The dhcploc command lets you see a computer’s DHCP traffic for a broadcast domain. Simply pass in the IP address of the machine from which you are running the command:

> dhcploc 192.168.32.24

You will not see any output from the command until it captures some DHCP traffic. You can try running ipconfig /renew to force some traffic to be generated. You can also press the “d” key when you have dhcploc running to have it generate a DISCOVER message.

Here is some sample output from the command:

9:34:58 (IP)0.0.0.0        NACK      (S)192.168.31.84     ***
 9:36:38 (IP)192.168.190.130 OFFER     (S)192.168.12.226   ***
 9:36:38 (IP)192.168.196.231 ACK       (S)192.168.13.53
 9:36:53 (IP)192.168.196.231 ACK       (S)192.168.13.53
 9:37:05 (IP)192.168.196.234 OFFER     (S)192.168.13.53
 9:37:05 (IP)192.168.193.232 OFFER     (S)192.168.12.198
 9:37:06 (IP)192.168.190.132 OFFER     (S)192.168.12.221   ***

The first column contains a timestamp, the second column is the IP address of the target computer, the third is the DHCP request type, the fourth is the IP address of the DHCP Server, and the fifth is a flag that indicates whether the DHCP Server is authorized. If it is not authorized, you’ll see three stars (***). In the previous output, you can see that 192.168.31.84, 192.168.12.226, and 192.168.12.221 are all unauthorized DHCP Servers.

dhcploc can also send alerts if it detects an unauthorized server. This allows you to start dhcploc, leave it running, and let it proactively notify you when it discovers an unauthorized server. To do so, specify the /a: option followed by the list of users to alert as shown here:

> dhcploc /a:"rallen" 192.168.32.24

dhcploc works by capturing all of the DHCP traffic it sees on the network. Since most DHCP traffic is sent via broadcast, every computer in the broadcast domain (e.g., all computers connected to a hub on a local segment), can look at DHCP traffic. Most computers simply discard the traffic unless it is destined for them, but dhcploc captures all DHCP traffic.

Do not run dhcploc from a DHCP Server. DHCP traffic will be delivered to dhcploc instead of the DHCP Server. By running the command directly on a DHCP Server, it is likely the server won’t be able to respond to any client requests.

Using a graphical user interface

1. Open the DHCP snap-in.
2. In the left pane, right-click on DHCP and select Add Server.
3. Type in the name of the DHCP Server you want to target and click OK.
4. Expand the server entry in the left pane.
5. Click on Server Options, then right-click it and select Configure Options.
6. Check the box beside the option you want to configure.
7. This will make the Data entry section of the dialog box active.
8. Enter the value(s) for the option and click OK.

Using a command-line interface

The following command configures a server option:

> netsh dhcp server \\ set optionvalue   

In the following example, the “domain name” option (code 015) is configured with a value of “rallencorp.com” on the DHCP server named dhcp01:

> netsh dhcp server \\dhcp01 set optionvalue 015 STRING rallencorp.com

You can view all of the server options that have been configured with this command:

> netsh dhcp server \\ show optionvalue

If the DHCP Server service is enabled on a domain controller, it is automatically authorized. A DHCP Server that is a member server of an Active Directory domain performs a query in Active Directory to determine whether it is authorized. If it is, it will respond to DHCP requests; if not, it will not respond to requests.

A standalone DHCP server that is not a member of an Active Directory domain sends out a DHCPINFORM message when it first initializes. If an authorized DHCP Server responds to the message, the standalone server will not respond to any further DHCP requests. If it does not receive a response from a DHCP Server, it will respond to client requests and give out leases.

DHCP servers are represented in Active Directory as objects of the dhcpClass class, which can be found in the cn=NetServices,cn=Services,cn=Configuratation, container. The relative distinguished name of these objects is the DHCP Server’s IP address. There is also an object in the same container named cn=dhcpRoot, which is created after the first DHCP Server is authorized. It has an attribute named dhcpServers that contains all authorized servers. I enumerated this attribute in the VBScript solution to display all authorized servers.

