# apt-get install bridge-utils 

https://wiki.debian.org/fr/BridgeNetworkConnections

openvpn --genkey --secret static.key

# serveur
openvpn --mktun --dev-type tap --dev tapthd
ip link set tapthd up
openvpn --dev-type tap --dev tapthd  --proto udp --verb 3 --daemon --keepalive 10 30 --persist-tun --secret static.key --port 1234

ip link set eth0 up

#brctl addbr br0
#brctl addif br0 eth0
#ip link set br0 up
# remplace par :
iface br0 inet static
        bridge_ports eth0 
        address 192.168.4.100/24

brctl addif br0 tapthd


#client
openvpn --mktun --dev-type tap --dev tapthd
ip link set tapthd up
openvpn --dev-type tap --dev tapthd  --proto udp --verb 3 --daemon --keepalive 10 30 --persist-tun --secret static.key --lport 0 --remote IP PORT 	

brctl addbr br0
brctl addif br0 eth0
ip link set br0 up

ip link set eth0 up

brctl addbr br0
brctl addif br0 eth0
brctl addif br0 tapthd
ip link set br0 up