MCSA Guide to Networking with Windows Server 2016, Exam 70-741 First Edition Chapter 4 Implementing DHCP 2018 Cengage. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part, except for use as permitted in a license distributed with a certain product or service or otherwise on a password-protected website for classroom use.
Objectives 4.1 Describe the DHCP protocol and process 4.2 Install and configure a DHCP server 4.3 Configure a DHCP server 4.4 Implement DHCPv6 4.5 Configure DHCP high availability
An Overview of Dynamic Host Configuration Protocol Dynamic Host Configuration Protocol (DHCP) is a component of the TCP/IP protocol suite Used to assign an IP address to a host automatically from a defined pool of addresses IP addresses assigned via DHCP are usually leased, not permanently assigned To prevent a disruption in communication, the client attempts to renew the lease when the lease interval is 50% expired DHCP is based on broadcast packets
The DHCP Address Assignment Process (1 of 2) The request for an IP address between a DHCP client and a DHCP server is a 4-packet sequence: 1. DHCPDISCOVER - client transmits a broadcast packet via UDP source port 68 and UDP destination port 67 to the network 2. DHCPOFFER - DHCP server received the DHCPDISCOVER packet and responds with an offer 3. DHCPREQUEST - client responds by requesting the offered address 4. DHCPACK - the server acknowledges the transaction and sends IP parameters to the client
The DHCP Address Assignment Process (2 of 2)
DHCP Address Renewal (1 of 2) A client uses unicast packets rather than broadcast packets in the lease renewal process The client already knows the address of the server A successful renewal is a 2-packet sequence: 1. DHCPREQUEST - when the lease is 50% expired, the client sends a unicast packet to request a renewal If server doesn t respond, client will try 3 more times 2. DHCPPACK - if server can honor the renewal request, will send a unicast packet granting and acknowledging the renewal request
DHCP Address Renewal (2 of 2) The renewal request may fail in these situations: The server responds but can t honor the renewal The requested address may have been deleted or deactivated from the scope The server doesn t respond 1. The client keeps its current address until 87.5% of the lease interval has expired and sends out a DHCPREQUEST broadcast packet 2. Two possible results from the DHCPREQUEST A DHCP server responds No DHCP server responds and client starts the process all over again
DHCP Messages Message Number Message name Description 1 DHCPDISCOVER Sent by a client to discover an available DHCP server and request a new IP address. 2 DHCPOFFER Sent by the server in response to a DHCPDISCOVER with an offer of an IP address. 3 DHCPREQUEST Sent by a client to request a lease on an offered IP address in response to a DHCPOFFER or to renew an existing lease. 4 DHCPDECLINE Sent by a client in response to a DHCPOFFER to decline an offered IP address. 5 DHCPACK Sent by the server to acknowledge a DHCPREQUEST or DHCPINFORM. 6 DHCPNAK Sent by the server in response to a DHCPREQUEST. 7 DHCPRELEASE Sent by a client to release a leased address. 8 DHCPINFORM Sent by a client to request additional configuration.
Installing and Configuring a DHCP Server (1 of 2) DHCP Server role can be installed by using the Add Roles and Features Wizard via Server Manager or the following PowerShell cmdlet: Install -WindowsFeature DHCP - IncludeManagementTools After installing, the DHCP console is available on the Tools menu in Server Manager In a Windows domain, the DHCP server must be authorized, and a scope must be created before the server can provide DHCP services
Installing and Configuring a DHCP Server (2 of 2)
DHCP Server Authorization DHCP clients have no way of determining whether a DHCP server is valid Reason for authorization: A rogue DHCP server could be installed on a network and incorrect IP address settings could be configured on client computers DHCP server authorization requires Enterprise Administrator credentials To authorize with PowerShell, type: Add -DhcpServerInDC
DHCP Scopes (1 of 2) DHCP scope - a pool of IP addresses and other IP configuration parameters that a DHCP server uses to lease addresses to DHCP clients A scope consists of the following required parameters: Scope name Start and end IP addresses Prefix length or subnet mask Lease duration - specifies how long a DHCP client can keep an address
DHCP Scopes (2 of 2)
Exclusion Ranges Addresses can be excluded in two ways: De facto exclusion - you set the start and end IP addresses in the scope so that several addresses in the subnet fall outside the scope s range Create an exclusion range - consists of one or more addresses in the scope that the DHCP server doesn t least to clients They can be created when the scope is created with the New Scope Wizard or afterward by right-clicking the Address Pool node under the scope and clicking New Exclusion Range
