How to check software installation path from Registry

Go To registry by regedit command and check the below path for the same. :


HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall

SAP not working

Copy services file from the running systems to the below path from the same path.

File name : services

C:\WINDOWS\system32\drivers\etc\

for Login entry copy sapmsg.ini and saplogon.ini file from the running system from below path.

C:\windows\sapmsg.ini
C:\windows\saplogon.ini

How to create outlook icon on Desktop

Save below content in TXT file and rename the extention .reg, eg. outlook.txt ----->  outlook.reg

double click on this registry file with amdin login. Outlook icon will create.

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Desktop\NameSpace\{00020D75-0000-0000-C000-000000000046}]

VMware Advantages and Disadvantages

Advantages of using VMware for your Test/Dev Environment
• Cost effective use of hardware
• Large portions of your production environment can be replicated
on a few servers
• Lower cost of hardware for the entire test environment
• Faster rollback during testing
• Faster deployment of a new test platform
• Test VMs can be decommissioned and even deleted after they
are not needed
Disadvantages of using VMware for your Test/Dev Environment
• Requires that your staff have (or learn) some basic VMware
skills
• VMs are not good for load testing if your production environment
is completely physical
As you can see, the advantages of using VMware in your development
or test environment easily outweigh the disadvantages. On top
of that, the second disadvantage can be completely mitigate by simply
keeping an few pieces of hardware for load testing if you plan on
maintaining physical production servers.

What is VMware

VMware, Inc. is a company providing virtualization software founded in 1998 and based in Palo Alto, California, USA. It is majorily owned by EMC Corporation.
VMware's desktop software runs on Microsoft Windows, Linux, and Mac OS X, while VMware's enterprise software hypervisors for servers, VMware ESX and VMware ESXi, are bare-metal embedded hypervisors that run directly on server hardware without requiring an additional underlying operating system


VMware software provides a completely virtualized set of hardware to the guest operating system^. VMware software virtualizes the hardware for a video adapter, a network adapter, and hard disk adapters. The host provides pass-through drivers for guest USB, serial, and parallel devices. In this way, VMware virtual machines become highly portable between computers, because every host looks nearly identical to the guest. In practice, a system administrator can pause operations on a virtual machine guest, move or copy that guest to another physical computer, and there resume execution exactly at the point of suspension. Alternatively, for enterprise servers, a feature called VMotion allows the migration of operational guest virtual machines between similar but separate hardware hosts sharing the same storage. Each of these transitions is completely transparent to any users on the virtual machine at the time it is being migrated.
VMware Workstation, Server, and ESX take a more optimized path to running target operating systems on the host than emulators (such as Bochs) which simulate the function of each CPU instruction on the target machine one-by-one, or dynamic recompilation which compiles blocks of machine-instructions the first time they execute, and then uses the translated code directly when the code runs subsequently (Microsoft Virtual PC for Mac OS X takes this approach.) VMware software does not emulate an instruction set for different hardware not physically present. This significantly boosts performance^, but can cause problems when moving virtual machine guests between hardware hosts using different instruction-sets (such as found in 64-bit Intel and AMD CPUs), or between hardware hosts with a differing number of CPUs. Stopping the virtual-machine guest before moving it to a different CPU type generally causes no issues.
VMware's products predate the virtualization extensions to the x86 instruction set, and do not require virtualization-enabled processors. On such older processors, they use the CPU to run code directly whenever possible (as, for example, when running user-mode and virtual 8086 mode code on x86). When direct execution cannot operate, such as with kernel-level and real-mode code, VMware products re-write the code dynamically, a process VMware calls "binary translation" or BT. The translated code gets stored in spare memory, typically at the end of the address space, which segmentation mechanisms can protect and make invisible. For these reasons, VMware operates dramatically faster than emulators, running at more than 80% of the speed that the virtual guest operating-system would run directly on the same hardware. In one study VMware claims a slowdown over native ranging from 0–6 percent for the VMware ESX Server.
VMware's approach avoids some of the difficulties of virtualization on x86-based platforms. Virtual machines may deal with offending instructions by replacing them, or by simply running kernel-code in user-mode. Replacing instructions runs the risk that the code may fail to find the expected content if it reads itself; one cannot protect code against reading while allowing normal execution, and replacing in-place becomes complicated. Running the code unmodified in user-mode will also fail, as most instructions which just read the machine-state do not cause an exception and will betray the real state of the program, and certain instructions silently change behavior in user-mode. One must always rewrite; performing a simulation of the current program counter in the original location when necessary and (notably) remapping hardware code breakpoints.
Although VMware virtual machines run in user-mode, VMware Workstation itself requires the installation of various drivers in the host operating-system, notably to dynamically switch the Global Descriptor Table (GDT) and the Interrupt Descriptor Table (IDT).
The VMware product line can also run different operating systems on a dual-boot system simultaneously by booting one partition natively while using the other as a guest within VMware Workstation.

Products

[VMware vSphere, a holistic view]

vCenter Server ($) (license manager)	Server Hardware	ESX ($) (vMotion, DRS, HA, Storage vMotion)			Guest OS Guest OS Guest OS...
		ESXi (freeware) (ESXi freeware is managed by the Virtual Infrastructure (or vSphere) Client) ESXi ($) (vMotion, DRS, HA, Storage vMotion)			Guest OS Guest OS Guest OS...
Workstation Hardware		Windows or Linux OS	VMware Server (freeware)		Guest OS Guest OS Guest OS...
			User Session	VMware Workstation ($) VMware Player (freeware)
				vSphere Client for managing ESX(i) hosts (freeware)

For more details on this topic, see List of VMware software.

Desktop software

• VMware Workstation (first product launched by VMware in 1999). This software suite allows users to run multiple instances of x86 or x86-64 -compatible operating systems on a single physical PC.
• VMware Fusion provides similar functionality for users of the Intel Mac platform, along with full compatibility with virtual machines created by other VMware products.
• VMware Player For users without a license to use VMware Workstation or VMware Fusion, VMware offers this software as freeware product for personal use. While initially not able to create virtual machines, this limitation was removed in version 3.0.1

Server software

VMware markets two virtualization products for servers:

• VMware ESX (formerly called "ESX Server"), an enterprise-level product, can deliver greater performance than the freeware VMware Server, due to lower system overhead. VMware ESX is a "bare-metal" product, running directly on the server hardware, allowing virtual servers to also use hardware more or less directly. In addition, VMware ESX integrates into VMware vCenter, which offers extra services to enhance the reliability and manageability of a server deployment, such as
  • VMotion - the capability to move a running virtual machine from one ESX host to another and faster than some other editions
  • Storage VMotion - the capability to move a running virtual machine from one storage device to another
  • DRS - Distributed Resource Scheduler - automatic load balancing of a ESX cluster using VMotion
  • HA - High Availability - In case of hardware failure in a cluster, the virtual servers will automatically restart on another host in the cluster

• VMware ESXi (formerly called "VMware ESX 3i"), is quite similar to ESX, but differentiates in that the Service Console is removed, and replaced with a minimal BusyBox installation. Disk space requirements are much lower than for ESX and the memory footprint is reduced. ESXi is intended to be run from flash disks in servers but can be run from normal disks. VMware ESXi hosts can't be managed directly from the console, all management is performed through a VirtualCenter Server.In July 2008, VMware decided to give away ESXi for free.

