Installing and Configuring the DHCPD Server

The DHCPD server is a server that allows DHCP clients to connect to the server, and request IP

addresses and gateway/DNS information. DHCP is used in most large networks as a means of easily

managing IP addresses. Linux has a server of its own, creatively called DHCPD. DHCPD is

available from the Internet Software Consortum’s website at isc.org. The server should also be

available from your distribution however, so check with your distribution first.

ISC only supplies standard tarball packages, so if your distribution does not supply the DHCPD

package, you will have to use the ISC’s package. Download the file and extract it using the

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following two commands:

gunzip dhcpd-version.tar.gz

tar -xvf dhcpd-version.tar

Make sure to replace version with the actual version. Now, perform the following commands:

cd dhcpd-version

./configure

make

make install

DHCPD should install flawlessly, if not then you should complain to the mailing list on the

ISC’s website. We will only have to perform three tasks with DHCPD, the first is to edit the

configuration file. Place the following text in your /etc/dhcpd.conf file:

# /etc/dhcpd.conf by Christopher Pace

ddns-update-style ad-hoc;

default-lease-time 259200;

max-lease-time 300000;

option subnet-mask 255.255.255.0;

option routers 192.168.0.1;

option domain-name-servers 192.168.0.1;

subnet 192.168.0.0 netmask 255.255.255.0 {

range 192.168.0.20 192.168.0.40;

range 192.168.0.50 192.168.0.90;

}

Of course you will want to substitute the routers, domain-name-servers, netmask, and range with

what is for your network. For instance, I have a network that I use DHCP to assign a total of 60

IP addresses. This range is from .20-.40, and from .50-.90. DHCPD will only assign IP addresses

within this range, as I like to keep .1-.19, .41-.49, and .91-.254 free for servers and such.

The ‘default-lease-time’ and ‘max-lease-time’ settings are used to specify how long the DHCP

lease will last if the client doesn’t request extra time (default), and if it requests the max

time (max). This time is in seconds. If you want to have a static IP assigned to a host, then

you can use the following syntax in your /etc/dhcpd.conf file:

host Joe {

hardware ethernet 00:c0:f0:25:b7:15;

fixed-address 192.168.0.205;

}

This will assign the IP of 192.168.0.205 to Joe each time that it requests an IP. The MAC

address is the ‘hardware ethernet’ address.

Now then, we move along to the next step, creating the /var/state/dhcp/dhcpd.leases file:

touch /var/state/dhcp/dhcpd.leases

Now, we will start DHCPD, to test it out. First, if you are currently using another DHCP server

on your network, disable that one. Next, run the following:

/usr/sbin/dhcpd

Finally, start up a DHCP client (if you are using Windows 98/2000/XP/NT, you can use the

ipconfig command to release the IP and then renew it by typing:

ipconfig /release_all

ipconfig /renew_all

This should take a while, as the DHCP client is searching for the original server. After a

while, it will time out, and then query the network for any DHCP servers, finding our Linux one.

Now, once you are sure that DHCPD works, we should create an init script for DHCPD. This is used

to start, restart, and stop the DHCPD service. Also, this init script will be automatically run

at boot to start DHCPD. Place the following text in /etc/init.d/dhcpd:

#!/bin/sh

# /etc/init.d/dhcpd file by Christopher Pace

case “$1″ in

start)

echo -n “Starting DHCPD Daemon: dhcpd”

start-stop-daemon –start –quiet –exec /usr/sbin/dhcpd

echo “.”

;;

stop)

echo -n “Stopping DHCPD Daemon: dhcpd”

killall -9 dhcpd

echo “.”

;;

restart)

echo -n “Restarting DHCPD Daemon: dhcpd”

killall -HUP dhcpd

echo “.”

;;

*)

echo “Usage: /etc/init.d/dhcpd {start|stop|restart|reload|force-reload}” >&2

exit 1

;;

esac

exit 0

You should now:

chmod 700 /etc/init.d/dhcpd

ntsysv < FOR REDHAT SYSTEMS

update-rc.d dhcpd defaults < FOR DEBIAN SYSTEMS

If you run a different system then listed, you should check with your distribution on how to

properly tell the system to use the init file you just made. On some systems, simply chmod-ing

the init file will work. Also, some systems only have a /etc/rc.d directory, where the init file

should be placed in the run levels associated with startup, halt, and so forth. Read the FAQs

that your distribution has as to which run levels are for which tasks, as some distributions

tend to go against POSIX.

DHCP is a useful client, but when routers are shipped with DHCP server capabilities, too often

the DHCP server is stripped-down, leaving the many options that DHCPD offers missing. Thus, it

is necessary to have DHCPD instead of these stripped-down servers, in order to satisfy

particular needs. For instance, I need 60 DHCP-assigned addresses, in two different IP ranges.

Thus, I would recommend DHCPD for anyone who needs a truly customizable DHCP server.
• Analyzing the basic intrusion
• Examining various protocols
• Detecting intrusion and its other role
• Analyzing fragment and IP routing
• Configuring snort and traffic analysis

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