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DNS/DHCP Servers and Routing

Other common edge services that are easily handled by Linux are Domain Name System/Service (DNS), Dynamic Host Configuration Protocol (DHCP), and routing. Again, both of these services, which are usually required in almost every large network, are great places to begin Linux implementation, as they do not directly affect end users or business applications and can be seamlessly integrated.

DNS

At a high level, DNS maps host or domain names to IP addresses. Because humans tend to use word-based names and computers require number-based addresses, DNS is given the job of matching them up. Every time you type in a domain name (such as http://www.novell.com), a DNS sever is employed to get you to the right host machine. DNS servers for individual users or small businesses are usually hosted by Internet service providers. In large organizations where subdomains are required (for example, support.novell.com, forge.novell.com, and so on), a DNS server that provides navigation addresses to domain specific machines is hosted by the company.

DNS is more than just simple mapping. It includes the methodology for navigating hierarchical trees based on domains and subdomains to determine actual IP addresses. DNS also accommodates multiple addresses being assigned to the same domain name (that is, load balancing) or when many domain names map to a single IP address (that is, virtual hosting).

DNS caching is particularly useful for increasing performance for website access. A local DNS cache stores the hostname/address mappings for the most commonly requested websites. Rather than search the full chain of external DNS servers to determine an IP address, the previously resolved name exists in the local cache for fast access.

Several open source DNS Linux solutions are available with the most popular being Berkeley Internet Name Domain (BIND). BIND is included with SUSE Linux and is usually configured at startup. It includes a domain name server, a domain name system resolver library, and tools for verifying the proper operation of a DNS server. DNS resolution is also key to routing of email, listing the email exchange servers that accept mail for each domain.

DHCP

DHCP is even more commonly used in organizations than DNS. In simple terms, every machine connecting to an IP network (internal or the Internet) must have an IP address. Static addresses (IP numbers or addresses permanently assigned to a single device) may be in short supply and/or are difficult to manage in dynamic organizations. DHCP simplifies the management and distribution of IP addresses.

DHCP provides several methods of IP address distribution, but the most common is dynamic allocation or leasing. A DHCP server acts as a leasing agent, providing IP addresses to any client on the network that requests one from a predetermined pool of addresses specified by the administrator. As a client (a Windows workstation, for example) boots up, the network adapter card submits an open request for an IP address to the network. The DHCP server responds, granting a specific address for a specified period of time. When the client shuts down or the time period expires, the address is again available in the pool for reuse.

DHCP allows multiple internal network clients to access the Internet without static or hard-to-come-by outside IP addresses. IP address management is greatly simplified without the need to manually configure every client. DHCP also includes information about DNS server addresses or gateways that need only be configured once for everyone on the entire network. DHCP is backward compatible with BOOTP, an earlier (now dated) version of IP address leasing.

Novell SUSE Linux includes the DHCP server dhcpd published by the Internet Software Consortium (ISC) and two DHCP client options, dhclient and dhcpcd. DHCP client support is part of the standard install for most Linux workstations—it is there unless you specifically decline the option to install it. With a DHCP server somewhere on the network, IP address assignment is automatic every time the workstation boots up.

Routing

In addition, a Linux server can be used as a common router. Static routing tables can be created that establish routes to a host, routes to a host via a gateway, or routes to a network. Complete and complex routing solutions can be created using Linux and other open source software. In fact, Linux is commonly used as the platform for commercial routing solutions available from original equipment manufacturers (OEMs).

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