Location Based DNS Switching For Internet vs. Intranet

I realized over the weekend that since I run my own mail server out of my home, I can configure my computer to download my mail over the intranet whenever I am on my home network. By doing this, I can drastically reduce my mail download times because it cuts the Internet out of the equation. Rather than using DNS + the Internet to get my mail, I can download it directly from internal IP address of the server.

To understand how to set this up, you have to understand that whenever you use a domain name (like michaeljaylissner.com), your computer does an IP lookup. First, it looks in /etc/hosts to see if it knows the IP of the domain locally. If it does, it will use the IP listed there. If it does not, it will ask your Internet provider what IP to use, and will use that. Thus, what we want to do is set up the computer so that when we are at home, /etc/hosts provides the internal IP of our server, and so when we are not at home, it does not.

When I am at home, I am always on a wireless network called, pizzapuppysantaclaus. Thus, by checking what wireless network I am connected to, I can check if I am at home, and make whatever changes are necessary. Conveniently, whenever you change network connections, you run all of the scripts located in /etc/network/if-up.d/. Thus, we will put a small script in there that checks what wireless network we are on, and then changes our /etc/hosts file if necessary.

To set up this configuration, I made three files. The first is the script mentioned above, which needs to be owned by root, and placed in /etc/network/if-up.d. You can name it whatever you want, and by changing pizzapuppysantaclaus to the name of your network, you can fit it to your needs. Here’s the contents of the script:

#First, we check if we are connected to pizzapuppysantaclaus

#If grep has a hit, we're connected, and $? will equal 0, if not, $? will equal 1
iwconfig 2> /dev/null | grep pizzapuppysantaclaus > /dev/null

if [ $? = 0 ]
then
  #Switch the /etc/hosts file with the other one
  cp -f /etc/hostsIntranet /etc/hosts

  else
  #Switch the /etc/hosts file with the other one
  cp -f /etc/hostsInternet /etc/hosts

fi

exit 0

This script simply performs a check of our wireless ID. If it’s pizzapuppysantaclaus, it switches /etc/hostsIntranet for /etc/hosts. If not, it switches /etc/hostsInternet for /etc/hosts.

The contents of /etc/hostsIntranet are:

192.168.1.132   michaeljaylissner.com
192.168.1.132   charityhikers.org
127.0.0.1   localhost
127.0.1.1   opal

# The following lines are desirable for IPv6 capable hosts
::1     localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
ff02::3 ip6-allhosts

And /etc/hostsInternet is just a copy of /etc/hosts.

So, to make this whole thing run, put the script in /etc/network/if-up.d, and set its owner to root with execute permission. Create a file called /etc/hostsIntranet, that contains your intranet configuration, as shown above. Make a copy of your normal /etc/hosts file called /etc/hostsInternet.

Once all that’s done, you should be all set. Any questions, please feel free to comment!