Configuring the Dynamic Host Configuration Protocol (DHCP)

Transcription

1 Chapter 1 Configuring the Dynamic Host Configuration Protocol (DHCP) The Dynamic Host Configuration Protocol allows a host on a network to receive configuration parameters including an IP address at boot time. DHCP is an extension of the Bootstrap Protocol (BOOTP), but differs in that DHCP can provide a complete set of TCP/IP configuration parameters and allows the dynamic assignment of IP addresses for a fixed lease period. DHCP s offers the following advantages to you as a network administrator: The ability to provide complete TCP/IP configuration information to any host connected to the network. An IP address can be assigned to a client for only as long as the client remains connected to the network. It allows a limited pool of IP addresses to be shared among a number of clients that do not require permanent IP addresses. Addresses become automatically available again at the expiration of a period of time, their lease period, without requiring you to take any action. SCO DHCP provides a graphical configuration utility, the DHCP Configuration Manager, which greatly reduces the administrative overhead in providing and maintaining a DHCP service. NOTE SCO DHCP does not provide a DHCP client capability. SCO DHCP implements the server aspects of the Dynamic Host Configuration Protocol as defined in the Internet standards: RFC 1533, RFC 1534, RFC 1541 and RFC 1542, together with the Internet Drafts: draft-ietf-dhc-dhcp-06.txt and draft-ietf-dhc-options-1533update-01.txt. 1

2 Configuring the Dynamic Host Configuration Protocol (DHCP) The SCO DHCP server receives IP addresses from the Address Allocation Server (AAS), which provides the same service to other network services that assign temporary IP addresses. See Chapter 2, Configuring the Address Allocation Server (AAS) (page 1). The configuration parameters supplied by the SCO DHCP server include those supported by BOOTP, the set of parameters specified for DHCP, and user-defined parameters. This chapter describes: When to use SCO DHCP (this page) How the SCO DHCP server works (page 3) Configuring DHCP (page 8) Troubleshooting DHCP (page 18) For more about DHCP (page 20) When to use SCO DHCP The purpose of DHCP is to deliver configuration information to individual network hosts and to reduce the administrative work involved in configuring larger IP networks. DHCP can provide configuration information to any host directly connected to the network, either permanently or temporarily. The type of IP address assignment, however, depends upon the type of host. Network hosts best suited to dynamic address assignment are desktop and laptop PCs and Macs that are routinely connected and disconnected from the network. Sales people and executives, who are the typical users of such machines, do not require constant connection to the network. Dynamic assignment gives them the address they need, for only the period of time that they need it without requiring any action by you. Other network hosts that provide a service on the network, such as routers, gateways or print servers are better suited to manual address assignment. This method gives them the same address everytime they boot and you only have to configure the address once. UNIX diskless workstations can also be configured with manual address assignment. NOTE DHCP does not support backup DHCP servers. If the DHCP server goes down, any machines about to boot or reboot, such as a router or print server, will not be able to receive their IP address and configuration parameters until the DHCP server is brought back up. DHCP cannot provide any type of IP address or configuration parameters to remote hosts connected to the network via PPP or SLIP. Address pools configured in /etc/ppppool for use by PPP clients must not be the same addresses found in the addresses pools configured in the Address Allocation Server for 2

3 How the SCO DHCP server works use by DHCP clients. How the SCO DHCP server works When the SCO DHCP server receives a DHCPDISCOVER message from a client requesting configuration parameters, it constructs the set of parameters corresponding to the client from the DHCP configuration file. This set of parameters can be built from any combination of global parameters, subnet-specific parameters, class-specific parameters, and client-specific parameters. See DHCP Options (page 5). If the client s entry in the configuration file contains an IP address, the DHCP server assigns that address to the client. See Manual assignment of IP addresses (this page). If there is no entry for the client in the configuration file, or if the client s entry does not contain an IP address, the DHCP server leases an IP address to the client from a pool of addresses consistent with the client s subnet. The Address Allocation Server (AAS) allocates dynamically assigned addresses to the DHCP server. See Dynamic assignment of IP addresses (page 4). The DHCP server constructs a DHCPOFFER message containing the client s configuration parameters and IP address and sends it to the client. The server then waits for a DHCPREQUEST from the client accepting the parameters and address. After the client accepts the offered parameters, the server sends a DHCPACK message completing the process. For more information on DHCP message types and message exchange, see RFC More than one SCO DHCP server can be configured on a network, if necessary, but these servers cannot share the same pool(s) of addresses. The DHCP server can configure clients on different subnets by using BOOTP relay agents as gateways. Manual assignment of IP addresses You can manually assign an IP address to a particular client by creating an entry for the client in the DHCP Configuration Manager and entering the IP address in the client s entry. The DHCP server always assigns this address to the client. Manually assigned addresses cannot belong to any address pools maintained by AAS. The lease for a manually assigned address is infinite. See Configuring DHCP clients (page 11). 3

