses a nameserver when it needs to
determine the network address that corresponds to a hostname. If your
network configuration changes, you may need to specify a new
nameserver or servers. Your ISP should provide you with the proper IP
address or addresses.
The network addresses of your system's name servers are
specified in the file
/etc/resolv.conf, which you
can edit by using
ae or another editor while logged
in as
root. The format of the file is simple,
though not as simple as that of the
/etc/hostname
file. To specify a name server, include a line of the form:
nameserver
xxx.xxx.xxx.xxx
where
xxx.xxx.xxx.xxx denotes the network
address (IP number) of the name server; for example, 192.168.1.1. You
can include as many as three such lines; when your system needs to
determine a network address, it will attempt to contact the name
server specified by the first such line. If that server is
unavailable, your system will attempt to contact the name server
specified in the second such line, if any. If that werver is
unavailable, your system will contact the name server specified in the
third such line, if any.
If your computer is part of a local area network attached to the
Internet, your computer doesn't generally send data packets directly
to Internet hosts. Instead, it sends data packets to a designated
computer - called the
gateway - on the
local area network. The gateway forwards data packets to the Internet
on behalf of your system. It also performs the complementary service,
forwarding data packets from Internet hosts to your system.
NOTE: If your system connects to the Internet via PPP, the PPP system
establishes a network configuation dynamically. You'll learn how this
works in the next chapter.
The information that describes your local area network is
contained in the file
/etc/init.d/network, which
you can easily edit. Here's a typical
/etc/init.d/network file:
#! /bin/sh
inconfig lo 127.0.0.1
route add -net 127.0.0.0
IPADDR=192.168.1.10
NETMASK=255.255.255.0
BROADCAST=192.168.1.255
GATEWAY=192.168.1.1
ifconfig eth0 ${IPADDR} netmask ${NETMASK} broadcast ${BROADCAST}
route add default qw ${GATEWAY} metric 1
The lines you're concerned with are the lines four through
eight, each of which has the following form:
variable=
ipnumber
The lines associate a name, given by
variable, with a network address, given by
ipnumber. The variables are referenced by the
following three lines, which are commands that configure
networking.
To change your network configuration, you need merely to
associate the proper IP number with each variable. You can do so by
logging in as
root and modifying the
/etc/init.d/network file by using a text
editor.
Table 10.1 describes each
variable. Your network administrator should be able to provide you
with the proper values.
Table 10.1: Network Configuration Variables
|
Variable |
Meaning |
|
IPADDR |
Specifies the network address of your system. |
|
NETMASK |
Specifies the network address of your network,
by indicating which bits of the 32-bit network address of your system
pertain to the network and which pertain to the system. Many local
area networks are so-called Class C networks, which require a netmask
of 255.255.255.0. |
|
BROADCAST |
Specifies the address used to send a message to
every system on the local area network. Often you can determine the
broadcast address of a local area network from the address of a system
on the local area network: simply replace the last of the four
components of the network address of the host by 255. |
|
GATEWAY |
Specifies the network address of the gateway
used by your system. |
Your Linux system can use as many as three methods to determine the IP
address that corresponds to a hostname. Your system can:
-
Query a DNS server (you configured your system's DNS client
earlier)
-
Read the contents of the file
/etc/hosts,
known as the
hosts file, which you'll learn about in the next subsection
-
Query an NIS (Network Information System) server
However, unless your system is part of a sophisticated local
area network, it's unlikely that an NIS server is
available. Therefore, most systems can query a DNS server and, failing
to obtain an answer, read the
/etc/hosts
file. Alternatively, most systems can read the
etc/hosts file and, failing to obtain an answer,
query a DNS server. The second alternative is generally better,
because reading the
/etc/hosts file takes less
time than querying a DNS server.
The
/etc/host.conf/ file specifies which of
these three operations are performed, and the order in which they're
attempted. You can edit this file by logging in as
root. Here's a typical file:
order hosts,bind
multi on
The
order line specifies that the system
should first consult the
/etc/hosts file and then
query a DNS server, referred to as
bind because of
the Berkeley Internet Name Daemon, an early DNS server.
The
multi line specifies that your system
will attempt to locate all possible names for a host when it reads the
/etc/hosts file. Unless that file is very large
(hundreds or thousands of lines), you should include the
multi line.
The hosts file,
/etc/hosts, lets your system
determine the network address number that corresponds to a hostname,
without querying a DNS server. Besides being faster than querying a
DNS server, the
/etc/hosts file is always
available.
Entries in the file have two parts:
By default, the hosts file contains an entry that associates the
hostname
localhost with the IP
address 127.0.0.1. It's not necessary that you include any other
entries in the
/etc/hosts file. However, most
system administrators include at least a second line, which associates
the local hostname with its network address. Here's a typical
file:
127.0.0.1 localhost
192.168.1.10 debian.mccarty.org debian
Notice that the second line gives both the fully qualified
hostname, consisting of the hostname and domain name, as well as the
hostname alone.
The
/etc/networks file, known as the
networks file, performs a function similar to that of the hosts file;
whereas the hosts file associates hostnames with network addresses,
the networks file associates networks' names with network
addresses. By default, the networks file contains a single line
associating the network address of the local area network with the
name localnet:
localnet 192.168.1.0
Generally, it's not necessary that you add other entries to the
networks file. However, by doing so, you can access frequently used
networks by name even if your DNS server is unavailable.
