Figure 7-8: Top-level domains for the Internet in Software

Draw EAN13 in Software Figure 7-8: Top-level domains for the Internet

Figure 7-8: Top-level domains for the Internet
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Just as you can locate files in a file system by following a path from the root, you can locate hosts or particular DNS servers by following their paths through the DNS hierarchy For example, a host called elvis in the domain stars within the commercial organization oldies is locatable by the name elvisstarsoldiescom In some ways the operation of DNS can be thought of as similar to the way routing tables work for routing packets through an internetwork No single DNS server has a complete picture of the whole hierarchy; it just knows what is the next DNS server in the chain to which it is to pass the request A particular domain is reachable when pointers for that domain exist in the domain above it Computers in the edu domain cannot access computers in the workcom domain until a pointer to the DNS server of the work subdomain is placed in the servers of the com domain A DNS database in the com DNS servers contains name server records that identify the names of the DNS servers for each domain directly under the com domain Let's clarify this by considering an example Supposing a domain under edu has been created called univ and a domain under com has been created called work (illustrated in Fig 7-8) Now, a computer in the univ domain (let's say it is named vax) needs to contact a machine in the workcom domain (which is called sun) The task to accomplish here is to provide to the machine vaxunivedu, the IP address of sunworkcom The process to complete this task is as follows: The vaxunivedu host is configured to have its DNS server as the machine with IP address 201123, so it sends a DNS request to that IP address This computer must be reachable by vaxunivedu Host vaxunivedu receives a reply stating that a machine named overtimeworkcom has all DNS information for the workcom domain, and the IP address of overtimeworkcom is included in the reply Host vaxunivedu will then send a query to the IP address of overtimeworkcom for the IP address of sunworkcom The computer overtimeworkcom replies with the requested IP address, so vaxunivedu can now contact sunworkcom
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DNS is normally implemented on a Unix machine via the Berkeley Internet Name Domain (BIND) programs BIND has two elements, the Name Server and the Resolver The Resolver forms
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through libraries) The Name Server answers queries, but only computers supplying DNS information need to run the Name Server On a Unix machine running BIND, the namedhosts file contains most of the domain information, converting host names to IP addresses and also containing information about the mail servers for that particular domain DNS and BIND are subjects that justify a complete book in their own right The preceding discussion is intended to give a very brief overview of how DNS can simplify administration of a large TCP/IP-based internetwork If you want to set up DNS on your network (I recommend that you do), refer to the publication DNS and BIND by Cricket Liu and Paul Albitz, published by O'Reilly and Associates If you can configure DNS and either BOOTP or DHCP, your life as administrator of a large TCP/IPbased internetwork will be much simpler
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Integrating the Operation of DNS and DHCP
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Cisco have released a product called the Cisco DNS/DHCP Manager, known as the CDDM for short What we'll look at in this section is an overview of the system components, how DNS and DHCP are coordinated by the CDDM, coping with multiple logical networks on the same physical wire, and creating a new domain CDDM System Components The key to understanding how this integration works is to realize that DHCP is the primary tool that allocates IP addresses and updates DNS database entries with new IP address information DNS is the passive receiver in this integrated product The CDDM consists of a Domain Name Manager, which is a graphical DNS management tool that runs on the usual platforms of Solaris, HP/UX and AIX, and a DHCP server that does the dynamic updating of DNS with IP addresses assigned to DHCP clients Using a graphical tool like the DNM to update your DNS configuration, rather than manually editing ASCII zone transfer files has many benefits Notably, the Domain Name Manager browser reduces common configuration errors by checking the syntax of each new entry The Cisco DHCP server automatically updates the Domain Name Manager with the IP address and domain name of the new nodes on the network The Domain Name Manager then propagates this information to DNS servers on the network As such, the Domain Name Manager replaces an organization's existing primary DNS server and becomes the source of DNS information for the entire network Many hosts that are accessed via the World Wide Web use information in DNS to verify that incoming connections are from a legitimate computer If both an A record and a PTR record are registered for the incoming client, the server assumes that a responsible network administrator has assigned this name and address If the information in the DNS is incomplete or missing, many servers will reject connections from the client To simplify the process of bringing a new host on-line, the Cisco Domain Name Manager automatically adds the PTR record into the DNS configuration The PTR record is the mapping between an IP address and a DNS name and is also known as reverse mapping Omitting the PTR record is one of the most common mistakes when managing a DNS server
