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Before going into detail about configuring DNS servers, you first need to learn exactly what DNS is and how it works. In this section, you ll read about the differences between DNS and other methods of resolving names. You ll also find out how the DNS hierarchy is structured and what roles the different types of DNS servers play in this hierarchy. DNS is not the only solution that you can use for name resolving. Let s have a quick look at two of the alternative methods: the /etc/hosts file and Sun s Network Information System (NIS).
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Managing Host Name Information with the /etc/hosts File
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Before centralized systems such as NIS and DNS were introduced, every host kept its own file that mapped IP addresses to names. In the days when the Internet was called (D)ARPANet and was still a very small network, this was a feasible solution, although the administrator had to
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CHAPTER 9 CONFIGURING NETWORK INFRASTRUCTURE SERVICES
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make sure that these files were updated properly. Such a mechanism still exists, but in the form of the /etc/hosts file. In this file, you can keep a list of commonly used names and their IP addresses. Ubuntu Server creates this file by default to make sure that the localhost can be resolved. Listing 9-1 shows an example of the file. Note that you can still use this file as an addition to DNS. Depending on the settings in /etc/nsswitch.conf, its contents will be checked first before any DNS lookup. Listing 9-1. Displaying the Contents of /etc/hosts root@RNA:~# cat /etc/hosts 127.0.0.1 localhost 127.0.1.1 RNA.lan RNA # The following lines are desirable for IPv6 capable hosts ::1 ip6-localhost ip6-loopback fe00::0 ip6-localnet ff00::0 ip6-mcastprefix ff02::1 ip6-allnodes ff02::2 ip6-allrouters ff02::3 ip6-allhosts
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Using NIS to Manage Name Resolution
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A more convenient method that you can use to keep mappings between host names and IP addresses is Sun s NIS, also known as the Yellow Pages. This system uses a database for important configuration files on a server, such as the /etc/hosts file, the /etc/passwd file, and /etc/shadow. As an administrator, you can determine for yourself what files to manage with NIS. These files are converted to NIS maps, which are the indexed files that comprise the NIS database. In NIS, one server is configured as the master server, which maintains the NIS database. All nodes are configured as NIS clients and send their name resolution requests to the NIS master. To provide redundancy, NIS can also use slave servers, which offer a read-only copy of the NIS master database. However, the master server is the single point of administration. Although NIS was a good solution to manage relevant information within a network, it never became very popular as an Internet-level name service mainly because NIS does not provide a hierarchical solution, only flat databases. All these flat databases are managed by local administrators, and there s no relation among the databases that are used in different NIS domains. The large amount of information on the Internet today makes it impossible to get quick results from a structure like NIS. For this reason, most organizations that still use NIS are phasing it out and configuring DNS to resolve host names to IP addresses and LDAP to manage user information (therefore, NIS is not covered in this book).
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Although DNS is the main system used for name resolution, it s not the only one. You can set it up in parallel with a NIS system and the /etc/hosts file. If you do this, the order in which the
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CHAPTER 9 CONFIGURING NETWORK INFRASTRUCTURE SERVICES
different systems are searched is important. The search order is determined by the /etc/nsswitch.conf file; see Listing 9-2 for an example. Listing 9-2. Contents of the /etc/nsswitch.conf File root@RNA:~# cat /etc/nsswitch.conf # /etc/nsswitch.conf # # Example configuration of GNU Name Service Switch functionality. # If you have the 'glibc-doc-reference' and 'info' packages installed, try: # 'info libc "Name Service Switch"' for information about this file. passwd: group: shadow: hosts: networks: protocols: services: ethers: rpc: netgroup: compat compat compat files dns files db db db db nis files files files files
For all the important information on your server, the nsswitch.conf file contains an indication of where it should be searched. In the case of hosts and network information, the example file is pretty clear: it first checks local configuration files and only after that does it check the DNS hierarchy. This means that you can use /etc/hosts to override information as defined in DNS.
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