Installing and Configuring DHCP with NetWare 5

Transcription

1 BY JOHN E. JOHNSTON Installing and Configuring DHCP with NetWare 5 The Dynamic Host Configuration Protocol (DHCP) function and its options are essential in the creation of a TCP/IP environment. The DHCP server not only automatically provides TCP/IP configuration parameters for the computers attached to the network, it can provide such configuration parameters as the computer's TCP/IP address, the default gateway address, WINS server address(es) and DNS server addresses. ONE of the fundamental tools required in a TCP/IP environment is the Dynamic Host Configuration Protocol (DHCP) server. The DHCP server automatically provides TCP/IP configuration parameters for the computers attached to the network. DHCP can provide such configuration parameters as the computer's TCP/IP address, the default gateway address, WINS server address(es) and DNS server addresses. When you are charged with supporting hundreds or even thousands of workstations, DHCP is a must. In a non-dhcp environment, simply adding a new WINS server to your network would require that you walk around and manually add the WINS server's address to each workstation's TCP/IP parameters. With DHCP, this change would be reflected in the DHCP server, and in turn this server would propagate the change to all of your workstations. Due to space limitations, I cannot discuss all of the available DHCP options; however, I will show you how to configure and utilize the following base DHCP functions: TCP/IP Address Allocation: I will explore the implementation of Dynamic DHCP address allocation. WINS Specification: If your network consists of NetWare and Windows NT Servers, you can use the Novell DHCP service to configure the addresses of your WINS servers on your workstations. Default Router (gateway): DHCP can be used to configure your workstations to point to a default TCP/IP router. DNS Servers: Most organizations have one or more DNS servers in their TCP/IP network. DHCP can be used to configure your workstations to point to these DNS servers. The entire list of DHCP server functions is included in the DHCP documentation included on the installation CD-ROM. Before you attempt to implement DHCP, you should have a firm understanding of the functions provided by the DHCP server. Once you grasp the concepts of the DHCP server, you should develop a plan for the implementation of DHCP. I will explore the DHCP concepts and special NetWare 5 considerations in the following sections. Then I will provide DHCP configuration examples that you can follow to set up your DHCP environment. Note: When you install the NetWare 5 DHCP service, you must also install the NetWare 5 DNS function. You can then enable either or both services. The installation of the DHCP/DNS services is described in the "Installing and Configuring DNS with NetWare 5" article in the September 1999 issue of Technical Support. NETWARE 5 DNS/DHCP NDS CONSIDERATIONS If your NetWare network is part of a Wide Area Network (WAN), special care must be taken with the NDS objects that are created by the DNS/DHCP services. If you have followed Novell's NDS design guidelines, you will have a separate NDS partition in each geographic location. Plan to implement a DNS and a DHCP server in each of these geographic locations. Failure to do so will cause a great deal of traffic to be generated on your WAN links as clients request DHCP and DNS services. The following two special NDS objects are created during the installation of the NetWare 5 DNS/DHCP services, which must be accessible by all DNS and DHPC servers on your network: DNS/DHCP Group Object DNS/DHCP Locator Object

2 FIGURE 1: CREATING THE DHCP SERVER OBJECT FIGURE 2: CREATING THE SUBNET OBJECT Novell recommends that a Organizational Unit (OU), Country (C) or Locality (L) container object be created near the top of the NDS tree and the DNS/DHCP Group and Locator objects be placed in that container. The partition that is to contain the DNS/DHCP Group and Locator objects should be replicated to all parts of your network. This replication will provide fault tolerance for your DNS and DHCP services. SUBNET PLANNING Most TCP/IP networks are split into multiple subnets. This subnetting provides a means to isolate traffic on different segments of a network, rather than having all traffic traverse the entire network. Each subnet will have its own IP addressing scheme and default router. Since the clients attached to the various subnets have unique TCP/IP addressing needs, you must create a DHCP Subnet object for each of your network's subnets. This Subnet object will contain a pool of TCP/IP addresses that will be available to the clients attached to that subnet. A single DHCP server can control all of the Subnet objects deployed in a LAN. TPC/IP ADDRESS ALLOCATION METHODS The DHCP server provides the ability to allocate client TCP/IP addresses