4 Configuring the Dynamic Host Configuration Protocol (DHCP) Dynamic assignment of IP addresses If there is no entry for the client in the configuration file, or if the client s entry does not contain an IP address, the DHCP server requests one from the Address Allocation Server. The AAS allocates an address from the address pool assigned to the client s subnet in the DHCP configuration file. If a subnet does not have an address pool configured, addresses cannot be allocated to clients on that subnet. Although there is no guarantee that the client will receive the same IP address it received last time, the AAS always attempts to do so. A DHCP client can also request a specific IP address. See IP address assignment priority (this page). Every address offered by the DHCP server has either a fixed or infinite lease period. See The lease period for an assigned IP address (this page). NOTE RFC 1541, Dynamic Host Configuration Protocol, also specifies an address allocation method called Automatic Allocation. In the SCO DHCP implementation, this is essentially a dynamically assigned address with an infinite lease. IP address assignment priority The DHCP server dynamically assigns IP addresses using the following priorities: 1. Previous Address The DHCP server always assigns the address previously assigned to the client unless the address is unavailable. 2. Requested Address If the previous address is unavailable, the DHCP server assigns the addressed requested by the client. (A DHCP client can request a specific IP address by using the Requested IP Address option in the DHCPDISCOVER message. In most cases, this is the previous address.) 3. Arbitrary Address If the previous and requested address(es) are unavailable, the DHCP server assigns an arbitrary address from the configured pool. The lease period for an assigned IP address When assigning an address pool to a subnet, you can also establish a default lease time and a maximum lease time for those addresses. The server uses the default lease time unless the client requests a specific lease time using the IP Address Lease Time option in either a DHCPDISCOVER or DHCPREQUEST message. If the client requests a lease time greater than the maximum, the server offers the maximum lease time. Both the default and maximum lease periods can be configured to a maximum of 136 years. There is no minimum lease period. Both lease periods can also be specified as infinite. 4

5 Clients can request an extension of the address lease. In addition to default and maximum lease times, you can configure the lease renewal and rebind time for the addresses in the pool. The DHCP server sends the lease renewal and rebind times to the client in the DHCPACK message. The server defines these values as percentages of the lease. The default renewal time in 50%, suggesting the client request a renewal when the lease is half over. The rebind time is the point at which the server suggests the client broadcast a renewal request. This is a second renewal attempt and accommodates multiple DHCP servers that may be sharing the address pool. The default is 90%. See Configuring subnets (page 10) for information about specifying lease periods and renewal information. To prevent an address from being assigned to more than one client at the same time, DHCP temporarily reserves the address until the client accepts or rejects it. You can configure this initial reservation time in the DHCP configuration database as a server parameter. See Specifying DHCP server parameters (page 17). After the client accepts the address, the DHCP server notifies AAS that the address has been leased. If the AAS is not notified, the address can be reallocated by the address server after the reservation time expires. A leased IP address becomes available again when either the DHCP server receives a DHCPRELEASE message from the client or the lease expires. To account for the possibility of drift between the clocks of the server and client, you can specify a Lease Padding parameter which extends the length of the lease period. The default is 1% of the lease period. The unpadded lease period is known to the client. The DHCP server knows the padded lease period so preventing the server from thinking that the lease period expired before the client does. See Specifying DHCP server parameters (page 17). DHCP Options How the SCO DHCP server works The DHCP server can configure a client with any of the options specified in RFC Since DHCP uses the same BOOTREQUEST and BOOTREPLY packet format as BOOTP, configuration options are carried in the portion of the BOOTP packet reserved for vendor extensions. This field has been renamed the options field and been expanded to 312 bytes from 64 bytes. You use the DHCP Configuration Manager to specify DHCP options and certain IP address parameters. See Configuring DHCP (page 8). Multiple sets of these DHCP options can be specified in the DHCP configuration database as global options, subnet options, vendor class options, user class options or client options. The set of global DHCP options, for example, applies to all clients, whereas the set of subnet options applies only to the clients on the specified subnet. If you configure the same option in more than one set of options, the DHCP server uses the following order of precedence 5

