secluded/content/posts/abusing-mesh-networks-for-easy-remote-support.md

---
title: "(Ab)using mesh networks for easy remote support"
author: ["Amolith"]
cover: ./cover.png
date: 2021-11-01T02:51:00-04:00
lastmod: 2023-01-18T09:33:39-05:00
tags: ["Mesh networking", "Open source", "Remote support"]
categories: ["Technology"]
draft: false
toc: true
---

One of the things many of us struggle with when setting friends and
family up with Linux is remote support. Commercial solutions like
[RealVNC](https://www.realvnc.com/) and [RustDesk](https://rustdesk.com/) do exist and function very well, but are often more
expensive than we would like for answering the odd "I can't get Facebook
open!" support call. I've been on the lookout for suitable alternatives
for a couple years but nothing has been satisfying. Because of this, I
have held off on setting others up with any Linux distribution, even the
particularly user-friendly options such as [Linux Mint](https://linuxmint.com/) and [elementary OS;](https://elementary.io/)
if I'm going drop someone in an unfamiliar environment, I want to be
able to help with any issue within a couple hours, not days and
_certainly_ not weeks.

[Episode 421 of LINUX Unplugged](https://linuxunplugged.com/421) gave me an awesome idea to use [Nebula,](https://github.com/slackhq/nebula) a
networking tool created by Slack, [X11vnc,](https://libvnc.github.io/) a very minimal VNC server, and
[Remmina,](https://remmina.org/) a libre remote access tool available in pretty much every Linux
distribution, to set up a scalable, secure, and simple setup reminiscent
of products like RealVNC.

## Nebula {#nebula}

The first part of our stack is Nebula, the tool that creates a network
between all of our devices. With traditional VPNs, you have a client
with a persistent connection to a central VPN server and other clients
can communicate with the first by going through that central server.
This works wonderfully in most situations, but there are a lot of
latency and bandwidth restrictions that would make remote support an
unpleasant experience. Instead of this model, what we want is a _mesh_
network, where each client can connect directly to one another _without_
going through a central system and slowing things down. This is where
Nebula comes in.

In Nebula's terminology, clients are referred to as _nodes_ and central
servers are referred to as _lighthouses_, so those are the terms I'll use
going forward.

Mesh networks are usually only possible when dealing with devices that
have static IP addresses. Each node has to know _how_ to connect with the
other nodes; John can't meet up with Bob when Bob moves every other day
without notifying anyone of his new address. This wouldn't be a problem
if Bob phoned Jill and told her where he was moving; John would call
Jill, Jill would tell him where Bob is, and the two would be able to
find each other

With Nebula, nodes are Bob and John and Jill is a lighthouse. Each node
connects to a lighthouse and the lighthouse tells the nodes how to
connect with one another when they ask. It _facilitates_ the P2P
connection then _backs out of the way_ so the two nodes can communicate
directly with each other.

It allows any node to connect with any other node on any network from
anywhere in the world, as long as one lighthouse is accessible that
knows the connection details for both peers.

### Getting started {#getting-started}

The _best_ resource is [the official documentation,](https://github.com/slackhq/nebula) but I'll describe the
process here as well.

After [installing the required packages,](https://github.com/slackhq/nebula#1-the-nebula-binaries-or-distribution-packages-for-your-specific-platform-specifically-youll-need-nebula-cert-and-the-specific-nebula-binary-for-each-platform-you-use) make sure you have a VPS with a
static IP address to use as a lighthouse. If you want something dirt
cheap, I would recommend one of the small plans from [BuyVM.](https://buyvm.net) I do have a
[referral link](https://my.frantech.ca/aff.php?aff=3783) if you want them to kick me a few dollars for your
purchase. [Hetzner](https://www.hetzner.com/cloud) (referral: `ckGrk4J45WdN`) or [netcup](https://www.netcup.eu/) (referral:
`36nc15758387844`) would also be very good options; I've used them all and
am very comfortable recommending them.

