DHCP principle , DHCP server and client communication process

1. Explain with principle: the DHCP server and client communication process

DHCP stands for Dynamic Host Configuration Protocol. It handles the automatic assignment of IP addresses and other configuration settings for devices on your network. DHCP automates Network and Sharing Center in your control panel. This is especially good for people who have laptops, aren't hooked up to the Internet all the time and often move from place to place with their portable devices. They can simply get a new IP address as needed without having to do it manually. DHCP is designed to make the assignment of IP addresses and other network configuration information faster and easier.

DHCP is a protocol that uses Level 4 on the OSI model. It communicates using User Datagram Protocol (UDP) datagrams through UDP Port 68. DHCP works with most current and past Windows clients, and also Linux, Macintosh, and many network-capable printers. DHCP is designed to make the assignment of IP addresses and other network configuration information faster and easier. 

Rather than going around to every device on your network and setting up its network configuration manually, you can use your DHCP server to set up pools of addresses, called scopes, from which clients can request a temporary IP address.

DHCP is a client-server protocol that uses DHCP servers and DHCP clients. A DHCP server is a machine that runs a service that can lease out IP addresses and other TCP/IP information to any client that requests them. The DHCP server typically has a pool of IP addresses that it is allowed to distribute to clients, and these clients lease an IP address from the pool for a specific period of time, usually several days. Once the lease is ready to expire, the client contacts the server to arrange for renewal.

DHCP clients are client machines that run special DHCP client software enabling them to communicate with DHCP servers. All versions of Windows include DHCP client software, which is installed when the TCP/IP protocol stack is installed on the machine. DHCP clients obtain a DHCP lease for an IP address, a subnet mask, and various DHCP options from DHCP servers in a four-step process:

1.      DHCPDISCOVER: The client broadcasts a request for a DHCP server.

2.      DHCPOFFER: DHCP servers on the network offer an address to the client

3.      DHCPREQUEST: The client broadcasts a request to lease an address from one of the offering DHCP servers.

4.      DHCPACK: The DHCP server that the client responds to acknowledges the client, assigns it any configured DHCP options, and updates its DHCP database. The client then initializes and binds its TCP/IP protocol stack and can begin network communication.

 1. Define DHCP. Explain the DHCP server configuration.(any Linux)

Ans: The dhcp package contains an ISC DHCP server. First, install the package as the superuser:

~]#yum install dhcp (sudo apt-get install isc-dhcp-server)

Installing the dhcp package creates a file, /etc/dhcp/dhcpd.conf, which is merely an

empty configuration file:

~]#cat /etc/dhcp/dhcpd.conf

#

# DHCP Server Configuration file.

# see /usr/share/doc/dhcp*/dhcpd.conf.sample

The sample configuration file can be found at

/usr/share/doc/dhcp-<version>/dhcpd.conf.sample. We should use this file to

help you configure /etc/dhcp/dhcpd.conf, which is explained in detail below.

Configuration File: The first step in configuring a DHCP server is to create the configuration file that stores the network information for the clients. Use this file to declare options and global options for client systems. The configuration file can contain extra tabs or blank lines for easier formatting. Keywords are case-insensitive and lines beginning with a hash sign (#) are considered comments.

There are two types of statements in the configuration file:

Parameters — State how to perform a task, whether to perform a task, or what network configuration options to send to the client.

Declarations — Describe the topology of the network, describe the clients, provide addresses for the clients, or apply a group of parameters to a group of declarations.

The parameters that start with the keyword option are referred to as options. These options control DHCP options; whereas, parameters configure values that are not optional or control how the DHCP server behaves Parameters (including options) declared before a section enclosed in curly brackets ({ }) are considered global parameters. Global parameters apply to all the sections below it. Restart the DHCP daemon for the changes to take effect If the configuration file is changed, the changes do not take effect until the DHCP daemon is restarted with the command service dhcpd restart (sudo systemctl restart isc-dhcp-server).

Check if our DHCP service has been started or not using command:

sudo systemctl status isc-dhcp-server

3. What are DHCP components. Explain them.

Ans: These components work together to automate IP addressing.

·         DHCP leases

·         DHCP scopes

·         DHCP reservations

·         DHCP options

·         DHCP relay agents

DHCP leases: 

Client type has to do with the type of devices on your network, such as desktop computers, mobile notebooks, and servers. If we have more than the usual number of laptops on our network, the length of lease duration should decrease.

we should estimate the connection time, or the average length of time our clients spend on the network. If this time is relatively low, such as 2 to 4 hours, your lease duration doesn't have to be as long. An important factor is the number of IP addresses you have available, as well as the number of clients who need DHCP's range of addresses.

If we have a lot of clients in relation to our number of available addresses, a shorter lease duration is advisable to avoid running out of IP addresses For most networks, the default settings should be sufficient. Shorter lease times would be necessary for networks with many clients that connect for less than a day. A college campus is a good example, since campuses usually offer free wireless access for their students, who are typically connected for a few hours a day.

DHCP Scope: 

A DHCP Scope is a range of IP addresses and related configuration information available by request from a DHCP client. These scopes usually represent a single subnet, or segment of a network. Each scope is a continuous range of IP addresses defined by a beginning IP address and an ending IP address. If we need to exclude IP addresses, we must create exclusions for those addresses. One reason for creating these addresses might be hardware with static IP addresses, like printers.

DHCP Reservations: 

Well, in some cases, a network device needs to have a static IP address. An example would be a server, a router, or a network printer.

 In the DHCP role console, we reserve these addresses using the list below. Use these option codes to reserve IP addresses in DHCP.

3: Router

6: DNS server

15: DNS domain name

42: NTP server

44: WINS server (NetBIOS name server)

45: NetBIOS datagram disribution server (NBDD)

46: WINS/NetBIOS node type

47: NetBIOS scope ID

51: Lease Time

53: DHCP message type

55: Special option type used to communicate a parameter request list to the DHCP server

58: Renewal time value (T1)

59: Rebind time value (T2)

DHCP Options: 

DHCP options are basic settings that a client needs for proper network communication. These options include an IP address, a subnet mask, a default gatewar, primary and secondary DNS servers, primary and secondary Windows Internet Name Service (WINS) if

applicable, and DHCP lease expiration. we can define these options when creating the scope or change them later. Server options are settings defined on each server that apply to all scopes on a specific DHCP server.

Scope options are settings defined on each scope that apply only to the scope to which they

are added. Router options are typically defined using scope options, which override server options. Server options are usually used for network resources whose IP addresses are the same for all scopes, such as DNS and WINS.

DHCP Relay Agent: 

DHCP requests are broadcast messages that cannot be routed, so they are limited to the subnet of the client requesting an IP address. You can choose one of two options to get around this. You can have a DHCP server on each subnet, which may be expensive and, therefore, implausible. Or, you can use a DHCP relay agent to forward DHCP requests. This agent accepts the broadcast packets and converts them to unicast packets that can traverse a routed network and vice versa when the DHCP server replies to the client requesting an IP address. Most modern routers support the passing of DHCP requests.