6 Configuring the Dynamic Host Configuration Protocol (DHCP) (from highest to lowest): 1. Client 2. User class 3. Vendor class 4. Subnet 5. Global For example, if the LPR Servers option is specified in both the client s subnet set of options and the global set of options, the DHCP server configures the client with the subnet s LPR Servers information. The different sets of configurable DHCP options are: Client options In addition to a set of DHCP options, you can specify an IP address for an individual client. The DHCP server automatically assigns this address to the client. An entry for a client can be created without an address; specifying only certain DHCP options for the client. The client s hardware type and hardware address identify the client s entry in the DHCP configuration file. User class options A set of DHCP options offered to any client that specifies the user class identifier in the DHCPDISCOVER message. You can create as many user classes as you need. Vendor class options A set of DHCP options offered to any client that specifies the vendor class identifier in the DHCPDISCOVER message. You can create as many vendor classes as you need. Subnet options The configurable options for a subnet include the pool of addresses from which the DHCP server draws on, the default and maximum lease periods for those addresses, and lease renewal and rebind times, along with a set of DHCP options specific to the subnet. Global options A set of DHCP options that apply to all DHCP clients. Although the DHCP client can specify a larger response packet size, the practical limit for DHCP options is 274 bytes. (An additional 192 bytes can be provided if the option overload feature is enabled. Option overload is a server parameter.) Each option consists of a 2-byte header and the option data, such as an integer or an array of IP addresses. A single option cannot exceed 255 bytes of data. 6

7 If the server finds that the configured options exceed the allowable limit, it uses as many options as possible starting from the top of the configured options list and sends an error message to /usr/adm/syslog. Clients can send a configured options list in the DHCPDISCOVER message. In the situation mentioned above, this Parameters Request List takes precedence over any other configured options. Some DHCP options have default values. If you configure an option that has a default value and do not configure a value for the opton, the DHCP server sends the default value. The following table lists the DHCP options that have default values. Option Name Default Value Broadcast address network or subnet number with all 1 s for the host Router discovery 1 (enable) IP forwarding 0 (disable) Non-local source routing 0 (disallow) TCP keepalive garbage 0 (disable) Default IP time to live 64 Ethernet encapsulation 0 (RFC 894, Ethernet version 2) Default TCP keepalive interval0 (no keepalives) See RFC 1533 for descriptions of these options. User defined options In addition to the DHCP options specified in RFC 1533, you can add new options. These user defined options become part of the total set of DHCP options configurable globally or for any client, subnet or class. To define a new option, specify: the option s name a description of the option the option s type (for example, IP address, string, integer, and so on) the minimum and maximum allowed values for the option (if the option is an integer) the minimum and maximum allowed length for the option (if the option is an IP address, string, binary or array) How SCO DHCP works with BOOTP How the SCO DHCP server works The SCO DHCP server, dhcpd, forwards BOOTP messages to the BOOTP server, bootpd. It does not support BOOTP clients directly. If you want to run both BOOTP and DHCP on the same host, use the -S option with bootpd to have it run in slave mode, and use the -a option to specify an alternate port for it to 7

