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- DHCP network tables
The Dynamic Host Configuration Protocol (DHCP) network tables are used to map the client identifiers of DHCP clients to IP addresses and the associated configuration parameters of that address. One DHCP network table exists for each network served by the DHCP server, and each table is named using the network's IP address. There is no table or file with the name dhcp_network.
The DHCP network tables can exist as ASCII text files, binary text files, or NIS+ tables, depending on the data store used. Since the format of the file could change, the preferred method of managing the DHCP network tables is through the use of dhcpmgr(1M) or the pntadm(1M) command.
The dhcp_network file is used as a policy mechanism for whether in.dhcpd(1M) leases addresses on a given network. If the DHCP server is not serving leases or information to a network, there should be no dhcp_network file for that network. To set the DHCP server in informational mode, where it responds to INFORM messages but does not lease addresses on that network, create an empty dhcp_network file for that network. For normal operations, where the DHCP server both leases addresses and responds to INFORM packets, create a dhcp_network file using dhcpmgr(1M) or pntadm(1M) and populate it with leasable addresses.
The format of the records in a DHCP network table depends on the data store used to maintain the table. However, an entry in a DHCP network table must contain the following fields:
The client identifier field, Client_ID, is an ASCII hexadecimal representation of the unique octet string value of the DHCP Client Identifier Option (code 61) which identifies a DHCP client. In the absence of the DHCP Client Identifier Option, the DHCP client is identified using the form given below for BOOTP clients. The number of characters in this field must be an even number, with a maximum length of 64 characters. Valid characters are 0 - 9 and A-F. Entries with values of 00 are freely available for dynamic allocation to requesting clients. BOOTP clients are identified by the concatenation of the network's hardware type (as defined by RFC 1340, titled "Assigned Numbers") and the client's hardware address. For example, the following BOOTP client has a hardware type of '01' (10mb ethernet) and a hardware address of 8:0:20:11:12:b7, so its client identifier would be: 010800201112B7
The Flags field is a decimal value, the bit fields of which can have a combination of the following values:
Evaluation of the Lease field is turned off (lease is permanent). If this bit is not set, Evaluation of the Lease field is enabled and the Lease is DYNAMIC.
This entry has a manual client ID binding (cannot be reclaimed by DHCP server). Client will not be allocated another address.
When set, this value means that either through ICMP echo or client DECLINE, this address has been found to be unusable. Can also be used by the network administrator to prevent a certain client from booting, if used in conjunction with the MANUAL flag.
This entry is reserved for allocation to BOOTP clients only.
The Client_IP field holds the IP address for this entry. This value must be unique in the database.
This field holds the IP address of the DHCP server which owns this client IP address, and thus is responsible for initial allocation to a requesting client. On a multi-homed DHCP server, this IP address must be the first address returned by gethostbyname(3NSL).
This numeric field holds the entry's absolute lease expiration time, and is in seconds since January 1, 1970. It can be decimal, or hexadecimal (if 0x prefixes number). The special value -1 is used to denote a permanent lease.
This ASCII text field contains the dhcptab macro name used to look up this entry's configuration parameters in the dhcptab(4) database.
This ASCII text field contains an optional comment.
This section describes how the DHCP/BOOTP server calculates a client's configuration lease using information contained in the dhcptab(4) and DHCP network tables. The server consults the LeaseTim and LeaseNeg symbols in the dhcptab, and the Flags and Lease fields of the chosen IP address record in the DHCP network table.
The server first examines the Flags field for the identified DHCP network table record. If the PERMANENT flag is on, then the client's lease is considered permanent.
If the PERMANENT flag is not on, the server checks if the client's lease as represented by the Lease field in the network table record has expired. If the lease is not expired, the server checks if the client has requested a new lease. If the LeaseNeg symbol has not been included in the client's dhcptab parameters, then the client's requested lease extension is ignored, and the lease is set to be the time remaining as shown by the Lease field. If the LeaseNeg symbol has been included, then the server will extend the client's lease to the value it requested if this requested lease is less than or equal to the current time plus the value of the client's LeaseTim dhcptab parameter.
If the client's requested lease is greater than policy allows (value of LeaseTim), then the client is given a lease equal to the current time plus the value of LeaseTim. If LeaseTim is not set, then the default LeaseTim value is one hour.
For more information about the dhcptab symbols, see dhcptab(4).
See attributes(5) for a description of the following attribute:
Reynolds, J. and J. Postel, Assigned Numbers, STD 2, RFC 1340, USC/Information Sciences Institute, July 1992.