• VMware Server (formerly called "GSX Server", now both obsolete) is also provided as freeware for non-commercial use, like VMware Player, and it is also possible to create virtual machines with it. It is a "hosted" application, which runs within an existing Linux or Windows operating system.

Run Commands Listed

Run Commands Listed below In Alphabetical Order

Program	Run Command
Accessibility Controls	access.cpl
Accessibility Wizard	accwiz
Add Hardware Wizard	hdwwiz.cpl
Add/Remove Programs	appwiz.cpl
Administrative Tools	control admintools
Adobe Acrobat ( if installed )	acrobat
Adobe Distiller ( if installed )	acrodist
Adobe ImageReady ( if installed )	imageready
Adobe Photoshop ( if installed )	photoshop
Automatic Updates	wuaucpl.cpl
Basic Media Player	mplay32
Bluetooth Transfer Wizard	fsquirt
Calculator	calc
Ccleaner ( if installed )	ccleaner
C: Drive	c:
Certificate Manager	cdrtmgr.msc
Character Map	charmap
Check Disk Utility	chkdsk
Clipboard Viewer	clipbrd
Command Prompt	cmd
Command Prompt	command
Component Services	dcomcnfg
Computer Management	compmgmt.msc
Compare Files	comp
Control Panel	control
Create a shared folder Wizard	shrpubw
Date and Time Properties	timedate.cpl
DDE Shares	ddeshare
Device Manager	devmgmt.msc
Direct X Control Panel ( if installed )	directx.cpl
Direct X Troubleshooter	dxdiag
Disk Cleanup Utility	cleanmgr
Disk Defragment	dfrg.msc
Disk Partition Manager	diskmgmt.msc
Display Properties	control desktop
Display Properties	desk.cpl
Display Properties (w/Appearance Tab Preselected )	control color
Dr. Watson System Troubleshooting Utility	drwtsn32
Driver Verifier Utility	verifier
Ethereal ( if installed )	ethereal
Event Viewer	eventvwr.msc
Files and Settings Transfer Tool	migwiz
File Signature Verification Tool	sigverif
Findfast	findfast.cpl
Firefox	firefox
Folders Properties	control folders
Fonts	fonts
Fonts Folder	fonts
Free Cell Card Game	freecell
Game Controllers	joy.cpl
Group Policy Editor ( xp pro )	gpedit.msc
Hearts Card Game	mshearts
Help and Support	helpctr
Hyperterminal	hypertrm
Hotline Client	hotlineclient
Iexpress Wizard	iexpress
Indexing Service	ciadv.msc
Internet Connection Wizard	icwonn1
Internet Properties	inetcpl.cpl
Internet Setup Wizard	inetwiz
IP Configuration (Display Connection Configuration)	ipconfig /all
IP Configuration (Display DNS Cache Contents)	ipconfig /displaydns
IP Configuration (Delete DNS Cache Contents)	ipconfig /flushdns
IP Configuration (Release All Connections)	ipconfig /release
IP Configuration (Renew All Connections)	ipconfig /renew
IP Configuration (Refreshes DHCP & Re-Registers DNS)	ipconfig /registerdns
IP Configuration (Display DHCP Class ID)	ipconfig /showclassid
IP Configuration (Modifies DHCP Class ID)	ipconfig /setclassid
Java Control Panel ( if installed )	jpicpl32.cpl
Java Control Panel ( if installed )	javaws
Keyboard Properties	control keyboard
Local Security Settings	secpol.msc
Local Users and Groups	lusrmgr.msc
Logs You Out of Windows	logoff
Malicious Software Removal Tool	mrt
Microsoft Access ( if installed )	access.cpl
Microsoft Chat	winchat
Microsoft Excel ( if installed )	excel
Microsoft Diskpart	diskpart
Microsoft Frontpage ( if installed )	frontpg
Microsoft Movie Maker	moviemk
Microsoft Management Console	mmc
Microsoft Narrator	narrator
Microsoft Paint	mspaint
Microsoft Powerpoint	powerpnt
Microsoft Word ( if installed )	winword
Microsoft Syncronization Tool	mobsync
Minesweeper Game	winmine
Mouse Properties	control mouse
Mouse Properties	main.cpl
MS-Dos Editor	edit
MS-Dos FTP	ftp
Nero ( if installed )	nero
Netmeeting	conf
Network Connections	control netconnections
Network Connections	ncpa.cpl
Network Setup Wizard	netsetup.cpl
Notepad	notepad
Nview Desktop Manager ( if installed )	nvtuicpl.cpl
Object Packager	packager
ODBC Data Source Administrator	odbccp32
ODBC Data Source Administrator	odbccp32.cpl
On Screen Keyboard	osk
Opens AC3 Filter ( if installed )	ac3filter.cpl
Outlook Express	msimn
Paint	pbrush
Password Properties	password.cpl
Performance Monitor	perfmon.msc
Performance Monitor	perfmon
Phone and Modem Options	telephon.cpl
Phone Dialer	dialer
Pinball Game	pinball
Power Configuration	powercfg.cpl
Printers and Faxes	control printers
Printers Folder	printers
Private Characters Editor	eudcedit
Quicktime ( if installed )	quicktime.cpl
Quicktime Player ( if installed )	quicktimeplayer
Real Player ( if installed )	realplay
Regional Settings	intl.cpl
Registry Editor	regedit
Registry Editor	regedit32
Remote Access Phonebook	rasphone
Remote Desktop	mstsc
Removable Storage	ntmsmgr.msc
Removable Storage Operator Requests	ntmsoprq.msc
Resultant Set of Policy ( xp pro )	rsop.msc
Scanners and Cameras	sticpl.cpl
Scheduled Tasks	control schedtasks
Security Center	wscui.cpl
Services	services.msc
Shared Folders	fsmgmt.msc
Sharing Session	rtcshare
Shuts Down Windows	shutdown
Sounds Recorder	sndrec32
Sounds and Audio	mmsys.cpl
Spider Solitare Card Game	spider
SQL Client Configuration	clicongf
System Configuration Editor	sysedit
System Configuration Utility	msconfig
System File Checker Utility ( Scan Immediately )	sfc /scannow
System File Checker Utility ( Scan Once At Next Boot )	sfc /scanonce
System File Checker Utility ( Scan On Every Boot )	sfc /scanboot
System File Checker Utility ( Return to Default Settings)	sfc /revert
System File Checker Utility ( Purge File Cache )	sfc /purgecache
System File Checker Utility ( Set Cache Size to Size x )	sfc /cachesize=x
System Information	msinfo32
System Properties	sysdm.cpl
Task Manager	taskmgr
TCP Tester	tcptest
Telnet Client	telnet
Tweak UI ( if installed )	tweakui
User Account Management	nusrmgr.cpl
Utility Manager	utilman
Volume Serial Number for C:	label
Volume Control	sndvol32
Windows Address Book	wab
Windows Address Book Import Utility	wabmig
Windows Backup Utility ( if installed )	ntbackup
Windows Explorer	explorer
Windows Firewall	firewall.cpl
Windows Installer Details	msiexec
Windows Magnifier	magnify
Windows Management Infrastructure	wmimgmt.msc
Windows Media Player	wmplayer
Windows Messenger	msnsgs
Windows Picture Import Wizard (Need camera connected)	wiaacmgr
Windows System Security Tool	syskey
Windows Script host settings	wscript
Widnows Update Launches	wupdmgr
Windows Version ( shows your windows version )	winver
Windows XP Tour Wizard	tourstart
Wordpad	write
Zoom Utility	igfxzoom

