The DHCP Server
The DHCP server runs as a daemon in Oracle Solaris on ahost system. The server has two basic functions:
Managing IP addresses – The DHCP server controls a range of IP addresses and allocates them to clients, either permanently or for a defined period of time. The server uses a lease mechanism to determine how long a client can use a nonpermanent address. When the address is no longer in use, it is returned to the pool and can be reassigned. The server maintains information about the binding of IP addresses to clients in its DHCP network tables, ensuring that no address is used by more than one client.
Providing network configuration for clients – The server assigns an IP address and provides other information for network configuration, such as a host name, broadcast address, network subnet mask, default gateway, name service, and potentially much more information. The network configuration information is obtained from the server's dhcptab database.
The DHCP server can also be configured to perform the following additional functions:
Responding to BOOTP client requests – The server listens for broadcasts from BOOTP clients discovering a BOOTP server and provides them with an IP address and boot parameters. The information must have been configured statically by an administrator. The DHCP server can simultaneously perform as a BOOTP server and as a DHCP server.
Relaying requests – The server relays BOOTP and DHCP requests to appropriate servers on other subnets. The server cannot provide DHCP or BOOTP service when configured as a BOOTP relay agent.
Providing network booting support for DHCP clients – The server can provide DHCP clients with information needed to boot over the network: an IP address, boot parameters, and network configuration information. The server can also provide information that DHCP clients need to boot and install over a wide area network (WAN).
Updating DNS tables for clients that supply a host name – For clients that provide a Hostname option and value in their requests for DHCP service, the server can attempt DNS updates on their behalf.
DHCP Server Management
As superuser, you can start, stop, and configure the DHCP server with DHCPManager or with command-line utilities described in DHCP Command-Line Utilities. Generally, the DHCP server isconfigured to start automatically when the system boots, and to stop when thesystem is shut down. You should not need to start and stop theserver manually under normal conditions.
DHCP Data Store
All the data used by the DHCP server is maintained in adata store. The data store might consist of plain text files, NIS+ tables, orbinary-format files. While configuring the DHCP service, you choose the type of datastore to be used. The section Choosing the DHCP Data Store describes the differences between the typesof data stores. You can convert a data store from one format toanother by using DHCP Manager or the dhcpconfig command.
You can also move data from one DHCP server's data store toanother server's data store. You can use export and import utilities that workwith the data stores, even if the servers are using different data storeformats. You can export and import the entire content of a data store,or just some of the data within it, using DHCP Manager or thedhcpconfig command.
Note - Any database or file format can be used for DHCP data storage ifyou develop your own code module to provide an interface between DHCP(server and management tools) and the database. For more information, see the Solaris DHCP Service Developer’s Guide.
Within the DHCP data store are two types of tables. You canview and manage the contents if these tables by using either DHCP Manageror the command-line utilities. The data tables are as follows:
dhcptab table – Table of configuration information that can be passed to clients.
DHCP network tables – Tables containing information about the DHCP and BOOTP clients that reside on the network specified in the table name. For example, the network 192.168.32.0 would have a table whose name includes 192_168_32_0.
The dhcptab Table
The dhcptab table contains all the information that clients can obtain from theDHCP server. The DHCP server scans the dhcptab table each time it starts. Thefile name of the dhcptab table varies according to the data store used.For example, the dhcptab table created by the NIS+ data store SUNWnisplus isSUNWnisplus1_dhcptab.
The DHCP protocol defines a number of standard items of information that canbe passed to clients. These items are referred to as parameters, symbols, oroptions. Options are defined in the DHCP protocol by numeric codes and textlabels, but without values. Some commonly used standard options are shown in thefollowing table.
Table12-1 Sample DHCP Standard Options
Code | Label | Description |
|---|---|---|
1 | Subnet | Subnet mask IP address |
3 | Router | IP address for the router |
6 | DNSserv | IP address for theDNS server |
12 | Hostname | Text string for the client host name |
15 | DNSdmain | DNS domain name |
Some options are automatically assigned values when you provide information during server configuration.You can also explicitly assign values to other options at a later time.Options and their values are passed to the client to provide configuration information.For example, the option/value pair, DNSdmain=Georgia.Peach.COM, sets the client's DNS domain nameto Georgia.Peach.COM.
Options can be grouped with other options in containers known as macros,which makes it easier to pass information to a client. Some macros arecreated automatically during server configuration and contain options that were assigned values duringconfiguration. Macros can also contain other macros.
