LAN (Local Area Network)

Senin, 23 Mei 2011

The DHCP server assigns a client an IP address

The DHCP server assigns a client an IP address taken from a predefined scope for a given amount of time. If an IP address is required for longer than the lease has been set for, the client must request an extension before the lease expires. If the client has not requested an extension on the lease time, the IP address will be considered free and can be assigned to another client. If the user wishes to change IP address then they can do so by typing "ipconfig /release", followed by "ipconfig /renew" in the command prompt. This will remove the current IP address and request a new one. Reservations can be defined on the DHCP server to allow certain clients to have their own IP address (this will be discussed a little later on). Addresses can be reserved for a MAC address or a host name so these clients will have a fixed IP address that is configured automatically. Most Internet Service Providers use DHCP to assign new IP addresses to client computers when a customer connects to the internet - this simplifies things at user level.

To export the DHCP server settings from a Windows NT 4.0–based server or Windows 2000–based server
Run DHCPExim.exe. This opens the DhcpExim dialog box.

To download and install DHCPExim.exe, see the Resource Kit Tools link on the Web Resources page at http://www.microsoft.com/windows/reskits/webresources.

In the DhcpExim dialog box, select Export configuration of the local service to a file.

In the DHCPEXIM Export To File dialog box, enter a file name and location to save the file, and then click OK.

The target location the file is shown in the DhcpExim Export dialog box.

In the DhcpExim Export dialog box, select all of the scopes on the list to migrate all settings on the server. Alternatively, you can only the specific scopes you want to export if you are not migrating the entire server. Select the Disable the selected scopes on local machine before export check box to disable the scopes on this server before exporting, and then click Export.

Tip

This step might take several minutes to complete, and there is no dialog box to indicate the progress of the export. You can view the process running on the Processes tab of the Windows Task Manager.

Click OK when a message appears that says "The operation was completed successfully."

Copy the exported file to a location where you can access it from the new Windows Server 2003–based server.

Setting up DHCP solaris

Setting up DHCP solaris

There are two ways of using DHCP:
DHCP has limited control
DHCP has full control

The first case may be where you want to use your own /etc/resolv.conf and so on, with a minimum of hassle.

The second case would be the normal situation, especially if your cable modem provider has a habit of changing DNS name server IP addresses on you (like mine does!), so I'll concentrate on that here. I have a script to automate the first method, should you want to use it. You'll need to change the DEFAULT_ADDR and INTERFACE variables as required.

The first thing to do is to create an empty /etc/hostname.le0, like this:

> /etc/hostname.le0

Creating this file ensures that the interface gets plumbed, ready for the DHCP software to do its stuff.

Next, you create /etc/dhcp.le0. This file can be empty if you want to accept the defaults, but may also contain one or both of these directives:
wait time, and
primary

By default, ifconfig will wait 30 seconds for the DHCP server to respond (after which time, the boot will continue, while the interface gets configured in the background). Specifying the wait directive tells ifconfig not to return until the DHCP has responded. time can be set to the special value of forever, with obvious meaning. I use a time value of 300, which seems to be long enough for my cable provider.

The primary directive indicates to ifconfig that the current interface is the primary one, if you have more than one interface under DHCP control. If you only have one interface under DHCP control, then it is automatically the primary one, so primary is redundant (although it's permissible).

With these files in place, subsequent reboots will place le0 under DHCP control: you're ready to go!

Creating IP Address Objects

Creating IP Address Objects

You use the DNS/DHCP Management Console to create and set up any IP Address objects to be assigned to specific devices or to be excluded from dynamic assignment. Create an IP Address object for each such device or address. Assigning a specific address to a client requires you to specify the client's media-access control (MAC) address or Client ID.

If you have set up subnets and subnet address ranges, you are not required to set up individual IP addresses unless you want to perform manual address assignment or exclude addresses from assignment.

To create and set up an IP Address object, complete the following steps:

Click DHCP Service tab of the DNS/DHCP Management Console.

Select the Subnet object of the target IP address, then click Create on the tool bar.