By default, only members of the Enterprise Admins group can authorize DHCP Servers. You, however, can delegate the rights to authorize a DHCP Server. Do the following to delegate the necessary permissions to a group called DHCP Admins:

1. Open ADSI Edit from the Support Tools while logged on as a member of the Enterprise Admins group.
2. In the left pane, expand the Configuration Container CN=Configuration CN=Services CN=NetServices.
3. Right-click on CN=NetServices and select Properties.
4. Select the Security tab.
5. Click the Advanced button.
6. Click the Add button.
7. Use the object picker to select the DHCP Admins group.
8. Check the boxes under Allow for Create dHCPClass objects and Delete dHCPClass objects.
9. Click OK until all dialog boxes are closed.
10. Back in the left pane of ADSI Edit, right-click on CN=dhcpRoot (if you’ve previously authorized DHCP Servers) and select Properties.
11. Select the Security tab.
12. Click the Advanced button.
13. Click the Add button.
14. Use the object picker to select the DHCP Admins group.
15. Check the boxes under Allow for Write.
16. Click OK until all dialog boxes are closed.

Using a graphical user interface

Windows 2000 DHCP Servers cannot be authorized with the Windows Server 2003 version of the DHCP snap-in unless the DHCP Server has Service Pack 2 or higher installed.

1. Open the DHCP snap-in.
2. In the left pane, right-click on DHCP and select Add Server.
3. Type in the name of the DHCP Server you want to target and click OK.
4. Click on the server entry in the left pane.
5. Right-click on the server and select Authorize.

If the DHCP Server is not a member of an Active Directory domain, you will not see the Authorize option.

Using a command-line interface

The following command authorizes a DHCP Server in Active Directory:

> netsh dhcp add server  

This example shows how to authorize the DHCP Server named dhcp01.rallencorp.com with IP 192.168.191.15:

> netsh dhcp add server dhcp01.rallencorp.com 192.168.191.15

Using VBScript

See the Introduction for more information on how to run the netsh command from within a script. The following script prints out the list of authorized DHCP Servers in Active Directory:

' ------ SCRIPT CONFIGURATION ------
 strForestRootDN = ""  ' e.g., dc=rallencorp,dc=com
 ' ------ END CONFIGURATION ---------
 set objCont = GetObject("LDAP://CN=DhcpRoot,CN=NetServices,CN=Services," & _
                         "CN=Configuration," & strForestRootDN)
 colDHCPServers = objCont.GetEx("dhcpServers")
 for each strDHCPServer in colDHCPServers
    Wscript.Echo strDHCPServer
 next

Using a graphical user interface

1. From the Control Panel, open the Add or Remove Programs applet.
2. Click Add/Remove Windows Components.
3. Double-click Network Services.
4. Check the box beside Dynamic Host Configuration Protocol (DHCP).
5. Click OK.
6. Click Next.
7. Click Finish.

Using a command-line interface

First, create a file using a text editor such as Notepad with the following contents:

[netoptionalcomponents]
 dhcpserver=1

Next, use the sysocmgr.exe utility with the following parameters (assuming the file you just created is named c:\dhcp_install.txt):

> sysocmgr /i:%windir%\inf\sysoc.inf /u:c:\dhcp_install.txt

If the server you installed DHCP on is multihomed (i.e., has multiple active network adapters), you’ll want to make sure the correct network adapters are enabled for use by the DHCP Server. You can enable or disable adapters for use by DHCP Server by doing the following:

1. Open the DHCP snap-in.
2. In the left pane, click on the server node.
3. From the menu, select Action > Properties.
4. Select the Advanced tab.
5. Click the Bindings button.
6. Make sure the interfaces where the DHCP Server should respond are checked.
7. Click OK until all dialog boxes are closed.

1. From the Control Panel, open the Network Connections applet.
2. Right-click the DHCP-enabled network connection you want to renew and select Repair. This will automatically attempt to renew the connection’s IP address.

Using a command-line interface

The following commands renew and release a DHCP IP address, respectively:

> ipconfig /renew

> ipconfig /release

With either of these commands, you can specify a pattern to match if you want to affect only a subset of adapters. The following command would release the IP address for any adapter that had “Con” (e.g., Local Area Connection 1) in its name:

> ipconfig /renew *Con*

Using VBScript

' This code releases all DHCP IP addresses.

' ------ SCRIPT CONFIGURATION ------

strComputer = "."