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For networks that use NT extensively as the applications and file/print services server, the CDDM provides enhanced TCP/IP services for NT that are sorely lacking in the base product These
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network devices including routers and a syslog server to log error messages from network devices over the network More frequently these days, organizations are deploying Ethernet switches that allow a reduction in network segment traffic, along with a flattening of the IP addressing scheme The main benefit of this is to use fewer router ports in an internetwork, which produces a cost saving Problems have occurred with traditional devices when DHCP is to be implemented on a segment that has more than 254 hosts and class C addressing is in use This is becoming more common, as address depletion of registered Internet addresses results in blocks of class C addresses being the only ones available It is feasible to have multiple class C networks assigned to the same physical segment, however, this does cause difficulties for many DHCP servers These DHCP servers expect one physical segment to be associated with only one network number By contrast, the Cisco DHCP server supports address pools that contain multiple logical networks on the same physical network Additionally, the Cisco DHCP server can combine pools of IP addresses from multiple networks into a single large pool of addresses The DHCP server also supports BOOTP to enable you to manage BOOTP and DHCP from one server The Specifics of CDDM Operation Having overviewed the operation of CDDM, let's look in more detail at how it is set up As previously stated, the cornerstone of the CDDM is the Domain Name Manager (DNM) Server, which lets you manage zone data, specifically, host names and addresses from across a network The zone data originates in ASCII text files called "zone files," which primary name servers read on start-up and propagate to "secondary" name servers via zone transfers Many network managers choose to advertise only their secondary name servers, and dedicate their primary name servers to perform zone transfers The CDDM supports this approach by assigning zone transfer and name resolution to separate servers The DNM server takes over the role of zone transfers but leaves the role of name resolution to DNS servers DNS servers configured as "secondary" name servers can obtain zone transfers from a DNM, but the DNM server must not be advertised as a name server with NS (name server) records because it does not resolve names Every time you modify the DNM server's database, the DNM server increments the appropriate zone serial numbers so that the corresponding secondary name servers can detect a change in the zones for which they are authoritative and request zone transfers Normally, you could not run a DNM server and a DNS server on the same host because both servers listen on port 53 for zone transfer requests The CDDM, however, includes an enhanced DNS server that can request zone transfers over any port The Cisco DNS server is based on BIND 495, so you can both use existing zone files and receive new zone data from your DNM server Cisco's on-line documentation uses an example of port 705 to receive data from a DNM server The DNM operates as a client server application on a network The DNM server maintains a single
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DNM client A special DNM client is called the DNM Browser which simplifies everyday DNS management tasks such as adding new hosts or changing host addresses and lets you configure DNM servers from remote hosts The DNM Browser presents a view of the domain name space in an outlinestyle layout that makes it easy to browse through domains The DNM Browser communicates with DNM servers over TCP/IP, so you can manage DNS names from remote hosts On UNIX platforms, the DNM Browser is an X client On Windows NT and Windows 95, the DNM Browser is a native Windows application As you grow your network, the DNM browser will automatically: Modify inverse mappings when you add new hosts, and propagates name server records appropriately when you create new subdomains Check for domain name conflicts Finds available IP addresses on specified networks Import existing DNS zone files and export zone files and