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101 lines
4.1 KiB
Markdown
101 lines
4.1 KiB
Markdown
---
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title: "Changing /etc/hosts based on network connection"
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date: 2023-11-22T09:23:09-05:00
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draft: false
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tags: []
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math: false
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medium_enabled: false
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---
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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:
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- If the client has multiple VPNs connected, only one DNS server can be set.
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- There may be more latency using a DNS server within a VPN than using a default one provided by the ISP.
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- You may not have permission to host the DNS server within the VPN network.
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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.
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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.
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Luckily, `NetworkManager-dispatcher` handles this for us. To get a complete understanding on how to write scripts for this, reference
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```bash
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man 8 networkmanager-dispatcher
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```
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In essence, scripts within `/etc/NetworkManger/dispatcher.d/` get executed in alphabetical order with two arguments set and a lot of environmental variables.
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What we'll care about in our scripts are:
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- `$1` the first argument passed to the script is the interface
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- `$2` the second argument refers to the *event* being triggered.
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- Possible options include: pre-up, up, pre-down, down, vpn-up, vpn-pre-up, vpn-pre-down, vpn-down, hostname, dhcp4-change, dhcp6-change, connectivity-change, reapply.
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- `$CONNECTION_UUID` refers to a particular connection profile in NetworkManager. This is so we know which `/etc/hosts` file to swap with which connection.
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Doing some quick in dirty tests, I found the following events were triggered when connecting to a particular network:
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` dhcp4-change -> up -> connectivity-change`
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And, the following events were triggered when disconnecting from a particular network:
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`connectivity-change -> down`
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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.
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For our example, here's what our default `/etc/hosts/` file will look like:
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```
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127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
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::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
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10.10.10.3 home-server.brandonrozek.com
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10.10.10.4 home-desktop.brandonrozek.com
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```
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When we're connected to my home network, we'll swap my `/etc/hosts/` to look like:
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```
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127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
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::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
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192.168.0.30 home-server.brandonrozek.com
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192.168.0.40 home-desktop.brandonrozek.com
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```
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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.
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```bash
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#!/usr/bin/env bash
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interface=$1
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event=$2
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if [[ $interface != "wlp0s20f3" ]] || [[ $event != "up" ]] then
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exit 0
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fi
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# This environmental variable is set on UP/DOWN events
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if [[ $CONNECTION_UUID != "901a1b68-e622-4be6-a61f-a8dc999212b3" ]] then
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exit 0
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fi
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cp /etc/NetworkManager/hosts.home /etc/hosts
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```
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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.
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Similarly, when we disconnect from the network, we'll need to set it back to our default `/etc/hosts` file.
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```bash
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#!/usr/bin/env bash
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interface=$1
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event=$2
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if [[ $interface != "wlp0s20f3" ]] || [[ $event != "down" ]] then
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exit 0
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fi
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cp /etc/NetworkManager/hosts.default /etc/hosts
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```
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