HP OpenVMS Systems Documentation

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Compaq TCP/IP Services for OpenVMS

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7.4.6 Methods to Configure DHCP Parameters

TCP/IP Services provides three methods for configuring server and client parameters:

  • An easy-to-use DHCP graphical user interface (GUI) to do the following:
    • Configure dynamic and static IP addressing for all clients. See Section 7.6.
    • Configure the client information appropriate for your client base. See Section 7.5.
    • Set DHCP parameters to customize the DHCP server. See Section 9.4.3.
  • Manually editing the DHCP configuration files and then signaling the DHCP server to read the files. See Section 7.7.
  • Using the DHCPDBMOD utility. See Section 7.8.2.

7.5 Using DHCP GUI to Configure DHCP

You can modify the default DHCP server settings and define additional characteristics by performing the following tasks:

Task Described in...
Define server and security parameters. Section 7.5.2
Define subnet parameters. Section
Define node parameters. Section
Define group parameters. Section

7.5.1 General Information

To use the DHCP GUI to configure DHCP:
  • You need the following system privileges:
  • If you have not already done so, execute the TCPIP$DHCP_SETUPCOMMANDS.COM command procedure to establish DHCP foreign commands .
  • Invoke the GUI by entering the following utility program command:


The system displays the configuration window with four tabs across the top of the window. The tabs allow you to configure the following sets of DHCP parameters:

Tab Function
Server/Security Defines the server configuration (see Section 7.5.2). You can set your IP address ranges, general server parameters, or view currently leased IP addresses and their lease time.
Subnets Assigns client configurations for entire subnets.
Nodes Adds and customizes specific machines on your network, usually for BOOTP clients.
Groups Defines a group of settings for predefined collections of machines.

Choose a tab for the category of parameters you want to configure. The window for each tab has three columns:

  • The left column lists the items that are configured for that category. The list always contains a [New Record] item to configure another machine. Choose an item from this list to enter or view its parameters.
  • The middle column lists the available parameters for the selected item along with the current specification or setting. Choose a parameter to enter or change the specification or setting.
  • The right column has fields for entering data. To add or change a parameter setting, select a parameter and enter the value for the parameter in the field to the right. You can enter values as:
    • Descriptive text, such as an IP address
    • Time in hours, minutes, and seconds
    • True or false statements

    When there is more than one value field, press Tab to move to the next field. To delete information in a field, select the text, then click Delete. Saving Information in a Record

If you add or revise information in a field, you need to save the information using one of the following methods:

  1. Choose Update from the File menu.
  2. Choose Exit from the File menu, then choose Save and Exit. This updates the database when you exit the program. Adding New Records

For some subjects, you can add more than one record. To add a new record:

  1. Choose [New Record] from the list on the left side of the window.
  2. Enter the information for the new record.
  3. Choose and enter parameter information as appropriate.

When only one record is possible, [New Record] disappears after you configure the first server.

7.5.2 Configuring Server and Security Parameters

Use the Server/Security tab to perform the following tasks:

  • Configure Server/Security parameters.
  • Configure IP ranges.
  • Set up the host name lists.
  • Use the Active IP Snapshot window.
  • Preload MAC addresses. Server/Security Parameters

To configure the server parameters using the Server/Security tab of the GUI, follow these steps:

  1. Click the Server/Security tab.
  2. Choose a parameter class from the drop-down list.
  3. Choose the parameter you want to change.

You can change any or all of the Server/Security parameters described in this section.

Accept Client Name

Specifies whether the server assigns names to client machines according to a policy that is established on the server by the system manager.

Even when this capability is enabled, the server ignores the client-suggested name if it is already in use by another client in the same domain.

If the server is unable to find a name for the client by applying this policy, it will take one of the following actions depending upon the specified value. The valid values are:

False: Assign a name from the NAMEPOOL. file if an IP address lookup does not return an associated name. Default.
True: Use the name the client suggests for itself, if specified.

Assign Name by Hardware Addr

Specifies whether you can assign host names by the hardware address. If you choose True, the client computer always has the same name, even if its IP address changes; however, to do so, the client must remain in the same domain.

This option is appropriate for sites supporting dynamic updating of the name service. When you select this policy, the server maintains a binding of the client's unique identifier to the name the client first acquires.

If the name service does not dynamically update, the new name-IP address mapping implied by this policy is not available to other clients until you bring the name service up to date by another mechanism. This means dumping data from the database and using it to update the name service manually. The following are valid values:

False: Disable assignment of host names by hardware addresses. Default.
True: Enable assignment of host names by hardware addresses. Use the naming method defined in the NAMEPOOL. file.

