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<td width="80%" align="center" valign="bottom">Chapter 5 Network Configuration</td>
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<div class="SECT1">
<h1 class="SECT1"><a id="NETWORK-CONFIGURATION-TCPIP"
name="NETWORK-CONFIGURATION-TCPIP">5.3 TCP/IP Configuration</a></h1>
<p>At this point, your network card should be physically installed in your computer, and
the relevant kernel modules should be loaded. You will not yet be able to communicate
over your network card, but information about the network device can be obtained with <tt
class="COMMAND">ifconfig -a</tt>.</p>
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<pre class="SCREEN">
<samp class="PROMPT">#</samp> <kbd class="USERINPUT">ifconfig -a</kbd>
eth0 Link encap:Ethernet HWaddr 00:A0:CC:3C:60:A4
UP BROADCAST NOTRAILERS RUNNING MULTICAST MTU:1500 Metric:1
RX packets:110081 errors:1 dropped:0 overruns:0 frame:0
TX packets:84931 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:114824506 (109.5 Mb) TX bytes:9337924 (8.9 Mb)
Interrupt:5 Base address:0x8400
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:2234 errors:0 dropped:0 overruns:0 frame:0
TX packets:2234 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:168758 (164.8 Kb) TX bytes:168758 (164.8 Kb)
</pre>
</td>
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</table>
<p>If you just typed <tt class="COMMAND">/sbin/ifconfig</tt> without the <var
class="OPTION">-a</var> suffix, you would not see the <tt class="FILENAME">eth0</tt>
interface, as your network card does not yet have a valid IP address or route.</p>
<p>While there are many different ways to setup and subnet a network, all of them can be
broken down into two types: Static and Dynamic. Static networks are setup such that each
node (geek lingo for thing with an IP address) always has the same IP address. Dynamic
networks are setup in such a way that the IP addresses for the nodes are controlled by a
single server called the DHCP server.</p>
<div class="SECT2">
<h2 class="SECT2"><a id="NETWORK-CONFIGURATION-TCPIP-DHCP"
name="NETWORK-CONFIGURATION-TCPIP-DHCP">5.3.1 DHCP</a></h2>
<p>DHCP (or Dynamic Host Configuration Protocol), is a means by which an IP address may
be assigned to a computer on boot. When the DHCP <span class="emphasis"><i
class="EMPHASIS">client</i></span> boots, it puts out a request on the Local Area Network
for a DHCP <span class="emphasis"><i class="EMPHASIS">server</i></span> to assign it an
IP address. The DHCP server has a pool (or <span class="emphasis"><i
class="EMPHASIS">scope</i></span>) of IP addresses available. The server will respond to
this request with an IP address from the pool, along with a <span class="emphasis"><i
class="EMPHASIS">lease time</i></span>. Once the lease time for a given IP address lease
has expired, the client must contact the server again and repeat the negotiation.</p>
<p>The client will then accept the IP address from the server and will configure the
requested interface with the IP address. There is one more handy trick that DHCP clients
use for negotiating the IP address that they will be assigned, however. The client will
remember it's last assigned IP address, and will request that the server re-assign that
IP address to the client again upon next negotiation. If possible, the server will do so,
but if not, a new address is assigned. So, the negotiation resembles the following:</p>
<p class="LITERALLAYOUT"> <span class="emphasis"><i
class="EMPHASIS">Client</i></span>: Is there a DHCP server available on the LAN?<br />
<span class="emphasis"><i
class="EMPHASIS">Server</i></span>: Yes, there is. Here I am.<br />
<span class="emphasis"><i
class="EMPHASIS">Client</i></span>: I need an IP address.<br />
<span class="emphasis"><i
class="EMPHASIS">Server</i></span>: You may take 192.168.10.10 for 19200 seconds.<br />
<span class="emphasis"><i
class="EMPHASIS">Client</i></span>: Thank you.</p>
<p class="LITERALLAYOUT"> <span class="emphasis"><i
class="EMPHASIS">Client</i></span>: Is there a DHCP server available on the LAN?<br />
<span class="emphasis"><i
class="EMPHASIS">Server</i></span>:Yes, there is. Here I am.<br />
<span class="emphasis"><i
class="EMPHASIS">Client</i></span>:I need an IP address. The last time we<br />
talked, I had 192.168.10.10;<br />
May I have it again?<br />
<span class="emphasis"><i
class="EMPHASIS">Server</i></span>:Yes, you may (or No, you may not: take 192.168.10.12 instead).<br />
<span class="emphasis"><i
class="EMPHASIS">Client</i></span>: Thank you.</p>
<p>The DHCP client in Linux is <tt class="COMMAND">/sbin/dhcpcd</tt>. If you load <tt
class="FILENAME">/etc/rc.d/rc.inet1</tt> in your favorite text editor, you will notice
that <tt class="COMMAND">/sbin/dhcpcd</tt> is called about midway through the script.
