website/content/blog/etc-hosts-connection.md

title	date	draft	tags	math	medium_enabled
Changing /etc/hosts based on network connection	2023-11-22T09:23:09-05:00	false		false	false

I use my laptop at home, university, and public locations. The IP address I use to connect to a particular resource changes depending on if I'm within the network it's hosted on or a VPN. A common solution is to have a DNS server within the VPN that all clients use. This, however, has a few issues:

• If the client has multiple VPNs connected, only one DNS server can be set.
• There may be more latency using a DNS server within a VPN than using a default one provided by the ISP.
• You may not have permission to host the DNS server within the VPN network.

To address these set of issues, we'll go over how to change /etc/hosts on the local client machine depending on which network it's connected to.

In this setup, we'll have a default /etc/hosts file. I'll show how to then swap it with one for a particular connection. To do this, we need a way for a script to run when NetworkManager connects or disconnects from a network.

Luckily, NetworkManager-dispatcher handles this for us. To get a complete understanding on how to write scripts for this, reference

man 8 networkmanager-dispatcher

In essence, scripts within /etc/NetworkManger/dispatcher.d/ get executed in alphabetical order with two arguments set and a lot of environmental variables.

What we'll care about in our scripts are:

• $1 the first argument passed to the script is the interface
• $2 the second argument refers to the event being triggered.
  • Possible options include: pre-up, up, pre-down, down, vpn-up, vpn-pre-up, vpn-pre-down, vpn-down, hostname, dhcp4-change, dhcp6-change, connectivity-change, reapply.
• $CONNECTION_UUID refers to a particular connection profile in NetworkManager. This is so we know which /etc/hosts file to swap with which connection.

Doing some quick in dirty tests, I found the following events were triggered when connecting to a particular network:

dhcp4-change -> up -> connectivity-change

And, the following events were triggered when disconnecting from a particular network:

connectivity-change -> down

My first instinct was to use the connectivity-change event, however, the CONNECTION_UUID variable is not set for those. Instead we'll use the up/down events.

For our example, here's what our default /etc/hosts/ file will look like:

127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4
::1         localhost localhost.localdomain localhost6 localhost6.localdomain6

10.10.10.3  home-server.brandonrozek.com
10.10.10.4  home-desktop.brandonrozek.com

When we're connected to my home network, we'll swap my /etc/hosts/ to look like:

127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4
::1         localhost localhost.localdomain localhost6 localhost6.localdomain6

192.168.0.30  home-server.brandonrozek.com
192.168.0.40  home-desktop.brandonrozek.com

The following script we'll store at /etc/NetworkManager/dispatcher.d/swap_home.sh which will swap the /etc/hosts file with the one stored at /etc/NetworkManager/hosts.home when I connect to my home network.

#!/usr/bin/env bash

interface=$1
event=$2

if [[ $interface != "wlp0s20f3" ]] || [[ $event != "up" ]] then
        exit 0
fi

# This environmental variable is set on UP/DOWN events
if [[ $CONNECTION_UUID != "901a1b68-e622-4be6-a61f-a8dc999212b3" ]] then
        exit 0
fi

cp /etc/NetworkManager/hosts.home /etc/hosts

In this script, you might have to replace wlp0s20f3 to reflect the interface that you're using for connecting to the network. Additionally, you'll have to replace the CONNECTION_UUID with the UUID of the connection you're trying to swap under. You can use nmcli c to show the UUIDs for each of your network connections.

Similarly, when we disconnect from the network, we'll need to set it back to our default /etc/hosts file.

#!/usr/bin/env bash

interface=$1
event=$2

if [[ $interface != "wlp0s20f3" ]] || [[ $event != "down" ]] then
        exit 0
fi

cp /etc/NetworkManager/hosts.default /etc/hosts