Since then I’ve actually extended this method into a tool that can manage multiple /etc/hosts files. Using this new method you can have a folder that you can add hosts configurations to, and those files become available for switching into /etc/hosts straight away.

There’s no reason why an adaptation of this method wouldn’t work in Linux as well.

Here’s how you do it:

1. Create the host configuration directory

We’ll store all the /etc/hosts type files in /usr/local/share/hs.

You’ll need to do all of this as a root user. So switch to root using sudo -s first (you’ll need to be an administrator to do this).

$ mkdir -p /usr/local/share/hs
$ chmod 755 /usr/local/share/hs

Now to start with, let’s make a copy of the existing /etc/hosts file in this directory.

$ cp /etc/hosts /usr/local/share/hs/hosts.normal
$ chmod 744 /usr/local/share/hs/hosts.normal

Then make the real /etc/hosts a symbolic link to that new file.

$ ln -sf /usr/local/share/hs/hosts.normal /etc/hosts

Any additional host configurations in this folder need to be named using a similar hosts.* pattern.

2. Create the host switching script

Take the following bash script and save it to a file located at /usr/local/bin/hs

#!/bin/bash

if [ $1 ]; then
	HFILE=hosts.$1
else
	HFILE=hosts.normal
fi

# Change this if you put your host files elsewhere
HPATH="/usr/local/share/hs/"

echo "Attempting to switch host file to $HPATH$HFILE"

if [ ! -e $HPATH$HFILE ]; then
	echo "--> File does not exist!"
	echo "--> Exiting with status 1"
	exit 1
fi

if [ -d $HPATH$HFILE ]; then
	echo "--> File is a directory!"
	echo "--> Exiting with status 2"
	exit 2
fi

if [ -h $HPATH$HFILE ]; then
	echo "--> File is a symbolic link!"
	echo "--> Exiting with status 3"
	exit 3
fi

if [ ! -h /etc/hosts ]; then
	echo "--> Current hosts file is a real file!"
	echo "--> Exiting with status 4"
	exit 4
fi

# Link it up
sudo ln -sf $HPATH$HFILE /etc/hosts

# Flush the system-wide DNS cache
if [ -x /usr/bin/dscacheutil ]; then
	# Mac OS X 10.5
	/usr/bin/dscacheutil -flushcache
elif [ -x /usr/bin/lookupd ]; then
	# Mac OS X Less than 10.5
	/usr/bin/lookupd -flushcache
elif [ -x /usr/sbin/lookupd ]; then
	# Mac OS X Less than 10.5 (alternative)
	/usr/sbin/lookupd -flushcache
fi

echo "--> Done"

The script does some basic validity checking on the file and also makes sure that /etc/hosts is just a symbolic link and not a real file.

You will need to adjust the permissions on this script so that only admins can run it.

$ chmod 750 /usr/local/bin/hs

You also need to make sure that /usr/local/bin is in your PATH so that the hs command can be found. We can test that.

$ echo $PATH

… will output something like …

/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin

If it’s not there then you need to add it in your users ~/.profile file. Add the following line to that file, you can create the file if it isn’t there.

export PATH=/usr/local/bin:$PATH

Now reload your profile and the hs command should be available.

$ . ~/.profile

You could also just close and open your terminal window to reload your profile.

3. Trying it out

The hs command takes one argument. The argument is used to specify the hosts configuration file to be loaded from /usr/local/share/hs. If no argument is given, then the assumed file is hosts.normal.

So for our first test we should add another hosts configuration.

$ cp /usr/local/share/hs/hosts.normal /usr/local/share/hs/hosts.test

Edit the new hosts configuration file to your liking.

Now we can try the switching command to switch to that configuration. You will need to supply your password if you are not already running as root.

$ hs test
Attempting to switch host file to /usr/local/share/hs/hosts.test
Password:
--> Done

The symbolic link at /etc/hosts should now be pointing to /usr/local/share/hs/hosts.test.

$ ls -l /etc/hosts
/etc/hosts -> /usr/local/share/hs/hosts.test

Now try switching back to your normal configuration.

$ hs
Attempting to switch host file to /usr/local/share/hs/hosts.normal
Password:
--> Done

Any more configurations that you add to /usr/local/share/hs are available this way.

Neat, huh?

4. Fancy schmancy tab completion

So that’s all great until you have 10 different hosts config files and you can’t remember their names.

The simple solution is to register additional commands so that command line tab completion can be used. The following additional script registers all the available hosts configs as separate commands. So instead of typing hs test we can type hstest. More importantly we can type hs then hit the “tab” key twice and get a full list of all available commands (one per configuration file).

Hard to explain, easier to just do. Add the following lines of code to the end of your ~/.profile file.