This will force the conversation shown above. <tt class="COMMAND">dhcpcd</tt> will also
track the amount of time left on the lease for the current IP address, and will
automatically contact the DHCP server with a request to renew the lease when necessary.
DHCP can also control related information, such as what ntp server to use, what route to
take, etc.</p>
<p>Setting up DHCP on Slackware is simple. Just run <tt class="COMMAND">netconfig</tt>
and select DHCP when offered. If you have more than one NIC and do not wish <tt
class="FILENAME">eth0</tt> to be configured by DHCP, just edit the <tt
class="FILENAME">/etc/rc.d/rc.inet1.conf</tt> file and change the related variable for
your NIC to “<var class="LITERAL">YES</var>”.</p>
</div>
<div class="SECT2">
<h2 class="SECT2"><a id="NETWORK-CONFIGURATION-TCPIP-STATIC"
name="NETWORK-CONFIGURATION-TCPIP-STATIC">5.3.2 Static IP</a></h2>
<p>Static IP addresses are fixed addresses that only change if manually told to. These
are used in any case where an administrator doesn't want the IP information to change,
such for internal servers on a LAN, any server connected to the Internet, and networked
routers. With static IP addressing, you assign an address and leave it at that. Other
machines know that you are always at that certain IP address and can contact you at that
address always.</p>
</div>
<div class="SECT2">
<h2 class="SECT2"><a id="NETWORK-CONFIGURATION-TCPIP-CONF"
name="NETWORK-CONFIGURATION-TCPIP-CONF">5.3.3 <tt
class="FILENAME">/etc/rc.d/rc.inet1.conf</tt></a></h2>
<p>If you plan on assigning an IP address to your new Slackware box, you may do so either
through the <tt class="FILENAME">netconfig</tt> script, or you may edit <tt
class="FILENAME">/etc/rc.d/rc.inet1.conf</tt>. In <tt
class="FILENAME">/etc/rc.d/rc.inet1.conf</tt> , you will notice:</p>
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<tr>
<td>
<pre class="PROGRAMLISTING">
# Primary network interface card (eth0)
IPADDR[0]=""
NETMASK[0]=""
USE_DHCP[0]=""
DHCP_HOSTNAME[0]=""
</pre>
</td>
</tr>
</table>
<p>Then further at the bottom:</p>
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<td>
<pre class="PROGRAMLISTING">
GATEWAY=""
</pre>
</td>
</tr>
</table>
<p>In this case, our task is merely to place the correct information between the
double-quotes. These variables are called by <tt class="FILENAME">/etc/rc.d/rc.inet1</tt>
at boot time to setup the nics. For each NIC, just enter the correct IP information, or
put “<var class="LITERAL">YES</var>” for <var class="LITERAL">USE_DHCP</var>.
