DHCP
From Antcor
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DHCP
The Dynamic Host Configuration Protocol (DHCP) provides configuration parameters to Internet hosts in a client-server model. DHCP server hosts allocate network addresses and deliver configuration parameters to other (client) hosts.
DHCP consists of two components: a protocol for delivering host-specific configuration parameters from a server to a host and a mechanism for allocation of network addresses to hosts.
To configure DHCP settings, select the DHCP tab, located under the Advanced Configuration of Node, Configuration tabs. The DHCP tab contains two sub-tabs: Server and Client, selected by clicking the corresponding option button.
Configuring a DHCP SERVER
The IKARUS DHCP server provides an extended set of configuration parameters while at the same time being effective and low resource consuming.
To configure a DHCP Server, select the interface from the interface tree. Its background turns blue. Only clients in the same physical interface will be able to acquire IP addresses from this DHCP server. If clients from other physical interfaces must acquire their IP addresses from the same server a bridge should be created, and those interfaces should be added under that bridge. Then, select that bridge as the DHCP server interface.
Setting DHCP Server Fields
To configure DHCP server settings, select the Server option button and select the Active check box. The Server tab becomes available.
After completing the required fields, click the Submit button. This uploads the configuration to the node without starting the server.
Start IP and End IP Type the appropriate IP addresses into the Start IP and End IP fields. These are the upper and lower limits for the DHCP server address pool.
Broadcast Type the appropriate IP address into the Broadcast field. This field contains the IP address clients will use. Broadcast IP should be one of the addresses the Subnet Mask permits. Subnet Mask Type the appropriate IP address into the Subnet Mask field. This is the subnet mask clients will use.
Domain Type the Domain name (if any) that will be allocated to clients into this text box.
Time Parameters For each of the following fields, type the appropriate value into the box.
- Lease
- The Lease field contains the number of seconds an allocated IP is valid. After expiration the client has to renegotiate for getting a new IP (which is usually the same). The expiration time that the client adopts depends on the operating system running on the client and the DHCP client configuration.
- Decline
- The Decline field contains the number of seconds that an IP will be reserved (leased) for if a DHCP decline message is received.
- Min Lease
- The Min Lease field contains the minimum number of seconds. If a lease to be given is below this value (sec), the full lease time is used instead.
- Conflict
- The Conflict field contains the amount of time (sec) that an IP address will be reserved (leased) if an ARP conflict (two clients with the same IP address) occurs.
- Max Lease
- The Max Lease field contains the maximum number of current leases (allocated IP addresses). After this limit is reached the server stops assigning IP addresses to new clients.
- Offer
- The Offer field contains the number of seconds an offered address is reserved (leased). This field specifies the number of seconds the DHCP server should cache the offers it has extended to discovering DHCP clients. The default value is 60 seconds. On fast network media this value can be decreased.
DNS Servers In the three DNS Servers fields (DNS 1, DNS 2 and DNS 3), type the IP addresses of the DNS servers that DHCP clients will use for dns requests.
WINS Servers If there are WINS servers that client should use, type the addresses in the WINS Servers fields (WINS 1 and WINS 2).
Routers In the Routers fields (Router 1 and Router 2), type the IP addresses of the routers (default gateways) the client can use.
Leases Info Click the Leases Info button to access the DHCP Leases dialog box that displays all the allocated leases.
Lease Time Strategies
One of the most common DHCP administration questions is, "What setting should I give my lease times?" As with many networking questions, the answer is, "It depends." The primary decision criterion is the desired frequency at which your clients update their configuration data.
If you are using DHCP only for randomized address assignments, having longer lease times will result in greater levels of stability. For example, if you use lease duration times of one month or longer, a temporary server outage is not likely to affect your normal operations much. However, if you are using DHCP for a variety of system-configuration options (such as default DNS servers and static routes), you will want to have shorter lease times so that changes to the network are recognized quickly by the DHCP clients. In this case, having lease times that are longer than a day or two can be problematic because clients that obtain a new lease just before a critical infrastructure change is made will not recognize this change until the lease expires or gets renewed.
For dynamic environments, there are two common lease-duration strategies. The first calls for leases to be renewed halfway through a working day (such as having them expire every eight hours, which will cause them to be renewed after four hours). Another strategy is to set the lease duration to a multiple of two and a half times the working day (that is, 20 hours for an eight-hour working day), causing the leases to completely expire overnight and thus be renegotiated every morning. The former strategy works well on networks that keep their machines running all of the time, while the latter strategy works well on networks where systems are powered down or otherwise removed from the network at night.
Be forewarned, however, that both strategies expose the network to problems if the DHCP server goes down or is on a remote network that is subject to outages. If the DHCP clients are getting their lease data from a remote DHCP server that is on the other side of a WAN link that is even minimally prone to failure, chances are good that short lease times will result in at least a few failed lease renewals.
Configuring a DHCP CLIENT
Configuration of the Dhcp Client application is simple. The only requirement is selection of the interface where the DHCP client will search for DHCP servers. Similar to DHCP server configuration, multiple instances of DHCP client on different interfaces are allowed.
To configure a DHCP Client, select the interface from the interface tree. Its background turns blue.
To configure DHCP client settings, select the Client option button and select the Active check box. The Client tab becomes available.
To prevent the changing of a client’s default system gateway and DNS addresses when the client receives an IP address from the server, select the Keep DNS and Gateway check box. This is useful when you already have set a static default gateway and DNS and want them to remain unchanged, or if they are to be configured from another application (e.g. PPPoE client). In most other cases this field should be remain unselected.
To complete the configuration, click the Submit button.
Configuring a DHCP Relay
DHCP does not require a server on each subnet. To allow for scale and economy, a relay agent can be installed listening to DHCP messages and forwarding them on (and onto other network segments). This eliminates the necessity of having a DHCP server on each physical network.
To configure a DHCP Relay, select the interface from the interface tree. Its background turns blue.
To view the full DHCP Relay pane, select the Relay option button, then select the Active check box. The Relay Configuration pane appears. The Relay Configuration pane represents the subnet (LAN) where a relay listens for client DHCP requests in order to forward them to DHCP servers Server 1, Server 2, Server 3 or Server 4. Type the appropriate IP addresses in these fields.
Interface where application relays on should has a valid ip and subnet mask and like the other DHCP apis, DHCP relay can have multiple instances on different interfaces.
To complete the configuration, click Submit.