Configure TCP/IP from the Command Prompt

Configure TCP/IP from the Command Prompt

In order to configure TCP/IP settings such as the IP address, Subnet Mask, Default Gateway, DNS and WINS addresses and many other options you can use Netsh.exe.

Netsh.exe is a command-line scripting utility that allows you to, either locally or remotely, display or modify the network configuration of a computer that is currently running. Netsh.exe also provides a scripting feature that allows you to run a group of commands in batch mode against a specified computer. Netsh.exe can also save a configuration script in a text file for archival purposes or to help you configure other servers.

Netsh.exe is available on Windows 2000, Windows XP and Windows Server 2003.

You can use the Netsh.exe tool to perform the following tasks:

• Configure interfaces
• Configure routing protocols
• Configure filters
• Configure routes
• Configure remote access behavior for Windows-based remote access routers that are running the Routing and Remote Access Server (RRAS) Service
• Display the configuration of a currently running router on any computer
• Use the scripting feature to run a collection of commands in batch mode against a specified router.

What can we do with Netsh.exe?

With Netsh.exe you can easily view your TCP/IP settings. Type the following command in a Command Prompt window (CMD.EXE):

netsh interface ip show config

With Netsh.exe, you can easily configure your computer's IP address and other TCP/IP related settings. For example:

The following command configures the interface named Local Area Connection with the static IP address 192.168.0.100, the subnet mask of 255.255.255.0, and a default gateway of 192.168.0.1:

netsh interface ip set address name="Local Area Connection" static 192.168.0.100 255.255.255.0 192.168.0.1 1

(The above line is one long line, copy paste it as one line)

Netsh.exe can be also useful in certain scenarios such as when you have a portable computer that needs to be relocated between 2 or more office locations, while still maintaining a specific and static IP address configuration. With Netsh.exe, you can easily save and restore the appropriate network configuration.

First, connect your portable computer to location #1, and then manually configure the required settings (such as the IP address, Subnet Mask, Default Gateway, DNS and WINS addresses).

Now, you need to export your current IP settings to a text file. Use the following command:

netsh -c interface dump > c:'location1.txt

When you reach location #2, do the same thing, only keep the new settings to a different file:

netsh -c interface dump > c:'location2.txt

You can go on with any other location you may need, but we'll keep it simple and only use 2 examples.

Now, whenever you need to quickly import your IP settings and change them between location #1 and location #2, just enter the following command in a Command Prompt window (CMD.EXE):

netsh -f c:'location1.txt

or

netsh -f c:'location2.txt

and so on.

You can also use the global EXEC switch instead of -F:

netsh exec c:'location2.txt

Netsh.exe can also be used to configure your NIC to automatically obtain an IP address from a DHCP server:

netsh interface ip set address "Local Area Connection" dhcp

Would you like to configure DNS and WINS addresses from the Command Prompt? You can. See this example for DNS:

netsh interface ip set dns "Local Area Connection" static 192.168.0.200

and this one for WINS:

netsh interface ip set wins "Local Area Connection" static 192.168.0.200

Or, if you want, you can configure your NIC to dynamically obtain it's DNS settings:

netsh interface ip set dns "Local Area Connection" dhcp

BTW, if you want to set a primary and secondary DNS address, add index=1 and index=2 respectively to the lines of Netsh command.

As you now see, Netsh.exe has many features you might find useful, and that goes beyond saying even without looking into the other valuable options that exist in the command.

Preboot Execution Environment (PXE)

The Preboot Execution Environment (PXE) is an industry standard client/server interface that allows networked computers that are not yet loaded with an operating system to be configured and booted remotely by an administrator. The PXE code is typically delivered with a new computer on a read-only memory chip or boot disk that allows the computer (a client) to communicate with the network server so that the client machine can be remotely configured and its operating system can be remotely booted. PXE provides three things:
1) The Dynamic Host Configuration Protocol (DHCP), which allows the client to receive an IP address to gain access to the network servers.
2) A set of application program interfaces (API) that are used by the client's Basic Input/Output Operating System (BIOS) or a downloaded Network Bootstrap Program (NBP) that automates the booting of the operating system and other configuration steps.
3) A standard method of initializing the PXE code in the PXE ROM chip or boot disk.
The PXE process consists of the client notifying the server that it uses PXE. If the server uses PXE, it sends the client a list of boot servers that contain the operating systems available. The client finds the boot server it needs and receives the name of the file to download. The client then downloads the file using Trivial File Transfer Protocol (Trivia File Transfer Protocol) and executes it, which loads the operating system. If a client is equipped with PXE and the server is not, the server ignores the PXE code preventing disruption in the DHCP and Bootstrap Protocol (BP) operations.
The advantages of using PXE include:

• The client machine does not necessarily need an operating system or even a hard disk.
• The client machine can be rebooted in the event of hardware or software failure. This allows the administrator to diagnose and perhaps fix the problem.
• Since PXE is vendor-independent, new types of computers can easily be added to the network.

What is DNS ?

What is DNS ?

Domain Name System (DNS) is the default name resolution service used in a Microsoft Windows Server 2003 network. DNS is part of the Windows Server 2003 TCP/IP protocol suite and all TCP/IP network connections are, by default, configured with the IP address of at least one DNS server in order to perform name resolution on the network. Windows Server 2003 components that require name resolution will attempt to use this DNS server before attempting to use the previous default Windows name resolution service, Windows Internet Name Service (WINS).
Typically, Windows Server 2003 DNS is deployed in support of Active Directory directory service. In this environment, DNS namespaces mirror the Active Directory forests and domains used by an organization. Network hosts and services are configured with DNS names so that they can be located in the network, and they are also configured with DNS servers that resolve the names of Active Directory domain controllers.

