70-410 Chapter 11
An exclusion is a range of IP addresses within a scope that you do not want to have used by DHCP in leasing addresses to clients. Typically, these are IP addresses of computers such as
servers configured with static IP addresses.
066 boot sector host name
specified hostname of the tftp server used in servicing pxe boot clients
015 DNS domain name
specifies the DNS domain name for all client reservations
006 DNS servers
specifies the IP address of the Network's DNS nameservers
044 WINS/NBNS servers
specifies the IP address of the networks NetBIOS name servers
Failure to obtain an IP address reservation
If you attempt to add a reservation for an IP address outside the range of a configured scope, you will receive a message stating The specified DHCP client is not a reserved client. You must use an IP address that is contained within an existing scope when configuring an IP address reservation. Use an exclusion range to ensure that another client does not receive the same IP address.
Clients are unable to receive an IP address
This might mean that all available IP addresses within the range of existing scopes have been allocated. You should extend the scope or create a new scope, according to the overall network configuration.
DHCP in Windows Server 2012 R2 provides option classes, which facilitate
the introduction of custom applications on enterprise networks.
When an IPv4 client computer starts up and discovers that it requires an IP address, it initializes a limited version of TCP/IP and then broadcasts a request for
the location of a DHCP server.
This broadcast indicates to the listening server that the client needs IP addressing information. The broadcast packet sent by the DHCP client at this stage contains the ___request, as well as the ___address for the client, 0.0.0.0, and the ___address, which is the broadcast address 255.255.255.255.
lease, source, destination
When using Windows Deployment Services (WDS) to install operating systems on new computers, the ___of the new computer must be compliant with the Preboot Execution Environment (PXE).
network interface card (NIC)
DHCP Scopes and Options After you have installed and started the DHCP Server service, you need to configure a scope of configuration information. Every DHCP server requires at least one scope with a pool
of IP addresses available for leasing to clients.
You can configure DHCP so that a DHCP server always assigns the same IP address to a client computer. This feature is known as a client
reservation.
The DHCP server is also smart enough to ___the IP address it has just offered. This ensures that it will not offer it to another DHCP client, thus avoiding duplicate IP addresses.
reserve
In Windows Server 2012 R2, DHCP is installed and configured as a server ___
role.
Note that many of the communications are in the form of broadcasts. The broadcast nature of these communications can present a major problem if ___on an internetwork are not capable of forwarding these DHCP messages.
routers
DHCP works by providing IP addressing information from a pool of addresses called a___, which is defined in the DHCP server's database.
scope
By specifying option classes, you can differentiate
groups of DHCP clients and specify customized options that apply only to the specified group of clients.
The packet also includes the client's ___address and computer name; recall that network communication often boils down to a hardware address of one kind or another.
hardware (MAC)
If the client is unable to renew its lease with the original DHCP server at this time, it continues to use
its currently leased IP address and any other configuration data it received from its DHCP server with the original lease.
It is worth remembering that the world does not stand still during this process—a DHCP server might be handling DHCP traffic from multiple clients at any given time. Therefore, it needs to
keep track of the IP addresses it is offering and dishing out.
Server
Specific for either IPv4 or IPv6, it serves as defaults for all scopes configured on the server.
003 router
Specifies the IP address of the default gateway
Client obtains an inappropriate IP address
Such an error can cause an inability to communicate on the network. This might occur if the client is connected to the wrong network or if the scope has been incorrectly configured. Check the scope configuration as well as the location of the client computer.
If a client cannot obtain a lease of an IP address from a DHCP server, it cannot initialize ___normally.
TCP/IP
The DHCPACK contains
a valid lease for an IP address and other configuration information, which a network administrator might have specified in a DHCP scope.
You can create multiple scopes to act as a ___method for other DHCP servers and for assigning IP addresses specific to a subnet, such as default gateway addresses.
backup
Similar to other DHCP messages, this packet is sent as a ___message, which is converted to the MAC address on the network.
broadcast
A client that receives a DHCPNACK must start the whole DHCP initialization process over from scratch. That is, it must
broadcast another DHCPDISCOVER packet looking for a fresh IP address from any available DHCP server.
047 netbios scope ID
specifies the local netbios scope ID. Hosts can communicate only with other hosts configured with the same scope ID
067 boot name file
specifies the name of a boot image file on the tftp server used in servicing pxe boot clients the
046 WINS/NBNS servers
specifies the type of NETBIOS over TCP/IP name resolution to be used by the client
060 vendor class identifier
specifies the vendor type and configuration of the DHCP client
When 50 percent of the lease time has expired, a client will attempt to renew its lease with the DHCP server that provided its lease and configuration information. It makes the renewal attempt by sending a DHCPREQUEST packet directly to
that DHCP server.
