1. What is DHCP
and explain it?
DHCP stands
for Dynamic Host
Configuration Protocol. DHCP is a network protocol that enables the
server to assign an IP addresses to the
clients in the network automatically from a defined range of IP addresses ie., scope configured for a given network.
DHCP allows a computer to join in an IP-based
network without having a pre-configured
IP address. DHCP is a protocol that assign unique IP addresses
to devices, then releases and
renews those addresses as devices leave
and rejoin in the network.
Internet Service
Providers (ISPs) usually use DHCP
to help customers join their networks with minimum setup effort
required. Likewise, home network
equipment like broadband routers
offers DHCP support to joining home computers to Local
Area Networks (LANs).
In simple terms
DHCP is used to assign the IP
addresses to the remote hosts
automatically. First client requests to the DHCP
server, then DHCP
server accepts the client's request
and assign the next
available IP address to the
requested DHCP client.
2. How
the DHCP works?
The process of requesting the IP address from the DHCP
clients and assign the
IP address by the DHCP server is called "D
O R A".
(i) When we switch on the system
with DHCP client,
the client system sends the broadcast request
looking for a DHCP
server to answer. This process is called DISCOVER or DHCP DISCOVER.
(ii) The
router directs the DISCOVER packet to the correct DHCP
server.
(iii) The server
receives the DISCOVER packet. Based on availability and
usage policies set on the server,
the server determines
an appropriate address (if any) to give to the client. The server then
temporarily reserves that
address for the client and sends back to the client an OFFER orDHCP
OFFER packet with that address information. The server also configures the client's DNS
servers, WINS servers,
NTP serves and sometimes
other services also.
(iv) Then
the Client sends a REQUEST
or DHCP REQUEST packet, letting the server know that it intends to use the address.
(v) Then
the server sends an ACK
or DHCP ACK packet, conforming that the client has been
given a
lease on the address for a server
specified period of
time.
3. What is the disadvantage to assign
the Static IP address?
When a
system uses a static
IP address, It means that the
system is manually configured to use a specific
IP address. One problem with
static assignment, which can result from user error or
inattention to detail, occurs when two systems are configured with
the same IP address. This creates a conflict that results in loss
of service. Using DHCP
to dynamically assign IP addresses
to avoid these
conflicts.
4. What is the profile of DHCP?
Package : dhcp*
Script file : /etc/init.d/dhcpd
Configuration file : /etc/dhcp/dhcpd.conf
Deamon : dhcpd
Port numbers : 67 (dhcp server)
and 68 (dhcp client)
Log messages : /var/log/messages
5. How to configure the DHCP
server?
(i) Assign a
static IP address to the
DHCP server.
(ii) Install the DHCP
package by # yum install dhcp*
-y command.
(iii) Open
the DHCP
configuration file by #
vim /etc/dhcp/dhcpd.conf command.
This file is empty and we have to copy the sample file
from /usr/share/doc/dhcp-4.25/dhcpd.conf.example to the
above location by #
cp -p
/usr/share/doc/dhcp-4.25/dhcpd.conf.example /etc/dhcp/dhcpd.conf command.
(iv) Now open the above DHCP
configuration file by # vim
/etc/dhcp/dhcpd.conf
command.
* Go to line number 47
and edit that line as below.
subnet
netmask {
range ;
default-lease-time 600; (the
minimum lease time to the client in
seconds)
max-lease-time 7200; (he
maximum lease time to the client in
seconds)
}
Example :
subnet 172.25.0.0 netmask
255.255.255.0 {
range 172.25.9.50 172.25.9.100;
default-lease-time 600;
max-lease-time 7200;
}
* Go to line number 51
and edit that as below.
option routes
;
option broadcast-address ;
Example :
option routes
172.25.9.11;
option broadcast-address 172.25.9.255; (save and
exit this file)
(v) Restart the
DHCP services in RHEL - 6
and RHEL - 7.
# service
dhcpd restart (to restart the DHCP
service in RHEL - 6)
# chkconfig
dhcpd on (to enable the DHCP service at next boot in
RHEL - 6)
# systemctl
restart dhcpd (to
restart the DHCP service
in RHEL - 7)
# systemctl enable
dhcpd (to enable the DHCP service at next boot in
RHEL - 7)
(vi) Add the DHCP
service to the IP tables and
Firewall.
In
RHEL - 6:
# iptables -A
INPUT -p udp
-i eth0 --deport
67 -j ACCEPT
# iptables -A
INPUT -p tcp
-i eth0 --deport
67 -j ACCEPT
# iptables -A
INPUT -p udp
-i eth0 --deport
68 -j ACCEPT
# iptables -A
INPUT -p tcp
-i eth0 --deport
68 -j ACCEPT
# iptables -A
OUTPUT -p udp
-i eth0 --deport
67 -j ACCEPT
# iptables -A
OUTPUT -p tcp
-i eth0 --deport
67 -j ACCEPT
# iptables -A
OUTPUT -p udp
-i eth0 --deport
68 -j ACCEPT
# iptables -A
OUTPUT -p tcp
-i eth0 --deport
68 -j ACCEPT
In
RHEL - 7 :
# firewall-cmd --permanent --add-service=dhcp
#
firewall-cmd --complete-reload
(vii) # cat
/var/lib/dhcpd/dhcpd.lease (to see the DHCP
lease message database
on DHCP server)
6. How to configure the DHCP
client?
(i) Change the IP addressing
from static to dynamic if it is configured as static.
In
RHEL - 6 :
#
setup
Network
Configuration -----> Press Enter
-----> Device Configuration ----->
Select eth0 ----->
Press Enter
-----> Select Use
DHCP -----> Press
Spacebar -----> OK
-----> Save ----->Save & Quit
----->
Quit
#
service NetworkManager restart
#
service network restart
In
RHEL - 7:
#
nmcli connection modify
"System eth0" ipv4.method auto or
dynamic
#
nmcli connection down
"System eth0"
#
nmcli connection up
"System eth0"
#
systemctl restart network
(ii) Open
/etc/sysconfig/network-scripts/ifcfg-eth0 file
and edit the
BOOTPROTO line.
#
vim
/etc/sysconfig/network-scripts/ifcfg-eth0
* Go
to BOOTPROTO line
and edit that line as below.
BOOTPROTO=dhcp (save and
exit this file)
(iii) Get
the IP address from the
DHCP server.
#
dhclient
#
ifdown eth0
#
ifup eth0
7. How to fix the IP address to the
client every time it requests or how to configure the MAC
binding?
The process
of assigning the same IP address
(fixed IP address)
to the DHCP
client every time it
booted is called
"MAC binding".
(i) Open the file /etc/dhcp/dhcpd.conf by #
vim /etc/dhcp/dhcpd.conf command.
* Go to line
number 76 and 77 and
edit those lines as below.
host {
hardware ethernet ;
fixed addresses ;
}
Example :
host client 1
{
hardware ethernet
2015:ac18::55;
fixed addresses
172.25.9.150;
} (save and
exit this file)
(ii) Restart the
DHCP services in RHEL - 6
and RHEL - 7.
#
service dhcpd restart (to
restart the DHCP service
in RHEL - 6)
#
chkconfig dhcpd on (to enable the DHCP service at next boot in
RHEL - 6)
#
systemctl restart dhcpd (to
restart the DHCP service
in RHEL - 7)
#
systemctl enable dhcpd (to enable the DHCP service at next boot in
RHEL - 7)
* Then
the above MAC address of the system will get the same IP
address every time it booted.
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