Posted by arlene

Mail clients use SMTP to send outgoing mail to an SMTP server. To retrieve mail messages, however, the Post Office Protocol (currently version 3) generally is used. POP3 is a stateful protocol, progressing from one state to another, depending on the results of the transaction in progress and the commands that are issued. The states are listed here:

• AUTHORIZATION—In this state, the user supplies a username and password to authenticate the client to the mail server. In its original implementation, the protocol supports clear-text for username and password transmissions. This, of course, can be a security problem, and other techniques should be used in a secure environment.

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Posted by arlene

BOOTP stands for the Bootstrap or BOOT Protocol. The standard method for booting a computer is to locate a boot block on a local drive and then go from there. During the 1980s, BOOTP was developed to allow diskless workstations to boot by downloading the operating system from another network node. Many operating systems at that time used this protocol because it allowed the use of cheaper desktop workstations on the network—long before PCs became a standard desktop item. At the time, the X-Windows workstation was popular in Unix and OpenVMS networks. BOOTP was an economical way to provide an X- Windows desktop with an operating system without the need to equip the machine with disk drives and other devices not necessary for a simple client. Other types of network clients adopted the BOOTP protocol, allowing network devices to also be suitably configured by use of the protocol. Read more…

Posted by arlene

Implicitly Releasing Configuration Information

In a DHCP environment, configuration information usually is granted for a set amount of time, called a lease. When the lease expires and the client has made no attempt to renew it, the configuration information can be used for another client. This can happen if a computer is moved from one subnet to another. Eventually, the original address it had obtained on the previous subnet will expire.

A client also can choose to implicitly release the addressing information. For example, a client that is gracefully shutting down can send a DHCPRELEASE message to the DHCP server, telling it that the lease is no longer needed. Read more…

Posted by arlene

Although based on the simple BOOTP protocol, the DHCP protocol client/server exchange is a little more complicated. Both sides communicate using a set of messages, as listed here:

• DHCPDISCOVER—The client broadcasts this message to locate DHCP servers.
• DHCPOFFER—The server uses this message type to offer a set of configuration parameters to the client.
• DHCPREQUEST—A client can use this message type to explicitly accept an offer from one server while implicitly implying that it is not going to use the offers made by other servers. This message type can also be used to confirm the configuration data when the client reboots or when it is attempting to extend a lease.

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Posted by arlene

Even though diskless workstations are a small percentage of the total number of network nodes in the world today, the concept of receiving configuration information from a central server has not gone away. The Dynamic Host Configuration Protocol (DHCP) was developed after BOOTP as a means for providing a workstation (or any other network device) not only with basic configuration information, but also with a lot of other configuration information, including the capability to add vendor-specific items to the networked computer. Read more…

Posted by arlene

Similar to BOOTP, DHCP uses a request/reply mechanism, and the packet format is almost the same for both to provide for backward compatibility. The layout of the packet used by DHCP looks very much like the layout of the BOOTP packet, with a few exceptions. The first 11 fields are the same. However, the last field, which is called the Vendor Extensions area in the BOOTP packet, is called the Options field in the DHCP packet. The format of the options is the same as it was for BOOTP. However, some of the options that are defined in RFC 2132 are specific only to DHCP. The options available for use with BOOTP clients are a subset of those available for use with DHCP clients. Although this field was limited to 64 bytes in the BOOTP packet, it now is a variable-length field that has a minimum of 312 bytes for DHCP options. Read more…

Posted by arlene

Creating an Address Scope

After you have authorized a server on the network, you can create a scope of addresses that the DHCP server can administer to clients. From the MMC utility, click once on the server you want to administer, and then select New Scope from the Action menu. The New Scope Wizard pops up. Alternatively, you can right-click the server and select New Scope. Click Next to dismiss the introductory dialog box and continue creating an address scope. The wizard then prompts you through the following steps: Read more…

Posted by arlene

Using the MMC Action Menu

To configure a server, click once to highlight it, and then click the Action menu. The Action menu allows you to perform the following tasks if you select a particular DHCP server object:

• Display Statistics—This shows statistical information about the selected server, such as the time the server started, the number of requests and offers, and the number (and percentage) of addresses in use, among other things.

• New Scope—Use this to create a new scope of IP addresses that the server can offer to clients. After the server is authorized in the Active Directory, this will be the first action you take to set up an address space the DHCP server can use.

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Posted by arlene

Installing a DHCP server on Windows 2000 or Windows 2003 Servers is just as simple as most application installs. However, you’ll need to have some information ready before you begin the installation. You will need to know the range of addresses that the server will administer and lease to clients. If you have any servers on the network that need static addresses, you’ll need to know those if they fall within the scope of the DHCP managed addresses. For example, DNS and WINS servers must have static IP addresses, and most DHCP servers do also. In a large network, you also should consider using multiple DHCP servers and enabling routers so that they can forward DHCP packets. Read more…

Posted by arlene

The Windows 2000/2003 DHCP servers provide support for BOOTP clients. The Default BOOTP user class of options is used to configure the information that is supplied to these clients. Although standard BOOTP servers require that the server be configured in advance with a table of client hardware addresses and corresponding IP addresses, Windows 2000/Server 2003 DHCP servers instead select the next available address to give to a BOOTP client. This matches the method the DHCP server useswhen granting IP address leases to its DHCP clients. Read more…

Posted by arlene

Many options that can be used for BOOTP and DHCP clients were discussed. The Windows 2000/2003 DHCP service enables you to configure which options will be offered to clients of the service. To configure the options, expand the MMC tree of DHCP servers to locate the server you want to manage. Click that server to get to the Options Folder for that server. After you have highlighted the Options Folder, click the Action menu.