Recall from earlier in this chapter that when a client computer starts up, it broadcasts a DHCPDISCOVER message to locate a DHCP server and obtain TCP/IP configuration information. What happens if there is no DHCP server on the subnet to which the client belongs? Broadcast messages do not cross routers unless the router can recognize and pass BOOTP broadcasts to other subnets, a feature known as
BOOTP-forwarding.
If the DHCP server and the IP address are both available, the server will renew the client's IP addressing information by sending a ___to the client with the renewed lease duration and any updated configuration information.
DHCPACK
The DHCP server responds to the client that made the selection by assigning IP addressing information to the client. After it has done so, it also acknowledges that it has assigned IP addressing information to that client through a special acknowledgement packet called a___, which it sends to the client.
DHCPACK
This server acts as a DHCP proxy by listening for DHCPDISCOVER broadcasts and translating them into ___messages that are directed to the IP addresses of all DHCP servers on adjacent subnets the DHCP relay agent knows about.
DHCPINFORM
The name of the actual packet sent is a ___packet.
DHCPREQUEST
General tab
Enables you to adjust the IP address range and lease duration, as shown in Figure 11-7.
The DHCPDISCOVER message indicates to the server that the client computer is ___-enabled, and after the client receives a valid IP address, it attempts to locate and connect to the WDS server to download a network boot program.
PXE
A DHCP relay agent is a Windows Server 2012 R2 computer configured with the ___role (also known as Routing and Remote Access) to pass BOOTP broadcasts
Remote Access
To extend an expiring IPv6 address lease, the DHCPv6 server will send a
Renew message.
To continue using its leased IP address, a DHCP client will attempt to renew its lease before it expires. This will occur at default intervals during the life of the lease. It sends the renewal request automatically if TCP/IP is still initialized on the client. The client receives a response if it is still on the same network or subnet and is
able to communicate with the DHCP server.
PXE enables a computer to
access an image across the network to simplify and streamline installations. The computer connects to a WDS server, which then installs the operating system via the network without the need for a CD, DVD, or other media..
A DHCP relay agent is
another means to forward BOOTP broadcasts to other subnets in search of a DHCP server.
In an Active Directory Domain Services (AD DS) domain, you must ___the DHCP server in Active Directory before it can lease IP addresses.
authorize
After you have configured WDS with appropriate images, you will need to ensure that your DHCP server is configured with options 066 and 067. These options enable clients to
connect to the WDS server using PXE. To configure the options, perform the following steps:
DHCP in Windows Server 2012 R2 provides for a___, to which all DHCP clients belong by default. Any options you define for this class automatically apply to all DHCP clients. user class Options assigned to this class can be overridden by options assigned to other user classes.
default
A DHCP server leases an IP address to the DHCP client for only a specified term. The default term is
eight days.
If a client accepts the address, it can use the address for a predefined period called a
lease.
In an Active Directory Domain Services (AD DS) domain, you must authorize the DHCP server in Active Directory before it can lease IP addresses. This is to prevent
rogue DHCP servers from leasing improper IP addresses that would result in communication problems.
3. The client sends a Request message to the server to request address and configuration settings (corresponding to the DHCPREQUEST message). If the client does not receive an Advertise message, it uses a stateless address configuration protocol to obtain IPv6 configuration information.
...
4. The DHCP server sends a Reply message (which corresponds to the DHCPACK message) to the client to confirm acceptance of the Request message and assignment of the IPv6 address and configuration settings.
...
The client must use the IP lease process when it is in any of the following states:
-A client configured to use DHCP is initializing TCP/IP for the very first time. -A client requests a specific IP address but the server has denied that IP address, as could occur if a DHCP server dropped a lease. -The client had previously leased an IP address but has since released that IP address and now requires a new lease. This can occur when the user has typed the ipconfig /release and ipconfig /renew commands.
The listening DHCP servers can make two possible responses:
-Any DHCP server can respond with a DHCPACK message to renew the lease. However, the renewal is most likely to come from the DHCP server that originally leased the IP address, although the location of an IP address or an IP address scope is not tied to one DHCP server forever. For the record, it is possible for a DHCP client to renew the same IP address lease from a different DHCP server. -Any DHCP server can respond with a DHCPNACK message. DHCPNACK messages force a client to reinitialize and to obtain a lease for a different IP address.
Occasionally, a DHCP lease request can be unsuccessful after the client has accepted a lease offered by the server. This can happen in situations such as the following:
-The IP address is not valid because the client has been moved to another subnet. -The client is attempting to lease its previous IP address and the IP address is no longer available.