UNIX-style host tables To manage a DNM server's database from a DNM client, you must have a DNM server account When you connect to a DNM server you must supply a valid user name and password If you use the DNM Browser, you enter your account information when prompted You can use the DHCP/BootP server to manage the DNM server, however, you must configure the DHCP/BootP server with valid account information to do so The previously mentioned Cisco DHCP/BootP server can be configured to behave as a DNM client, which allows you to automatically coordinate names in DNS with names in the DHCP/BootP server's database Note the DHCP/BootP server can only update the DNM server with information from its DHCP database The DHCP/BootP server does not propagate information from its BootP database to the DNM server Traditionally, DHCP and BootP databases have been managed independently of the DNS With most DHCP and BootP servers, every time you add a host entry to the DHCP database, you must also add corresponding domain names for the host: one in the parent domain and another in the in-addrarpa domain The Cisco DHCP/BootP server and DNM server eliminate the need to manually coordinate the DNS with your DHCP or BootP databases by dynamically updating zones dedicated to your DHCP clients
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in multiple domains If your network requires multiple DNM servers, you must configure a unique DHCP/BootP server for each DNM server Make sure that the DNM server is authoritative for both your dynamic host names and the corresponding addresses via the inverse domains under the inaddrarpa domain Of course if your network requires more than one DHCP/BootP server, make sure they each use a separate, unique domain One implementation recommendation is to define a dynamic domain for DHCP clients The DHCP/BootP server lets you choose the name of a domain to which it will add domain names for DHCP clients For example, if your domain is best-companycom, you might reserve the domain, dhcpbest-companycom, for DHCP clients All clients then acquire host names such as machine1dhcpbest-companycom when they accept DHCP leases from your DHCP/BootP server The DHCP/BootP server can support only one dynamic domain, but the DNM server can accommodate multiple DHCP/BootP servers if they are configured for different dynamic domains If you do implement a dynamic domain, you should avoid adding names to this domain by any means other than by the DHCP/BootP server If you manually add names to a dynamic domain, the DHCP server may delete them, depending on how you configure it There are two methods of specifying how the DHCP/BootP server chooses names for DHCP clients The first method is to create a fixed set of host names based on the DHCP database entry name for each IP address pool and propagate those names to the DNM server every time the DHCP/BootP server starts If a host already has the entry name (for example, test), subsequent hosts are assigned the entry name with a number appended to it (for example, test1 or test2) Each time you start the DHCP/BootP server, it checks its list of outstanding leases and, if necessary, it updates the DNM server to rebuild the dynamic domain, even if you did not change any of the corresponding DHCP entries The second method will let the DHCP client request a host name, and add a new domain name to the DNM server when the client accepts the DHCP server's offer If the client does not request a name, the DHCP/BootP server assigns a name based on the DHCP database entry much the same as method 1 The DHCP/BootP server only adds a name to the dynamic domain when a client accepts a lease Similarly, the DHCP/BootP server only deletes a name from the dynamic domain when the corresponding lease expires Supporting Multiple Logical Networks on the Same Physical Network The DHCP/BootP server lets you create a pool of IP addresses that spans multiple logical subnets, using the sc (subnet continuation) option tag, a functional extension of DHCP This option tag is useful when you need to pool addresses from different networks, such as two Class C networks or a Class B and a Class C network For example, suppose you need to offer a pool of 400 addresses and your network is composed of two Class C networks The sc option tag lets you combine the two subnets and put all 400 addresses in the pool
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Most IP routers let you forward DHCP/BootP requests received on a local interface to a specific host This forwarding feature is often called BootP helper or BootP forwarder On Cisco routers, you can control BootP forwarding with the Cisco IOS commands ip helper-address and set dhcp