8 Configuring the Dynamic Host Configuration Protocol (DHCP) listen for BOOTP packets. You can then configure dhcpd to forward BOOTP packets to the alternate port. See the dhcpd(admn) manual page for more information on configuring DHCP and BOOTP to run on the same host. Configuring DHCP You can start the DHCP Configuration Manager in any of the following ways: Double-click on the DHCP Configuration Manager icon in the System Administration window on the Desktop. Start SCOadmin by entering scoadmin on the command line. Select Networks followed by DHCP Configuration Manager. Enter scoadmin DHCP Configuration Manager on the command line (or an abbreviation of this such as scoadmin DHCP) For more information on using SCOadmin managers, see Administering your system with SCOadmin in thesco OpenServer Handbook. The DHCP Configuration Manager s top level and menus are illustrated here: 8

9 Configuring DHCP Depending upon the selection made in the View menu, the main screen shows subnet entries, client entries, user classes or vendor classes that you have configured. The Expand and Collapse items on the View menu allow you to examine the complete details for a selected entry in the list, or just to display its name and description. You can also expand and collapse individual entries by double-clicking on them. You can expand and collapse all entries using the Expand all and Collapse all items on the View menu. With the DHCP Configuration Manager, you can perform the following tasks: Configure the subnets on your network (page 10) for DHCP service by assigning address pools to the subnets, establishing lease parameters and assigning DHCP configuration options that are specific to particular subnets. Configure individual DHCP clients (page 11) with a static IP address and unique configuration options. Establish sets of DHCP options known as user classes (page 13) and vendor classes (page 14) which are supplied to DHCP clients requesting them. 9

10 Configuring the Dynamic Host Configuration Protocol (DHCP) Establish global DHCP options (page 15) that apply to all clients. Identical options configured specifically for subnets, individual clients or classes override global options. Add user-defined DHCP options (page 16) which can be configured for any subnet, client, class or globally. Configure DHCP server parameters (page 17). Configuring subnets You can add subnets configured on your network that have DHCP clients. For existing subnet entries, you can change the address pool, default lease parameters and DHCP options. You can also access the Address Allocation Server from the Subnet Entry screen and modify address pools to facilitate the changes you need to make. If you need to change a subnet s number, you must delete the subnet entry and add it again with the new subnet number Adding a subnet In the DHCP Configuration Manager: 1. Select Subnets from the View menu. 2. Select Add from the Entry menu. 3. On the Add Subnet Entry screen, enter the subnet s number in dot notation, the subnet mask (if subnetting is used), the address pool assigned to the subnet and a description, if desired. (If you press Return after entering the subnet number the subnet mask appears automatically in the Subnet mask field.) NOTE To modify an address pool, click on Configure address pool. The AAS Configuration Manager appears. See Chapter 2, Configuring the Address Allocation Server (AAS) (page 1). 4. Enter the default lease parameters: Default lease time To set an infinite default lease period, click on Unlimited. Otherwise, click on Limited and enter the lease period in days, hours and minutes. The lease period must be greater than 0, and less than or equal to 49,674 days (approximately 136 years). Maximum lease time To set an infinite maximum lease, click on Unlimited. Otherwise, click on Limited and enter the maximum lease period in days, hours and minutes. The lease period must be greater than 0, and less than or equal to 49,674 days (approximately 136 years). 10

11 Configuring DHCP Renewal time Enter the lease renewal time as a fraction of the lease period. The default is 500 or 50% of the lease period, suggesting the client request a renewal when the lease is half over. Rebind time Enter the lease rebind time as a fraction of the lease period. The rebind time is a second renewal attempt. The default is 900 or 90% of the lease period, suggesting the client make a second attempt to renew the lease when 10% of the lease period remains. 5. To configure options specifically for the subnet you are adding, click on Configure DHCP Options. See Configuring global DHCP options (page 15) for more information. 6. Click on Add. 7. To add more subnets, repeat steps 2 through 6 for every subnet you want to add. 8. Click on Close when you have finished adding subnets. Modifying a subnet 1. Select Subnets from the View menu. 2. Highlight the subnet you want to change and select Modify from the Entry menu. 3. On the Modify Subnet Entry screen, make the necessary changes and click on OK. NOTE You can change all fields except the Subnet number. 4. Click on OK. Deleting a subnet 1. Select Subnets from the View menu. 2. Highlight the subnet you want to delete. 3. Select Delete from the Entry menu. 4. When asked to confirm that you want to delete the subnet, click on OK. Configuring DHCP clients Machines such as networked printers, print servers, DNS servers, file servers, and, of course, DHCP servers, provide a service on the network and require fixed IP addresses so that other hosts can locate them. You can create client entries for these machines. If you have a client that requires a custom set of 11