The format of the dhcptab table is described in the dhcptab(4) man page. InDHCP Manager, all the information shown in the Options and Macros tabs comesfrom the dhcptab table. See About DHCP Options for more information about options. See About DHCP Macrosfor more information about macros.
Note that the dhcptab table should not be edited manually. You should useeither the dhtadm command or DHCP Manager to create, delete, or modify optionsand macros.
DHCP Network Tables
A DHCP network table maps client identifiers to IP addresses and the configurationparameters associated with each address. The format of the network tables is describedin the dhcp_network(4) man page. In DHCP Manager, all the information shown inthe Addresses tab comes from the network tables.
DHCP Manager
DHCP Manager is a graphical user interface (GUI) tool you can use toperform all management duties associated with the DHCP service. You can use itto manage the server as well as the data the server uses.You must be superuser when you run DHCP Manager.
You can use DHCP Manager with the server in the following ways:
Configuring and unconfiguring the DHCP server
Starting, stopping, and restarting the DHCP server
Disabling and enabling DHCP service
Customizing DHCP server settings
DHCP Manager enables you to manage the IP addresses, network configuration macros, andnetwork configuration options in the following ways:
Adding and deleting networks under DHCP management
Viewing, adding, modifying, deleting, and releasing IP addresses under DHCP management
Viewing, adding, modifying, and deleting network configuration macros
Viewing, adding, modifying, and deleting nonstandard network configuration options
DHCP Manager allows you to manage the DHCP data stores in thefollowing ways:
Convert data to a new data store format
Move DHCP data from one DHCP server to another by exporting it from the first server and importing it on the second server
DHCP Manager includes extensive online help for procedures you can perform with thetool. For more information, see About DHCP Manager.
DHCP Command-Line Utilities
All DHCP management functions can be performed by using command-line utilities. You can runthe utilities if you are logged in as superuser or as auser assigned to the DHCP Management profile. See Setting Up User Access to DHCP Commands.
The following table lists the utilities and describes the purpose of each utility.
Table12-2 DHCP Command-Line Utilities
Role-Based Access Control for DHCP Commands
Security for the dhcpconfig, dhtadm, and pntadm commands is determined by role-based access control(RBAC) settings. By default, the commands can be run only by superuser. Ifyou want to use the commands under another user name, you mustassign the user name to the DHCP Management profile as described in Setting Up User Access to DHCP Commands.
DHCP Server Configuration
You configure the DHCP server the first time you run DHCP Manager onthe system where you want to run the DHCP server.
DHCP Manager server configuration dialog boxes prompt you for essential information needed toenable and run the DHCP server on one network. Some default values areobtained from existing system files. If you have not configured the system forthe network, there are no default values. DHCP Manager prompts for the followinginformation:
Role of the server, either as the DHCP server or as the BOOTP relay agent
Data store type (files, binary files, NIS+, or something specific to your site)
Data store configuration parameters for the data store type you selected
Name service to use to update host records, if any (/etc/hosts, NIS+, or DNS)
Length of lease time and whether clients should be able to renew leases
DNS domain name and IP addresses of DNS servers
Network address and subnet mask for the first network you want to configure for DHCP service
Network type, either local area network (LAN) or point-to-point network
Router discovery or the IP address of a particular router
NIS domain name and IP address of NIS servers
NIS+ domain name and IP address of NIS+ servers
You can also configure the DHCP server using the dhcpconfig command. This utilityautomatically gathers information from existing system files to provide a useful initial configuration.Therefore, you must ensure that the files are correct before running dhcpconfig. Seethe dhcpconfig(1M) man page for information about the files that dhcpconfig uses to obtaininformation.
IP Address Allocation
The DHCP server supports the following types of IP address allocation:
Manual allocation – The server provides a specific IP address that you choose for a specific DHCP client. The address cannot be reclaimed or assigned to another client.
Automatic, or permanent, allocation – The server provides an IP address that has no expiration time, making it permanently associated with the client until you change the assignment or the client releases the address.
Dynamic allocation – The server provides an IP address to a requesting client, with a lease for a specific period of time. When the lease expires, the address is taken back by the server and can be assigned to another client. The period of time is determined by the lease time configured for the server.
Network Configuration Information
You determine what information to provide to DHCP clients. When you configure theDHCP server, you provide essential information about the network. Later, you can addmore information that you want to provide to clients.