' ------ END CONFIGURATION ---------

set objWMI = GetObject("winmgmts:\\" & strComputer & "\root\cimv2")

set objAdapterConfig = objWMI.Get("Win32_NetworkAdapterConfiguration")

intRC = objAdapterConfig.ReleaseDHCPLeaseAll( )

if intRC = 0 then

   WScript.Echo "Released all DHCP IP addresses"

elseif intRC = 1 then

   WScript.Echo "You must reboot to release all DHCP IP addresses"

else

   WScript.Echo "There was an error releasing all DHCP IP addresses: " & intRC

end if' This code shows performs the same function as the previous example

' but it performs a query for all DHCP enabled IP addresses.  Use this

' if you don't want to release all IP addresses.  Modify the WQL statement

' based on the criteria you need.

' ------ SCRIPT CONFIGURATION ------

strComputer = "."

' ------ END CONFIGURATION ---------

set objWMI = GetObject("winmgmts:\\" & strComputer & "\root\cimv2")

set colNetworkAdapters = objWMI.ExecQuery _

    ("Select * From Win32_NetworkAdapterConfiguration Where DHCPEnabled = True")

for each objNetworkConfig in colNetworkAdapters

   intRC = objNetworkConfig.ReleaseDHCPLease( )

   if intRC = 0 then

      WScript.Echo "Released IP address for " & objNetworkConfig.Description

   elseif intRC = 1 then

      WScript.Echo "You must reboot to release the IP address for " & _

                   objNetworkConfig.Description

   else

      WScript.Echo "There was an error releasing the IP address for " & _

                   objNetworkConfig.Description & ": " & intRC

   end if

next

' This code renews all DHCP IP addresses.

' ------ SCRIPT CONFIGURATION ------

strComputer = "."

' ------ END CONFIGURATION ---------

set objWMI = GetObject("winmgmts:\\" & strComputer & "\root\cimv2")

set objAdapterConfig = objWMI.Get("Win32_NetworkAdapterConfiguration")

intRC = objAdapterConfig.RenewDHCPLeaseAll( )

if intRC = 0 then

   WScript.Echo "Renewed all DHCP IP addresses"

elseif intRC = 1 then

   WScript.Echo "You must reboot to renew all DHCP IP addresses"

else

   WScript.Echo "There was an error renewing all DHCP IP addresses: " & intRC

end if

' This code renews all adapters made by Intel that are installed on the server.

' ------ SCRIPT CONFIGURATION ------

strComputer = "."

' ------ END CONFIGURATION ---------

set objWMI = GetObject("winmgmts:\\" & strComputer & "\root\cimv2")

set colNAs = objWMI.ExecQuery("select * " & _

                              " from Win32_NetworkAdapter    " & _

                              " where manufacturer = 'Intel' " )

for each objNA in colNAs

   set colSubNAConfig = objWMI.ExecQuery _

      ("ASSOCIATORS OF {Win32_NetworkAdapter.DeviceID='" & _

        objNA.DeviceID & "'} " & _

      " WHERE resultClass = win32_NetworkAdapterConfiguration ")

for each objNAConfig in colSubNAConfig

      if objNAConfig.DHCPEnabled = True then

         intRC = objNAConfig.RenewDHCPLease( )

         if intRC = 0 then

            WScript.Echo "Renewed IP address for " & objNA.Name

         elseif intRC = 1 then

            WScript.Echo "You must reboot to renew the IP address for " & _

                   objNA.Name

         else

            WScript.Echo "There was an error renewing the IP address for " & _

                   objNA.Name & ": " & intRC

         end if

      end if

   next

next

Fortunately, the whole DHCP release/renew process is automatic and not something you have to do manually. When a client receives a DHCP lease for an IP address, it will automatically attempt to renew that address after 50% of the lease duration. The DHCP Server can grant the renewal, after which the client restarts its lease timer. If the server doesn’t respond, the client tries again after 87.5% of the lease duration and then attempts to contact other DHCP Servers.

Even though this process is automatic, there may be times when you need to initiate it yourselfespecially if you’ve made network configuration changes. For example, let’s say you configured a reservation on your DHCP Server for a particular host. If that host already has an IP address, you’ll need to release the current lease and run the renew command to get the new address.

1. From the Control Panel, open the Network Connections applet.
2. Double-click the connection you want to configure.
3. Click the Properties button.
4. Double-click Internet Protocol (TCP/IP).
5. To enable DHCP, select Obtain an IP address automatically. To use a static address, select Use the following IP address. Then configure the IP address, subnet mask, and default gateway.
6. Click OK until all windows are closed.

