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Magic Hosts

Do you need to map some domain names to LAN addresses in your hosts file? Tired of having to manually edit /etc/hosts every time you change WiFi networks? This script automatically detects a target WiFi network and turns on/off LAN routing in your /etc/hosts file accordingly. It is also optimized to only make changes if you've actually changed networks, and it's 100% cron compatible.

Authors

  • Jason C. McDonald (CodeMouse92)

Compatibility

This should work on any Debian-based system, and may also work on other Linux systems. It is designed to work with any system that uses /etc/hosts for local DNS resolution.

If you encounter any problems with any Linux system, please report in Issues!

History

I run a local file server, which faces the world wide web via a Dynamic DNS service with a domain and several subdomains I own. Unfortunately, my router was not designed to handle the scenario where I would connect to that server using its external IP (and thus domain name) from a connection on the same router! After ruling out the viability of several common workarounds, I arrived at the conclusion I would need to edit the /etc/hosts file on every computer on our network.

Unfortunately, that meant changing /etc/hosts every time I connected from a different network, such as the WiFi at my favorite café. Finally, I decided to write this handy little cron script to do that for me.

Prerequisites

This script merely uses the features and native components of BASH, as well as iwconfig, which is on most Linux systems.

Installation

  1. Ensure your /etc/hosts file is already set up with a LAN routing - that is, it should include an entry that starts with 192.168. This line should be uncommented before starting the script.

  2. Place the magichosts file in a convenient place for scripts, and be sure the location is included in your environment path. Mark the script as executable via chmod +x magichosts.

Here's an example install, placing the script in your /usr/local/bin directory. Note that we only need the sudo because we're working in a system directory.

# Move to the location where the script will live.
$ cd /usr/local/bin
# Download the script file from GitHub directly.
$ sudo wget https://github.com/CodeMouse92/magichosts/raw/master/magichosts
# Make the script executable. (You are encouraged to read the file BEFORE doing this, so you know what it does.
$ sudo chmod +x magichosts
  1. Edit magichosts, and change the variable NETWORK on line 40 to be the WiFi network name where you want LAN routing enabled. For example, you might change it to...
NETWORK="FBIVanOutside"

If you have multiple WiFi networks on your router, but they all follow the same naming convention, you only need to include the part they have in common. For example, if you have MyGreatNet_2G and MyGreatNet_5G, just set NETWORK to "MyGreatNet".

  1. Run the script for the first time, and then verify that /etc/hosts/ is correct based on your network. The script MUST be run with sudo.
$ sudo magichosts
$ cat /etc/hosts

Ensure the output of /etc/hosts is correct, given your internet connection.

  1. Add the script to your root cron, via sudo crontab -e. Make sure you give the full path to the script.
* * * * * /usr/local/bin/magichosts

The script will run every minute (unless you modify the line).

Usage

Under normal circumstances, magichosts will run without you needing to do anything. If you want to force it to execute immediately, just run sudo magichosts.

The script follows these rules:

  1. If there is no WiFi connection, it won't do anything.

  2. If you have NOT switched between a target WiFi network and a non-target network since the last run, it won't try to make any edits.

  3. If you HAVE switched between a target WiFi network and a non-target network, it will either comment or uncomment all the LAN routing lines (those starting with 192.168 in your /etc/hosts file.

Known Issues

Right now, the script has the following limitations:

  • It only checks the SSID of the connected WiFi network. It won't make changes based on other internet connections, including ethernet and USB smartphone tether.

  • If no WiFi network is connected, no changes will be made to /etc/hosts. Ever.