Assign Name by IP Addr

Specifies whether you can assign host names by an IP address. If you choose True, the client receives its name from the name service as a result of a gethostbyaddr routine call. Also, when a client computer moves, it can receive a new name from the name service. The following are valid values:
False: Host names cannot be assigned by IP addresses. The DHCP server does not issue a gethostbyaddr routine call. Instead, the session uses the naming method defined in the NAMEPOOL. file.
True: Host names can be assigned by IP addresses. Default.

Auto Release Old Lease

Set this to True if you want to automatically delete leases when the client changes its network. For example, if the client:
  • Receives an address on subnet A
  • Then moves to subnet B

The server releases the leased IP address on subnet A even though the leased IP address on subnet A is still valid.

The default setting is False.


Some hardware configurations use a MAC address or client identifier that is the same regardless of which interface you are configuring. To the DHCP server, two interfaces of a client of this type can appear to be a single client that has changed networks. Do not autorelease these leases.

Auto Reread Config File

Instructs the server to see if the DHCPCAP. file has changed, indicated by the timestamp. This occurs each time a client requires a configuration. If the file changes, the server rereads and reparses the DHCPCAP. file.

The default is True.

Auto Synchronize Database

Choose True to flush the server database to disk after each update. This makes the server more reliable if there is a failure such as a system crash or unintentional power shutdown. Setting this parameter to True can slow down the server.

The default is False.

BOOTP Addr From Pool

Specifies whether the DHCP server does not require a preestablished binding for BOOTP clients. When none exists, the server allocates an address from the pool to the client. Because BOOTP does not understand the concept of lease times, all such allocations are permanent regardless of the lease times specified elsewhere in the database.

When you disable BOOTP Addr From Pool, the Server only supports BOOTP clients whose IP address is configured into the database. This means the binding of the IP address to the client must be preestablished. The address must be consistent with the network to which the client is attached. See Section 7.6 for information on how to preestablish a binding between a MAC address and an IP address. The following are valid values:

False: Do not pick an address from a pool. Requires a preestablished binding. Default.
True: Pick an address from a pool. Does not require a preestablished binding.

BOOTP Client Lease Extension

TCP/IP Services does not currently support this parameter.

When you set this parameter to a value above zero, the server grants Finite leases to BOOTP clients. BOOTP clients do not know this, so before the server can reuse these leases, it must ping the IP address. If the server hears a reply, it extends the lease by the time interval (in seconds) specified by this parameter.

The default value is 0 seconds.

BOOTP Compatibility

DHCP can serve BOOTP clients as well as DHCP clients. The following are valid values:
False: The server should act as a DHCP server only.
True: The server should also act as a BOOTP server. Default.

Bootfile not sent as option

Because the DHCP clients normally do not require bootfile names, the space reserved for this purpose (the "file" field) in reply packets is used by JOIN as an extension of the DHCP options field. This arrangement permits the client to receive more configuration information than would otherwise be possible in a standard-sized DHCP packet.

Enabling this parameter sends the bootfile name in the "file" field, instead of as a DHCP option. Certain network computers (NCs) expect to find the bootfile information in the "file" field and will not successfully load their OS images unless this parameter is set to True. Note: BOOTP clients always receive a bootfile name in the "file" field, regardless of the state of this option.

Canonical Name

Overrides the value normally returned by a gethostname routine call (default). Primarily used for multihomed hosts with a canonical name corresponding to an interface that is not recognized by DHCP (for example, ATM interfaces) and for high-availability servers that have per-service IP addresses that differ from a physical IP host address. The following are valid values:
False: Use the host name returned by a gethostname routine call. Default.
True: Use the specified canonical host name.

Check BOOTP Client Net

Before a BOOTP client is given a hard-wired IP address, the server makes sure that the client is connected to the logical IP network for which the address is valid. If the client is not connected, the server logs an error and does not send a response to the client.

For this to work properly, the NETMASKS. file must contain the network numbers and masks for any nonstandard IP Class A, B, or C configuration. The following are valid values:

False: Do not check the IP network of the address. Default.
True: Check the IP network of the address.

DNS expiration tracks DHCP lease

This parameter implies that SIG resource records for DNS are updated with expiration times that match the DHCP client's lease time. If a client sends a "DHCP release", the lease is prematurely expired and the SIG record is marked as expired. In order to reduce the amount of traffic between DHCP and DNS, the default value is False.

This policy affects only installations that use the Dynamic DNS option.

Default Lease Time

Specifies the value used on all leases for clients that have no other value explicitly configured. Enter the lease time of the IP address granted to a client.

The default lease time is one day.

Expand BOOTP Packet

Expands the BOOTP reply packet to 548 bytes. Applies to BOOTP clients only. The following are valid values:
False: All replies to BOOTP clients are 300 octets or a size equal to the size of the packet received, whichever is larger. Default.
True: All replies to BOOTP clients are expanded to 548 bytes.