Slackware will startup the interfaces with the information placed here in the order they
are found.</p>
<p>The <var class="LITERAL">DEFAULT_GW</var> variable sets up the default route for
Slackware. All communications between your computer and other computers on the Internet
must pass through that gateway if no other route is specified for them. If you are using
DHCP, you will usually not need to enter anything here, as the DHCP server will specify
what gateway to use.</p>
</div>
<div class="SECT2">
<h2 class="SECT2"><a id="NETWORK-CONFIGURATION-TCPIP-RESOLVER"
name="NETWORK-CONFIGURATION-TCPIP-RESOLVER">5.3.4 <tt
class="FILENAME">/etc/resolv.conf</tt></a></h2>
<p>Ok, so you've got an IP address, you've got a default gateway, you may even have ten
million dollars (give us some), but what good is that if you can't resolve names to IP
addresses? No one wants to type in <tt class="HOSTID">72.9.234.112</tt> into their web
browser to reach <tt class="HOSTID">www.slackbook.org</tt>. After all, who other than the
authors would memorize that IP address? We need to setup DNS, but how? That's where <tt
class="FILENAME">/etc/resolv.conf</tt> comes into play.</p>
<p>Chances are you already have the proper options in <tt
class="FILENAME">/etc/resolv.conf</tt>. If you setup your network connection using DHCP,
the DHCP server should handle updating this file for you. (Technically the DHCP server
just tells <tt class="COMMAND">dhcpcd</tt> what to put here, and it obeys.) If you need
to manually update your DNS server list though, you'll need to hand edit <tt
class="FILENAME">/etc/resolv.conf</tt>. Below is an example:</p>
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<td>
<pre class="SCREEN">
<samp class="PROMPT">#</samp> <kbd class="USERINPUT">cat /etc/resolv.conf</kbd>
nameserver 192.168.1.254
search lizella.net
</pre>
</td>
</tr>
</table>
<p>The first line is simple. The nameserver directive tells us what DNS servers to query.
By necessity these are always IP addresses. You may have as many listed there as you
like. Slackware will happily check one after the other until one returns a match.</p>
<p>The second line is a little more interesting. The search directive gives us a list of
domain names to assume whenever a DNS request is made. This allows you to contact a
machine by only the first part of its FQDN (Fully Qualified Domain Name). For example, if
“slackware.com” were in your search path, you could reach <tt
class="HOSTID">http://store.slackware.com</tt> by just pointing your web browser at <tt
class="HOSTID">http://store</tt>.</p>
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<td>
<pre class="SCREEN">
<samp class="PROMPT">#</samp> <kbd class="USERINPUT">ping -c 1 store</kbd>
PING store.slackware.com (69.50.233.153): 56 data bytes
64 bytes from 69.50.233.153 : icmp_seq=0 ttl=64 time=0.251 ms
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 0.251/0.251/0.251 ms
</pre>
</td>
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</table>
<br />
<br />
</div>
<div class="SECT2">
<h2 class="SECT2"><a id="NETWORK-CONFIGURATION-TCPIP-HOSTS"
name="NETWORK-CONFIGURATION-TCPIP-HOSTS">5.3.5 <tt
class="FILENAME">/etc/hosts</tt></a></h2>
<p>Now that we've got DNS working fine, what if we want to bypass our DNS server, or add
a DNS entry for a machine that isn't in DNS? Slackware includes the oft-loved <tt
class="FILENAME">/etc/hosts</tt> file which contains a local list of DNS names and IP
addresses they should match to.</p>
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<tr>
<td>
<pre class="SCREEN">
<samp class="PROMPT">#</samp> <kbd class="USERINPUT">cat /etc/hosts</kbd>
127.0.0.1 localhost locahost.localdomain
192.168.1.101 redtail
172.14.66.32 foobar.slackware.com
</pre>
</td>
</tr>
</table>
<p>Here you can see that localhost has an IP address of <tt class="HOSTID">127.0.0.1</tt>
(always reserved for localhost), redtail can be reached at <tt
class="HOSTID">192.168.1.101</tt>, and <tt class="HOSTID">foobar.slackware.com</tt> is
<tt class="HOSTID">172.14.66.32</tt>.</p>
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