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Nebula is a user-space mesh virtual private network (VPN) daemon that uses tunneling and encryption to create a secure private mesh network between participating hosts.

## Installation

Install the nebula package.

## Basic concepts and terminology

Nebula is a mesh VPN technology, inspired by tinc. In a mesh VPN, individual nodes form direct tunnels between each other. This allows for high speed direct communication between nodes, without the need to go through a central node. Nodes are authenticated using certificates signed by a certificate authority.

This is in contrast to WireGuard, which is a peer-to-peer VPN technology (although there exist mesh network managers for WireGuard, e.g. innernet and wesherAUR).

This is also different from OpenVPN, which uses a star topology (also called hub and spoke).

Certificate authority
  The certificate authority creates host certificates by signing it.
Lighthouse
  In a Nebula network, there is typically at least one lighthouse node that serves as an information hub for other nodes. Lighthouse nodes help other nodes find each other and form a network mesh.
Node
  A node in the Nebula network.
Nebula IP
  IP address of a node within the Nebula network. Also known as VPN IP.
Routable IP
  The "normal" or "native" IP address of a node. This can be a public IP address or a private IP address, depending on where the node is located and how its network is configured. A node can have multiple routable IP addresses.

## Example: Simple mesh VPN

### Network setup

In this example, we have 3 nodes:

- lighthouse
  - Nebula IP: 192.168.100.1
  - Routable IP: 12.34.56.78
- hostA
  - Nebula IP: 192.168.100.101
  - Routable IP: 10.0.0.22
- hostB
  - Nebula IP: 192.168.100.102
  - Routable IP: 23.45.67.89

The lighthouse has a public static IP address and is reachable by hostA and hostB. hostA lives behind a NAT. hostB has a public IP address.

In our case, we will use a /24 subnet for the VPN network. We will call this network "My Nebula Network".

### Certificate and key generation

First, generate the CA certificate and private key with `nebula-cert ca -name "My Nebula Network"`. This will create two files:

- `ca.crt`: The CA certificate file
- `ca.key`: the CA private key

Subsequently, generate the certificate and private key files for the nodes in the network:

    $ nebula-cert sign -name lighthouse -ip 192.168.100.1/24
    $ nebula-cert sign -name hostA -ip 192.168.100.101/24
    $ nebula-cert sign -name hostB -ip 192.168.100.102/24

Notice that we did not specify `ca.crt` and `ca.key`. By default, `nebula-cert` looks for those files in the current directory.

After this step, we will have these files:

- `lighthouse.crt`, `lighthouse.key`
- `hostA.crt`, `hostA.key`
- `hostB.crt`, `hostB.key`

### Configuration

Create this configuration file on the lighthouse node:

    /etc/nebula/config.yml

    pki:
      ca: /etc/nebula/ca.crt
      cert: /etc/nebula/lighthouse.crt
      key: /etc/nebula/lighthouse.key

    lighthouse:
      am_lighthouse: true

    firewall:
      outbound:
        - port: any
          proto: any
          host: any
      inbound:
        - port: any
          proto: any
          host: any

Create this configuration file on hostA:

    /etc/nebula/config.yml

    pki:
      ca: /etc/nebula/ca.crt
      cert: /etc/nebula/hostA.crt
      key: /etc/nebula/hostA.key

    static_host_map:
      "192.168.100.1": ["12.34.56.78:4242"]

    lighthouse:
      hosts:
        - "192.168.100.1"

    punchy:
      punch: true

    firewall:
      outbound:
        - port: any
          proto: any
          host: any
      inbound:
        - port: any
          proto: any
          host: any

Finally, use this configuration file for hostB:

    /etc/nebula/config.yml

    pki:
      ca: /etc/nebula/ca.crt
      cert: /etc/nebula/hostB.crt
      key: /etc/nebula/hostB.key

    static_host_map:
      "192.168.100.1": ["12.34.56.78:4242"]

    lighthouse:
      hosts:
        - "192.168.100.1"

    firewall:
      outbound:
        - port: any
          proto: any
          host: any
      inbound:
        - port: any
          proto: any
          host: any

### Distribute certificates and private keys

Because the certificates and private keys were generated by the certificate authority, they need to be distributed to each node. SCP and SFTP are suitable for this purpose.

Specifically:

- `ca.crt` should be copied to all 3 nodes: lighthouse, hostA, and hostB
- `lighthouse.crt` and `lighthouse.key` should be copied to the lighthouse node
- `hostA.crt` and `hostA.key` should be copied to hostA
- `hostB.crt` and `hostB.key` should be copied to hostB

Note: The `ca.key` file does not have to be copied over to any node. Keep it safe (do not lose it) and secure (do not leak it).