Using a command-line interface

The following command configures DHCP for a connection:

> netsh int ip set address name="<ConnectionName>" source=dhcp

Here is an example for configuring the connection named “Local Area Connection” to use DHCP:

> netsh int ip set address name="Local Area Connection" source=dhcp

This configures a connection with a static IP and default gateway:

> netsh int ip set address name="<ConnectionName>" source=static <IP> <Mask> <GateWayIP> <Metric>

This example configures a static IP address for “Local Area Connection”:

> netsh int ip set address name="Local Area Connection" source=static 10.3.53.3
255.255.255.0 10.3.53.1 1

Using VBScript

' This code enables DHCP for the specified connection.
' ------ SCRIPT CONFIGURATION ------
strComputer = "."
strConnection = "Local Area Connection"
' ------ END CONFIGURATION ---------
set objWMI = GetObject("winmgmts:\\" & strComputer & "\root\cimv2")
set colNA = objWMI.ExecQuery("select * " & _
                             " from Win32_NetworkAdapter " & _
                             " where NetConnectionID = '" & strConnection & "'" )
for each objNA in colNA
   set colNAConfig = objWMI.ExecQuery _
      ("ASSOCIATORS OF {Win32_NetworkAdapter.DeviceID='" & _
        objNA.DeviceID & "'} " & _
      " WHERE resultClass = win32_NetworkAdapterConfiguration ")
   for each objNAConfig in colNAConfig
      if objNAConfig.DHCPEnabled = True then
         WScript.Echo "DHCP already enabled for " & strConnection
      else
         intRC = objNAConfig.EnableDHCP( )
         if intRC = 0 then
            WScript.Echo "DHCP Enabled for " & strConnection
         elseif intRC = 1 then
            WScript.Echo "You must reboot to start using DHCP for " & _
                         strConnection
         else
            WScript.Echo "There was an error enabling DHCP for " & _
                         strconnection & ": " & intRC
         end if
      end if
   next
next

' This code configures an IP address, subnet mask, and default gateway
' for the specified connection.
' ------ SCRIPT CONFIGURATION ------
strComputer = "."
strConnection = "Local Area Connection"
strIP = Array("1.22.2.2")
strMask = Array("255.255.255.0")
strGatewayIP = Array("1.2.3.3")
' ------ END CONFIGURATION ---------
set objWMI = GetObject("winmgmts:\\" & strComputer & "\root\cimv2")
set colNA = objWMI.ExecQuery("select * " & _
                            " from Win32_NetworkAdapter " & _
                            " where NetConnectionID = '" & strConnection & "'" )
for each objNA in colNA
   set colNAConfig = objWMI.ExecQuery _
      ("ASSOCIATORS OF {Win32_NetworkAdapter.DeviceID='" & _
        objNA.DeviceID & "'} " & _
      " WHERE resultClass = win32_NetworkAdapterConfiguration ")
   for each objNAConfig in colNAConfig
      intRC = objNAConfig.EnableStatic(strIP,strMask)
      intRC2 = objNAConfig.SetGateways(strGatewayIP)
      if intRC = 0 and intRC2 = 0 then
         WScript.Echo "IP address configured for " & strConnection
      elseif intRC = 1 or intRC2 = 1 then
         WScript.Echo "You must reboot for the changes to take effect for " & _
                      strConnection
      else
         WScript.Echo "There was an error configuring IP for " & _
                      strconnection & ": " & intRC & " and " & intRC2
      end if
   next
next

If you use static IP addresses, any time you build a new server, you have to configure an IP address on that server. However, there is no reason why you can’t automate the process using either the netsh command shown in the command-line solution or WMI. You still have to find an available IP address, which may not be easy to automate depending on your environment, but at least you can provision the IP address in an automated fashion.

This leads to a discussion of using DHCP on servers. You’ll find some people dead set against it and others who wouldn’t do it any other way. Since network adapters are by default configured to use DHCP, you can remove the step of configuring network settings, such as IP address, DNS servers, etc., on your servers by using DHCP. After the server completes the build process and reboots, it will automatically request an IP address. But as you probably know, DHCP addresses aren’t set in stone. The next time the server reboots, it might obtain a different address. And since you more than likely want to add an A record (and possibly a PTR record) in DNS for this server, if it got a new address, those records would no longer be valid.