Force Broadcast Reply

The DHCP server generally sends unicast reply packets in response to client packets. This toggle tells the server to send broadcast reply packets instead of unicast reply packets. The following are valid values:
False: Forces the DHCP server to use unicast reply packets. Default.
True: Forces DHCP server to broadcast reply packets to the client, even when the server could use unicast replies.

Free List Size

Specifies the size of the internal array specifying the number of address blocks held on the free list. If this number is too high, the server will lose previous allocations of expired leases quickly. If this number is too low, performance can suffer.

The default setting is 8.

Ignore Hardware Type

This toggle tells the server to use the clients' hardware address as its identifier (for those clients that do not use DHCP client identifiers), but to ignore the hardware type field. In the DHCP DB the identifier is stored with a type field of zero (which is also the type for those clients which are using client identifiers).

Set this option to True only to work around problems introduced by clients that broadcast multiple DHCP requests with conflicting hardware types (for example, HP Jet Direct). The default value is False.

Ignore Name Owner

This parameter applies only if both "Assign Name by Hardware Address" and "Accept Client Name" are True. In such a case, a previously established name-hardware address binding with the same name will be overwritten with the MAC address of the requesting client in DHCP's internal name database.

Listen on PPP Interfaces

Not currently supported.

If True, the server will respond to DHCP requests on Point-to-Point Protocol (PPP) interfaces of the host. By default, DHCP ignores these interfaces.

Min BOOTP Packet Size

Specifies the minimum packet size for DHCP requests. Change this value to allow the Server to work with some noncompliant DHCP clients that send DHCP requests smaller than the minimum required packet length.

The default minimum packet size is 300 bytes.

Name Service

Specifies the implementation of the underlying name service. Name service authenticates, routes, addresses, and performs naming-related functions for other computers on the network.

DNS is the only name service available with TCP/IP Services.

Name Service Updatable

Choose True to have TCP/IP Services automatically update the name service with the assigned IP addresses and host names.

Ping BOOTP Clients

Before the DHCP server assigns an IP address to a BOOTP client, the server checks to see if the address is available by using ping to send an Internet Control Message Protocol (ICMP) echo request. If the server receives a reply, it logs an error. Then:
  • If the address was from the dynamic pool, the server marks it as unavailable, and selects a new address from the pool.
  • If the address was statically configured, the server refuses to configure the client.

The following are valid values:

False: Do not send an ICMP echo request to a BOOTP client before assigning an IP address. Default.
True: Send an ICMP echo request to a BOOTP client before assigning an IP address.

Ping Timeout

Specifies the duration (in milliseconds) of the ping timeout. Enter the amount of time the server is to wait before concluding no other host is using the IP address. After the timeout, the ping command stops checking.

If you do not want the server to ping before giving out an IP address, set the timeout value to 0.

The default is 500 milliseconds.

Provisional Time To Live

Specifies the maximum time period that an IP address can remain on the provisionally allocated list before it can be allocated to another client. The value should be limited to a few minutes.

The default is 1 minute.

Reply to Relay On Local Net

Specifies whether the server ignores packets forwarded to it from a relay agent on the same subnet as the server.

The following are valid values:

False: Do not reply (the server should hear the client broadcast directly). Default.
True: Reply no matter where the agent is located (the value in giaddr field).

Restrict to Known MAC Addresses

Specifies whether to restrict IP addresses that are assigned to a matching MAC address. When specified, you can manually assign a MAC address. This parameter indicates whether the server should respond to clients with a MAC address that is unknown to the server.

Choose True to have the server provide DHCP information to only those hosts that have a known MAC address. To register a known MAC address client, use Preload MAC Addresses feature from the Server/Security tab or use the DBREG utility.

The following are valid values:

False: Do not allow manual assignment of MAC addresses. Default.
True: Allow manual assignment of MAC addresses.

Send Options in DHCP Offer

Specifies whether the server is to send a complete configuration to a DHCP client. Resolving a client configuration can be time consuming. In a multiserver environment, the client can select another server.

The following are valid values:

False: Send a minimum configuration. Default.
True: Send a complete configuration.

Support Microsoft RAS Server

Specifies support for the Microsoft Proxy Remote Access Server (RAS). The RAS server generates a BOOTP packet with a MAC address of 16 octets.

The following are valid values:

False: Ignore a BOOTP packet with a MAC address of 16 octets. Default.
True: Recognize a BOOTP packet with a MAC address of 16 octets.

Use MAC addr as client ID

Specifies whether the server is to use the client ID to uniquely identify a client. If set to True, the server uses the client's MAC address as the client ID. BOOTP also uses the MAC address to uniquely identify a client.

The following are valid values:

False: Use client ID to identify clients. Default.
True: Use MAC address to identify clients.

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