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 renew 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 <DHCP server Network ID> netmask <subnetmask of the this network> {
range <starting IP address><ending IP address>;
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 <DHCP server IP address>;
option broadcast-address <DHCP server 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--->Enter
---> Use DHCP --->Press Spacebar --->OK--->Save--->Save & 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 <dhcp client hostname> {
hardware ethernet <MAC address of the Client's NIC card>;
fixed addresses <IP address>;
}
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.
No comments:
Post a Comment