Types of DNS records

Types of DNS records

A name server can act as a primary for multiple zones and as secondary for others. Some valid record types on a DNS server are stated as follows.

SOA record (Start of Authority), which consist of crucial information like the

SERIAL number, which is monitor by other name servers for change, which indicates to them a change in information for a zone, REFRESH which tell how often a secondary name server should check for a change in the serial number, RETRY is to inform a secondary server how long it should use it current entry if it is unable to perform a refresh and MINIMUM is how long the other name servers should hold these information.

NS record which show the authoritative DNS for the zone

A record which points a hostname to an IP address

CNAME record (canonical naming) which allows a node to be address using more than one hostname

MX record, which is used for message routing where there are multiple mail exchange hosts. A (A) record host address is needed for every MX record set.

PTR records, which are just the reverse of A records, it maps IP address to a hostname. These records can only be meaningful in-addr.arpa zones have been delegated to your control by your service provider of your IP block.

There are also some other lesser used record types like HINFO which indicate CPU and operating system types for mapping to specific hostnames and Text (TXT) record that provides a descriptive text associated with a domain name.

Understanding Zone Types

Understanding Zone Types

The DNS Server service provides for three types of zones:

• Primary zone
  
• Secondary zone
  
• Stub zone
  

Note
If the DNS server is also an Active Directory Domain Services (AD DS) domain controller, primary zones and stub zones can be stored in AD DS. See Understanding Active Directory Domain Services Integration for more information.

The following sections describe each of these zone types.

Primary zone

When a zone that this DNS server hosts is a primary zone, the DNS server is the primary source for information about this zone, and it stores the master copy of zone data in a local file or in AD DS. When the zone is stored in a file, by default the primary zone file is named zone_name.dns and it is located in the %windir%\System32\Dns folder on the server.

Secondary zone

When a zone that this DNS server hosts is a secondary zone, this DNS server is a secondary source for information about this zone. The zone at this server must be obtained from another remote DNS server computer that also hosts the zone. This DNS server must have network access to the remote DNS server that supplies this server with updated information about the zone. Because a secondary zone is merely a copy of a primary zone that is hosted on another server, it cannot be stored in AD DS.

Stub zone

When a zone that this DNS server hosts is a stub zone, this DNS server is a source only for information about the authoritative name servers for this zone. The zone at this server must be obtained from another DNS server that hosts the zone. This DNS server must have network access to the remote DNS server to copy the authoritative name server information about the zone.
You can use stub zones to:

• Keep delegated zone information current. By updating a stub zone for one of its child zones regularly, the DNS server that hosts both the parent zone and the stub zone will maintain a current list of authoritative DNS servers for the child zone.
  
• Improve name resolution. Stub zones enable a DNS server to perform recursion using the stub zone's list of name servers, without having to query the Internet or an internal root server for the DNS namespace.
  
• Simplify DNS administration. By using stub zones throughout your DNS infrastructure, you can distribute a list of the authoritative DNS servers for a zone without using secondary zones. However, stub zones do not serve the same purpose as secondary zones, and they are not an alternative for enhancing redundancy and load sharing.
  

There are two lists of DNS servers involved in the loading and maintenance of a stub zone:

• The list of master servers from which the DNS server loads and updates a stub zone. A master server may be a primary or secondary DNS server for the zone. In both cases, it will have a complete list of the DNS servers for the zone.
  
• The list of the authoritative DNS servers for a zone. This list is contained in the stub zone using name server (NS) resource records.
  

When a DNS server loads a stub zone, such as widgets.tailspintoys.com, it queries the master servers, which can be in different locations, for the necessary resource records of the authoritative servers for the zone widgets.tailspintoys.com. The list of master servers may contain a single server or multiple servers, and it can be changed anytime.

DNS Lookup Types

1. •   DNS lookup queries find different kinds of information.

     William Shakespeare once asked, "What's in a name?" With website domain names, additional information exists beneath the surface, and it's easy to access. The domain name system (DNS) translates user-friendly, easily memorized domain names like "Google.com" to unique numeric Internet Protocol (IP) addresses, such as 74.125.157.104, used by computers to connect to websites. The DNS also stores records containing information such as a website's server names and mail servers.

   Forward DNS Lookup

   • The forward DNS lookup, also known as an A (for address) record lookup, performs a query to a host server using the domain name. The server then responds, providing the associated unique IP address. A simple operation known as a "ping" can also perform a forward DNS lookup. Pinging a website sends a series of tiny packets of data to the domain name server to test its status and response time, and also returns the IP address for the requested domain name. Multiple free resources on the Internet provide tools to perform forward DNS lookups as well as other DNS queries.

   Reverse DNS Lookup

   • The reverse DNS lookup allows users to find the domain name of a numeric IP address through a server query similar to the forward DNS lookup. If the server has the proper record in place, this query will return the domain name for the IP address provided. Often used on mail servers, the reverse lookup authenticates emails and weeds out spammers. The mail server performs a reverse DNS lookup on the IP address located in the email's headers. If a corresponding domain name cannot be found--often the case with spammers using invalid IP addresses--the server then rejects and blocks the email.

   BIND Version Lookup

   • The BIND version lookup yields version information from a Unix/Linux Berkeley Internet Name Domain server. Security concerns may cause the queried server to either ignore or deny the lookup request, however.

   Additional DNS Lookups

   • A domain name server query can return several specific DNS records, including the A, or address, which maps the website's registered domain name to an IP address. This record enables the forward DNS lookup, while a PTR, or pointer, record created on a host server connects the IP address to the domain name, enabling reverse DNS lookups. An MX, or mail exchange, record query returns a list of mail servers associated with the domain name host server.
     The NS, or name server, record identifies the name of a host server that contains information about its associated domain name. The SOA, or start of authority, record identifies the domain name server as the best, most "authoritative" source of information about a given domain name, according to Microsoft's Technet website.

What Is WINS?

What Is WINS?

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Role of WINS in the Network

Although NetBIOS and NetBIOS names can be used with network protocols other than TCP/IP, WINS was designed specifically to support NetBIOS over TCP/IP (NetBT). WINS is required for any environment in which users access resources that have NetBIOS names. If you do not use WINS in such a network, you cannot connect to a remote network resource by using its NetBIOS name unless you use Lmhosts files, and you might be unable to establish file and print sharing connections.
The following figure illustrates the role of WINS for computers that use NetBIOS names. Typically, DHCP is used to assign IP addresses automatically.
WINS Name Registration and Resolution
In a typical scenario, the following occurs:

1. ClientA, which uses NetBIOS and is a WINS client, sends a name registration request to its configured primary WINS server (WINSA) when it starts up and joins the network. WINSA adds ClientA's NetBIOS name and IP address to the WINS database.
   
