genhost

Automating Hostfile Changes in Different Locations

If you find yourself with a laptop moving from one environment to another, you might find yourself needing to change your host file regularly to work in various environments. For instance, you might need internal ip addresses when you're inside a local network and external ip addresses when you're not. Making this kind of repetitive change is easy, but painful to do by hand, so it's helpful to have something to automate this. For my purposes, genhost is that automation.

Contents

The genhost project consists of three very small things:

• A template that contains all the hosts you might want to access.
• A configuration file for awk to manipulate the template in order to generate a host file for a particular location.
• A bash script that contains invokes awk on your behalf.

Installation

1. Back up your hosts file (/etc/hosts) because something could go wrong.
2. Make a copy of your hosts file (/etc/hosts) as /etc/hosts.template, and add markup to tell genhost what to do (see Configuration)
3. Put genhost and genhost.awk wherever you like, and add it to your path if you feel like. Personally, I've got them in ~/scripts, but that's your call.

Configuration

There are basically two simple patterns that genhost looks for:

• If you want genhost to include an entry when you're at a location, prefix the line with #<location>#
• If you want genhost to include an entry when you're not at a location, prefix the line with #!<location>#

By way of example you might end up with:

# Hosts File
127.0.0.1 localhost

# Work
#work#dev.myproject.mycompany.com 192.168.10.1
#!work#dev.myproject.mycompany.com 55.55.55.55
#work#staging.myproject.mycompany.com 192.168.10.2
#!work#staging.myproject.mycompany.com 55.55.55.56

# Home
#home#fileshare.home.local 192.168.10.1

So when you're at work, you'll find that your dev and staging environments point to internal addresses, and when you're at home, your dev and staging environments point to external addresses, and you'll have added your fileshare.

Using genhost

I'll assume that genhost is in your path, at which point all you need to do is invoke:

$ sudo genhost work
Password:
Generating hosts for location work.

Which, given the example above, will result in the following /etc/hosts:

# Hosts File
127.0.0.1 localhost

# Work
dev.myproject.mycompany.com 192.168.10.1
#dev.myproject.mycompany.com 55.55.55.55
staging.myproject.mycompany.com 192.168.10.2
#staging.myproject.mycompany.com 55.55.55.56

# Home
#fileshare.home.local 192.168.10.1

And if you were then to return home, you could invoke:

$ sudo genhost work
Password:
Generating hosts for location work.

Which would result in:

# Hosts File
127.0.0.1 localhost

# Work
#dev.myproject.mycompany.com 192.168.10.1
dev.myproject.mycompany.com 55.55.55.55
#staging.myproject.mycompany.com 192.168.10.2
staging.myproject.mycompany.com 55.55.55.56

# Home
fileshare.home.local 192.168.10.1

Now switching locations is a command away. If you want to automate it still further, and you're using Mac OS X, you might consider taking a look at Network Location to invoke genhost.