Add connection to server through IPv6 interface

Adds option to establish connection with openvpn server through IPv6. This way, if your ISP is already giving you an IPv6 address, you will still be able to connect to your server.
The IPv6 address is added on client's configuration file, below the IPv4 remote instruction. This way, a user will always try to connect to the IPv4 one, and only if it has no luck, it will then try to connect using IPv6.

Note: When using udp, client will need to wait until keepalive time elapses before it tries to connect through IPv6. If you would like this switch to be fast, I suggest you use TCP protocol instead.

openvpn-install.sh

@@ -198,7 +198,7 @@ function installQuestions () {
 	# Detect public IPv4 address and pre-fill for the user
 	IP=$(ip addr | grep 'inet' | grep -v inet6 | grep -vE '127\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}' | grep -oE '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}' | head -1)
 	read -rp "IP address: " -e -i "$IP" IP
-	# If $IP is a private IP address, the server must be behind NAT
+	# If $IP is a private IP address, the server must be behind NAT
 	if echo "$IP" | grep -qE '^(10\.|172\.1[6789]\.|172\.2[0-9]\.|172\.3[01]\.|192\.168)'; then
 		echo ""
 		echo "It seems this server is behind NAT. What is its public IPv4 address or hostname?"
@@ -229,6 +229,21 @@ function installQuestions () {
 	until [[ $IPV6_SUPPORT =~ (y|n) ]]; do
 		read -rp "Do you want to enable IPv6 support (NAT)? [y/n]: " -e -i $SUGGESTION IPV6_SUPPORT
 	done
+    echo ""
+    
+    # Ask the user for its public IPv6 address.
+    if [[ "$IPV6_SUPPORT" = 'y' ]]; then
+		until [[ $WANTS_CONNECTION_THROUGH_IPv6 =~ (y|n) ]]; do
+		    read -rp "Do you want to add connection to server through IPv6? [y/n]: " -e -i $SUGGESTION WANTS_CONNECTION_THROUGH_IPv6
+	    done
+    fi
+
+    if [[ "$WANTS_CONNECTION_THROUGH_IPv6" = 'y' ]]; then
+		until [[ "$PUBLICIPv6" != "" ]]; do
+			read -rp "Public IPv6 address or hostname: " -e PUBLICIPv6
+		done
+    fi
+        
 	echo ""
 	echo "What port do you want OpenVPN to listen to?"
 	echo "   1) Default: 1194"
@@ -883,13 +898,11 @@ ip6tables -D FORWARD -i tun0 -o $NIC -j ACCEPT" >> /etc/iptables/rm-openvpn-rule
 Description=iptables rules for OpenVPN
 Before=network-online.target
 Wants=network-online.target
-
 [Service]
 Type=oneshot
 ExecStart=/etc/iptables/add-openvpn-rules.sh
 ExecStop=/etc/iptables/rm-openvpn-rules.sh
 RemainAfterExit=yes
-
 [Install]
 WantedBy=multi-user.target" > /etc/systemd/system/iptables-openvpn.service
 
@@ -906,12 +919,17 @@ WantedBy=multi-user.target" > /etc/systemd/system/iptables-openvpn.service
 	# client-template.txt is created so we have a template to add further users later
 	echo "client" > /etc/openvpn/client-template.txt
 	if [[ "$PROTOCOL" = 'udp' ]]; then
-		echo "proto udp" >> /etc/openvpn/client-template.txt
+		PROTO4="udp"
+        PROTO6="udp6"
 	elif [[ "$PROTOCOL" = 'tcp' ]]; then
-		echo "proto tcp-client" >> /etc/openvpn/client-template.txt
+		PROTO4="tcp-client"
+        PROTO6="tcp6-client"
 	fi
-	echo "remote $IP $PORT
+	echo "remote $IP $PORT $PROTO4" >> /etc/openvpn/client-template.txt
-dev tun
+    if [[ "$WANTS_CONNECTION_THROUGH_IPv6" = 'y' ]]; then
+		echo "remote $PUBLICIPv6 $PORT $PROTO6" >> /etc/openvpn/client-template.txt
+    fi
+echo "dev tun
 resolv-retry infinite
 nobind
 persist-key