from a pool of addresses under the control of the DHCP server. This process makes the set up of client workstations much simpler and less error prone. The person setting up a client workstation is not required to obtain a TCP/IP address from a network administrator. The network administrator is not required to maintain a current list of client IP addresses, making the possibility of assigning a duplicate IP address non-existent. The following three methods can be used by the DHCP server to allocate IP addresses: Dynamic DHCP Manual or Static Dynamic BOOTP You can configure the DHCP server to use as many of these methods as required for your network. For example, you may wish to have the DHCP server provide static IP addresses for certain devices and use dynamic DHCP for all other devices. Dynamic DHCP Dynamic DHCP TCP/IP address allocation allows a client workstation to obtain an IP address from a pool of IP addresses maintained by the DHCP server. With Dynamic DHCP, the client does not permanently retain the IP address assigned by the DHCP server. This address is "leased" by the client for a specific period of time, which is configured in the DHCP server. When the lease is about to expire, the client will ask the DHCP server for a lease renewal. If the DHCP server allows the renewal, the client will retain the original TCP/IP address for another lease period. In most cases, the DHCP server would grant the lease renewal. Instances where the lease would not be renewed include: The client was turned off for an extended period of time (past the lease period) and the DHCP server has already allocated that IP address to another client. The DHCP server configuration was modified to assign addresses using a different method or from a different address range. Dynamic DHCP is useful in environments where a limited number of TCP/IP addresses are available and unused addresses must be reclaimed. Dynamic DHCP is also useful in environments where many client workstations will be temporarily connected to the network. Tip: An added benefit of Dynamic DHCP is when a lease is renewed, the DHCP server refreshes all of the parameters in the client's TCP/IP configuration. This allows the network administrator to modify the TCP/IP infrastructure (such as adding a new DNS server) and the clients will pick up this change at their next IP address lease renewal. You must plan the implementation of Dynamic DHPC or you may find that your implementation can't meet your future needs. The most important aspect of this planning is defining the pool of addresses that is to be used by Dynamic DHCP. You must also plan your lease renewal parameters. An improperly planned lease renewal can cause increased traffic on your network or the depletion of available IP addresses. Address Pool Planning Devices on your network, such as file servers, will require static TCP/IP addresses. You may also have devices that require BOOTP to obtain a TCP/IP address. You must leave enough reserved TCP/IP addresses for the devices that require static and BOOTP addresses out of the control of Dynamic DHCP. For example, if your network is using a Class B TCP/IP address range, such as , you may want to reserve the address range of through This reserves 2,530 TCP/IP addresses for static and BOOTP devices. If you are using a Class C IP address on your network, you may need to be a bit more frugal as to how many addresses you reserve.

3 FIGURE 3: SETTING THE DYNAMIC DHCP LEASE TIME This can be used on devices such as file servers, which must always have the same TCP/IP address. Dynamic BOOTP Address Allocation Dynamic BOOTP allows the DHCP server to assign a permanent IP address to clients that utilize BOOTP from a pool of addresses. This option allows the network administrator to support clients that use DHCP and clients that use BOOTP from a single server, thus reducing the complexity of the network. CREATING A DHCP SERVER OBJECT FIGURE 4: CREATING THE DYNAMIC DHCP ADDRESS POOL Lease Renewal Planning When you set up Dynamic DHCP, you will specify a lease time. This lease specifies the amount of time that a client can use the IP address assigned by the DHCP server. Keep the following points in mind when planning your Dynamic DHCP lease time: When a client's lease is about to expire, the client will attempt to renew their IP address lease from the DHCP server that provided the address. You control when the client will attempt to renew their lease by modifying the "Renewal Time (T1)" and "Rebinding Time (T2)" parameters. The "Renewal Time (T1)" parameter specifies the percentage of lease time remaining when the client should attempt to contact the DHCP server that originally issued its IP address. Most organizations specify a T1 value of 50 percent. The "Rebinding Time (T2)" field specifies the percentage of lease time remaining when the client should attempt to contact any DHCP server for a new lease. If