DHCP Server in Linux(RHEL or CentOS) – Installation, Configuration, Interview Questions, Tips

DHCP(Dynamic Host Configuration Protocol)

1. What is DHCP Server ?
Dynamic Host Configuration Protocol (DHCP) automatically assigns network configuration information including IP addresses and other information like subnetmask, broadcast address, etc to computers on a network.

2. How it works ?
DHCP Client & Server works in four steps below.
a. A computer configured as DHCP client sends broadcast request “DHCPDiscover” in the network.
b. DHCP server accepts the request and reply back with “DHCPOffer” with range of IPs available.
c. Client then sends the “DHCPRequest” requesting an IP address.
d. Server acknowledges and sends the DHCPAck packet with IP assignment to client.

After a specific time defined also known as lease time, the client again sends the request to renew the IP address.

3. DHCP Client Configuration
First we will look at what configuration required to become dhcp client, thereafter we will check dhcp server config.
a. Package required.:- DHCP-common & dhclient package is required for dhcp client and server both which is generally installed by default.

[root@nglinux ~]# rpm -qa | grep -i dhcp

b. Become DHCP Client temporarily

# To configure all interfaces.
[root@nglinux ~]# dhclient


### To configure a specific interface
[root@nglinux ~]# dhclient eth0

c. To permanently configure DHCP.
Permanent DHCP configuration is defined on specific interface file(s).
We need to define bootproto to dhcp and onboot yes to enable it on system boot.

[root@nglinux ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth2 
[root@nglinux ~]# 

d. Make sure networking is enabled.

[root@nglinux ~]# cat /etc/sysconfig/network
[root@nglinux ~]# 

4. DHCP Server Setup
After DHCP Client configuration, lets have a look how to configure DHCP Server.
a. First install DHCP server package.

[root@nglinux ~]# yum install dhcp
Failed to set locale, defaulting to C
Loaded plugins: auto-update-debuginfo, fastestmirror, refresh-packagekit, security
Setting up Install Process
Loading mirror speeds from cached hostfile
epel/metalink                                                                        |  16 kB     00:00     
epel-debuginfo/metalink                                                              |  14 kB     00:00     
 * base: repos.lax.quadranet.com
 * epel: mirrors.develooper.com
 * epel-debuginfo: mirrors.develooper.com
 * extras: mirror.fileplanet.com
 * updates: repos-lax.psychz.net
base                                                                                 | 3.7 kB     00:00     
base-debuginfo                                                                       | 2.5 kB     00:00     
epel                                                                                 | 3.2 kB     00:00     
epel-debuginfo                                                                       | 1.5 kB     00:00     
extras                                                                               | 3.3 kB     00:00     
updates                                                                              | 3.4 kB     00:00     
Resolving Dependencies
--> Running transaction check
---> Package dhcp.i686 12:4.1.1-61.P1.el6.centos will be installed
--> Processing Dependency: dhcp-common = 12:4.1.1-61.P1.el6.centos for package: 12:dhcp-4.1.1-61.P1.el6.centos.i686
--> Running transaction check
---> Package dhcp-common.i686 12:4.1.1-53.P1.el6.centos will be updated
--> Processing Dependency: dhcp-common = 12:4.1.1-53.P1.el6.centos for package: 12:dhclient-4.1.1-53.P1.el6.centos.i686
---> Package dhcp-common.i686 12:4.1.1-61.P1.el6.centos will be an update
--> Running transaction check
---> Package dhclient.i686 12:4.1.1-53.P1.el6.centos will be updated
---> Package dhclient.i686 12:4.1.1-61.P1.el6.centos will be an update
--> Finished Dependency Resolution

Dependencies Resolved

 Package                 Arch             Version                                 Repository           Size
 dhcp                    i686             12:4.1.1-61.P1.el6.centos               updates             826 k
Updating for dependencies:
 dhclient                i686             12:4.1.1-61.P1.el6.centos               updates             321 k
 dhcp-common             i686             12:4.1.1-61.P1.el6.centos               updates             145 k

Transaction Summary
Install       1 Package(s)
Upgrade       2 Package(s)