2. When ClientB needs to connect to ClientA by its name, it requests the IP address from the WINS server.
   
3. The WINS server locates the corresponding entry in its database and replies with ClientA's IP address.
   

Summary of WINS Benefits

WINS provides the following benefits over other NetBIOS name resolution methods:

• WINS name resolution reduces NetBIOS name query broadcast traffic because clients can query a WINS server directly instead of broadcasting queries.
  
• WINS enables the Computer Browser service to collect and distribute browse lists across IP routers.
  
• The WINS dynamic name-to-address database supports NetBIOS name registration and resolution in environments where DHCP-enabled clients are configured for dynamic TCP/IP address allocation.
  
• The WINS database also supports centralized management and replicates name-to-address mappings to other WINS servers.
  
• WINS and DNS can be used in the same environment to provide combined name searches in both namespaces.
  

WINS and DNS

WINS and DNS are both name resolution services for TCP/IP networks. While WINS resolves names in the NetBIOS namespace, DNS resolves names in the DNS domain namespace. WINS primarily supports clients that run older versions of Windows and applications that use NetBIOS. Windows 2000, Windows XP, and Windows Server 2003 use DNS names in addition to NetBIOS names. Environments that include some computers that use NetBIOS names and other computers that use domain names must include both WINS servers and DNS servers.

What Is DHCP?

What Is DHCP?

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

What is DHCP?

Dynamic Host Configuration Protocol (DHCP) is a client/server protocol that automatically provides an Internet Protocol (IP) host with its IP address and other related configuration information such as the subnet mask and default gateway. RFCs 2131 and 2132 define DHCP as an Internet Engineering Task Force (IETF) standard based on Bootstrap Protocol (BOOTP), a protocol with which DHCP shares many implementation details. DHCP allows hosts to obtain necessary TCP/IP configuration information from a DHCP server.
The Microsoft Windows Server 2003 operating system includes a DHCP Server service, which is an optional networking component. All Windows-based clients include the DHCP client as part of TCP/IP, including Windows Server 2003, Microsoft Windows XP, Windows 2000, Windows NT 4.0, Windows Millennium Edition (Windows Me), and Windows 98.
Note

• It is necessary to have an understanding of basic TCP/IP concepts, including a working knowledge of subnets before you can fully understand DHCP. For more information about TCP/IP, see “TCP/IP Technical Reference.”
  

Benefits of DHCP

In Windows Server 2003, the DHCP Server service provides the following benefits:

• Reliable IP address configuration. DHCP minimizes configuration errors caused by manual IP address configuration, such as typographical errors, or address conflicts caused by the assignment of an IP address to more than one computer at the same time.
  
• Reduced network administration. DHCP includes the following features to reduce network administration:
  
  
  • Centralized and automated TCP/IP configuration.
    
  • The ability to define TCP/IP configurations from a central location.
    
  • The ability to assign a full range of additional TCP/IP configuration values by means of DHCP options.
    
  • The efficient handling of IP address changes for clients that must be updated frequently, such as those for portable computers that move to different locations on a wireless network.
    
  • The forwarding of initial DHCP messages by using a DHCP relay agent, thus eliminating the need to have a DHCP server on every subnet.
    

Why use DHCP

Every device on a TCP/IP-based network must have a unique unicast IP address to access the network and its resources. Without DHCP, IP addresses must be configured manually for new computers or computers that are moved from one subnet to another, and manually reclaimed for computers that are removed from the network.
DHCP enables this entire process to be automated and managed centrally. The DHCP server maintains a pool of IP addresses and leases an address to any DHCP-enabled client when it starts up on the network. Because the IP addresses are dynamic (leased) rather than static (permanently assigned), addresses no longer in use are automatically returned to the pool for reallocation.
The network administrator establishes DHCP servers that maintain TCP/IP configuration information and provide address configuration to DHCP-enabled clients in the form of a lease offer. The DHCP server stores the configuration information in a database, which includes:

• Valid TCP/IP configuration parameters for all clients on the network.
  
• Valid IP addresses, maintained in a pool for assignment to clients, as well as excluded addresses.
  
• Reserved IP addresses associated with particular DHCP clients. This allows consistent assignment of a single IP address to a single DHCP client.
  
• The lease duration, or the length of time for which the IP address can be used before a lease renewal is required.
  

A DHCP-enabled client, upon accepting a lease offer, receives:

• A valid IP address for the subnet to which it is connecting.
  
• Requested DHCP options, which are additional parameters that a DHCP server is configured to assign to clients. Some examples of DHCP options are Router (default gateway), DNS Servers, and DNS Domain Name. For a full list of DHCP options, see “DHCP Tools and Settings.”
  

Terms and Definitions

The following table lists common terms associated with DHCP.
DHCP Terms and Definitions

 

Term	Definition
DHCP server	A computer running the DHCP Server service that holds information about available IP addresses and related configuration information as defined by the DHCP administrator and responds to requests from DHCP clients.
DHCP client	A computer that gets its IP configuration information by using DHCP.
Scope	A range of IP addresses that are available to be leased to DHCP clients by the DHCP Server service.
Subnetting	The process of partitioning a single TCP/IP network into a number of separate network segments called subnets.
DHCP option	Configuration parameters that a DHCP server assigns to clients. Most DHCP options are predefined, based on optional parameters defined in Request for Comments (RFC) 2132, although extended options can be added by vendors or users.
Option class	An additional set of options that can be provided to a DHCP client based on its computer class membership. The administrator can use option classes to submanage option values provided to DHCP clients. There are two types of options classes supported by a DHCP server running Windows Server 2003: vendor classes and user classes.
Lease	The length of time for which a DHCP client can use a DHCP-assigned IP address configuration.
Reservation	A specific IP address within a scope permanently set aside for leased use by a specific DHCP client. Client reservations are made in the DHCP database using the DHCP snap-in and are based on a unique client device identifier for each reserved entry.
Exclusion/exclusion range	One or more IP addresses within a DHCP scope that are not allocated by the DHCP Server service. Exclusions ensure that the specified IP addresses will not be offered to clients by the DHCP server as part of the general address pool.
DHCP relay agent	Either a host or an IP router that listens for DHCP client messages being broadcast on a subnet and then forwards those DHCP messages directly to a configured DHCP server. The DHCP server sends DHCP response messages directly back to the DHCP relay agent, which then forwards them to the DHCP client. The DHCP administrator uses DHCP relay agents to centralize DHCP servers, avoiding the need for a DHCP server on each subnet. Also referred to as a BOOTP relay agent.
Unauthorized DHCP server	A DHCP server that has not explicitly been authorized. Sometimes referred to as a rogue DHCP server. In a Windows Server 2003 domain environment, the DHCP Server service on an unauthorized server running Windows Server 2003 fails to initialize. The administrator must explicitly authorize all DHCP servers running Windows Server 2003 that operate in an Active Directory service domain environment. At initialization time, the DHCP Server service in Windows Server 2003 checks for authorization and stops itself if the server detects that it is in a domain environment and the server has not been explicitly authorized.
Automatic Private IP Addressing (APIPA)	A TCP/IP feature in Windows XP and Windows Server 2003 that automatically configures a unique IP address from the range 169.254.0.1 through 169.254.255.254 with a subnet mask of 255.255.0.0 when the TCP/IP protocol is configured for automatic addressing, the Automatic private IP address alternate configuration setting is selected, and a DHCP server is not available. The APIPA range of IP addresses is reserved by the Internet Assigned Numbers Authority (IANA) for use on a single subnet, and IP addresses within this range are not used on the Internet.
Superscope	A configuration that allows a DHCP server to provide leases from more than one scope to clients on a single physical network segment.
Multicast IP addresses	Multicast IP addresses allow multiple clients to receive data that is sent to a single IP address, enabling point-to-multipoint communication. This type of transmission is often used for streaming media transmissions, such as video conferencing.
Multicast Scope	A range of multicast IP addresses that can be assigned to DHCP clients. A multicast scope allows dynamic allocation of multicast IP addresses for use on the network by using the MADCAP protocol, as defined in RFC 2730.
BOOTP	An older protocol with similar functionality; DHCP is based on BOOTP. BOOTP is an established protocol standard used for configuring IP hosts. BOOTP was originally designed to enable boot configuration for diskless workstations. Most DHCP servers, including those running Windows Server 2003, can be configured to respond to both BOOTP requests and DHCP requests.