The DHCP server stores network configuration information in the dhcptab table, inthe form of option/value pairs and macros. Options are keywords for network datathat you want to supply to clients. Values are assigned to options andpassed to clients in DHCP messages. For example, the NIS server address ispassed by way of an option called NISservs. The NISservs option has avalue that is equal to a list of IP addresses, which is assignedby the DHCP server. Macros provide a convenient way to group together anynumber of options that you want to supply to clients. You can useDHCP Manager to create macros to group options and to assign values tothe options. If you prefer a command-line tool, you can use dhtadm, theDHCP configuration table management utility, to work with options and macros.
About DHCP Options
In DHCP, an option is a piece of network information to be passedto a client. The DHCP literature also refers to options as symbols ortags. An option is defined by a numeric code and a text label.An option receives a value when it is used in the DHCP service.
The DHCP protocol defines a large number of standard options for commonly specifiednetwork data: Subnet, Router, Broadcst, NIS+dom, Hostname, and LeaseTim are a fewexamples. A complete list of standard options is shown in the dhcp_inittab(4) manpage. You cannot modify the standard option keywords in any way. However, youcan assign values to the options that are relevant to your network whenyou include the options in macros.
You can create new options for data that is not represented bythe standard options. Options you create must be classified in one of threecategories:
Extended – Reserved for options that have become standard DHCP options but are not yet included in the DHCP server implementation. You might use an extended option if you know of a standard option that you want to use, but you do not want to upgrade your DHCP server.
Site – Reserved for options that are unique to your site. You create these options.
Vendor – Reserved for options that should apply only to clients of a particular class, such as a hardware or vendor platform. The DHCP implementation includes a number of vendor options for Oracle Solaris clients. For example, the option SrootIP4 is used to specify the IP address of a server that a client that boots from the network should use for its root (/) file system.
Chapter15, Administering DHCP (Tasks) includes procedures for creating, modifying, and deleting DHCP options.
About DHCP Macros
In the DHCP service, a macro is a collection of network configuration optionsand the values that you assign to them. Macros are created to groupoptions together to be passed to specific clients or types of clients. Forexample, a macro intended for all clients of a particular subnet might containoption/value pairs for subnet mask, router IP address, broadcast address, NIS+ domain, andlease time.
Macro Processing by the DHCP Server
When the DHCP server processes a macro, it places the network options andvalues defined in the macro in a DHCP message to a client.The server processes some macros automatically for clients of a particular type.
For the server to process a macro automatically, the name of the macromust comply with one of the categories shown in the following table.
Table12-3 DHCP Macro Categories for Automatic Processing
A macro with a name that does not use one of thecategories listed in Table12-3 can be processed only if one of the followingis true:
The macro is mapped to an IP address.
The macro is included in another macro that is processed automatically.
The macro is included in another macro that is mapped to an IP address.
Note - When you configure a server, a macro that is named to matchthe server's name is created by default. This server macro is not processed automaticallyfor any client because it is not named with one of thename types that cause automatic processing. When you later create IP addresses on theserver, the IP addresses are mapped to use the server macro bydefault.
Order of Macro Processing
When a DHCP client requests DHCP services, the DHCP server determines which macrosmatch the client. The server processes the macros, using the macro categories todetermine the order of processing. The most general category is processed first, andthe most specific category is processed last. The macros are processed in thefollowing order:
Client class macros – The most general category
Network address macros – More specific than Client class
Macros mapped to IP addresses – More specific than Network address
Client ID macros – The most specific category, pertaining to one client
A macro that is included in another macro is processed as partof the container macro.
If the same option is included in more than one macro, thevalue for that option in the macro with the most specific category isused because it is processed last. For example, if a Network address macrocontains the lease time option with a value of 24 hours, and aClient ID macro contains the lease time option with a value of 8hours, the client receives a lease time of 8 hours.
Size Limit for DHCP Macros
The sum total of the values assigned to all the options ina macro must not exceed 255 bytes, including the option codes and lengthinformation. This limit is dictated by the DHCP protocol.
The macros that are most likely to be impacted by this limitare macros that are used to pass paths to files on Oracle Solarisinstallation servers. Generally, you should pass the minimum amount of vendor information needed. Youshould use short path names in options that require path names. Ifyou create symbolic links to long paths, you can pass the shorter linknames.