Reservations Reservation - an IP address associated with the MAC address of a DHCP client to ensure that when the client requests an IP address, it always gets the same one The IP address in the reservation must fall within the same subnet and use the same subnet mask as the scope You can force a client to release a reserved address and get a different address by typing: ipconfig /release and then ipconfig /renew
Multiple Subnets, Multiple Scopes (1 of 2) A DHCP scope can service a single subnet DHCP is based on broadcasts, which can t traverse routers Three methods for handling a network with multiple subnets: Configure a DHCP server in each subnet Configure a single DHCP server with network interfaces connected to each subnet and scopes defined for each subnet Configure DHCP relay agents on subnets that don t have a DHCP server
Multiple Subnets, Multiple Scopes (2 of 2)
Configuring Superscopes and Multicast Scopes (1 of 3) Superscope - a special type of scope consisting of one or more member scopes that allows a DHCP server to service multiple IPv4 subnets on a single physical network Directs the DHCP server to draw addresses from both scopes Useful if the number of computers on a physical network exceeds the size of the original subnet Each scope that s part of a superscope is referred to as a member scope
Configuring Superscopes and Multicast Scopes (2 of 3)
Configuring Superscopes and Multicast Scopes (3 of 3) Create superscopes in the DHCP console by right-clicking the IPv4 node Click New Superscope and follow the New Superscope Wizard You can also create a superscope with the following PowerShell cmdlet: Add-DhcpServerv4SuperScope -SuperscopeName NewSuperScope -ScopeID 10.12.0.0,10.1.0.0
Configuring Multicast Scopes An IPv4 multicast packet is addressed to a group of hosts listening on a particular multicast IP address Multicast scope - allows assigning multicast addresses dynamically to multicast servers and clients with the Multicast Address Dynamic Client Allocation Protocol (MADCAP) Two common ranges of multicast addresses: Administrative scopes Global scopes Configure multicast scopes in the DHCP console or with PowerShell cmdlets
DHCP Options DHCP options can be assigned at the following levels: Server options - affect all scopes but can be overridden by a scope, policy, or reservation option Scope options - affect clients that get a lease from the scope in which the option is configured Policy options - allow an administrator to assign IP address options to clients based on client properties Reservation options - based on a computer s MAC address Take precedence over any conflicting options set at any other level
Common DHCP Options DHCP options are specified in the format NNNOptionName NNN represents a three-digit number that identifies the option in the DHCP packet Common options: 003 Router 006 DNS Servers 015 DNS Domain Name 044 WINS/NBNS Servers 046 WINS/NBT node type
Configuring Options (1 of 2) Server options are configured by clicking the IPv4 or IPv6 node in the DHCP console Right-click Server Options, click Configure Options The Server options dialog box has two tabs: General - has a list of available options in the upper pane Advanced - has the same list of options as well as a list box to choose the Vendor Class A field in the DHCP packet that is used to identify a device model or an OS version
Configuring Options (2 of 2)
DHCP Server Configuration (1 of 2) When performing DHCP server configuration tasks, right-click the server node and you can choose from the following tasks: Add/Remove Binding Backup Restore All Tasks Delete Refresh Properties
DHCP Server Configuration (2 of 2) Right-click the IPv4 node and you can choose from the following options: Display Statistics New Scope New Superscope New Multicast Scope Configure Failover and Replicate Failover Scopes Define User Classes and Define Vendor Classes Reconcile All Scopes Set Predefined Options Properties
Configuring IPv4 Server Properties (1 of 3) The IPv4 Properties dialog box has six tabs: General - specify statistics and logging parameters DNS - configure how DHCP interacts with a DNS server for making updates on behalf of DHCP clients Network Access Protection (NAP) - enable or disable NAP on all scopes Filters - configure MAC address filters to allow or deny DHCP services to computers Failover - configure and view failover status Advanced - offers many advanced options
Configuring IPv4 Server Properties (2 of 3)
Configuring IPv4 Server Properties (3 of 3)
DHCP Name Protection (1 of 2) Name squatting - A DNS problem that occurs when a non- Windows computer registers its name with a DNS server, but the name has already been registered by a Windows computer DHCP name protection - prevents name squatting by non- Windows computers by using a DHCP resource record called Dynamic Host Configuration Identifier (DHCID) To configure name protection, right-click the scope or server node in the DHCP console and click Properties Click the DNS tab Click the Configure button in the Name Protection section Click the Enable Name Protection check box to enable or disable name protection