a client has passed the T1 percentage of its lease and cannot contact the DHCP server that originally supplied its TCP/IP address, it will attempt to locate any DHCP server on the network for a new lease. You control when this occurs using the T2 parameter. The T2 parameter should be about 10 to 30 percent higher than the T1 parameter to give the client the opportunity to re-establish communications with the original DHCP server before looking for another. If TCP/IP addresses are in short supply on your network, you should specify a short lease time. This will create additional traffic on your network as well as increase the amount of NDS updates, but it will keep the maximum number of IP addresses available at all times. If you have more users than IP addresses, a short lease time is mandatory. When a lease is renewed, the client's IP configuration is refreshed. So, if you add a new DNS server to the network, the client's configuration will be modified to utilize this new DNS server at DHCP lease renewal. If you specify a long lease time, these types of changes will not be propagated in a timely manner. Manual (Static) Address Allocation You can configure the DHCP server to provide a static address to a host based upon the MAC address of the machine's NIC. Once you have the DNS/DHCP Service installed on your file server and the DHS/ DHCP Management Console is operational, you are ready to implement the DHCP server. For more information on installing the DNS/DHCP Service and the DNS/DHCP Management console, please refer to the article "Installing and Configuring DNS with NetWare 5," which appeared in the September 1999 issue of Technical Support. The first step to implementing the DHCP server is to create the DHCP Server NDS object. The following steps are used to create this object: 1. From the DNS/DHCP Management Console, click on the DHCP Services tab. 2. Click on the Create icon (the cube). The "Create New DHCP Record" dialog box will be displayed. 3. Highlight DHCP Server and click on OK. The "Create DHCP Server" dialog box will be displayed, as shown in Figure Browse through your NDS tree and select the NetWare 5 server that will be running the DHCP Server, then click on Create. CREATING A SUBNET OBJECT Once you have created the DHCP Server object, you are ready to create a Subnet Object for each subnet that this server is to control. To create a subnet object, perform the following: 1. From the DNS/DHCP Management Console, click on the DHCP Services tab. 2. Click on the Create icon (the cube). The "Create New DHCP Record" dialog box will be displayed. 3. Highlight Subnet and click on "OK." The "Create Subnet" dialog box will

4 FIGURE 5: TESTING THE DYNAMIC IP ADDRESS ASSIGNMENT of the lease that must expire before the client attempts to contact any DHCP server for a lease renewal. The T2 parameter should be higher than the T1 Time value to allow the client sufficient time to contact the original DHCP server before attempting to contact any other DHCP servers on the network. Create a Subnet Address Range Now we need to specify the actual pool of TCP/IP addresses that will be assigned to our clients. This pool is associated and controlled by the subnet object created previously. To create the Subnet Address Range, perform the following steps: be displayed, as shown in Figure 2. Enter a meaningful name for the subnet. 4. You must now select the NDS context for the Subnet Object. If this Subnet object is to be used at a WAN location, you should place the object in a branch of your NDS tree that is specific for that location. If this is a local segment, the Subnet object should be placed in a container close to the top of the tree. 5. Specify the TCP/IP address of the subnet. In this example, we are creating a subnet for a Class B network whose address is Enter the appropriate subnet mask. 7. In the "Default DHCP Server" field, select the DHCP server created in the previous step. 8. Click on the "Define Additional Properties" box, then click on "Create." Setting the Dynamic DHCP Lease Time Because you clicked on the "Define Additional Properties" button when creating the subnet object, the screen shown in Figure 3 will be displayed. There are several parameters in the subnet object that require attention. If you forgot to click on the "Define Additional Properties" button, you can get to this screen by clicking on the subnet object created previously. 1. Click on the Subnet Options tab. 2. Click on the "Timed" option under "Lease Type." 3. Specify the number of days, hours and/or minutes in the lease time fields. 4. Next, enter the T1 and T2 lease renewal parameters. Click on the "Other DHCP Options" tab. 5. Click on "Modify," then scroll down to and click on option number 58, "T1 Time Value." 