12 Configuring the Dynamic Host Configuration Protocol (DHCP) options but not a fixed address, you can create an entry without an IP address or hostname. You can also modify and delete client entries. Adding a client configuration In the DHCP Configuration Manager: 1. Select Clients from the View menu. 2. Select Add from the Entry menu. 3. On the Add Client Entry screen, enter the following information about the client: Hardware type Define the client s type of network hardware by selecting one of: IEEE 802 (type 6) Ethernet (type 1) Opaque (type 0) Other Use IEEE 802 (type 6) for any IEEE 802.x network; which includes IEEE (Ethernet), IEEE (bus) and IEEE (token ring). Ethernet (type 1) refers to Ethernet version 2.0. (IEEE uses the same hardware as Ethernet (type 1) but has a different packet format.) Use Opaque (type 0) for types of hardware that have a text string as their hardware address. Hardware address Enter the client s MAC address. Enter only the hexadecimal numbers: do not enter colons. For example, enter: 0000c034f152 not 00:00:c0:34:f1:52 For more information about MAC addresses, see?? Hardware address in Configuring Network Connections If you selected Opaque (type 0) for the hardware type, you must specify if the hardware address is a binary (hexadecimal) string or text string. IP address Enter either the client s IP address in dot notation or the client s hostname. (The IP address or hostname must not belong to an address pool that has been configured for dynamic address allocation.) Leave this field blank if you only want to assign specific DHCP options to the client. DHCP will then dynamically assign an address to this client. 12

13 Configuring DHCP Description A short description of the client. 4. To configure options specifically for this client, click on Configure DHCP Options. See Configuring global DHCP options (page 15) for more information. 5. Click on Add. 6. To add more clients, repeat steps 2 through 5 for every client you need to add. 7. Click on Close when you have finished adding clients. Modifying a client configuration 1. Select Clients from the View menu. 2. Highlight the client you want to change and select Modify from the Entry menu. 3. On the Modify Clients Entry screen, make the necessary changes and click on OK. NOTE You can change all fields except the Hardware address. 4. Click on OK. Deleting a client configuration 1. Select Clients from the View menu. 2. Highlight the client you want to delete. 3. Select Delete from the Entry menu. 4. When asked to confirm that you want to delete the client, click on OK. Specifying user class options If you need to group machines according departments, work groups, or physical location, and you want to provide the same options to all clients in the group, you can add user classes to achieve this. You might, for example, want different groups to use different print or DNS servers. Clients in these groups must request the user class. You can change options for a user class and delete user class entries. Adding a user class In the DHCP Configuration Manager: 13

14 Configuring the Dynamic Host Configuration Protocol (DHCP) 1. Select User classes from the View menu. 2. Select Add from the Entry menu. 3. On the Add User Class Specification screen, enter an Identifier (a text string) and a description for the entry. 4. Click on Configure DHCP Options to configure the options for this user class. See Configuring global DHCP options (page 15) for more information. 5. Click on Add. 6. To add more user classes, repeat steps 2 through 5 for every user class you need to add. 7. Click on Close when you have finished adding user classes. Modifying a user class 1. Select User classes from the View menu. 2. Highlight the user class you want to change and select Modify from the Entry menu. 3. On the Modify User Class Specification screen, make the necessary changes and click on OK. NOTE You can change the Description and any DHCP options. 4. Click on OK. Deleting a user class 1. Select User classes from the View menu. 2. Highlight the user class you want to delete. 3. Select Delete from the Entry menu. 4. When asked to confirm that you want to delete the user class, click on OK. Specifying vendor class options If you need to group machines according to their hardware or software configuration, and these machines need options other than standard options, you can add vendor classes to achieve this. A client of a particular vendor class will then receive the appropriate vendor-specific information and parameters. You can also change options for a vendor class and delete vendor class entries. 14