DHCP Name Protection (2 of 2)
Configuring Scope Properties (1 of 2) The Scope Properties dialog box has four tabs: General - change the scope name and the start and end IP addresses DNS - contains dynamic DNS configuration options Network Access Protection - enable or disable NAP on the scope Advanced - configure which type of clients the server responds to
Configuring Scope Properties (2 of 2)
Configuring Filters DHCP filters - allow administrators to restrict which computers on a network are leased IP addresses Filters use MAC addresses as the filtering criteria Filters are configured under the IPv4 node and are not available for IPv6 DHCP If you create an allow filter, only a device with a MAC address in the filter can lease an IP address from the DHCP server If you create a deny filter, all devices except those with a MAC address in the filter can lease an address
Configuring Policies (1 of 2) DHCP policies - give administrators more fine-tuned control over address lease options Contain conditions that specify one or more clients that IP address settings should be delivered to Policy conditions can be created with any combination of the following: Vendor Class User Class MAC address Client identifier Fully qualified domain name Relay agent information
Configuring Policies (2 of 2) Policies can be configured at the server level or the scope level Scope-level policies take precedence over server-level policies if both are configured and there s a conflict Server-level policies are limited to assigning DHCP options and lease duration to clients matching the policy conditions To create a policy, right-click the Policies node under the IPv4 node or the scope and click New Policy to start the DHCP Policy Configuration Wizard, then follow these steps: 1. Give the policy a name and description 2. Create one or more conditions that identify devices 3. Configure settings for the policy 4. Configure additional settings in the policy s Properties dialog box
Configuring DHCP for PXE Boot (1 of 2) If using WDS to install Windows OSs on computers You might need to configure DHCP to respond to Preboot Execution Environment (PXE) network interfaces PXE is a network environment built into many network interface cards (NICs) that allows a computer to boot from an image stored on a network server Common setups that require special DHCP configuration: The DHCP server is on a different server or a different subnet from the WDS server DHCP is installed on the same server as WDS, but it s not a Microsoft DHCP server, or the Microsoft DHCP server is installed after WDS was installed
Configuring DHCP for PXE Boot (2 of 2)
DHCP Relay Agents (1 of 3) DHCP relay agent - a device that listens for broadcast DHCPDISCOVER and DHCPREQUEST messages and forwards them to a DHCP server on another subnet You can configure a DHCP relay agent on a subnet that doesn t have a DHCP server You can still manage DHCP addresses from a central server without having to configure the DHCP server with network interfaces in each subnet
DHCP Relay Agents (2 of 3)
DHCP Relay Agents (3 of 3) DHCP relay process: 1. A client on the same subnet as the relay agent sends a DHCPDISCOVER broadcast 2. The relay agent forwards the message to the DHCP server s IP address as a unicast 3. The DHCP server receives the message. Since the relay agent s address is in the message, the server draws an address from scope matching the relay agent s address 4. The DHCP server sends a unicast DHCPOFFER to the subnet the message was received from 5. The relay agent forwards the DHCPOFFER as a broadcast to the subnet it was received from 6. The DHCP client broadcasts a DHCPREQUEST 7. The relay agent forwards it to the DHCP server 8. The server replies with a DHCPACK to the relay agent 9. The relay agent forwards the DHCPACK to the client 10. Renewal requests are unicast packets, so the client can communicate directly with the server
Installing a DHCP Relay Agent (1 of 2) The DHCP relay agent function is configured as part of the Routing role service under the Remote Access server role To make a Windows Server 2016 server a DHCP relay agent, follow these steps: 1. Install the Remote Access server role and include the Routing role service 2. In the Routing and Remote Access console, right-click the server node and click Configure and Enable Routing and Remote Access 3. Click Custom configuration in the Routing and Remote Access Server Setup Wizard 4. Click the LAN routing check box, click Next and then Finish and click Start service when prompted 5. In the Routing and Remote Access console, expand the IPv4 node and right-click General node and click New Routing Protocol 6. In the Routing and Remote Access console, right-click DHCP Relay Agent and click New Interface
