Tutorial on dhcp

04/13/2023 1

Advanced data communication

Dynamic Host Configuration Proto-col DHCP and Auto configuration

BY

SALAH AMEAN AHMMED

04/13/2023 2

Overview

Introduction

Basics

Case - We play DHCP Server◦ Problems ◦ Tasks◦ Solutions

Autoconfiguration ◦ Basics◦ Design goals ◦ Problems ◦ Solution◦ Example protocols

Discussion◦ Packet tracer demo

Conclusion

Questions and Answers

04/13/2023 3

DHCP Architecture

DHCP contains:◦ DHCP clients◦ DHCP servers◦ DHCP relay agents on a network

clients interact with servers using DHCP messages ◦ To obtain and renew IP address leases.

04/13/2023 4

Configuration need To be able to make use of TCP/IP protocol suite,

◦ Each host,and router need certain amount of configuration information ◦ Configuration information is used to assign local names to systems, and identifiers(IP) to interfaces◦ DNS and mobile IP agents

Many attempts to provides these info

Local broadcast

Outside communication(require routing)

04/13/2023 5

Reasons for such DHCP First , hosts move more likely than routers and servers

◦ Mechanisms are required to assign and reassign configuration for hosts

Second, server are expected to be autonomus in term of users movement◦ More confidentiality since configuration information is not dependant on other network services

Third,since devices are more than routers, ◦ Less error-prone

Fourth, in term of users experience,◦ Simple

04/13/2023 6

DHCP parts Address management:

◦ Handles the allocation of IP address and the lease to clients◦ Handles the address management

Delivery of configuration data:◦ DHCP protocol messages format and state machines

04/13/2023 7

DHCP Allocation Methods Dynamic allocation

◦ given IP address is revocable◦ IP address is given from a Pool (range of IP addresses) at the server

Automatic allocation◦ Same method is used in getting the address◦ Given IP address is never revoked

Manual allocation:◦ The address is allocated based on the user’s request◦ The address could be out of the DHCP allocated pool

Based on the identity of the node and the possibility of revoking the above methods differ

04/13/2023 8

Address pool and lease DHCP client requests IP

DHCP server responses with address from DHCP pool◦ Lease duration defines the time the user can utilize the address◦ When expired, user may request extension ◦ Can be few minutes to days

Lease time is tradeoff between the number of expected clients, the size of the address pool, and the desired stability of addresses

◦ Longer duration of lease, ◦ More stable ◦ But deplete the DHCP pool

Clients begin trying to renew lease tome after half time of the lease has passed

04/13/2023 9

Address pool and lease DHCP Client sends request of IP address

◦ Name of the client◦ Lease duration ◦ Copy of address it is using or last used◦ Other parameters

Server response with the IP address together with the lease duration◦ MAC address◦ Time ◦ Interface

Server maintain the address in non-volatile memory ◦ So configuration of host wont get lost if server shutdown

04/13/2023 10

DHCP & BOOTP Message For-mat

DHCP extends BOOTP

Compatibility is maintained

BOOTP relay agents can process DHCP messages,◦ BOOTP clients can use DHCP servers

04/13/2023 11

Similarities between BOOTP and DHCP

The format structure each uses to exchange messages between the server and clients

BOOTP and DHCP use nearly identical request messages (sent by clients) and reply messages (sent by servers). Messages in either of these protocols use a single User Datagram Protocol (UDP) datagram of 576 bytes to enclose each protocol message. Message headers are the same for both BOOTP and DHCP with one exception: the final message header field used to carry optional data. For BOOTP, this optional field is called the vendor-specific area and is limited to 64 octets. For DHCP, this area is called the options field and can carry up to 312 octets of DHCP options information.

Use of well-known UDP ports for client/server communication

Both BOOTP and DHCP use the same reserved protocol ports for sending and receiving messages between servers and clients. Both BOOTP and DHCP servers use UDP port 67 to listen for and receive client request messages. BOOTP and DHCP clients typically reserve UDP port 68 for accepting message replies from either a BOOTP server or DHCP server.

Because DHCP and BOOTP messages use nearly identical format types and packet structures, and typically use the same well-known service ports, BOOTP or DHCP re -lay agent programs usually treat BOOTP and DHCP messages as essentially the same message type, without differentiating between them.

IP address distribution as an integral part of configuration service

Although both BOOTP and DHCP allocate IP addresses to clients during startup, they use different methods of allocation. BOOTP typically provides fixed allocation of a single IP address for each client, permanently reserving this address in the BOOTP server database. DHCP typically provides dynamic, leased allocation of available IP addresses, reserving each DHCP client address temporarily in the DHCP server database.