15 Adding a vendor class In the DHCP Configuration Manager: 1. Select Vendor classes from the View menu. 2. Select Add from the Entry menu. 3. On the Add Vendor Class Specification screen, enter this entry s Identifier. This can be either in binary (hexadecimal) or a text string. 4. Select the Type of string: Binary or Text. 5. Enter a description of the vendor class. 6. Click on Configure DHCP Options to configure the options for this vendor class. See Configuring global DHCP options (this page) for more information. 7. Click on Add. 8. To add more vendor classes, repeat steps 2 through 7 for every user class you need to add. 9. Click on Close when you have finished adding vendor classes. Modifying a vendor class 1. Select Vendor classes from the View menu. 2. Highlight the vendor class you want to change and select Modify from the Entry menu. 3. On the Modify Vendor Class Specification screen, make the necessary changes and click on OK. You can change the Type, Description, and any DHCP options. 4. Click on OK. Deleting a vendor class 1. Select Vendor classes from the View menu. 2. Highlight the vendor class you want to delete. 3. Select Delete from the Entry menu. 4. When asked to confirm that you want to delete the vendor class, click on OK. Configuring global DHCP options Configuring DHCP Global options are those parameters that are not subnet-specific and would be the same for all hosts, like the DNS server or Time Offset. 15

16 Configuring the Dynamic Host Configuration Protocol (DHCP) In the DHCP Configuration Manager: 1. Select Global from the Options menu. 2. Configure the options you want to apply to all DHCP clients by moving them from the Unused list to the Configured list. To add an option, highlight the option in the Unused list and click on Add. Continue from step 3. To modify a configured option, highlight the option in the Configured list and click on Edit value. Continue from step 3. To delete an option, highlight the option in the Configured list and click on Remove. 3. On the Edit DHCP Option screen, enter the value for the selected option. 4. Click on OK. 5. To configure more global options, repeat steps 2 through 4 for every global option you want to configure. 6. Click on OK when you have finished configuring global options. Specifying user defined options You can define new options not available in the set of global options. In the DHCP Configuration Manager: 1. Select User defined from the Options menu. 2. On the User Defined Options Specifications screen, configure the additional options you want to apply to one or more DHCP clients. To add an option, click on the Add button. Define the option in the Add User Defined Option screen and click on OK. Identifier Numeric identifier for the option in the range 1 to 254. Name Text string to identify the option. Description Additional text string to describe the option. Type Select the type of value for the option from the list. Array If you selected any integer types or IP address, click on the Array button if the option can be comprised of more than one value. An array can be a set of individual values or a range of values. 16

17 Configuring DHCP Minimum value If the value is an integer, enter the minimum value allowed. Maximum value If the value is an integer, enter the maximum value allowed. Minimum length If the value type is a string, binary or an array, enter the minimum length allowed for the option. Maximum length If the value type is a string, binary or array, enter the maximum value length allowed for the option. To modify an option, highlight the option in the list and click on the Modify button. Make the necessary changes and click on OK. You can change all fields except Identifier and Name. To delete an option, highlight the option in list and click on the Delete button. 3. When asked to confirm that you want to delete the option, click on OK. 4. Click on OK when you have finished configuring options. Specifying DHCP server parameters To specify DHCP server parameters: 1. Select Server from the Options menu. 2. Specify the server s parameters: Initial lease reservation time Enter the time to reserve an address while the server offers it to a client. The lease period begins when the client accepts the address. This reservation period prevents and address from being offered to more than one client at the same time. Lease padding time Enter the amount of time, as a fraction of the lease period, that you want to extend the lease to allow for clock drift between the server and the client. The default is 10 or 1% of the lease period. The unpadded lease period is known to the client. The DHCP server knows the padded lease period so preventing the server from thinking that the lease period has expired before the client does. Probe address Specify if you want the server to ping an address before it is allocated to make sure it is free. DHCP option overload Specify if you wish to use option overload. Enabling this option allows the server to use the sname or file fields to carry DHCP options if the 17