Installing a DHCP Relay Agent (2 of 2) To make a Windows Server 2016 server a DHCP relay agent, follow these steps (continued): 7. In the DHCP Relay Properties dialog box, accept the default settings and click OK 8. In the Routing and Remote Access console, right-click the DHCP Relay Agent and click Properties Type the address of the DHCP server the relay agent should forward DHCP messages to and click Add, click OK
Server Migration, Export, and Import (1 of 2) To migrate a DHCP server from Server1 to Server2, follow these steps: 1. On Server1, create a folder named C:\Export or similar and change the directory to that folder. Export the DHCP server configuration and database using one of the following methods: Use the following PowerShell cmdlet: Export-DhcpServer -File Dhcp.xml -Leases Use the following Command Prompt command: netsh dhcp server export Dhcp.txt all 2. On Server1, copy the exported file to Server2. 3. Unauthorize the DHCP server, stop the DHCP service, or uninstall the DHCP Server role on Server1. 4. On Server2, install the DHCP server role and authorize it, if necessary
Server Migration, Export, and Import (2 of 2) To migrate a DHCP server from Server1 to Server2, follow these steps (continued): 5. On Server2, import the exported file using one of the following methods: Use the following PowerShell cmdlet: Import-DhcpServer -File C:\Dhcp.xml -Leases - BackupPath C:\dhcpback Use the following Command Prompt command: netsh dhcp server import C:\Dhcp.txt all 6. On Server2, verify that the scope and existing leases were imported and that the DHCP service isrunning.
Troubleshooting DHCP DHCP is a generally reliable protocol, but things can and do go wrong from time to time: A client is not receiving a DHCP address A client is receiving an incorrect DHCP address IP address conflicts are occurring The DHCP server service is not starting No addresses are being leased DHCP Troubleshooting Tools Reviewing DHCP server statistics and reconciling scopes Third-party protocol analyzer (such as Wireshark) The built-in DHCP audit logging feature Enabled by default
Implementing DHCPv6 DHCPv6 enables you to: Manage IPv6 address assignment better See which addresses are being used on the network Control IPv6 address options
IPv6 Address Structure Uses 128 bits, instead of IPv4 s 32 bits, for an address IPv6 addresses are written as eight 16-bit hexadecimal numbers separated by colons: Fe80:0:0:0:18ff:0024:8e5a:60 Things to note about IPv6 addresses: One or more consecutive 0 values can be written as a double colon, but only one double colon can exist in an IPv6 address Leading 0s are optional Hexadecimal numbers are easier to convert to binary
The IPv6 Interface ID The interface ID of an IPv6 is typically the last 64 bits of an IPv6 address Can be assigned to a host in these ways: Using the 48-bit MAC address - first two zeros in a MAC address are replaced with 02 This autoconfigured 64-bit host ID is referred to as an Extended Unique Identifier (EUI)-64 interface ID A randomly generated permanent interface identifier A temporary interface identifier
IPv6 Autoconfiguration IPv6 autoconfiguration occurs by two methods: Stateless autoconfiguration - the node listens for router advertisement messages from a local router Stateful autoconfiguration - the node uses an autoconfiguration protocol, such as DHCPv6, to obtain its IPv6 address and other configuration information
How Autoconfiguration Works on Windows Hosts The Windows autoconfiguration steps: 1. At initialization, a link-local address is determined 2. The link-local address is verified as unique by using duplicate address detection 3. If the address is verified as unique, the address is assigned to the interface 4. The host transmits a router solicitation message 5. If no router advertisement messages are received in response to the solicitation message, the host attempts to use DHCPv6 to get an address 6. If router advertisement message is received, the prefix in the router advertisement is used along with the interface ID to configure the IPv6 address on the interface
Configuring DHCPv6 Scopes (1 of 2) Configure DHCPv6 scopes in the DHCP console or with the Add-DhcpServerv6Scope PowerShell cmdlet To configure a DHCPv6 scope you need to provide the following information: Scope name Prefix Preference Exclusions Scope lease
Configuring DHCPv6 Scopes (2 of 2)
DHCPv6 Operation DHCPv6 uses a DHCP Unique Identifier (DUID) to lease an address A hexadecimal number, usually derived from the network interface s MAC address After a DHCPv6 scope is created and activated The DHCPv6 server can assign IPv6 addresses Dynamic assignment of IPv6 addresses is similar to the process for IPv4 addresses DHCPv6 client listens on port 546 and the server listens on port 547
