When you have multiple servers or client computers under your control it's nice to have a centralized way of monitoring them for health or unexpected errors.
There are a few different solutions out there but we'll have a go at installing and configuring [munin](https://munin-monitoring.org/).
## Munin
I set one up on [this](https://munin.86thumbs.net/) domain for demonstration purposes.
You can log in with the account details I'll hand out in class.
### Architecture
Munin has a server client architecture where they refer to the server as **master** and the clients as **nodes**.
When monitoring for example ten workstations or servers you will have **one** master and on each workstation or server you'll install **one** node but in total you'll have **ten** nodes connected to your **single** master.
First we'll install one munin **master** for everybody in the classroom.
The installation documentation on the official [website](http://guide.munin-monitoring.org/en/latest/installation/install.html) is very good so we'll follow that one.
The overall processes has three components.
1. install munin
2. configure the master to query some future nodes
3. install a web server to serve the monitor pages
Nginx is really easy to install on Debian. Just type `sudo apt install nginx` and when you get your prompt back, you'll have a web server running.
To verify it's working we can use any web browser we want.
The main configuration directory for nginx can be found at `/etc/nginx`.
Nginx is a bit different compared to most configuration we've done up until now.
The server itself is configured via the `/etc/nginx/nginx.conf` file and the `/etc/nginx/conf.d/` folder but the **websites** your server serves to the outside world are stored in `/etc/nginx/sites-available` and `/etc/nginx/sites-enabled`.
Each site should have an actual configuration file in the `/etc/nginx/sites-available` folder and if you want the lite to be *live* you should create a [symbolic link](https://stackoverflow.com/questions/39080375/nginx-symbolic-link) to this site in the `/etc/nginx/sites-enabled` folder.
First have a look at the `/etc/nginx/sitex-available/default` file to understand how the configuration works.
**Can you change something on the default webpage please?**
We can add a new file in the `/etc/nginx/sites-available` directory called `munin`.
You can name this any which way you want.
For example, on my vps that runs our domain I have the following files which offer websites:
```
➜ sites-enabled ls -l
total 36
-rw-r--r-- 1 root root 2049 Mar 25 20:56 86thumbs.net
-rw-r--r-- 1 root root 1813 Mar 25 12:42 gitea.86thumbs.net
-rw-r--r-- 1 root root 1646 May 24 12:00 ipcdb.86thumbs.net
The nodes have to be in the same network in order to be contactable by the master.
There are nifty ways to overcome this *problem* but that is for a later date.
If you do feel like diving deeper into it, have a look [here](https://guide.munin-monitoring.org/en/latest/tutorial/monitoring-the-unreachable.html#unreachable-index).
On any virtual machine you have around you can install the node by simply doing `sudo apt install munin-node`.
The node is now up, running and listening for incoming connections on port 4949 (by default).
It is however quite picky in accepting incoming connections!
The configuration file for the node is located at `/etc/munin/munin-node.conf`.
Open it up and have a read.
You should see two lines very similar to the one below.
```
allow ^127\.0\.0\.1$
allow ^::1$
```
Can you tell me what those are?
Yes, that's a **regex**!
By default the node only accepts incoming connections from the localhost.
We can add as many **masters** as we like to this list but we must respect the regex syntax.
Munin works with [plugins](https://guide.munin-monitoring.org/en/latest/plugin/use.html#plugin-use) to supply the master with data to plot.
You can install extra plugins or even write your own in [bash](https://guide.munin-monitoring.org/en/latest/develop/plugins/howto-write-plugins.html#howto-write-plugins) or in [python](https://aouyar.github.io/PyMunin/).
To have your node auto detect which plugins should be used you can execute the following command as root (not sudo).
```
munin-node-configure --shell --families=contrib,auto | sh -x
```
## Other operating systems
Because the munin server client model is so basic you can find node software for most operating systems.
I've run it successfully on [freeNAS](https://www.truenas.com/community/threads/install-munin-node.79224/#post-549934) but you can try a [windows](https://github.com/munin-monitoring/munin-node-win32) node as well.
## Alternatives
There are quite a few alternatives out there to choose from.
One of the better known is [nagios](https://www.nagios.org/) which can be installed on Debian as well.
There are ways to have munin and nagios work together that are described [here](https://guide.munin-monitoring.org/en/latest/tutorial/nagios.html?highlight=nagios).
You might feel munin is too simple or basic because it does not offer any tweaking via it's web GUI but that's the main reason why I *like* and use it so much.
Because it's so simple, it's not a real security risk.
I would however **not** expose a master to the internet without **SSL** and a good **htpasswd**.
There are some known exploits in the CGI part of munin.
* if your dynamic zoom is not working have a look [here](https://serverfault.com/questions/615048/munin-dynamic-graph-zoom-dynazoom-not-working-nginx-php-fpm)
