From 772c2b05df94fa55dfaccf80e642885582a34111 Mon Sep 17 00:00:00 2001 From: Brandon Rozek Date: Wed, 22 Nov 2023 10:36:33 -0500 Subject: [PATCH] New post --- content/blog/etc-hosts-connection.md | 101 +++++++++++++++++++++++++++ 1 file changed, 101 insertions(+) create mode 100644 content/blog/etc-hosts-connection.md diff --git a/content/blog/etc-hosts-connection.md b/content/blog/etc-hosts-connection.md new file mode 100644 index 0000000..7def5e4 --- /dev/null +++ b/content/blog/etc-hosts-connection.md @@ -0,0 +1,101 @@ +--- +title: "Changing /etc/hosts based on network connection" +date: 2023-11-22T09:23:09-05:00 +draft: false +tags: [] +math: false +medium_enabled: 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 + +```bash +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. + +```bash +#!/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. + +```bash +#!/usr/bin/env bash + +interface=$1 +event=$2 + +if [[ $interface != "wlp0s20f3" ]] || [[ $event != "down" ]] then + exit 0 +fi + +cp /etc/NetworkManager/hosts.default /etc/hosts +``` +