About
A host file is a database name in the form of a file on the local file system.
It is a computer file used by the operating system resolver to map an IP name to an IP address.
The mapping information have a higher priority that the DNS of your network.
Format
The hosts file contains lines of text consisting of:
- an IP address in the first text field
- followed by the fully qualified format
- followed by one or more short name
Each field is separated by white space or tabs
Example:
127.0.0.1 localhost
127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
127.0.0.1 fqdn name1 name2
OS
Windows
It is located in the directory for windows:
C:\WINDOWS\system32\drivers\etc\hosts
Linux
cat /etc/hosts
# The HOSTFILE shell variable may have the location
echo $HOSTFILE
Example
When you call:
- localhost,
- or gerardnico.com
Windows transform it by the IP 127.0.0.1 with the host file below:
# 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
127.0.0.1 localhost1
127.0.0.1 localhost2
10.10.10.10 ngerard.nf.newfrontiers.nl ngerard
127.0.0.1 gerardnico.com
The entry (127.0.0.1 gerardnico.com) for instance, permit me to have a copy of my website on my computer. When I ask http://gerardnico.com in my browser (and that Apache is also configured with a virtual host for gerardnico.com), I will see my local copy of my internet website.
Management
Powershell
Powershell script for adding/removing/showing entries to the hosts file.
From https://gist.github.com/markembling/173887
# Known limitations:
# - does not handle entries with comments afterwards ("<ip> <host> # comment")
#
$file = "C:\Windows\System32\drivers\etc\hosts"
function add-host([string]$filename, [string]$ip, [string]$hostname) {
remove-host $filename $hostname
$ip + "`t`t" + $hostname | Out-File -encoding ASCII -append $filename
}
function remove-host([string]$filename, [string]$hostname) {
$c = Get-Content $filename
$newLines = @()
foreach ($line in $c) {
$bits = [regex]::Split($line, "\t+")
if ($bits.count -eq 2) {
if ($bits[1] -ne $hostname) {
$newLines += $line
}
} else {
$newLines += $line
}
}
# Write file
Clear-Content $filename
foreach ($line in $newLines) {
$line | Out-File -encoding ASCII -append $filename
}
}
function print-hosts([string]$filename) {
$c = Get-Content $filename
foreach ($line in $c) {
$bits = [regex]::Split($line, "\t+")
if ($bits.count -eq 2) {
Write-Host $bits[0] `t`t $bits[1]
}
}
}
try {
if ($args[0] -eq "add") {
if ($args.count -lt 3) {
throw "Not enough arguments for add."
} else {
add-host $file $args[1] $args[2]
}
} elseif ($args[0] -eq "remove") {
if ($args.count -lt 2) {
throw "Not enough arguments for remove."
} else {
remove-host $file $args[1]
}
} elseif ($args[0] -eq "show") {
print-hosts $file
} else {
throw "Invalid operation '" + $args[0] + "' - must be one of 'add', 'remove', 'show'."
}
} catch {
Write-Host $error[0]
Write-Host "`nUsage: hosts add <ip> <hostname>`n hosts remove <hostname>`n hosts show"
}
Ansible
- name: Add mappings to /etc/hosts
blockinfile:
path: /etc/hosts
block: |
{{ item.ip }} {{ item.name }}
marker: "# {mark} ANSIBLE MANAGED BLOCK {{ item.name }}"
with_items:
- { name: host1, ip: 10.10.1.10 }
- { name: host2, ip: 10.10.1.11 }
- { name: host3, ip: 10.10.1.12 }