DHCPv6 Options You configure DHCPv6 options just as you do for standard DHCPv4 There s no option in DHCPv6 to assign a default gateway (router) This task is handled by router advertisements sent by routers on the network You can configure options at the server level, the scope level, and the reservation level There are no policies or filters for DHCPv6
DHCP High Availability Microsoft offers the following ways to achieve high availability for DHCP: Split scopes DHCP failover DHCP server cluster Hot standby Most recommended methods for high availability and fault tolerance: Split scopes and DHCP failover
DHCP Split Scopes (1 of 2) Split scope - a fault-tolerant DHCP configuration in which two DHCP servers share the same scope information Allows both servers to offer services to clients Create a split scope by using the wizard as follows: 1. Install the DHCP Server role on two servers 2. Create a scope on the first server and activate it The wizard configures the scope on the second server 3. Run the wizard on the first server 4. The wizard creates the scope on the secondary server and creates the necessary exclusion range According to the split percentage on both servers 5. Create reservations on both servers
DHCP Split Scopes (2 of 2)
DHCP Failover DHCP failover - allows two DHCP servers to share the pool of addresses in a scope Lease information is replicated between servers If one server goes down, the other server maintains lease information Only available in IPv4 scopes Two modes for DHCP failover: Load-balancing mode (default mode) Hot standby mode
Configuring Load-Balancing Modes (1 of 2) Configure DHCP failover in the DHCP console Right-click the IPv4 node or the target scope Click Configure Failover The Configure Failover Wizard will guide you through the process If you configure failover from the IPv4 node, all scopes are listed and selected by default You will choose the partner server Must be an authorized server that already has the DHCP Server service configured You will name the failover relationship and choose whether the failover configuration will be load balancing or hot standby
Configuring Load-Balancing Modes (2 of 2)
Configuring Hot Standby Mode The process for configuring hot standby mode is almost identical to configuring load balancing mode, with the following exceptions: Select Hot standby option for failover mode Instead of choosing load balancing percentage, specify whether the failover partner is the active server or the standby server, assign a percentage
Editing or Deleting a Failover Configuration If you need to edit or delete a failover configuration, right-click the IPv4 node and click Properties
Chapter Summary (1 of 3) Dynamic Host Configuration Protocol (DHCP) is a component of the TCP/IP protocol suite that is used to assign an IP address to a host automatically from a defined pool of addresses The process of a DHCP client requesting an IP address and a DHCP server fulfilling the request is a four-packet sequence of broadcasts The DHCP service is installed as a server role named DHCP Server A DHCP scope is a pool of IP addresses and other IP configuration parameters that a DHCP server uses to lease addresses to DHCP clients A split scope is a fault-tolerant DHCP configuration in which two DHCP servers share the same scope information Almost every network requires a DNS server s IP address for name resolution and a default gateway to communicate with other subnets and the Internet
Chapter Summary (2 of 3) You can perform several DHCP server configuration tasks in the DHCP console The IPv4 server properties include statistics and logging parameters, dynamic DNS configuration, NAP configuration, filters, conflict detection, and configuration of DNS registration credentials DHCP filters allow administrators to restrict which computers on a network are leased an IP address based on the client MAC address DHCP policies give you more fine-tuned control of address lease options than you have with server, scope, and reservation options If you re using Windows Deployment Services to install Windows OSs on computers, you might need to configure DHCP to respond to Preboot Execution Environment network interfaces A DHCP relay agent is a device that listens for broadcast DHCPDISCOVER and DHCPREQUEST messages and forwards them to a DHCP server on another subnet
Chapter Summary (3 of 3) Windows Server 2016 allows you to migrate the DHCP server role to another server DHCP is a fairly reliable protocol, but at times, you may encounter basic problems The DHCP audit logging feature, which is enabled by default, provides a logging file that shows information about when addresses were leased, renewed, and released as well as information about DNS updates attempted by the DHCP server Ways to achieve high availability: split scopes, DHCP failover, DHCP server clusters, and hot standby DHCP failover allows two DHCP servers to share the pool of addresses in a scope