Total download size: 1.3 M
Is this ok [y/N]: y
Downloading Packages:
(1/3): dhclient-4.1.1-61.P1.el6.centos.i686.rpm                                      | 321 kB     00:01     
(2/3): dhcp-4.1.1-61.P1.el6.centos.i686.rpm                                          | 826 kB     00:02     
(3/3): dhcp-common-4.1.1-61.P1.el6.centos.i686.rpm                                   | 145 kB     00:01     
Total                                                                       192 kB/s | 1.3 MB     00:06     
Running rpm_check_debug
Running Transaction Test
Transaction Test Succeeded
Running Transaction
  Updating   : 12:dhcp-common-4.1.1-61.P1.el6.centos.i686                                               1/5 
  Installing : 12:dhcp-4.1.1-61.P1.el6.centos.i686                                                      2/5 
  Updating   : 12:dhclient-4.1.1-61.P1.el6.centos.i686                                                  3/5 
  Cleanup    : 12:dhclient-4.1.1-53.P1.el6.centos.i686                                                  4/5 
  Cleanup    : 12:dhcp-common-4.1.1-53.P1.el6.centos.i686                                               5/5 
  Verifying  : 12:dhcp-4.1.1-61.P1.el6.centos.i686                                                      1/5 
  Verifying  : 12:dhcp-common-4.1.1-61.P1.el6.centos.i686                                               2/5 
  Verifying  : 12:dhclient-4.1.1-61.P1.el6.centos.i686                                                  3/5 
  Verifying  : 12:dhclient-4.1.1-53.P1.el6.centos.i686                                                  4/5 
  Verifying  : 12:dhcp-common-4.1.1-53.P1.el6.centos.i686                                               5/5 
  dhcp.i686 12:4.1.1-61.P1.el6.centos                                                                       
Dependency Updated:
  dhclient.i686 12:4.1.1-61.P1.el6.centos             dhcp-common.i686 12:4.1.1-61.P1.el6.centos            
[root@nglinux ~]#

b. Now update DHCPD interface.

### DHCPD by default file is empty.
[root@nglinux ~]# cat /etc/sysconfig/dhcpd
# Command line options here
[root@nglinux ~]# 

### Now enter the interface name in the dhcpd file.
[root@nglinux ~]# vim /etc/sysconfig/dhcpd

[root@nglinux ~]# cat /etc/sysconfig/dhcpd
# Command line options here
[root@nglinux ~]# 

c. Default DHCPD configuration File.

[root@nglinux ~]# cat /etc/dhcp/dhcpd.conf 
# DHCP Server Configuration file.
#   see /usr/share/doc/dhcp*/dhcpd.conf.sample
#   see 'man 5 dhcpd.conf'
[root@nglinux ~]# 

### As advised above, copy the sample configuration file.
[root@nglinux ~]# cp -p /usr/share/doc/dhcp-4.1.1/dhcpd.conf.sample /etc/dhcp/dhcpd.conf 
cp: overwrite `/etc/dhcp/dhcpd.conf'? y

[root@nglinux ~]# more /etc/dhcp/dhcpd.conf 
# dhcpd.conf
# Sample configuration file for ISC dhcpd

5. DHCPD Server Configuration

# cat /etc/dhcp/dhcpd.conf

### Global DHCP Configuration
option domain-name "ngelinux.com";
option domain-name-servers ns1.ngelinux.com, ns2.ngelinux.com;
default-lease-time 3600; 
max-lease-time 7200;

### General Configuration of subnet and IP range
subnet netmask {
        option routers        ;
        option subnet-mask    ;
        option domain-search            "ngelinux.com";
        option domain-name-servers;

### Assigning static IPs
host redhat-node {
	 hardware  ethernet 00:e0:j4:6y:23:0a;

host oracle-node {
	 hardware  ethernet 00:4q:8a:12:7h:3b;

6. Last step to get DHCP working.

a. Start and enable services.
###  On CentOS / RHEL 7 
# systemctl start dhcpd
# systemctl enable dhcpd

### On CentOS / RHEL 6 
# service dhcpd start
# chkconfig dhcpd on

b. Add rule in Firewall
### On CentOS / RHEL 7 
# firewall-cmd --add-service=dhcp --permanent 
# firewall-cmd --reload 

### On CentOS / RHEL 6 
# iptables -A INPUT -p tcp -m state --state NEW --dport 67 -j ACCEPT
# service iptables save

7. Interview Questions Of DHCP

1. What is DHCP ?
DHCP is a protocol(dynamic host configuration protocol) responsible for assigning network settings(which includes assigning ip address, subnet mask, gateway, dns servers, etc) to a host.

2. Ports used by DHCP.
Requests sent on UDP port 68, Server replies on UDP 67 .

3. How DHCP works ?
DHCP woks on DORA model: - Discover, Offer, Request, and Acknowledgement

4. What is DHCP Scope ?
Range of possible IP Addresses that the DHCP server can lease to clients on a subnet.

5. What is Super Scope ?
Collection of all individual scopes that are configured/ managed by a single server/unit in network.

6. What is default lease time to a client ?
8 days

7. What is reservation in DHCP ?
When a server is mapped to a fix IP address.

8. What is DHCP relay ?
DHCP relay is an intermediate host that can forward dhcp client requests to a server in cases when the server is not directly reachable.

9. Can we assign static IP address to hosts ?
Yes we can assign static IP address to a particular host based on MAC address.

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