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8.3.1. Name Resolution<br />

The mechanism for name resolution in Linux is modular and can use various sources of information<br />

declared in the /etc/nsswitch.conf file. The entry that involves host name resolution<br />

is hosts. By default, it contains dns files, which means that the system consults the /etc/hosts<br />

file first, then DNS servers. NIS/NIS+ or LDAP servers are other possible sources.<br />

NOTE<br />

NSS and DNS<br />

Be aware that the commands specifically intended to query DNS (especially<br />

host) do not use the standard name resolution mechanism (NSS). As a consequence,<br />

they do not take into consideration /etc/nsswitch.conf, and thus,<br />

not /etc/hosts either.<br />

8.3.1.1. Configuring DNS Servers<br />

DNS (Domain Name Service) is a distributed and hierarchical service mapping names to IP addresses,<br />

and vice-versa. Specifically, it can turn a human-friendly name such as www.eyrolles.<br />

com into the actual IP address, 213.244.11.247.<br />

To access DNS information, a DNS server must be available to relay requests. Falcot Corp has its<br />

own, but an individual user is more likely to use the DNS servers provided by their ISP.<br />

The DNS servers to be used are indicated in the /etc/resolv.conf, one per line, with the nam<br />

eserver keyword preceding an IP address, as in the following example.<br />

nameserver 212.27.32.176<br />

nameserver 212.27.32.177<br />

nameserver 8.8.8.8<br />

8.3.1.2. The /etc/hosts file<br />

If there is no name server on the local network, it is still possible to establish a small table<br />

mapping IP addresses and machine hostnames in the /etc/hosts file, usually reserved for local<br />

network stations. The syntax of this file is very simple: each line indicates a specific IP address<br />

followed by the list of any associated names (the first being “completely qualified”, meaning it<br />

includes the domain name).<br />

This file is available even during network outages or when DNS servers are unreachable, but<br />

will only really be useful when duplicated on all the machines on the network. The slightest<br />

alteration in correspondence will require the file to be updated everywhere. This is why /etc/<br />

hosts generally only contains the most important entries.<br />

This file will be sufficient for a small network not connected to the Internet, but with 5 machines<br />

or more, it is recommended to install a proper DNS server.<br />

Chapter 8 — Basic Configuration: Network, Accounts, Printing…<br />

153

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