04/13/2023 12

BOOTP and DHCP

BOOTP DHCP

Designed prior to DHCP. Designed after BOOTP.

Intended to configure diskless workstations with limited boot capabilities. Intended to configure frequently relocated networked computers (such as portables) that have local hard drives and full boot capabilities.

Dynamic BOOTP has a default 30-day expiration on IP address leases. DHCP has a default eight-day expiration on IP address leases.

Supports a limited number of client configuration parameters calledvendor extensions. Supports a larger and extensible set of client configuration parameters called options.

Describes a two-phase bootstrap configuration process, as follows:•Clients contact BOOTP servers to perform address determination and boot file name selection.

•Clients contact Trivial File Transfer Protocol (TFTP) servers to perform file transfer of their boot image.

Describes a single-phase boot configuration process whereby a DHCP client negotiates with a DHCP server to determine its IP address and obtain any other initial configuration details it needs for network operation.

BOOTP clients do not rebind or renew configuration with the BOOTP server except when the system restarts.

DHCP clients do not require a system restart to rebind or renew configuration with the DHCP server. In-stead, clients automatically enter a rebinding state at set timed intervals to renew their leased address allocation with the DHCP server. This process occurs in the background and is transparent to the user.

04/13/2023 13

Differences Between BOOTP and DHCP BOOTP supports a limited number of client configuration parameters called vendor extensions , while DHCP supports a larger and extensible set of client configuration parameters called options.

BOOTP uses a two-phase bootstrap configuration process in which clients contact BOOTP servers to perform address determination and boot file name selection, and clients contact Trivial File Transfer Protocol (TFTP) servers to perform file transfer of their boot image. DHCP uses a single-phase boot configuration process whereby a DHCP client negotiates with a DHCP server to determine its IP address and obtain any other initial configuration details it needs for network operation.

BOOTP clients do not rebind or renew configuration with the BOOTP server except when the system restarts, while DHCP clients do not require a system restart to re-bind or renew configuration with the DHCP server. Instead, clients automatically en-ter the Rebinding state at set timed intervals to renew their leased address alloca-tion with the DHCP server. This process occurs in the background and is transparent to the user.

04/13/2023 14

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

04/13/2023 15

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.

04/13/2023 16

Message Format

Vendor specific is replaced with options

17

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

04/13/2023 18

Message Types DHCPDISCOVER: Broadcast by a client to find available DHCP servers.

DHCPOFFER: Response from a server to a DHCPDISCOVER and offering IP address and other parameters.

DHCPREQUEST: Message from a client to servers that does one of the following:◦ Requests the parameters offered by one of the servers and declines all other offers.◦ Verifies a previously allocated address after a system or network change (a reboot for

example).◦ Requests the extension of a lease on a particular address.

04/13/2023 19

Contd. DHCPACK: Acknowledgement from server to client with parameters,

including IP address.

DHCPNACK: Negative acknowledgement from server to client, indicating that the client's lease has expired or that a requested IP address is incorrect.

DHCPDECLINE: Message from client to server indicating that the offered address is already in use.

DHCPRELEASE: Message from client to server canceling remainder of a lease and relinquishing network ad-dress.

DHCPINFORM: Message from a client that already has an IP address (manually configured for example), re-questing further configuration parameters from the DHCP server.

04/13/2023 20

DHCP operation Client discover server and address offered by them

Client uses broadcast to request address

The selected server will respond to the client(others will be rejected)

If the client knows the address the process in simplified by ◦ Starting with REQUEST and ACK messages

The offer msg includes the lease upper bound to renew its address◦ T1=t/2◦ T2=7T/8

DHCPREQUEST is broadcast to all servers and only one server will respond

04/13/2023 21

DHCP operation Several servers could be there

Client choose any server

Other servers declined

04/13/2023 22

DHCP state machine DHCP client have few states

◦ INIT: no address yet◦ Selecting: processing address◦ Requesting: after selecting◦ Bound address with T1 and T2

T1 client attempt renewing address

T1 expires client reacquires address from a server

04/13/2023 23

DHCP OperationDHCP Client

00:a0:24:71:e4:44 DHCP Server

DHCPDISCOVERSent to 255.255.255.255

DHCP Server

DCHP DISCOVER

DHCP Client00:a0:24:71:e4:44 DHCP Server

DHCP Server

DHCPOFFER

DHCPOFFER

DCHP OFFER

04/13/2023 24

DHCP OperationDHCP Client

00:a0:24:71:e4:44 DHCP Server

DHCP Server

DHCPREQUEST

DHCPACKDCHP DISCOVER

At this time, the DHCP client can start to use the IP address

DHCP Client00:a0:24:71:e4:44 DHCP Server

DHCP Server

DHCPREQUEST

DHCPACKRenewing a Lease(sent when 50% of lease

has expired)If DHCP server sends DHCPNACK, then ad-dress is released.