How DHCP Technology Works

How DHCP Technology Works

 

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

How DHCP Works

DHCP provides an automated way to distribute and update IP addresses and other configuration information on a network. A DHCP server provides this information to a DHCP client through the exchange of a series of messages, known as the DHCP conversation or the DHCP transaction. If the DHCP server and DHCP clients are located on different subnets, a DHCP relay agent is used to facilitate the conversation.
Note

• It is necessary to have an understanding of basic TCP/IP concepts, including working knowledge of subnets before you can have a full understanding of DHCP. For more information about TCP/IP, see “TCP/IP Technical Reference.”
  

In this section

• DHCP Architecture
  
  
• DHCP Protocols
  
  
• DHCP Processes and Interactions
  
  

DHCP Architecture

The DHCP architecture consists of DHCP clients, DHCP servers, and DHCP relay agents on a network. The clients interact with servers using DHCP messages in a DHCP conversation to obtain and renew IP address leases.

DHCP Client Functionality

A DHCP client is any network-enabled device that supports the ability to communicate with a DHCP server in compliance with RFC 2131, for the purpose of obtaining dynamic leased IP configuration and related optional information.
DHCP provides support for client computers running any of the following Microsoft operating systems:

• Windows NT version 4.0
  
• Windows 2000
  
• Windows XP
  
• Windows Server 2003
  
• Windows 98
  
• Windows Millennium Edition
  

Automatic IP Configuration

DHCP supports Automatic Private IP Addressing (APIPA), which enables computers running Windows 2000, Windows XP, and Windows Server 2003 to configure an IP address and subnet mask if a DHCP server is unavailable at system startup and the Automatic private IP address Alternate Configuration setting is selected. This feature is useful for clients on small private networks, such as a small-business office or a home office.
The DHCP Client service on a computer running Windows XP and Windows Server 2003 uses the following process to auto-configure the client:

1. The DHCP client attempts to locate a DHCP server and obtain an IP address and configuration.
   
2. If a DHCP server cannot be found or does not respond after one minute, the DHCP client checks the settings on the Alternate Configuration tab of the properties of the TCP/IP protocol.
   
   If Automatic private IP address is selected, the DHCP client auto-configures its IP address and subnet mask by using a selected address from the Microsoft-reserved Class B network, 169.254.0.0, with the subnet mask 255.255.0.0. The DHCP client tests for an address conflict to ensure that the IP address is not in use on the network. If a conflict is found, the client selects another IP address. The client retries auto-configuration up to 10 times.
   
   If User Configured is selected, the DHCP client configures a static IP address configuration. The DHCP client tests for an address conflict to ensure that the IP address is not already in use on the network. If a conflict is found, the DHCP client indicates the error condition to the user.
   
3. When the DHCP client succeeds in self-selecting an address, it configures its network interface with the IP address. The client then continues to check for a DHCP server in the background every five minutes. If a DHCP server responds, the DHCP client abandons its self-selected IP address and uses the address offered by the DHCP server (and any other DHCP option information that the server provides) to update its IP configuration settings.
   

If the DHCP client obtained a lease from a DHCP server on a previous occasion, and the lease is still valid (not expired) at system startup, the client tries to renew its lease. If, during the renewal attempt, the client fails to locate any DHCP server, it attempts to ping the default gateway listed in the lease, and proceeds in one of the following ways:

• If the ping is successful, the DHCP client assumes that it is still located on the same network where it obtained its current lease, and continues to use the lease as long as the lease is still valid. By default the client then attempts, in the background, to renew its lease when 50 percent of its assigned lease time has expired.
  
• If the ping fails, the DHCP client assumes that it has been moved to a network where a DHCP server is not available. The client then auto-configures its IP address by using the settings on the Alternate Configuration tab. When the client is auto-configured, it attempts to locate a DHCP server and obtain a lease every five minutes.
  

Local Storage

Windows Server 2003 DHCP supports local storage, which allows clients to store DHCP information on their own hard disks. Local storage is useful because it enables the client to store its last leased IP address, so that when the client starts it first attempts to renew the lease of its previous IP address. Local storage also enables a client to be shut down and restarted and it will use its previously leased address and configuration, even if the DHCP server is unreachable or offline at the time that the client computer is restarted.

DHCP Server Responsibilities

The DHCP servers maintain scopes, reservations, and options as set by the administrator.

Scopes

A scope must be properly defined and activated before DHCP clients can use the DHCP server for automatic TCP/IP configuration. A DHCP scope is an administrative collection of IP addresses and TCP/IP configuration parameters that are available for lease to DHCP clients of a specific subnet. The network administrator creates a scope for each subnet.
A scope has the following properties:

• A scope name, assigned when the scope is created.
  
• A range of possible IP addresses from which to include or exclude addresses used in DHCP lease offers.
  
• A unique subnet mask, which determines the network ID for an IP address in the scope.
  
• Lease duration values.
  

Each DHCP scope can have a single continuous range of IP addresses. To use several address ranges within a single scope you must first define the entire address range for the scope, and then set exclusion ranges.

Lease Durations

When a scope is created, the lease duration is set to eight days by default. However there are situations when the administrator might want to change the lease duration. The following are examples of adjusting the lease duration due to individual network consideration:

• An organization has a large number of IP addresses available and configurations that rarely change. The administrator increases the lease duration to reduce the frequency of lease renewal exchanges between clients and the DHCP server. Because the DHCP clients are renewing their leases less frequently, DHCP-related network traffic is reduced.
  
