Localhost file in linux

Format of /etc/hosts on Linux (different from Windows?)

Pasted below this question is a sample of a /etc/hosts file from a Linux (CentOS) and a Windows machine. The Linux file has two tabbed entries after the IP address (that is localhost.localdomain localhost) and Windows has only one. If I want to edit the hosts file in Windows to have the machine name (etest) instead of localhost, I simply replace the word localhost with the machine name I want. The machine need not be part of a domain. In a Linux machine, the two entries localhost.localdomain and localhost seems to indicate that I will need the machine to be part of a domain. Is this true? Can I simply edit both entries to etest so that it will read:

or is it required that I substitute one entry with a domain name? Additionally, please let me know what the second line of the /etc/hosts file on the Linux machine is for.

::1 localhost6.localdomain6 localhost6 

# Do not remove the following line, or various programs # that require network functionality will fail. 127.0.0.1 localhost.localdomain localhost ::1 localhost6.localdomain6 localhost6 

# Copyright (c) 1993-1999 Microsoft Corp. # # This is a sample HOSTS file used by Microsoft TCP/IP for Windows. # # This file contains the mappings of IP addresses to host names. Each # entry should be kept on an individual line. The IP address should # be placed in the first column followed by the corresponding host name. # The IP address and the host name should be separated by at least one # space. # # Additionally, comments (such as these) may be inserted on individual # lines or following the machine name denoted by a '#' symbol. # # For example: # # 102.54.94.97 rhino.acme.com # source server # 38.25.63.10 x.acme.com # x client host 127.0.0.1 localhost 

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The /etc/hosts File Complete Guide for Linux

What Is the /etc/hosts File and What is its Purpose?

The /etc/hosts or simply the Hosts file is a plain text file that maps IP addresses with their corresponding hostnames. Primarily, the Hosts file is useful when you are working on a local network of computers. It provides some simple sort of hostname resolution.

The /etc/hosts file is usually useful when we are not using DNS or NIS service for resolving IP addresses. In fact, when DNS did not exist, there were no centralized systems for resolving hostnames. Local networks and computers relied on their Hosts file. This file holds the entries for all the known hostnames and their corresponding IP addresses.

After DNS appeared on the scene, computers did not need this file and started using the DNS service for fetching hostnames. Despite these advances, the Hosts file is still relevant on modern operating systems.

What will we Cover?

In this guide, we will see what a Hosts file in Linux is, some use cases of this file, and an application of this file based on the use cases.

Use Cases of the Hosts File

We can assign a domain name to an IP address using the Hosts file. However, these changes are local and will work on the local computer.

The Hosts file is still relevant today, there are some specific use cases for the Hosts file as mentioned below:

1. We have built a website, but it is not live on the web since we have not registered the domain name for it. However, if we have a registered IP address from our hosting, we can map this IP to a dummy or non-existing domain name and continue building our website. Similarly, in a software testing environment, many web applications run on the local hosts address i.e. addresses of the type 127.0.0.1. Again, we can manage this using the Hosts file.
2. In case we have migrated our website from one hosting to another one and we want to check the new hosting performance, we can connect our domain to the new hosting without closing our old hosting account. In this way, we can see how our website is loading from the new hosting.
3. Suppose you want to block a website, like blocking Facebook on an educational system. We can easily do this by mapping the target domain name to an invalid address IP like 0.0.0.0. This will create a loopback. This is usually helpful for blocking non-educational websites in universities or blocking mature content for kids.
4. When edited in a proper manner, the Hosts file can operate as a security firewall for a system.

Format of the /etc/hosts File

The Hosts file is located inside the /etc folder. The entries of the file are per line basis. For instance, every single line consists of a hostname followed by its IP address:

The IP addresses used here are either IPv4 or IPv6. These addresses and hostnames maintain a distance of any number white space or a distance of a tab character.

On all operating systems, the Hosts file has the same format. Let us see the contents of the file on a Linux system:

# 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

The Hosts file contains the entries for both the IPv4 and IPv6 addresses.

Application of the Hosts File

As mentioned earlier, we can use the Hosts file for blocking a domain. Let us take the case of blocking Youtube. Open the file:

Use the tab character to insert space between the IP address and its corresponding domain name.