04/13/2023 25

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

04/13/2023 26

Client Server Interactions The client broadcasts a DHCPDISCOVER message on its local physical subnet.

◦ The DHCPDISCOVER message may include some options such as network address sugges-tion or lease duration.

Each server may respond with a DHCPOFFER message that includes an available net-work address (your IP address) and other configuration options.

◦ The servers record the address as offered to the client to prevent the same address being of-fered to other clients in the event of further DHCPDISCOVER messages being received before the first client has completed its configuration.

04/13/2023 27

Contd. The client receives one or more DHCPOFFER messages from one or more servers.

◦ The client chooses one based on the configuration parameters offered and broad-casts a DHCPREQUEST message that includes the server identifier option to indicate which message it has selected and the requested IP address option, taken from your IP address in the selected offer.

◦ In the event that no offers are received, if the client has knowledge of a previous net-work address, the client may reuse that address if its lease is still valid, until the lease expires.

04/13/2023 28

Contd. The servers receive the DHCPREQUEST broadcast from the client.

◦ Those servers not selected by the DHCPREQUEST message use the message as notification that the client has declined that server's offer.

◦ The server selected in the DHCPREQUEST message commits the binding for the client to persistent storage and responds with a DHC-PACK message containing the configuration parameters for the re-questing client.

04/13/2023 29

Contd. The combination of client hardware and assigned network address constitute a unique identifier for the client's lease and are used by both the client and server to identify a lease referred to in any DHCP messages.

The your IP address field in the DHCPACK messages is filled in with the selected net-work address.

04/13/2023 30

Contd. The client receives the DHCPACK message with configuration parameters.

◦ The client performs a final check on the parameters, for example with ARP for allo-cated network address, and notes the duration of the lease and the lease identifica-tion cookie specified in the DHCPACK message. At this point, the client is configured.

◦ If the client detects a problem with the parameters in the DHCPACK message (the address is already in use on the network, for example), the client sends a DHCPDE-CLINE message to the server and restarts the configuration process.

04/13/2023 31

Contd. The client should wait a minimum of ten seconds before restarting the configuration process to avoid excessive network traffic in case of looping.

On receipt of a DHCPDECLINE, the server must mark the offered address as unavail-able (and possibly inform the system administrator that there is a configuration problem).

If the client receives a DHCPNAK message, the client restarts the configuration process.

04/13/2023 32

Contd. The client may choose to relinquish its lease on a network address by sending a DHCPRELEASE message to the server.

The client identifies the lease to be released by including its network address and its hardware address.

04/13/2023 33

Lease Renewal When a server sends the DHCPACK to a client with IP address and configuration parameters, it also registers the start of the lease time for that address.

This lease time is passed to the client as one of the options in the DHCPACK message, together with two timer values, T1 and T2.

The client is rightfully entitled to use the given address for the duration of the lease time.

04/13/2023 34

Contd. On applying the receive configuration, the client also starts the timers T1 and T2. At this time, the client is in the BOUND state.

Times T1 and T2 are options configurable by the server but T1 must be less than T2, and T2 must be less than the lease time.

According to RFC 2132, T1 defaults to (0.5 * lease time) and T2 defaults to (0.875 * lease time).

04/13/2023 35

Contd. When timer T1 expires, the client will send a DHCPREQUEST (unicast) to the server that offered the address, asking to extend the lease for the given configuration. The client is now in the RENEWING state

The server would usually respond with a DHCPACK message indicating the new lease time, and timers T1 and T2 are reset at the client accordingly.

The server also resets its record of the lease time.

Under normal circumstances, an active client would continually renew its lease in this way indefinitely, without the lease ever expiring.

04/13/2023 36

Contd. If no DHCPACK is received until timer T2 expires, the client enters the REBINDING state.

Client now broadcasts a DHCPREQUEST message to extend its lease.

This request can be confirmed by a DHCPACK message from any DHCP server on the network.