From the Action menu, select Configure Options. You can see the default dialog box used for configuring options. Note that this dialog box has a General and an Advanced tab. Read more…

Posted by arlene

If a client is configured to use DHCP, what happens if no DHCP server is available on the network? Starting with Windows 98, Microsoft Windows clients can use Automatic Private IP Addressing (APIPA). This is not a solution for a large network. It is for use on small LANs, such as a home office with 25 or fewer network nodes.

Simply configure each client computer to use DHCP in the properties page for TCP/IP, and reboot. When the client computer realizes that no DHCP server is on the network (because it’s not receiving any replies from its broadcasts), it will timeout and begin to use APIPA. The scheme in which addresses are allocated is not that complicated. Read more…

Posted by arlene

Some computers or other networked devices, such as routers or printers, might need to keep the same IP address all the time. For example, Microsoft very strongly suggests you be sure that your DHCP server has a static, unchanging address. There are two ways you can be sure a particular computer or device keeps the same static address. The first method is to manually configure the client using the client’s software. For example, when you configure a Microsoft Windows 2000 Professional client or a Windows XP client, you can specify a static IP address (along with other network information) using the TCP/IP properties page for the client. Read more…

Posted by arlene

Microsoft’s version of DNS supports dynamic updates, as specified in RFC 2136, “Dynamic Updates in the Domain Name System (DNS UPDATE).” Windows 2000 clients can send dynamic updates after having received configuration information from a DHCP server. When a DHCP lease expires, the client will send an update to deregister the addressing information.

To register with DNS, the client first contacts a name server. If the name server is just a local serverand is not authoritative for the zone, it will return the address of the authoritative server to the client. The client then will contact the primary authoritative server to send it the updated addressing information. If it’s successful, a reply is sent back to the client. Read more…

Posted by arlene

If you are using Windows 2000 Advanced Server or Windows Server 2003 Enterprise or Datacenter servers, you can use the clustering feature for DHCP. This allows two separate DHCP servers to be administered as a single DHCP server. Windows 2000/2003 clustering supports a failover mode in which a service running on one computer can be monitored. If the node that is supplying a DHCP service supported by the clustering software fails, another node that runs the same service can be activated to take over for the failed node. By clustering DHCP services between two nodes, you will make the network less prone to downtime due to problems with your DHCP server. Read more…

Posted by arlene

Many small- to medium-sized networks use the router that provides Internet access as a DHCP server, using it to provide network addresses to client PCs. Configuring a router to act as a DHCP server is not difficult (most routers are configured to provide IP addresses by default), but you should be familiar with the process so you can make changes to the configuration, and, most important in the case of a wireless access point (WAP) or wireless router, limit the number of IP addresses provided. By limiting the number of IP addresses to the number of clients on your network, you can help minimize the possibilities of unauthorized users borrowing your network for Internet access or hacking. Read more…

Posted by eric

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Posted by arlene

Configuring WINS Clients

To use WINS, a client must first be configured with the address of a WINS server. This can be done in one of two ways. First, you could configure the client manually, using a network properties page for the particular Windows operating system. Second, you could configure a DHCP server to provide the name of one or more WINS servers to a client if you use DHCP in your. network.

If your network consists of nothing but Windows 2000, Windows XP, or Windows Server 2003 computers, you won’t need the WINS service. The Active Directory combined with DHCP and a dynamic DNS server provide all the name registration and resolution functionality you need. However, Windows 2000 clients can still be configured to use WINS. This might be necessary if you have a mixed network using clients from Windows 98, Windows NT, or even Unix or Linux clients that have Samba installed. Read more…

Posted by arlene

WINS is Microsoft’s NetBIOS Name Server (NBNS) developed according to the details set forth in RFCs 1001 and 1002, and is based on a client/server architecture. Only Windows server operating systems can run the WINS service, and the server does not have to be a domain controller. In the traditional DNS server that originated on Unix systems, an administrator is responsible for manually editing files to maintain address-to-name mappings. WINS is a dynamic database. Name registrations are performed by unicast messages (direct contact) between the server and the WINS client. Because the WINS server does not have to be on the same network segment as the client, and because no broadcast messages clutter up network medium, WINS is a more efficient method of name resolution when compared to b-node functionality. Read more…

Posted by arlene

Using the Action Menu to Manage Individual WINS Servers

You can perform most of the basic management tasks associated with WINS servers by selecting the WINS server from those you’ve added to the tree structure in the left pane and then using options from the Action menu. Alternatively, you can right-click on a server and select the same options from the menu that pops up. The things you can manage from the Action menu for any server are

• Display Server Statistics—This option enables you to see when the server was started, statistics about replication, name registrations, releases, and so on.

• Scavenge Database—This starts the scavenging process. You can determine when this process has finished by looking in the event log.

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