### Creating a Certificate Authority {#creating-a-certificate-authority}

After picking a device with a static IP address, it needs to be set up
as a lighthouse. This is done by first creating a Certificate Authority
(CA) that will be used for signing keys and certificates that allow our
other devices into the network. The `.key` file produced by the following
command is incredibly sensitive; with it, anyone can authorise a new
device and give it access to your network. Store it in a safe,
preferably encrypted location.

```bash
nebula-cert ca -name "nebula.example.com"
```

I'll explain why we used a Fully-Qualified Domain Name (FQDN) as the
CA's name in a later section. If you have your own domain, feel free to
use that instead; it doesn't really matter what domain is used as long
as the format is valid.

### Generating lighthouse credentials {#generating-lighthouse-credentials}

Now that we have the CA's `.crt` and `.key` files, we can create and sign
keys and certificates for the lighthouse.

```bash
nebula-cert sign -name "buyvm.lh.nebula.example.com" -ip "192.168.100.1/24"
```

Here, we're using a FQDN for the same reason as we did in the CA. You
can use whatever naming scheme you like, I just prefer
`<vps-host>.lh.nebula...` for my lighthouses. The IP address can be on any
of the following private IP ranges, I just happened to use `192.168.100.X`
for my network.

| IP Range                      | Number of addresses |
| ----------------------------- | ------------------- |
| 10.0.0.0 – 10.255.255.255     | 16 777 216          |
| 172.16.0.0 – 172.31.255.255   | 10 48 576           |
| 192.168.0.0 – 192.168.255.255 | 65 536              |

### Creating a config file {#creating-a-config-file}

The next step is creating our lighthouse's config file. The reference
config can be found in [Nebula's repo.](https://github.com/slackhq/nebula/blob/master/examples/config.yml) We only need to change a few of
the lines for the lighthouse to work properly. If I don't mention a
specific section here, I've left the default values.

The section below is where we'll define certificates and keys. `ca.crt`
will remain `ca.crt` when we copy it over but I like to leave the node's
cert and key files named as they were when generated; this makes it easy
to identify nodes by their configs. Once we copy everything over to the
server, we'll add the proper paths to the `cert` and `key` fields.

```yaml
pki:
  ca: /etc/nebula/ca.crt
  cert: /etc/nebula/
  key: /etc/nebula/
```

The next section is for identifying and mapping your lighthouses. This
needs to be present in _all_ of the configs on _all_ nodes, otherwise they
won't know how to reach the lighthouses and will never actually join the
network. Make sure you replace `XX.XX.XX.XX` with whatever your VPS's
public IP address is. If you've used a different private network range,
those changes need to be reflected here as well.

```yaml
static_host_map:
  "192.168.100.1": ["XX.XX.XX.XX:4242"]
```

Below, we're specifying how the node should behave. It is a lighthouse,
it should answer DNS requests, the DNS server should listen on all
interfaces on port 53, it sends its IP address to lighthouses every 60
seconds (this option doesn't actually have any effect when `am_lighthouse`
is set to `true` though), and this lighthouse should not send reports to
other lighthouses. The bit about DNS will be discussed later.

```yaml
lighthouse:
  am_lighthouse: true
  serve_dns: true
  dns:
    host: 0.0.0.0
    port: 53
  interval: 60
  hosts:
```

The next bit is about [hole punching](https://en.wikipedia.org/wiki/Hole_punching_%28networking%29), also called _NAT punching_, _NAT
busting_, and a few other variations. Make sure you read the comments for
better explanations than I'll give here. `punch: true` enables hole
punching. I also like to enable `respond` just in case nodes are on
particularly troublesome networks; because we're using this as a support
system, we have no idea what networks our nodes will actually be
connected to. We want to make sure devices are available no matter where
they are.

```yaml
punchy:
  punch: true
  respond: true
  delay: 1s
```

`cipher` is a big one. The value _must_ be identical on _all_ nodes _and_
lighthouses. `chachapoly` is more compatible so it's used by default. The
devices _I_ want to connect to are all x86 Linux, so I can switch to `aes`
and benefit from [a small performance boost.](https://www.reddit.com/r/networking/comments/iksyuu/comment/g3ra5cv/?utm_source=share&utm_medium=web2x&context=3) Unless you know _for sure_
that you won't need to work with _anything_ else, I recommend leaving it
set to `chachapoly`.

```yaml
cipher: chachapoly
```

The last bit I modify is the firewall section. I leave most everything
default but _remove_ the bits after `port: 443`. I don't _need_ the `laptop` and
`home` groups (groups will be explained later) to access port `443` on this
node, so I shouldn't include the statement. If you have different needs,
take a look at the comment explaining how the firewall portion works and
make those changes.