Now, save and close the file. Open any web browser and try to navigate to youtube and see what happens:

We can see youtube.com is blocked and we are getting an ‘Unable to connect’ message. To unblock the site, we have to simply remove the above entry from the /etc/hosts file.

Access Control Files

Besides the plain Hosts file, we also have a /etc/hosts.allow file and a /etc/hosts.deny file.

These files, referred to as hostess access files, are used by TCP wrappers. These files decide whether a client machine can connect to a host.

When a TCP wrapped service gets a client request, it performs the actions below:

1. Refer to the hosts.allow file: it sequentially reads this file and executes the first rule stated for that service.
2. Refer to the hosts.deny file: it sequentially reads this file and if a matching rule is found, it does not accept the connection request. Otherwise access is granted to the requesting service.

The hosts.allow file decides which IP addresses can connect to a host. The /etc/hosts.deny file is used in conjunction with it.

Security Issues of the Hosts File

Although the Hosts file looks like a simple file, in some cases, for example, on Windows systems, malware like adware or spyware modifies the Hosts file to take the users to malicious websites.

If you are unsure about the health of your Hosts file, you can use Lynis system auditor for Linux. Lynis has its built-in test build for checking the security strength of your Linux system.

Also, try to keep the Hosts file at a minimum length. If you have too many systems declared inside the Hosts file, you should consider placing them in a different DNS zone.

Conclusion

The Hosts file is an old file on Linux, but still, it is a powerful utility. Right from local name resolving, it is an important part of many software and their development process. In this article, we have seen a basic introduction to the Hosts file in Linux. We have also shown how to block a website using the Hosts file. Also, we can use it to block banners, adware, and other third party page counters.

After reading this article, you should now know how to use the Hosts file for managing DNS queries as per our requirements.

About the author

Ali Imran Nagori

Ali imran is a technical writer and Linux enthusiast who loves to write about Linux system administration and related technologies. You can connect with him on LinkedIn
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Localhost file in linux

NAME

hosts - static table lookup for hostnames

SYNOPSIS

DESCRIPTION

This manual page describes the format of the /etc/hosts file. This file is a simple text file that associates IP addresses with hostnames, one line per IP address. For each host a single line should be present with the following information: IP_address canonical_hostname [aliases. ] Fields of the entry are separated by any number of blanks and/or tab characters. Text from a "#" character until the end of the line is a comment, and is ignored. Host names may contain only alphanumeric characters, minus signs ("-"), and periods ("."). They must begin with an alphabetic character and end with an alphanumeric character. Optional aliases provide for name changes, alternate spellings, shorter hostnames, or generic hostnames (for example, localhost). The Berkeley Internet Name Domain (BIND) Server implements the Internet name server for UNIX systems. It augments or replaces the /etc/hosts file or hostname lookup, and frees a host from relying on /etc/hosts being up to date and complete. In modern systems, even though the host table has been superseded by DNS, it is still widely used for: bootstrapping Most systems have a small host table containing the name and address information for important hosts on the local network. This is useful when DNS is not running, for example during system bootup. NIS Sites that use NIS use the host table as input to the NIS host database. Even though NIS can be used with DNS, most NIS sites still use the host table with an entry for all local hosts as a backup. isolated nodes Very small sites that are isolated from the network use the host table instead of DNS. If the local information rarely changes, and the network is not connected to the Internet, DNS offers little advantage.

FILES

NOTES

Modifications to this file normally take effect immediately, except in cases where the file is cached by applications. Historical notes RFC 952 gave the original format for the host table, though it has since changed. Before the advent of DNS, the host table was the only way of resolving hostnames on the fledgling Internet. Indeed, this file could be created from the official host data base maintained at the Network Information Control Center (NIC), though local changes were often required to bring it up to date regarding unofficial aliases and/or unknown hosts. The NIC no longer maintains the hosts.txt files, though looking around at the time of writing (circa 2000), there are historical hosts.txt files on the WWW. I just found three, from 92, 94, and 95.

EXAMPLE

127.0.0.1 localhost 192.168.1.10 foo.mydomain.org foo 192.168.1.13 bar.mydomain.org bar 146.82.138.7 master.debian.org master 209.237.226.90 www.opensource.org

SEE ALSO

hostname(1), resolver(3), resolver(5), hostname(7), named(8) Internet RFC 952

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