27. What features or restrictions can a DHCP server have?
While the DHCP server protocol is designed to support dynamic management of
IP addresses, there is nothing to stop someone from implementing a server that
uses the DHCP protocol, but does not provide that kind of support. In particular,
the maintainer of a BOOTP server-implementation might find it helpful to
enhance their BOOTP server to allow DHCP clients that cannot speak "BOOTP" to
retrieve statically defined addresses via DHCP. The following terminology has
become common to describe three kinds of IP address allocation/management.
These are independent "features": a particular server can offer or not offer any
of them:
· Manual allocation: the server's administrator creates a configuration for
the server that includes the MAC address and IP address of each DHCP
client that will be able to get an address: functionally equivalent to
BOOTP though the protocol is incompatible.
· Automatic allocation: the server's administrator creates a configuration
for the server that includes only IP addresses, which it gives out to
clients. An IP address, once associated with a MAC address, is
permanently associated with it until the server's administrator intervenes.
· Dynamic allocation: like automatic allocation except that the server will
track leases and give IP addresses whose lease has expired to other
DHCP clients.
Other features which a DHCP server may or may not have:
· Support for BOOTP clients.
· Support for the broadcast bit.
· Administrator-settable lease times.
· Administrator-settable lease times on manually allocated addresses.
· Ability to limit what MAC addresses will be served with dynamic
addresses.
· Allows administrator to configure additional DHCP option-types.
· Interaction with a DNS server. Note that there are a number of
interactions that one might support and that a standard set & method is
in the works.
· Interaction with some other type of name server, e.g. NIS.
· Allows manual allocation of two or more alternative IP numbers to a
single MAC address, whose use depends upon the gateway address
through which the request is relayed.
· Ability to define the pool/pools of addresses that can be allocated
dynamically. This is pretty obvious, though someone might have a server
that forces the pool to be a whole subnet or network. Ideally, the server
does not force such a pool to consist of contiguous IP addresses.
· Ability to associate two or more dynamic address pools on separate IP
networks (or subnets) with a single gateway address. This is the basic
support for "secondary nets", e.g. a router that is acting as a BOOTP
relay for an interface which has addresses for more than one IP network
or subnet.
· Ability to configure groups of clients based upon client-supplied user
and/or vendor class. Note: this is a feature that might be used to assign
different client-groups on the same physical LAN to different logical
subnets.
· Administrator-settable T1/T2 lengths.
· Interaction with another DHCP server. Note that there are a number of
interactions that one might support and that a standard set & method is
in the works.
· Use of PING (ICMP Echo Request) to check an address prior to
dynamically allocating it.
· Server grace period on lease times.
· Ability to force client(s) to get a new address rather than renew.
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