18 Configuring the Dynamic Host Configuration Protocol (DHCP) options field is full. (Microsoft Windows NT clients do not support Option Overload.) Address allocation server location If the address server runs on a different host, click on Remote and enter the host s IPaddress or hostname and the password for access to the address server host. Otherwise, click on Local. 3. Click on OK. Troubleshooting DHCP Procedures for solving common problems with DHCP. Can t boot client If a particular client will not boot, the problem may center on IP address assignment. If the DHCP server dynamically assigns an address to the client, first check /usr/adm/syslog and search for the out of addresses message. This indicates that all the addresses in the pool assigned to the client s subnet have been allocated. There are a number of actions you could take: 1. Reconfigure the address pool with more addresses, provided you have unused addresses available on the subnet. Use the Configure address pool button in the Modify Subnet Entry screen. This button gives you access to the AAS Configuration Manager. 2. Review the address pool s current allocation information in the AAS Configuration Manager and look for any disabled addresses (state= d ). Enabling these addresses will make them available again. See UNRESOLVED XREF-assT.enable_addr (page 15 xxx) in "Configuring the Address Allocation Server (AAS)". 3. Review the allocated addresses (state= a ) in the address pool s current allocation information and try to identify addresses that you know are not really in use even though they are allocated. These would probably be addresses with long or infinite leases (Lease Duration) which were allocated to temporary network machines such as laptop PCs. (This is the result of setting a long or infinite Default Lease Time when configuring the subnet for DHCP.) Release any allocated addresses that are not in use. See UNRESOLVED XREF-assT.release_addr (page 15 xxx) in "Configuring the Address Allocation Server (AAS)". After you have increased the number of addresses available in the pool, restart the DHCP client. 18

19 Troubleshooting DHCP DHCP can t find AAS If the DHCP server cannot find the Address Allocation Server, that server may be down and needs to be restarted using the AAS Configuration Manager. Clients not being configured with all options If a client has been configured with an IP address and DHCP options, but not with all the options you specified in the DHCP Configuration Manager, too many options may have been specified for the client. The DHCP server has a 274 byte practical limit for DHCP options (466 bytes if the Option Overload feature is enabled). Check /usr/adm/syslog for the packet size exceeded message. This indicates that the DHCP server did not send all the options specified for the client because the total size of all the client s options exceeded the options field size. Follow the steps below to determine why there may be too many options. 1. Check to see if the Option Overload parameter is specified in the DHCP server parameters. Setting this parameter allows the server to use the sname and file fields of the DHCPOFFER message, which provides an additional 192 bytes for options. See Specifying DHCP server parameters (page 17). 2. Check the values configured for the options sent to the client in the appropriate DHCP option list (subnet, client, user class, vendor class or global). You can specify an array of values for some options, such as an array of IP addresses for the Routers option. Arrays that consist of many nonsequential values consume space in the options field which excludes options found farther down on the configured list from being sent in the DHCPOFFER message. For arrays of IP addresses, a single range of addresses consumes the least space. 3. Determine if the client included a parameter request list in the DHCPDIS- COVER message by examining the client s DHCP configuration. These options take precedence over the options you have configured for the client, subnet and classes in the DHCP Configuration Manager. If the options requested by the client are different from the options configured by you, the total group of options may exceed the size limitation. Depending upon the total size of all the configured options, the options you ve configured which are toward the bottom of the list may be excluded from the DHCPOFFER message sent to the client. If this scenario is true, you will have to select the most important options for the client and eliminate the less important ones. 19

20 Configuring the Dynamic Host Configuration Protocol (DHCP) For more about DHCP To obtain more information about DHCP, see the following manual pages: Manual page aasd(admn) bootpd(admn) dhcpd(admn) Information provided Address Allocation Server daemon Internet Boot Protocol server and gateway daemons Dynamic Host Configuration Protocol daemon To get more information about DHCP, see the following Request for Comments (RFCs). For information on how to obtain RFCs, see Obtaining RFCs from the Internet in thenetworking Guide. RFC Title 1122 Requirements for Internet Hosts Communication Layers 1533 DHCP Options and BOOTP Vendor Extensions 1534 Interoperation Between DHCP and BOOTP 1541 Dynamic Host Configuration Protocol 1542 Clarifications and Extensions for the Bootstrap Protocol 20