8.3. Setting the Hostname and Configuring the Name Service

The purpose of assigning names to IP numbers is to make them easier for people to remember. In reality, an IP address identifies a network interface associated with a device such as a network card. Since each machine can have several network cards, and several interfaces on each card, one single computer can have several names in the domain name system.

Each machine is, however, identified by a main (or “canonical”) name, stored in the /etc/hostname file and communicated to the Linux kernel by initialization scripts through the hostname command. The current value is available in a virtual filesystem, and you can get it with the cat /proc/sys/kernel/hostname command.

BACK TO BASICS /proc/ and /sys/, virtual filesystems

The /proc/ and /sys/ file trees are generated by “virtual” filesystems. This is a practical means of recovering information from the kernel (by listing virtual files) and communicating them to it (by writing to virtual files).

/sys/ in particular is designed to provide access to internal kernel objects, especially those representing the various devices in the system. The kernel can, thus, share various pieces of information: the status of each device (for example, if it is in energy saving mode), whether it is a removable device, etc. Note that /sys/ has only existed since kernel version 2.6.

Surprisingly, the domain name is not managed in the same way, but comes from the complete name of the machine, acquired through name resolution. You can change it in the /etc/hosts file; simply write a complete name for the machine there at the beginning of the list of names associated with the address of the machine, as in the following example:

127.0.0.1     localhost
192.168.0.1   arrakis.falcot.com arrakis

8.3.1. Name Resolution

The mechanism for name resolution in Linux is modular and can use various sources of information declared in the /etc/nsswitch.conf file. The entry that involves host name resolution is hosts. By default, it contains files dns, which means that the system consults the /etc/hosts file first, then DNS servers. NIS/NIS+ or LDAP servers are other possible sources.

8.3.1.1. Configuring DNS Servers

DNS (Domain Name Service) is a distributed and hierarchical service mapping names to IP addresses, and vice-versa. Specifically, it can turn a human-friendly name such as www.eyrolles.com into the actual IP address, 213.244.11.247.

To access DNS information, a DNS server must be available to relay requests. Falcot Corp has its own, but an individual user is more likely to use the DNS servers provided by their ISP.

The DNS servers to be used are indicated in the /etc/resolv.conf, one per line, with the nameserver keyword preceding an IP address, as in the following example:

nameserver 212.27.32.176
nameserver 212.27.32.177
nameserver 8.8.8.8

Note that the /etc/resolv.conf file may be handled automatically (and overwritten) when the network is managed by NetworkManager or configured via DHCP.

8.3.1.2. The `/etc/hosts` file

If there is no name server on the local network, it is still possible to establish a small table mapping IP addresses and machine hostnames in the /etc/hosts file, usually reserved for local network stations. The syntax of this file is very simple: each line indicates a specific IP address followed by the list of any associated names (the first being “completely qualified”, meaning it includes the domain name).

This file is available even during network outages or when DNS servers are unreachable, but will only really be useful when duplicated on all the machines on the network. The slightest alteration in correspondence will require the file to be updated everywhere. This is why /etc/hosts generally only contains the most important entries.

This file will be sufficient for a small network not connected to the Internet, but with 5 machines or more, it is recommended to install a proper DNS server.

TIP Bypassing DNS

Since applications check the /etc/hosts file before querying DNS, it is possible to include information in there that is different from what the DNS would return, and therefore to bypass normal DNS-based name resolution.

This allows, in the event of DNS changes not yet propagated, to test access to a website with the intended name even if this name is not properly mapped to the correct IP address yet.

Another possible use is to redirect traffic intended for a specific host to the localhost, thus preventing any communication with the given host. For example, hostnames of servers dedicated to serving ads could be diverted which would bypass these ads resulting in more fluid, less distracting, navigation.