To further ensure that there is no confusion over which server's offer the client is accepting, the client includes the following additional information in the request packet:
-The IP address of the server, whose offer it accepted -The client's hardware address -The IP address the client is accepting
To be able to respond to a DHCPDISCOVER packet, a DHCP server must have valid IP configuration information for the client. Any DHCP server with valid IP information can respond to the client with a DHCPOFFER packet containing the following information:
-The client's hardware address (allows the unique identification of the DHCP client) -An offered IP address An appropriate subnet mask -A duration for the lease -A server ID, which would be the IP address of the DHCP server
1. The client sends a Solicit message from its link-local address to the All_DHCP_Relay_Agents_and_Servers address of FF02::1:
...
2. This message corresponds to the DHCPDISCOVER message used with IPv4. 2. A DHCP server receiving this message sends an Advertise message (which corresponds to the DHCPOFFER message) to the client. This message informs the client that the server can provide address and configuration settings.
...
An IPv6 client attempts DHCPv6 configuration according to the values of the M and O flags in router advertisement messages. If the M flag is set to 1, the client requesting addressing information participates in a four-step process similar to that used with IPv4, as follows:
...
Any option applied at a later stage in this sequence always overwrites a conflicting option that was applied earlier in this sequence. For example, server options are overwritten by any conflicting option applied at any of the other levels, and client options always overwrite other options applied at any level.
...
As already noted, the Create Scope Wizard enables you to specify several options related to the scope. DHCP in Windows Server 2012 R2 provides a considerable range of options you can use to specify additional TCP/IP-related parameters to client computers on your network. You can specify these options at any of the following four levels:
...
Configuring DHCP Scope Properties Each scope you create on your DHCP server has a set of properties associated with it. Right-click the scope and select Properties to display the dialog box shown in Figure 11-7. The four tabs enable you to configure the following properties:
...
IPv6 clients can use address autoconfiguration to automatically configure themselves without DHCPv6 using a link-local address and router discovery. This enables the host to determine the addresses of routers, servers, and other configuration parameters. The following types of address configuration can be used:
...
The following are a few problems you might encounter with a DHCP server, together with suggestions for resolution:
...
The following two types of option classes are available in Windows Server 2012 R2:
...
This can be useful if you have a collection of servers, perhaps a development pool of virtual machines you don't want to receive IP addresses from DHCP Server A, for example.
...
Two address autoconfiguration flags are used to determine the use of stateful and stateless address autoconfiguration on an IPv6 network:
...
As with any other server role, things can and do go wrong with a DHCP server. If problems occur with a DHCP server, client computers might not receive proper IP leases and network communication errors might result. Further, it is important that you know whether your DHCP server is encountering performance bottlenecks or other issues. You can obtain DHCP server monitoring data from any of the following three locations:
... -DHCP statistics -Statistical information provided by Performance Monitor -Events recorded by Event Viewer
Table 11 - 3 common DHCP options
...4
If this is also not possible, it resorts to ___to give itself an IP address when the lease expires.
APIPA
After restoring the DHCP server from backup, clients receive IP addresses that are already in use
After restoring the DHCP server from backup, it might be unaware of which IP addresses it has leased from a given scope. The Conflict detection parameter is provided for this purpose. By default, this parameter is set to 0, which disables conflict detection. When this parameter is set to a nonzero value, the DHCP server uses the ping utility to test an IP address before leasing it to a client; the value represents the number of times the server performs this test. The value can range from 0 to 6. Higher values perform a more thorough test at the expense of server resources. To specify a value for this parameter, right-click the DHCP server in the console tree of the DHCP snap-in. From the Advanced tab of the Properties dialog box, type a value between 1 and 6 in the Conflict detection attempts text box, and then click OK.
Scope
Applies only to the scope for which it is specified.
Client
Applies only to the specified client computer.
Option class
Applies to all computers belonging to the defined option class. Option classes are discussed later in this chapter.
If the client cannot make contact with a DHCP server and consequently fails to maintain its lease, the client must discontinue use of the IP address and begin the entire process again by issuing a ___packet.
DHCPDISCOVER
The inclusion of the hardware address reveals the origin of the request to DHCP servers. The actual packet that the client issues to request an IP address from a DHCP server is called a ___packet.
DHCPDISCOVER
In either of these situations, the server would broadcast an unsuccessful (negative) acknowledgment packet or
DHCPNACK.
The DHCP client selects the IP addressing information it requires from the first ___packet it receives—that is, the offer from the server quickest off the mark to supply the information.
DHCPOFFER
The client can still use the address because only 50 percent of the lease duration has expired. If a client still does not have a renewed lease after 87.5 percent of the active lease period has gone by, it will attempt to communicate with any DHCP server on the network to secure IP addressing and configuration information by broadcasting a ___packet.
