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PC Encyclo \ D
DHCP by Chuck Bryan
Aug 07 2011
Network Aug 07 2011
DHCP
Dynamic Host Control Protocol: Automatically Assigning IP Addresses
Dynamic Host Control Protocol ( DHCP ) is a system that manages the dynamic allocation and
assignment of IP Addresses.
Note: This is an important first step whether on a big PC , wired ( network cable ) network or using a
small iPod Touch on a wireless wi-Fi network!
Example: My new iPod Touch uses Wi-Fi for wireless network access but I still
need an IP Address, and that is where DHCP comes in, to automatically get me
one from my wireless router!
DHCP Server built-in to Router: Home network
When it comes to your average DHCP Server for a Home Newtork, it typically is simply built-in to
Router:
Network Aug 07 2011
DHCP LAN Addressing 192.168.x.x
LAN Addressing 192.168xx scheme:
If you have a LAN with a Router, you probably have DHCP LAN Addressing for any new PC to be
plugged into the network.
This is characterized by the “standard” 192.168.x.x LAN addressing scheme
( as opossed to the APIPA ( Automatic Private IP Addressing ) scheme characterized by the “standard”
169.254.x.x APIPA Addressing scheme )
APIPA Addressing: 169.254.x.x( Automatic Private IP Addressing )
Network Crossover Cable and IPCONFIG test:
The two are easily checked on a Home Network, with the use of an RJ-45 Network Crossover Cable
connected between two PCs and doing an “ipconfig” at the Command Prompt window. Follow that
by reconnecting the network cable and doing the same “ipconfig” command for the new address. You
will notice the address changing.
If nothing else, it’s a good exercise when discussing IP Addresses, addressing schemes, and Home
Network\LANs!
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External DHCP Server:
Example:When you connect thru ADSL to your ISP, there is a DHCP Server
involved that assigns you an IP Address.
Network Aug 07 2011
DHCP Broadcast Query
Broadcast Query to DHCP Server:
When a DHCP-configured client ( PC or network-aware device ) connects to a network, the DHCP
Client sends a Broadcast query requesting information from a DHCP Server.
DHCP uses two ports: Check Firewall ports!
67/udp for the server side
68/udp for the client(PC) side
This could be important in case of firewall settings.
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The DHCP Server manges a pool of IP Addresses as well as the default gateway, DNS Servers and so
on.
DHCP Server Response:
Upon receipt of a valid request, a DHCP Server will assign the PC a:
IP Address
Lease (time duration)
Subnet mask
Default Gateway
Network Aug 07 2011
DHCP Hacker Attack
DHCP Hacker Attacks:
Thus far, Aug 2011, the basic DHCP protocol includes no security features. That’s good news for
hackers and it translates into three basic vulnerabilities:
Unauthorized DHCP Servers: Hijacking the DHCP process can configure the “client”
victim to use a malicious DNS Server ( DNS Cache Poisoning )
Unauthorized DHCP Client: By masquerading as a legitimate client, an unauthorized
client can gain access to “network configuraytion”, right up to getting an IP Address on
the network!
DHCP DOS Attack: Flood the DHCP Server with requests for IP Addresses!
Network Aug 07 2011
DHCP iPod Touch Wi-Fi
DHCP used in iPod Touch : Wi-Fi Network under IP Address > DHCP:
Example: This lets my iPod Touch acce4ss my HomeW network so Safari can
access Google and give me the day’s news on my iPod Touch
Network Aug 07 2011
DHCP Port usage
DHCP uses two ports:
DHCP uses two ports:
67/udp for the server side
68/udp for the client(PC) side
This could be important in case of firweall settings.
DHCP was initially defined in October 1993, and updated in 1997 as the current DHCP definition for
IPv4 networks.
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DHCP Apr 18 2009
DHCP Registry Entry
Your DHCP is entered in the Windows Registry:
HKLM\System\CurrentControlSet\Services\VxD\DHCP
Windows XP Apr 18 2009
DHCP Domain
DhcpDomain in the Registry:
As shown below, your DHCP Domain ( workgroup ) is stored in the Windows Registry as well as the
DHCP Name Server IP Address ( 192.168.2.1 ) in the example shown below:
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Network Aug 07 2011
DHCP Wireless
DHCP and Mac’s Wireless Access:
Even Mac’s use DHCP! Surprise. If you are a typical Mac user, you may have a lapptop, using Airport (
Mac’s MacBook Pro built-in Wireless Network Access.)
Mac’s Network Window:
The Network window lets you view the Mac’s basic network settings. As shown below, the Mac’s use
TCP/IP just like PC!
Mac’s TCP-IP Settings:
Fortunately, Mac’s can connect to other than Apple White Boxes! I did it with the MacBook Pro and
my “Windows” Home Network.
Mac and PC : Using WINS:
Mac’s can talk with PC and even can have a PC “NetBIOS Name”. Talk about playing nicely in the
sandbox!
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