you can control BootP forwarding with the Cisco IOS commands ip helper-address and set dhcp relay The action of these commands is to pick up a UDP broadcast on a local segment and direct them to the IP address specified in the ip helper-address command When routers forward DHCP/BootP requests, they place their own IP addresses in the DHCP/BootP packet in a field called "GIADDR" The router inserts the IP address of the interface (called the "primary" interface) on which it received the original DHCP/BootP request The DHCP/BootP server uses the address in the GIADDR field to determine the IP subnet from which the request originated so it can determine which pool of addresses to use before allocating an IP address to the DHCP/BootP client When you run multiple IP network numbers on the same physical network, you typically assign multiple IP addresses to a router interface by use of the secondary IP address interface configuration command Because the DHCP/BootP server only allocates addresses based on the GIADDR field, and because it only receives the router's primary address in the GIADDR field, you must configure the DHCP/BootP server to associate the other network addresses with the primary subnet using the Subnet Continuation parameter (sc option tag) You can specify an arbitrary number of secondary address pools in the DHCP configuration, to make all addresses in the primary and secondary entries available to DHCP clients on the corresponding network segments DHCP entries that contain sc option tags must appear after the entry for the subnet they specify in the DHCP configuration editor's entry list Creating a New Domain To complete our discussion of the CDDM and its components, we'll look at the specifics of creating a new domain Clearly as new revisions of the CDDM these steps will change, but the following procedure should give you a good feel for what it takes to operate the CDDM First we'll create a test domain using the DNM Browser, then we'll configure the DNS server as a secondary name server for the test domain Next we'll configure the Syslog service for troubleshooting DHCP and BootP service, and look at how to manage the DNM server via the DHCP server and finally how to configure the BootP service So, let's get started by using the DNM Browser to create and propagate the best-companycom domain Using a Windows NT system as an example, double-click the DNM Browser icon in the directory in which you installed CDDM, and choose DNM Browser from the Windows Start menu Next, in the Authentication for local host dialog box, enter admin in both the Username and in the Password fields, and click OK Next, choose Add from the Edit menu in the DNM Browser main window and enter bestcompanycom in the Fully Qualified Name field Check that the Modify records box is enabled, and click OK When the Modify Resource Records window appears, select the Authority tab Click Reset to Suggested Values, at which point the DNM Browser inserts a set of suggested SOA values You can then change the Primary Name Server field to name1best-companycom Once this is completed, change the Responsible Person Mailbox field to my_e-mail@best-companycom (insert an appropriate e-mail address for my_e-mail)
Now, click the Name servers "+" button in the Name Server Records group, type name1bestcompanycom in the Name servers field, and click OK The best-companycom domain should now
companycom in the Name servers field, and click OK The best-companycom domain should now appear in the DNM Browser Next choose Add from the Browser's Edit menu, and in the Fully Qualified Name field of the Add dialog box, type name1best-companycom, then make sure that the Modify records box is enabled, and click OK In the Basic tab of the Modify Resource Records window, click the "+" button in the Address Records group In the Add IP Address dialog box, type 1724511 (or whatever IP address is appropriate) in the Starting IP Address field, and click OK When the Modify Resource Records dialog box is active again, click OK, name1 now appears in the DNM Browser You can add a host (called for example hostbest-companycom) by repeating the above To refresh the DNM Browser's display, choose Reload from the Edit menu The address 1724512 should be automatically assigned Note that although the DNM server automatically creates the "reverse" pointer records, it does not create a Start of Authority (SOA) record To add the SOA records for the 1110in-addrarpa domain, make sure 1110in-addrarpa is selected in the DNM browser window, then choose Modify from the Edit menu Once the Modify Resource Records dialog box appears, select Authority in the Modify Resource Records window, and click Reset to Suggested Values The DNM Browser inserts a set of suggested SOA values Next you must change the Primary Name Server field to name1best-companycom, change the Responsible Person Mailbox field to an appropriate e-mail address and click the Name servers "+" button in the Name Server Records group to enter name1best-companycom, then click OK The icon for 1110in-addrarpa in the DNM Browser now indicates the new SOA record with a red triangle There are lots of other tasks associated with the CDDM that you can complete, like setting up a secondary system As you gain familiarity with this very useful tool, you can expand the scope of the CDDM activities to largely automate the assigning of hostnames and IP addresses on your network
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