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DHCP Server Overview
The Sambar Dynamic Host Configuration Protocol (DHCP) Server was created
to supply each network client with an IP addresse, subnet mask,
default gateway, and IP address for a DNS server.
Using DHCP a host is dynamically configured
to the network using a simple handshake that is build-in to Windows
and Unix operating systems; the client requests a network address
and various additional configuration parameters, and the server replies
with an address that can be used for a specified duration (lease period).
The Sambar DHCP Server only implements a subset of the DHCP protocol
necessary to serve:
- IP address (and subnet)
- Gateway address
- DNS Server address (Sambar DNS Server, if enabled;
otherwise DNS Primary/Secondary from [common] section of the
config.ini file.)
- Domain name
- WINS (NetBIOS Name Server)
In addition, ethernet addresses can be reserved in the dhcp.ini file.
The DHCP server works with Windows 98/2000/NT and other DHCP clients.
If Windows 95, NT or Unix machines are configured to use DHCP, the machines
will have no networking ability unless the DHCP server is up and
running at the time the machines boot. Windows 98 and 2000 machines do
not have this limitation because the use a special ("169.x.x.x" range)
temporary address until a defined IP address can be assigned by the
DHCP server.
The Sambar DHCP server is limited to serving a single range of IP addresses.
So, if you have available addresses 10.0.0.2 through 10.0.0.100, your
network components are scattered throughout this range. The DHCP server
attempts to PING an address before assigning it for use, so machines
that are inadvertently configured for a specific address within the range,
or machines that have not freed their IP address after a lease expires
can safely continue to operate.
Initialization
- From the System Administration -> Server Configuration
form set the Act as DHCP Server parameter to Yes and then
update the server configuration.
- Restart the Sambar Server.
On the main System Administration page, a DHCP Server
entry should appear along with various configuration options.
dhcp.ini Configuration File
If Act As DHCP Server is set to true in the
config.ini the DHCP Server is initialized at startup.
The parameters of the dhcp.ini configuration file (below)
guides the DHCP Server operation.
[dhcp]
| Subnet |
The subnet from which to serve the IP addresses.
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| Router |
The gateway to use for communication outside the subnet (this is
typically your router).
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| Domain |
The domain name to serve to hosts for fully qualified domain name lookups.
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| Force IP Bind |
This allows you to configure the DHCP listener to bind to a specific
interface on a multi-homed machine.
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| IP Range Lower Limit |
The lowest IP address that should be served by this DHCP server.
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| IP Range Upper Limit |
The highest IP address that should be served by this DHCP server.
Note: A check is performed to ensure that the lowest and highest IP
address ranges conform with the subnet mask specified.
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| Default Lease Time |
The default time, in seconds that a DHCP lease should be provided a host.
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| Maximum Lease Time |
The maximum time, in seconds that a DHCP lease should be provided a host.
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| Trace DHCP |
Display DHCP requests via the GUI display (if enabled).
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DHCP Protocol
DHCP operations fall into four basic phases. These phases are IP lease request, IP lease offer, IP lease selection, and IP lease acknowledgement.
IP Lease Request
Whenever a computer comes on line, it checks to see if it currently has an
IP address leased. If it doesn't, it requests a lease from a DHCP server.
Because the client computer doesn't know the address of a DHCP server, it
broadcasts a DHCPDISCOVER message across the network. This message consists
of the client computer's Media Access Control (MAC) address (the hardware
address built into the network card) and its NetBIOS name.
IP Lease Offer
When a DHCP server receives an IP lease request from a client, it extends
an IP lease offer. This is done by reserving an IP address for the client
and broadcasting a DHCPOFFER message across the network. This message
contains the client's MAC address, followed by the IP address that the
server is offering, the subnet mask, the lease duration, and the IP address
of the DHCP server making the offer.
IP Lease Selection
When the client PC receives an IP lease offer, it must tell all the other
DHCP servers that it has accepted an offer. To do this, the client broadcasts
a DHCPREQUEST message containing the IP address of the server that made
the offer. When the other DHCP servers receive this message, they withdraw
any offers that they might have made to the client. They then return the
address that they had reserved for the client back to the pool of valid
addresses that they can offer to another computer. Any number of DHCP
servers can respond to an IP lease request, but the client can only accept
one offer per network interface card.
IP Lease Acknowledgement
When the DHCP server receives the DHCPREQUEST message from the client, it
initiates the final phase of the configuration process. This acknowledgement
phase involves sending a DHCPACK packet to the client. This packet includes
the lease duration and any other configuration information that the client
might have requested. At this point, the TCP/IP configuration process is
complete.
Address Ranges
Dynamic DNS
When the DHCP and DNS servers are both enabled in the Sambar Server,
the DNS server will resolve host names to IP addresses without any
manual intervention. Every time a computer comes on-line or drops
off-line the DNS tables are updated automatically.
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