The Create New DHCP Object dialog box is displayed.

Select IP Address and click OK.

The Create IP Address dialog box is displayed.

Enter the IP address to be assigned or excluded, select the assignment type, then click Create.

If you choose Manual Assignment Type, you must provide information for either the Client Identifier or the MAC Address fields. You can also specify the MAC Type by clicking in the field; the default is FF Any.

DHCP router overview

In this section you will learn the DHCP router overview, dynamic host configuration protocol overview, network tutorials, troubleshooting tips, subnetting, IP addressing. DHCP or Dynamic Host Configuration Protocol is a network protocol, which is configured in such a way that is provides the IP addresses to the network computers automatically. There is no need to assign the IP address to the client computers individually. DHCP assigns the IP addresses from the given range of the numbers also called DHCP scope.

On the other end, client computers are configured in such a way that they get the IP addresses automatically from the DHCP server during the boot up process. Additionally, DNS and WINS servers can also be configured with the DHCP server.
When a computer starts up it gets the IP addresses from the DHCP server from the defined pool of addresses.
The assigning of the IP address by the DHCP server can be categorized as follows

1. A user turns on the computer with the DHCP client enabled on it.
2. The client PC sends broadcast request (known as DHCP DISCOVER) and then look for the DHCP server to answer.
3. The DHCP server receives the DISCOVER packet and based on the availability and defined settings, the server choose any available IP address and then give to the client. Then DHCP server sends back to the client with DHCP OFFER with that available address information.

4. Again client sends a request to DHCP server known as DHCP REQUEST, in which it lets the server know that it is using the address offered by the DHCP.

5. The DHCP server then again send a acknowledgement to the client known as DHCP ACK, in which it confirm the client that the specific IP address has been assigned to it for a given period of time.

This process of assigning the IP addresses by the DHCP server also known as DORA (Discover, Offer, Request, and Acknowledgement).

When a computer uses a static IP address there can be the chances of error and conflict when two computers use the same IP address. By using the DHCP server there are no chances of such kind of conflicts and errors.

Along with the IP addresses, the DHCP server also enables the client computers to extract all the settings and configurations from the DHCP server on an IP network. These settings include the Firewall, Router, DNS, NAT, WINS, Gateway and Subnet masks settings.

The overall purpose of the DHCP server is to reduce the workload and error margins, which can occur in assigning the IP addresses manually.

The IP address assigned by the DHCP server is for the limited or leased period of time and if a client needs an to extend that leased period for the IP address then client must send a extension request to the DHCP server before this period expires. If the client do not send an extension request, then this IP address becomes free on expiration and is assigned to another client. If a user wants to change the IP address given by the DHCP server then he/she do it by giving these commands on the command prompt.

“IPconfig/release then IPconfig/renew”.

This command will remove the current IP address and assign a new IP address to the client.
There can also be some reserved addresses based on the MAC address or hostname of the client. These reserved addresses are fixed addresses and can only be assigned to the fixed clients.

Solaris DHCP

Solaris DHCP daemon is started automatically during bootup if file /etc/dhcp.interface exists for at lest one interface Only interfaces with a corresponding /etc/dhcp.interface file are automatically configured during boot.

Like DNS there can be primary and secondary DHCP servers but secondary DHCP servers are not a requirement and they are not that common. Primary and secondary DHCP servers must have access to the exact same data. Copies cannot be used. This can be achieved by via NIS and NFS. Both run in.dhcpd daemon:
primary server assign IP addresses to clients. The IP address range for DHCP zone is defined during the installation. A primary DHCP server assigns an IP address to any client which requested a new configuration. Assigned IP-addresses always belong to the DHCP zone for which the server responsible. Multiple primary servers can exist on the same network as long as each server is responsible for a different DHCP address range.

Configuration for the DHCP server daemon is stored in the /etc/inet/dhcpsvc.conf file.

