Lab 04: DHCP - devinziegler/Devin-Tech-Journal GitHub Wiki

Assignment: DHCP

Objectives: Install and configure DHCP service on CENTOS.

Remoting into a system:

SSH:

To use SSH open a terminal and type the following command:

ssh <user_name@hostname

PUTTY:

PuTTY can be downloaded here: chiark.greenend.org.uk

PuTTY is pretty straight forward and the setup is similar to the ssh command listed above. Instead of putting the username in, all you need is a the system hostname.

DHCP:

Installing DHCP:

The following command will install the DHCP package:

yum install dhcpd -y

Configuring DHCP:

Becoming the system user for a brief time using the following command:

sudo -i

This is helpful when you are doing work that requires elevated privileges for a long period of time. Don't forget to exit when finished.

  • This is the path to the target config:
/etc/dhcp/dhcpd.conf
  • The following information is required in dhcpd.conf
subnet <network_address> netmask <netmask> {
     option routers <default_gateway
     option subnet-mask <subnet_mask>
     option domain-name <domain_name>
     range <first_usable> <last_usable>
}

Lease time can be added to the config as well:

default-lease-time <timeinseconds>
max-lease-time <timeinseconds>

Starting DHCP service:

Start the service:

systemctl start dhcpd

View DHCP service status:

systemctl status dhcpd

Start service on boot:

systemctl enable dhcpd

Firewall Configuration to allow DHCP:

list default firewall rules:

firewall-cmd --list-all

Add DHCP service to the firewall rules:

fiewall-cmd --add-service-dhcp --permanent

command output should be success.

  • Configure workstation so it uses DHCP.

DHCP Options:

  • These options can be added to the dhcpd.conf file and allow for some cool features.

Reserve IP:

In order to reserve an IP address the host hardware address is needed.

The following changes can be made to dhcpd.conf:

host <host_name> {
     hardware ethernet <host_mac>;
     fixed-address <address_to_reserve>;
}

Looking at the changes in Wireshark, something interesting is happening. When the host IP is released, the same one is requested. Dhcp01 however, knows that another address is reserved for that workstation. The DHCP server offers the new IP THREE times before the workstation finally accepts the new IP address.

dhcp_wirehsark

Client Hardware Address Padding:

In all packets of the DHCP process captured, there is client hardware address padding in the DHCP header.

image

The purpose of this padding is to insure proper alignment in the data fields.

DHCP options:

In every DHCP packet there are options that determine what it is. For example this is a discover packet with the corresponding option of 53:

image

A source to find out other options can be found here

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