There are two ways to work around this. First, you can configure a reservation on the DHCP server for a specific IP address. You only need to retrieve the MAC address of the server’s network adapter order to configure a reservation (you can get this via the ipconfig /all command). With this configuration, the server will always receive the same IP address even though it is getting it from DHCP. The benefit of this is that you don’t have to manually configure the other settings such as DNS and WINS servers directly on the server. The potential drawback is that if you have to change network adapters for any reason, the reservation would also have to be updated.

The second option is to use dynamic DNS to have the server automatically update DNS based on its current IP address. In fact, you could have the sever itself send the dynamic updates or use DHCP to do that. That way, even if the server gets a new IP address after every reboot, DNS will be updated automatically. The benefit to this is that it is the most automated solution. There is very little configuration you have to do on a per-server basis (and it is therefore attractive for large environments). The downside is that DNS could have outdated information for a period of time until the server dynamically updates its new information.

Yes, you surely can. You’ll need the MAC address of your network card, which you can find with ifconfig:

$ /sbin/ifconfigeth0      Link encap:Ethernet  HWaddr 00:03:6D:00:83:CF

inet addr:192.168.1.5  Bcast:192.168.1.255  Mask:255.255.255.0

UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

.....

You want the HWaddr value.

On Windows 95/98/ME, open a DOS prompt and run winipcfg. On NT/2000/XP, run ipconfig.

Make an entry in dhcpd.conf like this:

host mail1 {   hardware ethernet 00:03:6D:00:83:CF;

fixed-address 192.168.1.100;

}

Note that multiple-line directives must be enclosed in curly braces.

Name servers have their own directive in dhcpd.conf, so they don’t need MAC addresses:

option domain-name-servers 192.168.1.10, 192.168.1.11

That’s all there is to it. Use this for any machine you wish to have a static IP address.

You can still use /etc/hosts for important servers, as a fallback for local users. Remember, with /etc/hosts the network does not break when a single server goes down.

Every network card ever made has a unique 48-bit Media Access Control (MAC address). The Institute of Electrical and Electronics Engineers, Inc. (IEEE) assigns the first 24 bits, then the manufacturer assigns the remaining 24 bits.

It’s just as easy in Linux; the only hard part is that every distribution puts the configuration file in a different place. In Red Hat and Fedora, edit /etc/sysconfig/network-scripts/ifcfg-eth0 as follows:

TYPE=EthernetDEVICE=eth0

BOOTPROTO=dhcp

ONBOOT=yes

DHCP_HOSTNAME=stinkpad

On Debian, edit /etc/network/interfaces as follows:

auto loiface lo inet loopback

auto eth0

iface eth0 inet dhcp

You may wish to use a graphical configuration tool. On Red Hat and Fedora, use system-config-network. KDE and Gnome have their own GUI configuration tools (as does practically every Linux distribution), so you won’t be short of choices. Another option is netGo, a system-independent network configuration utility that lets you create profiles for easily connecting to different networks

A dhcp server can feed all network configuration data to the clients. The configuration file is /etc/dhcpd.conf. Here is a sample configuration:

# /etc/dhcpd.conf

default-lease-time 259200;

max-lease-time 518400;

subnet 192.168.1.0 netmask 255.255.255.0 {

   option subnet-mask 255.255.255.0;

   option broadcast-address 192.168.1.255;

   option routers 192.168.1.1;

   option domain-name "test.net";

   range 192.168.1.50 192.168.1.100;

   option domain-name-servers 152.163.199.56, 198.83.210.28;

}

This is pretty straightforward. The lease times are in seconds, so the minimum and maximum in this example are three days and six days. "Option routers" points to your Internet gateway, or the gateway to the subnet. A pool of 50 addresses is made available in the "range." The name servers are either your own private caching server or servers, or the name servers at your ISP.

This example uses private, nonroutable IPv4 (Internet Protocol Version 4) addresses. Here are the private IPv4 address classes, in both dotted-quad and Classless Inter-Domain Routing (CIDR) notation:

10.0.0.0     - 10.255.255.255  (10/8)

172.16.0.0   - 172.31.255.255  (172.16/12)

192.168.0.0  - 192.168.255.255 (192.168/16)

These are for use on private networks, so you’ll select your subnet ranges from these.