Again, I _remove_ the following bit from the config.

```yaml
- port: 443
  proto: tcp
  groups:
    - laptop
    - home
```

### Setting the lighthouse up {#setting-the-lighthouse-up}

We've got the config, the certificates, and the keys. Now we're ready to
actually set it up. After SSHing into the server, grab the [latest
release of Nebula for your platform,](https://github.com/slackhq/nebula/releases/latest) unpack it, make the `nebula` binary
executable, then move it to `/usr/local/bin` (or some other location
fitting for your platform).

```bash
wget https://github.com/slackhq/nebula/releases/download/vX.X.X/nebula-PLATFORM-ARCH.tar.gz
tar -xvf nebula-*
chmod +x nebula
mv nebula /usr/local/bin/
rm nebula-*
```

Now we need a place to store our config file, keys, and certificates.

```bash
mkdir /etc/nebula/
```

The next step is copying the config, keys, and certificates to the
server. I use `rsync` but you can use whatever you're comfortable with.
The following four files need to be uploaded to the server.

- `config.yml`
- `ca.crt`
- `buyvm.lh.nebula.example.com.crt`
- `buyvm.lh.nebula.example.com.key`

With `rsync`, that would look something like this. Make sure `rsync` is also
installed on the VPS before attempting to run the commands though;
you'll get an error otherwise.

```bash
rsync -avmzz ca.crt user@example.com:
rsync -avmzz config.yml user@example.com:
rsync -avmzz buyvm.lh.* user@example.com:
```

SSH back into the server and move everything to `/etc/nebula/`.

```bash
mv ca.crt /etc/nebula/
mv config.yml /etc/nebula/
mv buyvm.lh* /etc/nebula/
```

Edit the config file and ensure the `pki:` section looks something like
this, modified to match your hostnames of course.

```yaml
pki:
  ca: /etc/nebula/ca.crt
  cert: /etc/nebula/buyvm.lh.nebula.example.com.crt
  key: /etc/nebula/buyvm.lh.nebula.example.com.key
```

Run the following command to make sure everything works properly.

```bash
nebula -config /etc/nebula/config.yml
```

The last step is daemonizing Nebula so it runs every time the server
boots. If you're on a machine using systemd, dropping the following
snippet into `/etc/systemd/system/nebula.service` should be sufficient. If
you're using something else, check the [the examples directory](https://github.com/slackhq/nebula/tree/master/examples/) for more
options.

```text
[Unit]
Description=nebula
Wants=basic.target
After=basic.target network.target
Before=sshd.service

[Service]
SyslogIdentifier=nebula
ExecReload=/bin/kill -HUP $MAINPID
ExecStart=/usr/local/bin/nebula -config /etc/nebula/config.yml
Restart=always

[Install]
WantedBy=multi-user.target
```

We're almost done!

### Setting individual nodes up {#setting-individual-nodes-up}

This process is almost exactly the same as setting lighthouses up. All
you'll need to do is generate a couple of certs and keys then tweak the
configs a bit.

The following command creates a new cert/key for USER's node with the IP
address `192.168.100.2`. The resulting files would go on the _remote_ node
not yours. Replace `HOST` and `USER` with fitting values.

```bash
nebula-cert sign -name "HOST.USER.nebula.example.com" -ip "192.168.100.2/24"
```

The following command will create a _similar_ cert/key but it will be part
of the `support` group. The files resulting from this should go on _your_
nodes. With the config we'll create next, nodes in the `support` group
will be able to VNC and SSH into other nodes. Your nodes need to be in
the `support` group so you'll have access to the others.

```bash
nebula-cert sign -name "HOST.USER.nebula.example.com" -ip "192.168.100.2/24" -groups "support"
```

On to the config now. This tells the node that it is _not_ a lighthouse,
it should _not_ resolve DNS requests, it _should_ ping the lighthouses and
tell them its IP address every 60 seconds, and the node at `192.168.100.1`
is one of the lighthouses it should report to and query from. If you
have more than one lighthouse, add them to the list as well.

```yaml
lighthouse:
  am_lighthouse: false
  #serve_dns: false
  #dns:
  #host: 0.0.0.0
  #port: 53
  interval: 60
  hosts:
    - "192.168.100.1"
```

The other bit that should be modified is the `firewall:` section and this
is where the groups we created earlier are important. Review its
comments and make sure you understand how it works before proceeding.

