# Intro

This note describes how to connect two networks/devices/VMs over public network using Wireguard with Layer 2 support (ARP, IPv6 link-local, etc).

This can also be achieved using SSH and its "tap" tunnel, however, it does not provide the same level of latency and bandwidth as full-blown VPN such as Wireguard.

In addition, this note describes how to tunnel Wireguard over TCP connection. This may be of use if you encounter firewall in-between so, for instance, you can use TCP port 443 only.

# Objective

Let's draw a network diagram of what we would like to have in as a result.

 
 Host on private LAN                                         Host on the Internet
 +---------------------------------+                       +-------------------------------+
 |   brtun bridge                  |                       |                  brtun bridge |
 | +-------------+                 |                       |                 +--------+    |
 | | ethX gretun |<->wg0<->udp2raw | <-Internet, TCP 443-> | udp2raw<->wg0<->| gretun |    |
 | +-------------+                 |                       |                 +--------+    |
 +---------------------------------+                       +-------------------------------+
 
 brtun: 192.168.0.200/24                                    brtun: 192.168.0.50/24
 wg0: 12.12.12.2/24                                         wg0: 12.12.12.1/24
 gretun: 12.12.12.2->12.12.12.1                             gretun: 12.12.12.1->12.12.12.2
 


# Setting things up

Distinguishing between server and client here is based on the answer to the question: who can connect to Wireguard listener socket.

## Server

Generating a key:

 wg genkey | tee wgkeyprivs | wg pubkey > wgkeypubs

Creating a Wireguard interface, setting the private key and a unique private IPv4 address:

 ip l a wg0 type wireguard
 wg set wg0 private-key ./wgkeyprivs
 ip a a 12.12.12.1/24 dev wg0

In case we want Wireguard over TCP, we have to decrease MTU:

 ip l set dev wg0 mtu 1200

Configuring peer's public key and setting interface up (you can restrict allowed-ips to the range you want):
 
 ip l set dev wg0 up
 wg set wg0 listen-port 51820 peer $(cat wgkeypubc) allowed-ips 0.0.0.0/0

Creating GRETAP interface which will provide Layer 2 over our Wireguard tunnel:

 ip l a gretun type gretap local 12.12.12.1 remote 12.12.12.2
 ip l s gretun up

Configuring a bridge which includes GRETAP interface. This step is in fact optional, but it allows you some flexibility:  you can assign an IP address to this bridge interface and/or you can add additional interfaces to this bridge.

 ip l add name brtun type bridge
 ip l set dev brtun up
 ip l set gretun master brtun

Let's assign an IP address from the target network to this bridge:

 sudo ip a a 192.168.0.51/24 dev brtun

If we want Wireguard over TCP, launch UDP2RAW listener which will do its magic. You can get UDP2RAW from this Git repo: https://github.com/wangyu-/udp2raw-tunnel/releases/

 ./udp2raw_amd64 -s -l YOUR_PUBLIC_IP:443 -r 127.0.0.1:51820 -a --fix-gro

## Client

Client is very similar to the server described above.

Generating a key:

 wg genkey | tee wgkeyprivc | wg pubkey > wgkeypubc

Creating a Wireguard interface, setting the private key and a unique private IPv4 address:

 ip l a wg0 type wireguard
 wg set wg0 private-key ./wgkeyprivc
 ip a a 12.12.12.2/24 dev wg0

In case we want Wireguard over TCP, we have to decrease MTU:

 ip l set dev wg0 mtu 1200

Configuring peer's public key, endpoint address and setting interface up (you can restrict allowed-ips to the range you want):
 
 ip l set dev wg0 up
 wg set wg0 listen-port 51820 peer $(cat wgkeypubs) allowed-ips 0.0.0.0/0 endpoint YOUR_PUBLIC_IP:51820 persistent-keepalive 15

In case you want Wireguard over TCP, set endpoint address to a localhost interface:

 wg set wg0 listen-port 51820 peer $(cat wgkeypubs) allowed-ips 0.0.0.0/0 endpoint 127.0.0.1:7777 persistent-keepalive 15

Creating GRETAP interface which will provide Layer 2 over our Wireguard tunnel:

 ip l a gretun type gretap local 12.12.12.2 remote 12.12.12.1
 ip l s gretun up

Configuring a bridge which includes GRETAP interface.

 ip l add name brtun type bridge
 ip l set dev brtun up
 ip l set gretun master brtun

Let's add a physical interface belonging to a LAN network to the bridge and start DHCP client:

 ip l set ethX master brtun
 dhcpcd ethX

If we want Wireguard over TCP, launch UDP2RAW listener on the localhost interface which connects to the corresponding public listener.

 ./udp2raw_amd64 -c -l 127.0.0.1:7777 -r YOUR_PUBLIC_IP:443 -a --fix-gro

# Results

That's it. Now you will have Layer 2 connectivity between two separated machines over Wireguard VPN (optionally, over TCP). Enjoy your ARP and IPv6 link-local experience.

# References

https://notes.superlogical.ch/pages/note_wg/nolayer2/
https://lafibre.info/vpn/tunnel-gre-sur-wireguard/&prev=search&pto=aue