Dynamic Host Configuration Protocol DHCP

• NFS is highly suitable for intranet file service. It provides data consistency intranet−wide, and makes the data backup and restore easier. For not−UNIX clients DOS, NT, Mac, a number of emulation software products is available, like free Samba or other professional packages Xinet, Totalnet, etc.. For more details about NFS see Chapter 18. • Intranet is ideal for networked backup implementations, like free Amanda, or professional products as Legato Networker or Veritas NetBackup. • E−mail is certainly needed within the intranet, as well as with external Internet users. There is no difference between the two; e−mail traffic must be routed through the firewall. The inbound e−mail traffic has to be scanned for viruses and other malicious codes. Today e−mail is the main transporter of nasty offenders, and the intranet must be protected accordingly. This is efficiently provided by the intranet viruswall. For more details about e−mail see Chapter 20. • Intranet community consists of trusted hosts. So internal remote login and other remote commands including remshing should be quite safe, as well as telnet, ftp, and other insecure network services. Despite internal intranet safety, the secure remote commands and secure shelling SSH are recommended even within the intranet. Today SSH should be standard on every computer, especially on the UNIX platform. • Intranet actually means the small Internet confined within a company or organization. This also means that internal Web services are also very useful within the intranet. All internal information could be published on the internal Web site, and they will be available companywide. But everything remains among the intranet users. Technically, internal Web service is the same as the external one. The difference is only that the audience is restricted strictly to the intranet community. • Most other Internet services are also possible within the intranet. The only issue is what are the benefits of their implementations. Each new service has a price, and it is not worth it to support senseless network services. • Among all mandatory, recommended, possible, and senseless network services, there is one that sounds like it was invented for the intranet environment. This is the dynamic host configuration protocol DHCP. We will elaborate on this service in more detail. •

25.3.2.1 Dynamic Host Configuration Protocol DHCP

DHCP dynamic host configuration protocol is a protocol that allows the centralized and automatic assignment of IP configurations on a computer network. Each participant in the network communication requires a unique IP address. Up to now we have assumed a manual IP address assignment, and we learned about the corresponding UNIX configuration behind it. A manually assigned IP address is known as a static IP address and it is almost standard on the UNIX platform. UNIX hosts are assumed to run primarily as network servers and they need in most cases a static IP address. But intranet is not exclusively UNIX based. Desktop computers are the most numerous participants in the Intranet, and mostly they are not UNIX hosts at all. Although the manual IP configuration always can be implemented, the ability to assign IP client configurations automatically can alleviate the painful process of intranet IP address management. Network administrators have quickly appreciated the importance, flexibility, and ease−of−use offered in DHCP. 648 For every computer to be online, a unique IP address is required. What makes an intranet unique is the fact that the majority of computers should not be online continuously. The truth is that they spend more time offline or inactive. Also, when mobile computer users travel between sites, they have had to relive this process for each different site from which they connected to a network. So there are many reasons to automate the process of adding machines to a network and assigning unique IP addresses. How does DHCP work? When a client needs to start up TCPIP operations, it broadcasts a request for address information. The DHCP server receives the request, assigns a new address for a specific time period called a lease period, and sends it to the client together with the other required configuration information. This information is acknowledged by the client and used to set up its configuration. The DHCP server will not reallocate the address during the lease period and will attempt to return the same address every time the client requests an address. The client may extend its lease with subsequent requests, and may send a message to the server before the lease expires telling it that it no longer needs the address so it can be released and assigned to another client on the network. The use of DHCP in the intranet is extremely useful and efficient. Manual configuration requires the careful input of a unique IP address, subnet mask, default router address, and a DNS server address. In an ideal world, manually assigning client addresses should be relatively straightforward and error free. Unfortunately, we do not live in an ideal world; computers are frequently moved and new machines get added to a network. Also if other intranet resources such as routers change, this could mean changing many system configurations. For an administrator, this process can be time−consuming, tedious, and error prone. DHCP has several major advantages over manual configurations. Each intranet computer gets its configuration from a pool of available IP addreses automatically for a specific leasing period, meaning no wasted IP numbers. When a computer has finished with the address, it is released for another computer to use. Configuration information can be administered from a single point. And major network resource changes require only the DHCP server to be updated with the new information, rather than each computer in the intranet. The benefits of dynamic addressing are especially helpful in mobile computing environments where users frequently change locations. New mobile users in the intranet simply plug in their laptop to the network, and receive their required configuration automatically. When moving to a different network using another DHCP server, then that networks server will supply the configuration. No manual reconfiguration is required at all. At the same time, DHCP servers are easy to administer and can be set up in just a few minutes. The DHCP servers have to run continuously as they must be available at all times when clients need IP access. Some machines in the intranet need to be at fixed addresses. For example, all servers, routers, printers, and similar devices that have to be accessible by all clients. The changes in their IP addresses would disable the corresponding services. The DHCP server should be capable of assigning pre−allocated IP addresses to these specific machines. To avoid conflicts between addresses assigned by the DHCP server and those assigned manually, 649

25.3.3 Virtual Private Network VPN