module18-dhcp server concept dan definition.ppt

A DHCP server automatically sends the required network parameters for clients to properly communicate on the network. Without it, the network administrator has to manually set up every client that joins the network, which can be cumbersome, especially in large networks. DHCP servers usually assign each client with a unique dynamic IP address, which changes when the client’s lease for that IP address has expired. When to use a router/switch as your DHCP Server
There are many enterprise companies who are still using DHCP for IPv4 on their routers/switches. This is typically done by the network administrator who needs to get a DHCP capability up and running quickly but does not have access to a DHCP server. Most routers/switches have the ability to provide the following DHCP server support:

a DHCP client and obtain an interface IPv4 address from an upstream DHCP service
a DHCP relay and forward UDP DHCP messages from clients on a LAN to and from a DHCP server
a DHCP server whereby the router/switch services DHCP requests directly. However, there are limitations to using a router/switch as a DHCP server
Running a DHCP server on a router/switch consumes resources on the network device. These DHCP packets are handled in software (not hardware accelerated forwarding). The resources required make this practice not suitable for a network with a large number (> 150) of DHCP clients.
Does not support dynamic DNS. The router/switch DHCP server cannot create an entry into DNS on behalf of the client based on the IPv4 address that was leased to the client.
No ability to e asily manage the scope and see the current DHCP bindings and leases across multiple routers. Administrator must log into the switch/router individually to get information about DHCP bindings.
No high availability or redundancy of the DHCP bindings. This could cause problems if the current DHCP server and default gateway fails.
It is more difficult to configure DHCP options on router/switch platform.
The DHCP service running on a router/switch is not integrated with IP address management (IPAM) for address tracking and scope utilization or security forensics. The Benefits of a dedicated DHCP Server
A better approach than trying to use DHCP on your router/switch is to use a centralized DHCP server. This is particularly true for network environments that require support of both DHCP for IPv4 and DHCP for IPv6 at the same time. Virtually all DHCP server vendors support both protocols so you can use the same management interface for IPv4 and IPv6. There are several benefits that make it advantageous for an enterprise to use DHCPv6. Having a DHCPv6 server that is integrated into your IP Address Management (IPAM) system for IPv6 gives visibility to the IPv6-enabled client nodes.
You also would want this same functionality for IPv4. As IPv4 address space becomes increasingly constrained, you will want to keep track of your DHCP scopes and determine if your lease time is adequate with the pl

1. 1
Dynamic Host Configuration Protocol
(DHCP)

2. 2
Dynamic Assignment of IP addresses
•Dynamic assignment of IP addresses is desirable for several
reasons:
–IP addresses are assigned on-demand
–Avoid manual IP configuration
–Support mobility of laptops

3. 3
Solutions for dynamic assignment of IP addresses
•Reverse Address Resolution Protocol (RARP)
–Works similar to ARP
–Broadcast a request for the IP address associated
with a given MAC address
–RARP server responds with an IP address
–Only assigns IP address (not the default router and
subnetmask)RARP
Ethernet MAC
address
(48 bit)
ARP
IP address
(32 bit)

4. 4
BOOTP
•BOOTstrap Protocol (BOOTP)
•From 1985
•Host can configure its IP parameters at boot time.
•3 services.
–IP address assignment.
–Detection of the IP address for a serving machine.
–The name of a file to be loaded and executed by the client machine
(boot file name)
–Not only assign IP address, but also default router, network mask, etc.
–Sent as UDP messages (UDP Port 67 (server) and 68 (host))
–Use limited broadcast address (255.255.255.255):
•These addresses are never forwarded

5. 5
Dynamic Host Configuration Protocol
(DHCP)
Relates to Lab 7.
Module about dynamic assignment of IP addresses with DHCP.