6. Click on "Add" and you will be prompted to enter a value for the T1 timer. Remember, the T1 timer specifies the percentage of the lease that must expire before the client attempts to contact the original DHCP server for a lease renewal. Perform these same steps to specify the T2 Time value (option number 59). Remember, the T2 Time value specifies the percentage 1. Click on the Subnet Object, then click on the Create icon (the cube). 2. Select "Subnet Address Range" from the "Create New DHCP Record" dialog box. 3. Enter a meaningful name for the subnet range object, then enter the starting and ending TCP/IP addresses for the pool, as shown in Figure When completed, click on Create. Testing Dynamic DHCP We are now ready to test the DYNAMIC DHCP function. You should exit from the DNS/DHCP Management Console to make sure all of your changes have been saved. Next, enter the following command from the system console of the NetWare 5 file server that is associated with the DHCP Server object created earlier: LOAD DHCPSRVR After DHCPSRVR has loaded, you should receive the following two messages: DHCPSRVR : Main thread for UI and Lease Expiration processing started. DHCPSRVR : DHCP Server is ready. To modify a test Windows 95/98 or NT Workstation to use the DHCP function, perform the following steps: 1. Click on Settings > Control Panel > Network > TCP/IP > IP Address. 2. Click on the "Obtain IP Address Automatically" box. You will need to reboot the workstation after making this change. 3. After the workstation reboots, check the IP configuration to see if the workstation was assigned an address from the new pool we created. From a Windows NT Workstation, check this by running "IPCONFIG /ALL" from a command prompt. From a Windows 95/98 workstation, click on Start > Run > WINIPCFG. Figure 5 shows a Windows 95 workstation that is using our new Dynamic DHCP address pool. You can also see this assignment from the DNS/DHCP Management Console. To do this, expand the Subnet Address Range object. You should see an entry for your test workstation, as shown in Figure 6. When you click on this entry, you will see detailed information about the client.

5 FIGURE 6: MONITORING DYNAMIC DHCP ADDRESS ASSIGNMENT FROM THE MANAGEMENT CONSOLE FIGURE 7: COMPLETED DHCP OPTIONS TO SPECIFY WINS SERVER ADDRESSES The following steps are required to configure your DHCP server on the subnet level to assign WINS server addresses to your clients: 1. From the DNS/DHCP Management Console, click on the "DHCP" tab. 2. Click on the Subnet object to be modified. 3. Click on the "Other DHCP Options" tab. 4. Click on "Modify" and scroll down to option number 44, "NetBIOS over TCP/IP Name Server." 5. Click on "Add" and you will be prompted for the IP address of your WINS server. If you have multiple WINS servers, click on "Add" again and you will be prompted for the next WINS server address. 6. Click on Modify again and scroll down to option number 46, "NetBIOS over TCP/IP Node Type." Click on "Add" and you will be prompted for a node type. Select "H-Node." All Windows NT networks use H-Node (or hybrid node) types. When completed, your DHCP options should look like the ones shown in Figure 7. SPECIFYING A WINS SERVER FOR DYNAMIC DHCP USERS Many corporate networks consist of both NetWare and Windows NT Servers and most medium- to large-sized shops utilize WINS servers on the Windows NT side of their networks. You can (and should) configure your DHCP server to pass the address(es) of your WINS server(s) to your clients. This flexibility will allow you to add WINS servers and change WINS server addresses without reconfiguring all of your workstations. Before I describe how to make your NetWare 5 DHCP server deliver WINS server addresses to your clients, I need to discuss DHCP Global Options. As stated before, there are many DHCP options available to you. One of these options is the specification of WINS servers. All of these DHCP options can either be set on the subnet object or set globally. When a DHCP option is set on the subnet level only the workstations attached to that subnet will have these options available to them. This allows you to specify subnet specific options, such as the default gateway on the subnet level, where it belongs. When DHCP options are globally set, all clients on all subnets controlled by that DHCP server receive the same options. There may be some DHCP options that you desire to have administered globally, such as DHCP option number 46, which specifies the NetBIOS node type. However, it is much simpler to manage DHCP options on the subnet level. When you manage DHCP options at the subnet level, you can see all of the options set for that subnet on a single panel within the DNS/DHCP Management Console. When you have some global and some subnet level options set, you must remember to check your global options when modifying your subnet level options to ensure that the global options are not regressed by the subnet options. Tip: To set Global DHCP Options, click on the "Global Preferences" icon in the DNS/DHCP Management Console. This is the sixth icon from the left and is a picture of a globe with a checklist by it. Tip: There are some bugs in the DHCP server and Management Console. If your changes (such as adding WINS servers) do not take effect immediately, close the DNS/DHCP Management Console, then unload and reload the DHCPSRVR NLM. The client settings must be modified to ask the DHCP server for the WINS server addresses. This is accomplished in Windows using the following steps: 1. Click on Start > Settings > Control Panel > Network > TCP/IP > WINS Configuration. 