• A limited number of IP addresses are available and client configurations change frequently or clients move often in or out of the network. The administrator reduces the lease duration. This increases the rate at which unused addresses are returned to the available address pool for reassignment.
  

For example, consider the ratio between connected computers and available IP addresses. If 40 computers share 254 available addresses, the demand for reusing addresses is low. A long lease time, such as a few months, might be appropriate in such a situation. However, if 230 computers must share the same address pool, demand for available addresses is greater, and a shorter lease time, for example a few days, is more appropriate.
Note

• Although it is possible to configure a client with infinite lease duration, use infinite lease durations with caution. Even relatively stable environments have a certain amount of client turnover. At a minimum, computers might be added and removed, moved from one office to another, or network adapters might be replaced. If a client with an infinite lease is removed from the network without releasing its lease, the DHCP server is not notified, and the IP address is not automatically reused. Also, when using an infinite lease, options set on the DHCP server are not automatically updated on the DHCP client, because the client is never required to renew its lease and obtain the new options. It is recommended that reservations be used rather than infinite lease durations.
  

Exclusion Ranges

When you create a new scope, immediately exclude the addresses of existing statically configured computers from the scope. By using exclusion ranges, you can exclude specific IP address ranges within a scope so that those addresses are not offered to clients. Assign IP addresses within exclusion ranges to computers or devices that must have a static IP address, such as servers, firewalls, or routers.
You can use excluded IP addresses on your network by manually configuring these addresses at computers that do not use DHCP to obtain an address, or by configuring reservations for these addresses.

Reservations

You can reserve IP addresses for assignment to specified computers or devices on the network. Reservations ensure that a specified hardware device on a subnet always receives the same IP address lease. Use reservations for DHCP-enabled devices that must always have the same IP address on your network, such as servers that do not support Domain Name System (DNS) dynamic update.
Note

• If multiple DHCP servers are each configured with scopes that cover addresses that must be reserved, the reservations must be specified on each DHCP server. Otherwise, the client might receive an IP address from one of the DHCP servers that does not contain the reservation, and therefore might not receive the IP address reserved for the client.
  

Superscopes

A superscope allows a DHCP server to provide leases from more than one scope to clients on a single physical subnet. Before you can create a superscope, you must use the DHCP Microsoft Management Console (MMC) snap-in to define at least one of the scopes to be included in the superscope. Scopes added to a superscope are called member scopes. Superscopes can resolve DHCP Server service issues in several different ways; these issues include situations in which:

• Support is needed for DHCP clients on a single physical network segment — such as a single Ethernet LAN segment — where multiple logical IP networks are used. When more than one logical IP network is used on a physical network, these configurations are also known as multinets. In a situation where multinets are used, clients might not be able to communicate directly with each other, because the clients might be on different logical subnets, even if they are on the same physical network segment. In this case, routing must be enabled to allow the clients to communicate with each other. Also, a router or BOOTP/DHCP relay agent must be configured on the subnet to allow DHCP messages to travel between the logical subnets.
  
• Support is needed for DHCP clients that are in a multinet located on the other side of BOOTP relay agents.
  
• Clients need to be migrated to a new scope.
  

Interactions between Client and Server

DHCP servers and DHCP clients communicate through a series of DHCP messages. To obtain a lease, the DHCP client initiates a conversation with a DHCP server using a series of these DHCP messages.

DHCP Messages

The following list includes the eight types of messages that can be sent between DHCP clients and servers. For more information about the structure and specifics of each of these packets, see “DHCP Message Format” later in this section.

DHCPDiscover

Broadcast by a DHCP client when it first attempts to connect to the network. The DHCPDiscover message requests IP address information from a DHCP server.

DHCPOffer

Broadcast by each DHCP server that receives the client DHCPDiscover message and has an IP address configuration to offer to the client. The DHCPOffer message contains an unleased IP address and additional TCP/IP configuration information, such as the subnet mask and default gateway. More than one DHCP server can respond with a DHCPOffer message. The client accepts the best offer, which for a Windows DHCP client is the first DHCPOffer message that it receives.

DHCPRequest

Broadcast by a DHCP client after it selects a DHCPOffer. The DHCPRequest message contains the IP address from the DHCPOffer that it selected. If the client is renewing or rebinding to a previous lease, this packet might be unicast directly to the server.

DHCPAck

Broadcast by a DHCP server to a DHCP client acknowledging the DHCPRequest message. At this time, the server also forwards any options. Upon receipt of the DHCPAck, the client can use the leased IP address to participate in the TCP/IP network and complete its system startup. This message is typically broadcast, because the DHCP client does not officially have an IP address that it can use at this point. If the DHCPAck is in response to a DHCPInform, then the message is unicast directly to the host that sent the DHCPInform message.

DHCPNack

Broadcast by a DHCP server to a DHCP client denying the client’s DHCPRequest message. This might occur if the requested address is incorrect because the client moved to a new subnet or because the DHCP client’s lease has expired and cannot be renewed.

DHCPDecline

Broadcast by a DHCP client to a DHCP server, informing the server that the offered IP address is declined because it appears to be in use by another computer.

DHCPRelease

Sent by a DHCP client to a DHCP server, relinquishing an IP address and canceling the remaining lease. This is unicast to the server that provided the lease.

DHCPInform

Sent from a DHCP client to a DHCP server, asking only for additional local configuration parameters; the client already has a configured IP address. This message type is also used by DHCP servers running Windows Server 2003 to detect unauthorized DHCP servers.

DHCP Lease Process

A DHCP-enabled client obtains a lease for an IP address from a DHCP server. Before the lease expires, the DHCP client must renew the lease or obtain a new lease. Leases are retained in the DHCP server database for a period of time after expiration. By default, this grace period is four hours and cleanup occurs once an hour for a DHCP server running Windows Server 2003. This protects a clients lease in case the client and server are in different time zones, the internal clocks of the client and server computers are not synchronized, or the client is off the network when the lease expires.

Obtaining a New Lease

A DHCP client initiates a conversation with a DHCP server when it is seeking a new lease, renewing a lease, rebinding, or restarting. The DHCP conversation consists of a series of DHCP messages passed between the DHCP client and DHCP servers. The following figure shows an overview of this process when the DHCP server and DHCP client are on the same subnet.
DHCP Lease Process Overview

1. The DHCP client requests an IP address by broadcasting a DHCPDiscover message to the local subnet.
   
2. The client is offered an address when a DHCP server responds with a DHCPOffer message containing an IP address and configuration information for lease to the client. If no DHCP server responds to the client request, the client sends DHCPDiscover messages at intervals of 0, 4, 8, 16, and 32 seconds, plus a random interval of between -1 second and 1 second. If there is no response from a DHCP server after one minute, the client can proceed in one of two ways:
   
   
   • If the client is using the Automatic Private IP Addressing (APIPA) alternate configuration, the client self-configures an IP address for its interface.
     