6. 6
DHCP
•Dynamic Host Configuration Protocol (DHCP)
–From 1993
–An extension of BOOTP, very similar to DHCP
–Same port numbers as BOOTP
–Extensions:
•Supports temporary allocation (“leases”) of IP addresses
•DHCP client can acquire all IP configuration parameters
needed to operate
–DHCP is the preferred mechanism for dynamic assignment
of IP addresses
–DHCP can interoperate with BOOTP clients.

7. 7
BOOTP Interaction
•BOOTP can be used for
downloading memory
image for diskless
workstations
•Assignment of IP addresses
to hosts is staticArgon
00:a0:24:71:e4:44 BOOTP Server
BOOTP Request
00:a0:24:71:e4:44
Sent to 255.255.255.255 Argon
128.143.137.144
00:a0:24:71:e4:44 DHCP Server
BOOTP Response:
IP address: 128.143.137.144
Server IP address: 128.143.137.100
Boot file name: filename
(a)
(b)
(c)

8. 8
DHCP Interaction (simplified)Argon
128.143.137.144
00:a0:24:71:e4:44 DHCP Server
DHCP Response:
IP address: 128.143.137.144
Default gateway: 128.143.137.1
Netmask: 255.255.0.0

9. 9
BOOTP/DHCP Message FormatNumber of Seconds
OpCode Hardware Type
Your IP address
Unused (in BOOTP)
Flags (in DHCP)
Gateway IP address
Client IP address
Server IP address
Hardware Address
Length
Hop Count
Server host name (64 bytes)
Client hardware address (16 bytes)
Boot file name (128 bytes)
Transaction ID
Options
(There are >100 different options)

10. 10
BOOTP/DHCP
•OpCode: 1 (Request), 2(Reply)
Note: DHCP message type is sent in an option
•Hardware Type: 1 (for Ethernet)
•Hardware address length: 6 (for Ethernet)
•Hop count: set to 0 by client
•Transaction ID: Integer (used to match reply to response)
•Seconds:number of seconds since the client started to boot
•Client IP address, Your IP address, server IP address,
Gateway IP address, client hardware address, server
host name, boot file name:
client fills in the information that it has, leaves rest blank

11. 11
DHCP Message Type
•Message type is sent as an
option.
Value Message Type
1 DHCPDISCOVER
2 DHCPOFFER
3 DHCPREQUEST
4 DHCPDECLINE
5 DHCPACK
6 DHCPNAK
7 DHCPRELEASE
8 DHCPINFORM

12. 12
Other options (selection)
•Other DHCP information that is sent as an option:
Subnet Mask, Name Server, Hostname, Domain Name,
Forward On/Off, Default IP TTL, Broadcast Address, Static
Route, Ethernet Encapsulation, X Window Manager, X
Window Font, DHCP Msg Type, DHCP Renewal Time, DHCP
Rebinding, Time SMTP-Server, SMTP-Server, Client FQDN,
Printer Name, …

13. 13
DHCP Operation
•DCHP DISCOVERDHCP Client
00:a0:24:71:e4:44 DHCP Server
DHCPDISCOVER
Sent to 255.255.255.255
DHCP Server DHCP Client
00:a0:24:71:e4:44 DHCP Server
DHCP Server
DHCPOFFER
DHCPOFFER
•DCHP OFFER

14. 14
DHCP OperationDHCP Client
00:a0:24:71:e4:44 DHCP Server
DHCP Server
DHCPREQUEST
DHCPACK
•DCHP DISCOVER
At this time, the DHCP
client can start to use the IP
addressDHCP Client
00:a0:24:71:e4:44 DHCP Server
DHCP Server
DHCPREQUEST
DHCPACK
•Renewing a Lease
(sent when 50% of lease
has expired)
If DHCP server sends
DHCPNACK, then
address is released.

15. 15
DHCP OperationDHCP Client
00:a0:24:71:e4:44 DHCP Server
DHCP Server
DHCPRELEASE
•DCHP RELEASE
At this time, the DHCP
client has released the IP
address