The Sambar 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)

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.
Router	The gateway to use for communication outside the subnet (this is typically your router).
Domain	The domain name to serve to hosts for fully qualified domain name lookups.
Force IP Bind	This allows you to configure the DHCP listener to bind to a specific interface on a multi-homed machine.
IP Range Lower Limit	The lowest IP address that should be served by this DHCP server.
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.
Default Lease Time	The default time, in seconds that a DHCP lease should be provided a host.
Maximum Lease Time	The maximum time, in seconds that a DHCP lease should be provided a host.
Trace DHCP	Display DHCP requests via the GUI display (if enabled).

Integration with DNS
Domain Name System (DNS) servers provide name resolution for network resources and are closely related to DHCP services. When the Sambar DNS Server is enabled, DHCP clients are automatically registered with the DNS server. Using this mechanism, the DNS server can respond to pointer (PTR) and address (A) resource records on behalf of DHCP-enabled clients.

The tight integration between the DHCP and DNS servers is quite compelling; if both servers are 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.

Leases
A lease is the length of time that a DHCP server specifies that a client computer can use an assigned IP address. When a lease is made to a client, it is described as active. At half-lease period, the client must renew its address lease assignment with the server. The duration of leases affects how often clients attempt to renew those leases that have been assigned with the DHCP server.

The DHCP leases are stored in the config/dhcp.dat. Leases are saved (one lease per line) in the format:

Host-name:IP-address:MAC-address:lease-duration:lease-time

Reserving Leases
Reservations allow permanent address lease assignments by the DHCP server. Where reservations are used, they ensure that a specific hardware device on the subnetwork can always use the same IP address. To pre-define/reserve an IP-address for a given host such as print servers or other special devices, you must create a lease record entry as specified above (using the MAC-address), and set the lease-time to 0. This will result in the lease never expiring for the device.

To obtain MAC address on Windows NT/2000/XP based clients, type ipconfig /all at the comand prompt and view the Physical Address field. For Windows 95/98/ME based clients, run Winipcfg.exe, and view the Adapter Address field.

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.