04/13/2023 37

Contd. If the client does not receive a DHCPACK message after its lease has expired, it has to stop using its current TCP/IP configuration.

The client may then return to the INIT state, issuing a DHCPDISCOVER broadcast to try and obtain any valid address.

04/13/2023 38

Reusing a Previously allocated address

The client broadcasts a DHCPREQUEST message on its local subnet.◦ The DHCPREQUEST message includes the client's previously used network address.

If the client’s lease is still current, the server with knowledge of the client's configuration parameters responds with a DHCPACK message to the client, renewing the lease at the same time.

◦ The client must then proceed to test for the IP address.

If the client's lease has expired, the server with knowledge of the client responds with DHCPNACK.

◦ The client then must initiate a new IP address allocation process.

04/13/2023 39

DHCP Pros It relieves the network administrator of a great deal of manual configuration work.

The ability for a device to be moved from network to network and to automatically obtain valid configuration parameters for the current network can be of great benefit to mobile users.

Because IP addresses are only allocated when clients are actually active, it is possible, by the use of reasonably short lease times and the fact that mobile clients do not need to be allocated more than one address, to reduce the total number of addresses in use in an organization.

04/13/2023 40

Ipv6 auto configuration

IPv6 supports the following types of auto-configuration:

Stateful auto-configuration.

This type of configuration requires a certain level of human intervention because it needs a Dynamic Host Con-figuration Protocol for IPv6 (DHCPv6) server for the installation and administration of the nodes. The DHCPv6 server keeps a list of nodes to which it supplies configuration information. It also maintains state information so the server knows how long each address is in use, and when it might be available for reassignment.

Stateless auto-configuration. This type of configuration is suitable for small organizations and individuals. In this case, each host determines its addresses from the contents of received router advertisements. Using the IEEE EUI-64 standard to define the network ID portion of the address, it is reasonable to assume the unique-ness of the host address on the link.

Regardless of how the address is determined, the node must verify that its potential address is unique to the local link. This is done by sending a neighbor solicitation message to the potential address. If the node receives any response, it knows that the address is already in use and must determine another address.

04/13/2023 41

Attacks on DHCP An Authorised users/devices connection to the network

◦ DOS :◦ Involve resource exhaustion of netwok resources◦ Requesting all the IP addresses in the server’s pocket

◦ So when the authorised user comes ◦ «Sold out»

IP was designed based on trust

WPA2

04/13/2023 42

Supplement-Packet tracer con-figuration

en

conf t

host name R1

inte fa0/0

ip address 192.168.10.1 255.255.255.0

no shutdown

exit

conf t

ip dhcp pool ip10

net 192.168.10.0 255.255.255.0

default 192.168.10.1

exit

ip dhcp execlude 192.168.10.1 192.168.10.10

exit

copy run star

04/13/2023 43

supplement Best practice

04/13/2023 44

04/13/2023 45

With DHCP not enabled Assuming that no DHCP is enabled,

◦ Request from any hosts will result in failure

04/13/2023 46

Binding addresses

04/13/2023 47

DHCP provided address an d Leases

04/13/2023 48

DHCPDISCOVER Client is trying to re-request address

04/13/2023 49

DHCP DISCOVER This Message contains rich info

◦ Parameters ◦ Configuration the client seeks◦ Auto configuration enquiry

04/13/2023 50

DHCPNAK DHCP server address 10.0.0.1

Server rejects the request for address◦ Request IP address: 172.16.1.34◦ The client should not attempt to request for it

Transaction ID uniquify client’s request◦ So the response reaches the specific client

04/13/2023 51

DHCP Request Client switched networks and

◦ Attempts to request its old address◦ It has switched networks◦ 172.16.1.34 is old address

Depends on server’s response

04/13/2023 52

DHCP OFFER DHCP offer is sent from server 10.0.0.1

◦ Offering IP 10.0.0.57 for up to 12 hours

04/13/2023 53

References[1] Fall, K.R. and Stevens, W.R..(2011). TCP/IP Illustrated, Volume 1: The Protocols. ddison-Wesley Professional Computing Series, Pearson Education. http://book -s.google.co.kr/books?id=a23OAn5i8R0C

[2](n. d.). BOOTP and DHCP. Microsoft Technet. accessed on 10/03/2014 from http://technet.microsoft.com/en-us/library/cc781243(v=ws.10).aspx

[3] Droms, R.( March,1997). “Dynamic Host Configuration Protocol ”, RFC 2131.

[4] dhcpcd.http://roy.marples.name/man/html8/dhcpcd.html

Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.

