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HOSTS file for Linux

History behind the hosts file

During the 70s all the domain name to IP address was contained in a single file called the HOSTS.TXT file. A single for only a few hundred machines on the internet. Of course the load began to grow and eventually a different system supplanted it. The "/etc/hosts" file (without the txt) is now used primarily as a backup (or a cache) if the name servers are inaccessible.

The format of each entry is:
[internet address] [domain name] [alias]
127.0.0.1 www.elouai.com localhost

internet address - a standard IP address, i.e. 127.0.0.1
domain name - A Fully Qualified Domain Name, FQDN, i.e. www.elouai.com
alias - this field is optional, i.e. localhost

Proceeding an entry with a number sign '#' will comment it out.

There is a lookup order which can be changed in /etc/host.conf
The default order is hosts, bind:

hosts - first the host file is checked (local cache)
bind - if it does not find then it checks with the name servers

The line is formatted simply as:
order hosts,bind

/etc/hosts.deny

Describes the name of the hosts which are NOT allowed to use the local INET services. This file is called by "/usr/sbin/tcpd"

/etc/hosts.allow

Describes the name of the hosts which are ALLOWED to use local INET services. This file is called by "/usr/sbin/tcpd"

references:
Redhat Linux 8.0: Managing Hosts

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