# Setup hs* commands
HSPATH=/usr/local/share/hs
if [ -d $HSPATH ]; then
	HSFILES=(`ls $HSPATH`)
	HSCOUNT=${#HSFILES[*]}
	for (( HSI=0 ; $HSI < $HSCOUNT ; HSI=$HSI+1 )); do
		HSFILE=${HSFILES[HSI]}
		HSFILE=${HSFILE:6}
		alias hs$HSFILE='hs '$HSFILE
	done
fi
unset HSPATH HSFILES HSCOUNT HSI HSFILE

This script loops through all the files in /usr/local/share/hs and adds a command for each. Once that is added, reload your profile to make the script work.

$ . ~/.profile

Now you should be able to type hs then hit the “tab” key twice and get a list of the available commands.

$ hs TAB TAB
hs            hsnormal      hstest

If you add any new configuration files to /usr/local/share/hs you need to reload your profile for it to turn up as a new command (or just relaunch your terminal window).

Enjoy!

UPDATE: This method has been extended and superseded, find out about the “new improved” method here.

Web developers often have reason to fake a host name’s associated IP address by editing their /etc/hosts file.

The peculiarities of the development environment we use here at WordPress.com mean I find myself switching back and forth between two /etc/hosts configurations quite frequently. It started getting annoying to manually go into the /etc/hosts file and uncomment or re-comment entries all the time, so I came up with a quick and dirty solution to shortcut the process.

This solution involves creating two (or more) /etc/hosts files and writing a very short script for each which activates them.

Duplicate existing /etc/hosts file

First move your existing /etc/hosts file to a new location and symlink it back to the original location.

$ sudo mv /etc/hosts /etc/hosts.normal
$ sudo ln -sf /etc/hosts.normal /etc/hosts

This file should contain your “normal” set of entries. We will now copy it to create a file where we can have our custom entries.

$ sudo cp /etc/hosts.normal /etc/hosts.custom

Now we can edit the “custom” file to add our custom entries.

$ sudo nano -w /etc/hosts.custom

Add your entries just after the localhost entries.

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##
127.0.0.1		localhost
255.255.255.255		broadcasthost
::1			localhost
fe80::1%lo0		localhost

# Custom entries
10.0.1.4		host.example.com
192.168.4.2		anotherhost.example.com

Save your changes on exit (ctrl-x) and then we can set up the activation scripts.

Set up activation scripts

We will now setup an activation script for each file, this could be more cleverly done using a single script, but then we would need to do tricky things to get tab command completion to work, and I’m not too keen on that.

Create a new file in a location that is in your PATH. You can see the directories that are in your path by doing the following.

$ echo $PATH

The line it spits out is your path. Usually something like this:

/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin

I decided to put my scripts in /usr/local/bin, if the directory doesn’t already exist, you can create it.

$ mkdir -p /usr/local/bin

Now fire up a text editor and create the first script, this script will be the one that switches you to your “normal” /etc/hosts configuration.

nano -w /usr/local/bin/hosts-normal

In the file add the following:

#!/bin/bash
sudo ln -sf /etc/hosts.normal /etc/hosts
dscacheutil -flushcache

The dscacheutil command will flush the existing local DNS cache and make the changes available right away. If you happen to be using OS X 10.4, then replace dscacheutil with the older command lookupd to achieve the same result.

Save the file and change it’s permissions.

$ sudo chown root:wheel /usr/local/bin/hosts-normal
$ sudo chmod 750 /usr/local/bin/hosts-normal

Now we can copy this script to create another for the custom configuration.

$ sudo cp /usr/local/bin/hosts-normal /usr/local/bin/hosts-custom

Edit the new script to point to the custom hosts file created earlier.

$ sudo nano -w /usr/local/bin/hosts-custom

Just change the target of the symlink command.

#!/bin/bash
sudo ln -sf /etc/hosts.custom /etc/hosts
dscacheutil -flushcache

Check the permissions on both files, they should look somthing like this:

$ ls -la /usr/local/bin
total 16
drwxr-xr-x   4 root  wheel  136 14 Feb 22:49 .
drwxr-xr-x  12 root  wheel  408 05 Mar 20:40 ..
-rwxr-x---   1 root  wheel   77 05 Mar 22:49 hosts-normal
-rwxr-x---   1 root  wheel   78 05 Mar 22:53 hosts-custom

Finally, using your scripts

That was quite a lot of work, but here’s the pay-off. To switch to your “normal” hosts just call your hosts-normal script.

$ hosts-normal
password:

You will be prompted for your admin password as the script makes the symlink using a sudo command. The process is similar to switch to your “custom” hosts configuration.

$ hosts-custom
password:

You can create additional sets of host configurations and scripts to match as you require them.