DHCPREQUEST
Other Stateful Configuration (O) flag
Determines how additional IPv6 configuration parameters are obtained. This includes such settings as the IPv6 addresses of Domain Name System (DNS) servers. When set to 1, DHCPv6 is used to obtain these types of information. If the M flag is set to 0 and the O flag is set to 1, a combination known as DHCPv6 stateless is being used, where DHCPv6 is assigning additional stateless configuration settings but not stateful addresses to IPv6 clients.
Managed Address Configuration (M) flag
Determines when DHCPv6 is used to obtain IPv6 stateful addresses. When set to 0, DHCPv6 is not used and stateless addresses are obtained. When set to 1, DHCPv6 is used to assign stateful addresses to IPv6 clients.
DNS tab
Enables you to automatically update the host (A) and pointer (PTR) records on the authoritative DNS servers. More information on this process is provided in Chapter 12, "Deploying and Configuring Domain Name System (DNS)."
Advanced tab
Enables you to select whether the scope provides IP addressing information to DHCP clients, BOOTP clients, or both. You can adjust the lease duration for BOOTP clients, which is 30 days by default.
Network Access Protection tab
Enables you to set up Network Access Protection (NAP) settings for the scope. NAP is discussed in the Cert Guide for exam 70-411.
You can create a scope for each physical subnet on your network and use this scope to define networking parameters such as the range of ___and their subnet masks, lease duration values, scope options, and client reservation options.
IP addresses
The DHCPDISCOVER packet and all that it contains represent the client's
IP lease request.
Client reservation maps the IP address to a specific ___address and is useful for configuring servers that must always be reached at the same IP address with a specified set of options. You can include client-specific options that override conflicting server- and scope-based options.
MAC
Using the DHCP filter function, you can specify a
MAC address or range and allow or deny IP address assignment on the network.
Using the filter function on your DHCP server, you can add the MAC addresses for each virtual server and create a deny rule. On the other hand, perhaps you have created a development DHCP server and want it to hand out addresses to only specific hosts. You can choose to create an allow rule for the specific MAC addresses. Although this process might work, it's not a guarantee because
MAC addresses can be spoofed.
When the computer initializes a PXE boot, the PXE ROM requests an IP address from the DHCP server using the normal ___process described earlier in this chapter.
four-step
IP address conflicts
This can occur if a user has manually configured a computer with a static IP address that is within the range of a DHCP scope. Ensure that the user selects the Obtain an IP address automatically option If there is a reason that the chosen IP address must be retained, configure an exclusion so that the DHCP server does not lease this address. This error might also occur if more than one DHCP server is configured with overlapping scopes.
User classes
Used to differentiate clients according to their type, such as desktop, laptop, or server computer. For example, you can group mobile computers into a specific class and apply options such as shorter lease times to only these computers by supplying them with the relevant class ID. You can also define class identifiers that specify information such as a client's software configuration, physical location within a building, operating system in use, and so on.
Vendor classes
Used to identify a client's vendor type and configuration when obtaining a DHCP lease. You can use the vendor class ID option (code 60) to specify vendor classes. This option includes an identifier with a string of character data readable by the DHCP servers. Often used with vendor classes are standard reserved hardware and operating system codes defined in RFC 1700.
Both types of addresses
Uses Router Advertisement messages that include address prefixes and stateful address configuration protocols.
Stateless address autoconfiguration
Uses Router Advertisement messages to configure link-local addresses and additional addresses by exchanging Router Solicitation and Router Advertisement messages with neighboring routers.
Stateful address autoconfiguration
Uses a stateful address configuration protocol such as DHCPv6 to obtain non-link-local addresses and other IPv6 configuration parameters.
The DHCP server and the DHCP client both need to go through a ___-phase process before DHCP configures the client with a working set of TCP/IP parameters.
four
At this time, it broadcasts this information out onto the network. In this broadcast, the client requests the IP address the server has proposed for it. The explanation for including the IP address request in the broadcast is that
the client could have received more than one offer if there are other DHCP servers on the network. By broadcasting its request, the client announces to any other DHCP servers that this client will not be accepting their offers.
The server sends a DHCPOFFER packet to the client's hardware (MAC) address because
the client does not yet have an IP address.
Any DHCP server with valid IP addressing information will respond to a needy DHCP client with an offer of IP addressing information. It responds with one of the ___IP addresses from a scope of addresses that are valid for that specific host.
unassigned
Upon receipt of the DHCPACK, the client
updates its configuration.
In addition to requesting a specific IP address from a DHCP server, this packet asks other DHCP servers on the network to ___their offers of an IP address if they have made any.
withdraw
