Extending l2 networks using wireguard & gretap

At some cases, you might need to extend a vlan over l3 links, and for some reason while; 
 
 you don't want to spend money on devices 
 change routing/interface configurations on firewalls. 
 No vxlan capable devices in use (i know, there vpls, l2tpv3 and some other solutions but hey it's linux so why not ?) 
 
 Then here is your solution, with a simple dnat entry you can extend your vlan. 
 In my case, customer was in need to backup their vm's using VMWare's replication to their DR center and whenever they needed, they do like to use the DR as Active data center. And changing configs on a disaster situation was the least they want to deal with. 
 Of course this is only a half of a complete DR solution may less, but restoring a services directly from DR was the requirement as i've been told. 
 Requirements: 
 
 2 ubuntu 18.04 machines on both data centers 
 for each ubuntu vm 1 interface for l3 tunnel as underlay and one interface to use as tap to extend vlan 
 2 cores were quite powerfull for 200 mbits of througput 
 1 gb ram per machine 
 Allowing forged ethernet frames on both tap ports 
 1 cup of coffee and a slice of good chestnut cake 
 
 Some caveats ; 
 
 good cpu = good throughput, and the opposite 
 do not loop ! 
 be carefull with the Xstp or you might have a lame root bridge 
 My config was interface per vlan, so somone might have a trunk solution. 
 No HA solution, those machines needs to be monitored closely, use observium and it's agent 
 
 install wireguard 
 To install wireguard vpn on ubuntu 18.04 
 #sudo apt update
#sudo apt-get install libmnl-dev libelf-dev linux-headers-$(uname -r) build-essential pkg-config
#sudo apt install wireguard 
 
 After installation you'll need to restart operating system and check for installed module, output should be as shown below. 
 #lsmod| grep wireguard
udp_tunnel	16384 1 wireguard
ip6_udp_tunnel 16384 1 wireguard
 
 generete keys 
 Wireguard need public and private keys to operate, there is a tool called wg which can generate them as shown below 
 #wg genkey | sudo tee /etc/wireguard/privatekey | wg pubkey | sudo tee /etc/wireguard/publickey
 
 You can find the keys at /etc/wireguard 
 create wireguard0.conf 
 You'll need to create a file called wireguard0.conf at /etc/wireguard 
 Contents of the file should be like this ; 
 For initiator: 
 [interface]
private_key= the_private_key_you_generated
address= 10.10.10.1/24 #ip address of the wireguard0 

[peer]
PublicKey = #he public key that you generated on the responder host
AllowedIPs = 10.10.10.0/24 #and the other networks that you'd like route through
EndPoint = internet_address:port 
PersistenKeepAlive = 15 #seconds
 
 For responder: 
 [interface]
private_key= the_private_key_you_generated
address= 10.10.10.2/24 #ip address of the wireguard0 
listenPort = 4900

[peer]
PublicKey = #he public key that you generated on the initiator host
AllowedIPs = 10.10.10.0/24 #nd the other networks that you'd like route through
PersistenKeepAlive = 15 #seconds
 
 initiate tunnel 
 to start the tunnel 
 wg-quick up wireguard0
 
 to check the status 
 wg show wireguard0
interface: wireguard0
 public key: +/1R3JqLKlszbaGUSBtckoxNOMuSvLYKUCl03ShoFw8=
 private key: (hidden)
 listening port: 4900

peer: 2f/RmbuvKtR/L2ZFlQBHsVGkTXkA6d1pJO1ay5EjwSQ=
 endpoint: 172.21.23.111:49792
 allowed ips: 10.10.10.0/24, 192.168.5.0/24
 latest handshake: 1 minute, 31 seconds ago
 transfer: 19.10 MiB received, 11.95 MiB sent
 
 install bridge-utils 
 To install bridge utils 
 #apt install bridge-utils
 
 enable br_netfilter 
 to enable 
 #modprobe br_netfilter
 
 to keep loading on boot 
 #sudo sh -c 'echo "br_netfilter" > /etc/modules-load.d/br_netfilter.conf'
#cat net.bridge.bridge-nf-call-ip6tables = 1 >> /etc/sysctl.d/bridge.conf
 
 enable routing 
 To enable routing on the fly 
 sysctl -w net.ipv4.ip_forward=1
 
 to make it permanent, add lines below to /etc/sysctl.conf ; 
 # Uncomment the next line to enable packet forwarding for IPv4
net.ipv4.ip_forward=1
 
 fix mss on exit interface 
 When you are going to use ip tunnel for underlay and gretap for overlay, there will be some serious mss/mtu size problems to fix that, we need the br_netfilter module that we installed before and a special chain to limit the mss size to the max mtu of interface 
 #iptables -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
 
 We specially use -I to put it at the top of the forward chain. 
 configure gretap interface 
 For the initiator: 
 ip link add gretap0 type gretap local 172.16.5.2 remote 192.168.5.2
 
 For responder 
 ip link add gretap0 type gretap local 192.168.5.2 remote 172.16.5.2
 
 configure bridge 
 Now you will add the interfaces to bridge 
 #brctl addbr br0
 
 add interfaces to bridge br0 
 #brctl addif br0 ens4
#brctl addif br0 gretap0
 
 Bring everything up 
 #wp-quick up wireguard0
#ip link set up dev br0
#ip link set up dev gretap0
#ip link et up dev ens4
 
 tests 
 do pings
i've checked that dhcp is working, 
 what needs to be done are ; 
 
 Multicast packets 
 the situation about protocols like vrrp and hsrp 
 The effect of broadcast packets to cpu