We want to allow inbound connections on ports 5900, the standard port
for VNC, and 22, the standard for SSH. Additionally, we _only_ want to
allow connections from nodes in the `support` group. Any _other_ nodes
should be denied access.

Note that including this section is not necessary on _your_ nodes, those
in the `support` group. It's only necessary on the remote nodes that
you'll be connecting to. As long as the `outbound:` section in the config
on _your_ node allows any outbound connection, you'll be able to access
other nodes.

```yaml
- port: 5900
  proto: tcp
  groups:
    - support

- port: 22
  proto: tcp
  groups:
    - support
```

The certs, key, config, binary, and systemd service should all be copied
to the same places on all of these nodes as on the lighthouse.

## X11vnc {#x11vnc}

_Alright._ The hardest part is finished. Now on to setting `x11vnc` up on
the nodes you'll be supporting.

All you should need to do is install `x11vnc` using the package manager
your distro ships with, generate a 20 character password with `pwgen -s
20 1`, run the following command, paste the password, wait for `x11vnc` to
start up, make sure it's running correctly, press `Ctrl` + `C`, then add the
command to the DE's startup applications!

```bash
x11vnc --loop -usepw -listen <nebula-ip> -display :0
```

`--loop` tells `x11vnc` to restart once you disconnect from the session.
`-usepw` is pretty self-explanatory. `-listen <nebula-ip>` is important; it
tells `x11vnc` to only listen on the node's Nebula IP address. This
prevents randos in a coffee shop from seeing an open VNC port and trying
to brute-force the credentials. `-display :0` just defines which X11
server display to connect to.

Some distributions like elementaryOS and those that use KDE and GNOME
will surface a dialogue for managing startup applications if you just
press the Windows (Super) key and type `startup`. If that doesn't work,
you'll have to root around in the settings menus, consult the
distribution's documentation, or ask someone else that might know.

After adding it to the startup application, log out and back in to make
sure it's running in the background.

## Remmina {#remmina}

Now that our network is functioning properly and the VNC server is set
up, we need something that connects to the VNC server over the fancy
mesh network. Enter [Remmina.](https://remmina.org/) This one goes on _your_ nodes.

Remmina is a multi-protocol remote access tool available in pretty much
ever distribution's package archive as `remmina`. Install it, launch it,
add a new connection profile in the top left, give the profile a
friendly name (I like to use the name of the person I'll be supporting),
assign it to a group, such as `Family` or `Friends`, set the Protocol to
`Remmina VNC Plugin`, enter the node's Nebula IP address in the Server
field, then enter their username and the 20 character password you
generated earlier. I recommend setting the quality to Poor, but Nebula
is generally performant enough that any of the options are suitable. I
just don't want to have to disconnect and reconnect with a lower quality
if the other person happens to be on a slow network.

Save and test the connection!

If all goes well and you see the other device's desktop, you're done
with the VNC section! Now on to SSH.

## SSH {#ssh}

First off, make sure `openssh-server` is installed on the remote node;
`openssh-client` would also be good to have, but from what I can tell,
it's not strictly necessary. You _will_ need `openssh-client` on _your_ node,
however. If you already have an SSH key, copy it over to
`~/.ssh/authorized_keys` on the remote node. If you don't, generate one
with `ssh-keygen -t ed25519`. This will create an Ed25519 SSH key pair.
Ed25519 keys are shorter and faster than RSA and more secure than ECDSA
or DSA. If that means nothing to you, don't worry about it. Just note
than this key might not interact well with older SSH servers; you'll
know if you need to stick with the default RSA. Otherwise, Ed25519 is
the better option. After key generation has finished, copy
`~/.ssh/id_ed25519.pub` (note the `.pub` extension) from your node to
`~/.ssh/authorized_keys` on the remote node. The file _without_ `.pub` is your
_private_ key. Like the Nebula CA certificate we generated earlier, this
is extremely sensitive and should never be shared with anyone else.

