Learn about Bluetooth Technology, Slides of Mobile Computing

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DYNAMIC HOST CONFIGURATION PROTOCOL (DHCP) Introduction ◼ Dynamic Host Configuration Protocol (DHCP) is a protocol used by networked devices (clients) to obtain various parameters necessary for the them to operate in an Internet Protocol (IP) network. ◼ By using this protocol, system administration workload greatly decreases, and devices can be added to the network with minimal or no manual configurations. ◼ It automates the assignment of IP addresses, subnet masks, default gateway, and other IP parameters to clients that do not require permanent IP address ◼ The characteristics of DHCP includes ❑ Centralized IP address administration ❑ Backward compatibility with BOOTP ❑ Supports multiple servers ❑ Provides dynamic address assignment ❑ Allows static address assignment ❑ Does not interact with domain name service Basic DHCP configuration Server Client Client Relay DHCPDISCOVER DHCPDISCOVER DHCP Databases ◼ DHCP servers use two databases ❑ One database acquires IP address manually (statically) and binds them permanently to hardware addresses ❑ The second database use a block of temporary IP addresses ( address pool ) that are dynamically ( automatically) assigned to clients on first-come, first-serve basis ❑ When a DHCP client sends a request to a DHCP server for a temporary IP address, the server first checks its static database. If it contains an entry that permanently binds an IP address to the hardware address of the requesting client, the server simply returns the permanent IP address to the client. If the static address does not contain a permanent assignment, the server assigns the client an IP address from the pool of available temporary IP addresses for a specified length of time (lease) Address Acquisition Process ◼ Client broadcasts a DHCPDISCOVER message into the subnet ◼ Servers reply to the client ’ s request with DHCPOFFER and offer a list of configuration parameters ◼ Client accepts one configuration and rejects others using DHCPREQUEST ◼ Server with rejected DHCPREQUEST free the reserved configuration for other possible clients ◼ Server with accepted configuration confirms the configuration with DHCPACK ◼ This completes initialization phase Address Acquisition Process : States and Transitions ◼ Initialization state to selection state: While a client is initially booting up, it enters the first state, which is the initialization state. While in the initialization state, the client broadcasts a DHCP Discover message onto the network that places the client in the selection state. ◼ Selection state to request state: One or more of the DHCP servers on the network responds to the broadcast with a DHCP Offer message. The requesting client may receive more than one response. ◼ The client chooses one of the responses and negotiates with the respective server for an address lease by sending a DHCP Request message, which puts the client in the request state. ◼ Request state to bound state: The server responds to the request message with a positive acknowledgment (DHCP ACK), which begins the lease time and puts the client in the bound state. The client will remain in the bound state until the lease expires or the client releases the IP address. Address Renewal Process : States and Transitions ◼ When a client’s lease near its expiration time, the client must initiate an address renewal procedure if it wishes to maintain an IP address and continue communicating over the network ◼ While in bound state, it sets three timers relating to lease renewal and records the time the address was received. ◼ The DHCP server determines explicit values for the timers when it assigns an IP address to a client Address Renewal Process : States and Transitions Initialization state Rebind state Bound state Renew state Lease reaches 50% of expiration, DHCP Request DHCP ACK DHCP NAK Lease reaches 87.5% of expiration, DHCP Request DHCP ACK DHCP NAK or Lease expires DHCP Release Address Renewal Process : States and Transitions ◼ When a client is in the normal state of operation (i.e. bound state), and determines that it no longer needs an IP address, DHCP allows the client to release its current IP addresses without waiting for the lease time to expire ◼ This is called as early termination and is useful when there are limited number of IP addresses available ◼ Early termination allow servers to provide IP addresses to more clients than it has IP addresses SECURITY ◼ Due to its standardization before Internet security became an issue, the basic DHCP protocol does not include any security provisions, potentially exposing it to two types of attacks: ❑ Unauthorized DHCP Servers ❑ Unauthorized DHCP Clients Unauthorized DHCP Servers ◼ A client cannot specify the server it wants; hence an unauthorized server can respond to client requests sending the client network configuration values that are beneficial to a hijacker. ◼ As an example, a hacker can configure the DHCP server to configure clients to a DNS server which has been poisoned.