   • If the client does not support alternate configuration, such as APIPA, or if IP auto-configuration has been disabled, the client network initialization fails.
     
   In both cases, the client begins a new cycle of DHCPDiscover messages in the background every five minutes, using the same intervals as before (0, 4, 8, 16, and 32 seconds), until it receives a DHCPOffer message from a DHCP server.
   
3. The client indicates acceptance of the offer by selecting the offered address and broadcasting a DHCPRequest message in response.
   
4. The client is assigned the address and the DHCP server broadcasts a DHCPAck message in response, finalizing the terms of the lease.
   

When the client receives acknowledgment, it configures its TCP/IP properties by using the DHCP option information in the reply, and completes its initialization of TCP/IP.
In rare cases, a DHCP server might return a negative acknowledgment to the client. This can happen if a client requests an invalid or duplicate address. If a client receives a negative acknowledgment (DHCPNack), the client must begin the entire lease process again.
When the DHCP client and the DHCP server are on the same IP broadcast subnet, the DHCPDiscover, DHCPOffer, DHCPRequest, and DHCPAck messages are sent to identify clients by means of IP-level broadcasts sent to the limited broadcast address and the media access control (MAC) broadcast address.
When the DHCP server and DHCP client are not on the same subnet either a router or a host on the DHCP client’s subnet must act as a DHCP relay agent to support the forwarding of DHCP messages between the DHCP client and the DHCP server.

Renewing a Lease

The DHCP client first attempts to renew its lease when 50 percent of the original lease time, known as T1, has passed. At this point the DHCP client sends a unicast DHCPRequest message to the DHCP server that originally granted its lease. If the server is available, and the lease is still available, the server responds with a unicast DHCPAck message and the lease is renewed.
If the original DHCP server is available, but the client’s current lease is no longer available, the DHCP server responds with a DHCPNack message, and the client immediately starts the process to obtain a new lease. This can happen if the client has changed subnets or if the DHCP server cannot fulfill the lease request for some other reason.
If there is no response from the DHCP server, the client waits until 87.5 percent of the lease time has passed (known as T2). At T2, the client enters the rebinding state, and broadcasts a DHCPRequest message to attempt to renew the lease from any available DHCP server. If no DHCP server is available by the time the lease expires, the client immediately unbinds itself from the existing lease and starts the process to obtain a new lease, beginning with a DHCPDiscover message.

Preventing Address Conflicts

Windows Server 2003 DHCP has both server-side and client-side conflict detection to prevent duplicate IP addresses on your network.

Client Conflict Detection

Client computers running Windows Server 2003, Windows XP, Windows 2000, Windows NT 4.0, Windows Millennium Edition, and Windows 98 automatically check to determine if an IP address is already in use before using it.
After the DHCP client receives a lease from the DHCP server, the client sends an Address Resolution Protocol (ARP) request to the address that it has been assigned. If a reply to the ARP request is received, the client has detected a conflict and sends a DHCPDecline message to the DHCP server. The DHCP server attaches a BAD_ADDRESS value to the IP address in the scope for the length of the lease. The client then begins the lease process again, and is offered the next available address in the scope.
Note

• ARP requests do not traverse routers. Clients use ARP requests rather than pings (ICMP Echo messages) because pings require the sender to have an IP address.
  

Server Conflict Detection

If your network includes older DHCP clients that do not perform conflict detection themselves, you can enable conflict detection on the DHCP server. By default, the Windows Server 2003 DHCP Server service does not perform any conflict detection.
To detect conflicts, the DHCP server pings (sends an ICMP Echo message to) an IP address before offering that address to clients in a new lease. The DHCP server only pings addresses that have not been successfully and previously leased. If a client requests a lease on an IP address that it already had or is requesting a renewal, the DHCP server does not ping the IP address.
If conflict detection is enabled, an administrator-defined number of pings are sent. The server waits 1 second for a reply. Because the time required for a client to obtain a lease is equal to the number of pings used, choose this value carefully because it directly impacts the overall performance of the server. In general, one ping is sufficient.
If a response to the ping is received, a conflict is registered and that address is not offered to clients requesting a lease from the server. The DHCP server then attaches a BAD_ADDRESS value to that IP address in the scope. The DHCP server then tries to lease the next available address. If the duplicate address is removed from the network, the BAD_ADDRESS value attached to the IP address can be deleted from the scope’s list of active leases, and then the address returns to the pool. Addresses are marked as BAD_ADDRESS for the length of the lease for which the scope is configured. If the BAD_ADDRESS entry is not manually removed, it will automatically be removed after a period of time equal to the lease time for the scope.
Note

• In general, use server conflict detection only as a troubleshooting aid when you suspect that duplicate IP addresses are in use on your network. Each additional conflict detection attempt adds to the time needed to negotiate leases for DHCP clients.
  

DHCP Options

DHCP options are additional configuration parameters that a DHCP server assigns to clients. Options can also be used for DHCP communication between the server computer and client computers.
The most specific options take precedence over the least specific options. This simplifies DHCP management and allows a flexible administration that can range from per-server default settings to common settings for a specific subnet and individualized client settings when needed for special circumstances. In most cases, the option values are specified in the Options dialog box on the DHCP server, scope, or reservation.
DHCP options can be configured for specific values and enabled for assignment and distribution to DHCP clients based on:

• Server options. These options apply globally for all scopes and classes defined at each DHCP server and any clients that it services. Configured server option values always apply unless they are overridden by options assigned to other scope, class, or client reservation.
  
• Scope options. These options apply to any clients that obtain a lease within that particular scope. Configured scope option values always apply to all computers obtaining a lease in a given scope unless they are overridden by options assigned to class or client reservation.
  
• Class options. These options apply to any clients that specify that particular DHCP Class ID value when obtaining a scope lease. Configured class option values always apply to all computers configured as members in a specified DHCP option class unless they are overridden by options assigned to a client reservation.
  
• Reserved client options. These options apply only to the client corresponding to the reservation. Reserved client option values override all other server, scope, or class assigned option values.
  

Options are typically applied at each DHCP server at the server or scope level. To precisely manage or customize option settings for a group or class of computers, specify either a user or vendor class assignment that overrides the broader server or scope option defaults.
For special requirements, such as clients with special functions, assign options for specific reserved clients.
Options can also be used to separate and distribute appropriate options for clients with similar or special configuration needs. For example, DHCP clients on the same floor of a building can be configured with the same DHCP Class ID value to assign them membership in the same option class. You can then distribute additional or varied option data to that class during the lease process, overriding any scope or globally provided default options.
Note
Statically configured values on a client override any DHCP options of any type or level.