CONFIGURING NETWORK INFRASTRUCTURE SERVICES in Font

Paint QR-Code in Font CONFIGURING NETWORK INFRASTRUCTURE SERVICES

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 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 of the important information on your server, the nsswitch.conf file contains an indication where it should be searched for. 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.

The most important advantage offered by DNS is the fact that it s organized in a hierarchical manner. This makes the system very scalable because it can be extended by simply adding another branch to the tree-like hierarchy. On top of the hierarchy are the root servers, which have one purpose only: provide information about the top-level domains (TLDs). Some fixed domain names are used for top-level domains, including .com, .org, and .info. Top-level domains exist for all countries as well, such as .nl, .uk, .fr, and so on. Within these top-level domains, persons and organizations can create their own domains, which may contain subdomains as well. For example, an organization could create a domain called example.com and, within the structure of example.com, it could create some subdomains as well, such as east.example.com and west.example.com.

The number of subdomains is virtually unlimited, although it becomes hard to work with more than four or five levels of domains. No one wants to type www.servers.east.nl. sandervanvugt.com all the time, do they Figure 9-1 provides an example of the partial DNS hierarchy.

Within the DNS hierarchy, different servers are responsible for the data in specific domains and sometimes subdomains as well. These are the so-called name servers and the part of the hierarchy that they are responsible for is the zone. A zone can include more than just one domain; for example, if one name server is responsible for everything in sandervanvugt.nl, including the subdomain s servers and workstations, then the complete zone is sandervanvugt.nl. If, however, there s a subdomain called sales.sandervanvugt.com that has its own name server, that subdomain would be a zone by itself which is not part of sandervanvugt.com. Speaking in a very generic way, a zone is just a branch of the DNS hierarchy. All zones should have at least two name servers. The first is the master name server, which is ultimately responsible for the data in a zone. For fault tolerance and to make the information more accessible, it s a good idea to use one or more slave servers as well. These slave servers will periodically get an update of all the data on the master server by means of a zone transfer: this is the process the master server uses to update the database on the slave server. Note that DNS uses a single-master model: updates are performed on the master server and nowhere else, and the databases on the slave servers are read-only. You should also know that the name servers do not need to be in the zone that they are responsible for. For example, the name server of a given domain will often be hosted by the Internet provider that (of course) has its own domain. You can maintain your own DNS server, and it s useful to do so if your organization is larger than average, but you don t have to. You can also just purchase a domain and have your Internet server do the name server maintenance work.