Next is configuring SSH to only listen on Nebula's interface; as with
`x11vnc`, this prevents randos in a coffee shop from seeing an open SSH
port and trying to brute-force their way in. Set the `ListenAddress`
option in `/etc/ssh/sshd_config` to the remote node's Nebula IP address.
If you want to take security a step further, search for
`PasswordAuthentication` and set it to `no`. This means your SSH key is
_required_ for gaining access via SSH. If you mess up Nebula's firewall
rules and accidentally give other Nebula devices access to this machine,
they still won't be able to get in unless they have your SSH key. I
_personally_ recommend disabling password authentication, but it's not
absolutely necessary. After making these changes, run `systemctl restart
sshd` to apply them.

Now that the SSH server is listening on Nebula's interface, it will
actually fail to start when the machine (re)boots. The SSH server starts
faster than Nebula does, so it will look for the interface before Nebula
has even had a chance to connect. We need to make sure systemd waits for
Nebula to start up and connect before it tells SSH to start; run
`systemctl edit --full sshd` and add the following line in the `[Unit]`
section, above `[Service]`.

```text
After=nebula.service
```

Even now, there's still a bit of a hiccup. Systemd won't start SSH until
Nebula is up and running, which is good. Unfortunately, even after
Nebula has started, it still takes a minute to bring the interface up,
causing SSH to crash. To fix _this_, add the following line directly below
`[Service]`.

```text
ExecStartPre=/usr/bin/sleep 30
```

If the `sleep` executable is stored in a different location, make sure you
use that path instead. You can check by running `which sleep`.

When the SSH _service_ starts up, it will now wait an additional 30
seconds before actually starting the SSH _daemon_. It's a bit of a hacky
solution but it works™. If you come up with something better, please
send it to me and I'll include it in the post! My contact information is
at the bottom of [this site's home page.](/)

After you've made these changes, run `systemctl daemon-reload` to make
sure systemd picks up on the modified service file, then run `systemctl
restart sshd`. You should be able to connect to the remote node from your
node using the following command.

```bash
ssh USER@<nebula-ip>
```

If you want to make the command a little simpler so you don't have to
remember the IP every time, create `~/.ssh/config` on your node and add
these lines to it.

```text
Host USER
  Hostname <nebula-ip>
  User USER
```

Now you can just run `ssh USER` to get in. If you duplicate the above
block for all of the remote nodes you need to support, you'll only have
to remember the person's username to SSH into their machine.

## Going further with Nebula {#going-further-with-nebula}

This section explains why we used FQDNs in the certs and why the DNS
resolver is enabled on the lighthouse.

Nebula ships with a built-in resolver meant specifically for mapping
Nebula node hostnames to their Nebula IP addresses. Running a public DNS
resolver is very much discouraged because it can be abused in terrible
ways. However, the Nebula resolver mitigates this risk because it _only_
answers queries for Nebula nodes. It doesn't forward requests to any
other servers nor does it attempt to resolve any domain other than what
was defined in its certificate. If you use the example I gave above,
that would be `nebula.example.com`; the lighthouse will attempt to resolve
any subdomain of `nebula.example.com` but it will just ignore `example.com`,
`nebula.duckduckgo.com`, `live.secluded.site`, etc.

Taking advantage of this resolver requires setting it as your secondary
resolver on any device you want to be able to resolve hostnames from.
If you were to add the lighthouse's IP address as your secondary
resolver on your PC, you could enter `host.user.nebula.example.com` in
Remmina's server settings _instead of_ `192.168.1.2`.

But how you do so is beyond the scope of this post!

If you're up for some _more_ shenanigans later on down the line, you could
set up a Pi-Hole instance backed by Unbound and configure Nebula as
Unbound's secondary resolver. With this setup, you'd get DNS-level ad
blocking _and_ the ability to resolve Nebula hostname. Pi-Hole would query
Unbound for `host.user.nebula.example.com`, Unbound would receive no
answer from the root servers because the domain doesn't exist outside of
your VPN, Unbound would fall back to Nebula, Nebula would give it an
answer, Unbound would cache the answer, tell Pi-Hole, Pi-Hole would
cache the answer, tell your device, then your device would cache the
answer, and you can now resolve any Nebula host!

Exactly how you do _that_ is **_definitely_** beyond the scope of this post :P

If you set any of this up, I would be interested to hear how it goes! As
stated earlier, my contact information is at the bottom of the site's
home page :)