04/13/2023 54

Address Polling/defense Mecha-nism

Mechanism for discovery of addresses that are already in use.

ARP is used by the client to check the uniqueness of it provided address◦ Avoid duplicate address

04/13/2023 55

DHCP supports mechanisms

automatic allocation", DHCP assigns a permanent IP address to a

client. In "dynamic allocation", DHCP assigns an IP address to a

client for a limited period of time (or until the client explicitly

relinquishes the address). In "manual allocation", a client's IP

address is assigned by the network administrator, and DHCP is used

simply to convey the assigned address to the client. A particular

network will use one or more of these mechanisms, depending on the

policies of the network administrator.

Dynamic allocation is theonly one the three mechanisms that allows reuse of an address when this address in no longer needed by the client to which it was as-signed

Dynamic allocation is the only one of the three mechanisms that allows automatic reuse of an address that is no longer needed by the client to which it was assigned

04/13/2023 56

DHCP support mechanism Dynamic allocation

◦ Allows the reassignment of addresses if they are no longer used

Automatic allocation

Manual allocation

04/13/2023 57

DHCP supports mechanisms

dynamic allocation is particularly useful for assigning an address to a client that will be

connected to the network only temporarily or for sharing a limited

pool of IP addresses among a group of clients that do not need

permanent IP addresses. Dynamic allocation may also be a good choice

for assigning an IP address to a new client being permanently

connected to a network where IP addresses are sufficiently scarce

that it is important to reclaim them when old clients are retired.

04/13/2023 58

The manual configuration sometimes for some reason is neede

Manual allocation allows DHCP to be used to eliminate the error-prone process of manually configuring hosts with IP addresses in environments where (for whatever reasons) it is desirable to manage IP address assignment outside of the DHCP mechanisms.

04/13/2023 59

The format of DHCP messages is based on the format of BOOTP messages,

to capture the BOOTP relay agent behavior described as part of the

BOOTP specification [7, 21] and to allow interoperability of existing

BOOTP clients with DHCP servers. Using BOOTP relay agents eliminates

the necessity of having a DHCP server on each physical network

segment.

04/13/2023 60

Life capture

DHCP packet https://www.cloudshark.org/captures/0009d5398f37

https://www.cloudshark.org/captures/000731192129

https://www.cloudshark.org/analysis/000731192129/conversations?proto=udp&window=false

Lease query

https://www.cloudshark.org/captures/0004fd9466b7

04/13/2023 61

BOOTP and RARP Client and server

RARP can solve the IP address. Why do we need BOOTP?

The RARP client and server must be in the same network.The BOOTP client and server can be in the different networks.

Client and Server in the same network

Destination IP addressSource IP address

Port number

04/13/2023 62

Client and server in the same network

Operations of BOOTP for client and server in the same network:1. The BOOTP server issues a passive open command on UDP port number

67 and waits for a client. 2. A booted client issues an active open command on port number 68. The

message is encapsulated in a UDP user datagram, using the destination and source port number 67 and 68. The UDP user datagram is encapsulatedin an IP datagram. The client uses all 0s as the source IP address and all 1s as the destination IP address.

3. The server responds with either a broadcast or a unicast messageusing a UDP source and destination port numbers 67 and 68.

04/13/2023 63

Client and server on two different networks

An IP address with all 1s is broadcast within a network. A host or a router needs to be configured as a relay agent to relay the message to other networks.The relay agent knows the unicast address of the BOOTP server. When therelay agent receives a broadcast request message, it sends the message tothe BOOTP server and send the reply back when it gets the replay message from the server.

04/13/2023 64

BOOTP packet format

Operation code definesthe BOOTP packet type:(1) request(2) reply

04/13/2023 65

DHCP for IPv6 DHCPv6

04/13/2023 66

Link-local address

04/13/2023 67

Zero-configuration networking

Bonjour

UPnP

04/13/2023 68

DHCP in linux dhcpcd is an implementation of the DHCP client.

dhcpcd gets the host information (IP address, routes, etc) from a DHCP server and configures the network interface of the machine on which it is running

04/13/2023

Dynamic Host Configuration Protocol (DHCP)

69

04/13/2023 70

Dynamic Assignment of IP ad-dresses

Dynamic assignment of IP addresses is desirable for several reasons:◦ IP addresses are assigned on-demand◦ Avoid manual IP configuration◦ Support mobility of laptops

71

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 MACaddress(48 bit)

ARPIP address(32 bit)