2. Click on the "Use DHCP for WINS Resolution." 3. You should now test the configuration on a workstation. If you are testing from a Windows 95/98 workstation, you can run the WINIPCFG command and click on the "Release All" then the "Renew All" button to have your lease renewed, which causes all of the client s DHCP options to be refreshed. Make sure that no applications are running on the workstation before performing the Release and Renew functions. If you are testing from a Windows NT Workstation, release the configuration using the "ipconfig /release" command followed by the

6 "ipconfig /renew" command to refresh the client's DHCP options. ADDING A DEFAULT GATEWAY As you can see in the following steps, all DHCP options are implemented in the same manner: 1. You first decide if you want the option to be defined globally, or on the subnet level. 2. Next, add the DHCP option with its corresponding parameter, such as the IP address of a WINS server. 3. Test the change by "releasing" and "renewing" the lease using the WINIPCFG utility. 4. If the test fails, you must unload, then reload the DHCPSRVR.NLM and re-test. To add a default gateway, perform the aforementioned procedure for DHCP option number 3, "Router." The Router, or Default Gateway, should definitely be performed on the subnet level since, in most cases, each subnet will have a different Default Gateway address. SPECIFYING DNS SERVERS DHCP can and should be configured to pass out the address(es) of your DNS servers to the network clients. The DNS servers are added using DHCP option number 6, "Domain Name Server." You can implement the DNS server specification in either the global or subnet levels. You must also modify the client's TCP/IP configuration to "Disable DNS." When DSN is disabled on the client's TCP/IP configuration and DNS is configured on the DHCP server, the client will pick up and utilize the DNS server addresses from the DHCP server. ADVANCED CONFIGURATION OPTIONS There are many advanced configuration options available with the NetWare 5 DHCP server. Due to space restrictions, I'm unable to illustrate all of the DHCP options available. Some of the most useful advanced configuration options are listed below, and you should investigate the use of these options to see if they would be beneficial in your environment. Excluding Hardware Addresses from DHCP: You can exclude certain NIC address from participating in the DHCP arena. This is useful in cases where you do not want a particular host from utilizing your DHCP server. DHCP Option 85: This is a NetWarespecific DHCP option that provides the TCP/IP address of one or more NDS servers the workstation should contact to log in. DHCP Option 86: This is a NetWarespecific DHCP option that provides the NDS tree name the client will be accessing. DHCP Option 87: This is a NetWarespecific option that specifies the NDS context that the client should use to login to the NDS tree. Auditing: You can enable auditing on the DHCP server. Auditing allows you to monitor the actions taken by your NetWare 5 DHCP server. Dynamic DNS: When Dynamic DNS is enabled, the NetWare 5 DNS is updated to reflect the name and IP address of each client to which a TCP/IP lease is provided. SNMP Event Generation: You can configure DHCP to generate SNMP alerts if the DHCP server detects critical, major, warning or minor events. Import Configuration Information: You can use the Management Console to import existing DHCP configuration information. This is useful if you are planning to replace an existing DHCP server with the NetWare 5 DHCP service. CONCLUSION The NetWare 5 DHCP service is very robust and shows Novell's commitment to the TCP/IP protocol. The Java-based Management Console provides an excellent interface to both the NetWare 5 DHCP and DNS services. If you have had any experience with the older NetWare/IP DHCP server, you will definitely appreciate the improved NetWare 5 DHCP service. The Management Console provides a much simpler and more intuitive interface than the previous NetWare/IP DHCP server. NaSPA member John E. Johnston is the owner of a web development company that specializes in e-commerce. John also performs contract work on Novell, NT and UNIX networks. John can be contacted at johnj@fast.net Technical Enterprises, Inc. For reprints of this document contact editor@naspa.net.