### Start the nebula daemon

On each node, start `nebula.service`. Optionally, enable it so that it will be started on boot.

Note that it does not matter which node starts the nebula daemon. The lighthouse node can even be started last. Each individual node always tries to connect to the list of known lighthouse nodes, so any network interruption can be rectified quickly.

### Test for mesh functionality

With a mesh network, every node is directly connected to every other node. So, even if the connection between lighthouse and both hostA and hostB is slow, traffic between hostA and hostB can be fast, as long as there is a direct link between those two.

This can be demonstrated by a simple ping test on hostA:

    $ ping -c 5 12.34.56.78

    PING 12.34.56.78 (12.34.56.78) 56(84) bytes of data.
    64 bytes from 12.34.56.78: icmp_seq=1 ttl=56 time=457 ms
    64 bytes from 12.34.56.78: icmp_seq=2 ttl=56 time=480 ms
    64 bytes from 12.34.56.78: icmp_seq=3 ttl=56 time=262 ms
    64 bytes from 12.34.56.78: icmp_seq=4 ttl=56 time=199 ms
    64 bytes from 12.34.56.78: icmp_seq=5 ttl=56 time=344 ms

    --- 12.34.56.78 ping statistics ---
    5 packets transmitted, 5 received, 0% packet loss, time 4004ms
    rtt min/avg/max/mdev = 199.141/348.555/480.349/108.654 ms

    $ ping -c 5 192.168.100.1

    PING 192.168.100.1 (192.168.100.1) 56(84) bytes of data.
    64 bytes from 192.168.100.1: icmp_seq=1 ttl=64 time=218 ms
    64 bytes from 192.168.100.1: icmp_seq=2 ttl=64 time=241 ms
    64 bytes from 192.168.100.1: icmp_seq=3 ttl=64 time=264 ms
    64 bytes from 192.168.100.1: icmp_seq=4 ttl=64 time=288 ms
    64 bytes from 192.168.100.1: icmp_seq=5 ttl=64 time=163 ms

    --- 192.168.100.1 ping statistics ---
    5 packets transmitted, 5 received, 0% packet loss, time 4004ms
    rtt min/avg/max/mdev = 162.776/234.874/288.073/42.902 ms

    $ ping -c 5 192.168.100.102

    PING 192.168.100.102 (192.168.100.102) 56(84) bytes of data.
    64 bytes from 192.168.100.102: icmp_seq=1 ttl=64 time=106 ms
    64 bytes from 192.168.100.102: icmp_seq=2 ttl=64 time=2.14 ms
    64 bytes from 192.168.100.102: icmp_seq=3 ttl=64 time=4.53 ms
    64 bytes from 192.168.100.102: icmp_seq=4 ttl=64 time=4.29 ms
    64 bytes from 192.168.100.102: icmp_seq=5 ttl=64 time=5.39 ms

    --- 192.168.100.102 ping statistics ---
    5 packets transmitted, 5 received, 0% packet loss, time 4006ms
    rtt min/avg/max/mdev = 2.136/24.535/106.344/40.918 ms

Notice that the connection between hostA and lighthouse is slow, but the connection between hostA and hostB is very fast. Also notice that the first packet between hostA and hostB is delayed a bit, but subsequent packets take almost no time at all.

## Configuration options

`listen.port`
  This is the listening port for the nebula daemon, which by default is 4242. On a lighthouse node, or a node with a static IP address, set this to any other number in order to personalize your setup and reduce the chances of unwanted service discovery and DDoS attacks on that port. Then update `static_host_map` to reflect the change.
  On a node with a dynamic IP address, it is recommended to set this to 0, such the nebula daemon will use a random port for communication.
`logging.level`
  By default, the nebula daemon logs INFO-level messages. Thus handshakes are printed, and this can generate a lot of log messages. Set it to `warning` in order to reduce the amount of messages logged.
`relay`
  This option can be used if a node cannot be reached directly from another node. Relay nodes help forward the communication between such nodes.
`firewall`
  This option can be used to allow only certain traffic to and from a node.

## Troubleshooting

### My lighthouse node takes forever to handshake

If your lighthouse node needs a long time to handshake, and it prints multiple handshake messages all at once when handshake is completed, maybe it does not support `recvmmsg()`. To get around this issue, add this configuration option:

    /etc/nebula/config.yml

    listen:
      batch: 1

This problem usually happens if your Linux kernel is too old (<2.6.34). The proper solution is to upgrade it.

## See also

- Nebula Project
- Blog post explaining Nebula VPN

Category:

- Virtual Private Network