The information about DHCP address range and allocated addresses is stored in two tables: dhcptab and dhcp_network
dhcptab file contains all the information that clients can obtain from the DHCP server. The DHCP server scans the file each time it starts. The DHCP protocol defines a number of standard items of information that can be passed to clients. These items are referred to as parameters, symbols, or options. Options are defined in the DHCP protocol by numeric codes and text labels, but without values. For example, some commonly used standard options are shown in the following table.
dhcp_network
secondary server only can confirms existing configurations previously supplied by a primary server when the primary server is not available. Every primary server can also act as a secondary server.

There are two alterative ways (command line based and GUI-based) to configure DHCP servers (and BOOTP relay servers):
Command line configuration using dhcpconfig. Can be used in scripts. Does not check the validity of user input as it is entered. The dhcpconfig utility is the fastest way to configure the server. It also checks the validity of user input as it is entered.
GUI-based configuration using dhcpmgr (DHCP manager). The dhcpmgr utility speeds up the configuration process by omitting prompts for nonessential server options by using default values for them. You can change nonessential options after the initial configuration.

Both utilities permit setting of startup options, configure the DHCP service database type (binary or text) and location, as well as initialize the dhcptab and dhcp_network tables for the DHCP zone that the server controls.
/etc/inet/dhcpsvc.conf File

The /etc/inet/dhcpsvc.conf file contains configuration information and typically is populated using utilities that we mentioned above, although nothing prevents you for editing it manually. It the configuration file is not present, then the server does not run DHCP daemon. The content is a typical Unix-style configuration file with keyword-value pairs:

cat /etc/inet/dhcpsvc.conf

DAEMON_ENABLED=TRUE
RUN_MODE=server
RESOURCE=SUNWfiles
PATH=/var/dhcp
CONVER=1
VERBOSE=TRUE
ICMP_VERIFY=TRUE
INTERFACES=hme0,qfe0
UPDATE_TIMEOUT=15
LOGGING_FACILITY=7
BOOTP_COMPAT=automatic

Dynamic Host Configuration Protocol (DHCP) is a method by which each computer

When an IP-based local area network is installed, each computer must be configured with an IP address. If the computers need to access the Internet, they should also be configured with a gateway address and one or more DNS server addresses. As an alternative to manual configuration, Dynamic Host Configuration Protocol (DHCP) is a method by which each computer on the network can automatically obtain this configuration information. A device on the network may act as a DHCP server. The DHCP server stores a list or pool of IP addresses, along with other information (such as gateway and DNS addresses) that it may assign to the other devices on the network. Some NETGEAR products can act as DHCP servers.

Some NETGEAR products also function as DHCP clients when connecting to the ISP. Such NETGEAR products can automatically obtain an IP address, subnet mask, DNS server addresses, and a gateway address if the ISP provides this information by DHCP.

Dynamic Host Configuration Protocol (DHCP)

In the Internet Engineering Task Force (IETF), several authentication methods for Dynamic Host Configuration Protocol (DHCP) messages have been proposed. These were published and circulated as the IETF Internet Drafts. However, they have several drawbacks. One is that users can reuse addresses illegally by using an expired address that was allocated to a host. This may cause serious security problems. We propose a new access control method to be used as the DHCP message authentication mechanism.

We designed and developed the DAG (DHCP Access Control Gateway) as an example of the proposed method. The DAG is a gateway program that passes network accesses only by clients with addresses formally allocated from the DHCP server. In order to determine the address allocation formally, the DAG observes DHCP interactions between servers and clients and gathers information on address allocations made by the observed servers. The DAG can authenticate the DHCP server based on the message authentication code (MAC), which is included in the DHCP message. When the new address is allocated to the client, the DAG resets the gateway filters.

The gateway made by this method can be used even with DHCP servers lacking authentication mechanisms. Even if a regular DHCP server and the DAG are combined, network security can be improved. By combining a DHCP server and a DHCP client that supports authentication schemes such as IETF Internet Draft, the DAG can offer a mechanism whereby only a specific client may access the network.

.::BY JUMBHO MY AT HOME IN THE JEPARA CITY OF BEAUTIFUL::.