Configure IPSec VPN Gateway to Connect to External Systems using Strongswan

1. Create a new Virtual Machine on the same network as the rest of the cluster nodes. This machine will be used as a gateway to access the external IPs. This machine will need a public IP. The preferred OS is Ubuntu Server 20.04 or higher.

2. The rest of this guide will assume the following:

   1. 10.10.0.0/24 - the local network subnet.

   2. 192.168.0.0/24 - the external network subnet which we are trying to reach over VPN.

   3. 10.10.0.15 - the internal IP of the VPN gateway machine from Step 1.

   4. 3.10.x.x - Public IP of the VPN gateway machine from Step 1.

   5. 4.10.y.y - Public IP of VPN tunnel of the external Network.

3. VPN Gateway Setup:

   1. Enable IP Forwarding on the node.

      1. Create a file /etc/sysctl.d/60-ip-forward.conf with the following contents:

         net.ipv4.ip_forward = 1
         net.ipv6.conf.all.forwarding = 1

      2. Run this to apply the above config:

         sudo systctl --system

   2. Install and configure Strongswan.

      1. Install Strongswan, run:

         sudo apt install strongswan libcharon-extra-plugins libcharon-extauth-plugins libstrongswan-extra-plugins

      2. Take backup of ipsec.conf, run:

         sudo cp /etc/ipsec.conf /etc/ipsec.conf.orig

      3. Edit the /etc/ipsec.conf with the following contents:

         config setup
                 charondebug="all"
                 uniqueids=yes
         conn openg2p-to-external-vpn
                 type=tunnel
                 auto=start
                 keyexchange=ikev2
                 authby=psk
                 # Phase 1
                 ike=aes256-sha256-ecp521
                 ikelifetime=28800s
                 # Phase 2
                 esp=aes256-sha256-ecp256
                 lifetime=3600s
                 aggressive=no
                 keyingtries=%forever
                 rekeymargin=3m
                 left=10.10.0.15
                 leftsubnet=10.10.0.15/32
                 leftid=3.10.x.x
                 right=4.10.y.y
                 rightsubnet=192.168.0.0/24
                 rightid=4.10.y.y
                 dpddelay=30s
                 dpdtimeout=120s
                 dpdaction=restart

      4. Create /etc/ipsec.secrets with the following content:

         10.10.0.15 4.10.y.y : PSK "<PSK Value>"

      5. Start strongswan tunnel, run:

         sudo systemctl enable ipsec
         sudo systemctl start ipsec

      6. Check status by running:

         sudo ipsec statusall

   3. Configure iptables (firewall).

      1. Install iptables-persistent , run:

         sudo apt install iptables-persistent

      2. Set default forward policy as DROP, run:

         sudo iptables -P FORWARD DROP

      3. For each node that is allowed to access the external network, run the following: (The following is only an example, change it according to your system. To get the network interface names run: ip link )

         sudo iptables -A FORWARD -o <primary_network_interface_name> -s <10.10.node1.internalip> -j ACCEPT
         sudo iptables -A FORWARD -i <primary_network_interface_name> -d <10.10.node1.internalip> -j ACCEPT
         
         sudo iptables -A FORWARD -o <primary_network_interface_name> -s <10.10.node2.internalip> -j ACCEPT
         sudo iptables -A FORWARD -i <primary_network_interface_name> -d <10.10.node2.internalip> -j ACCEPT

      4. Enable NAT forwarding; run

         sudo iptables -A POSTROUTING -t nat -o <primary_network_interface_name> -j MASQUERADE

      5. Save the iptables changes for the next boot, run: (Make sure to run this whenever you change something on iptables)

         sudo bash -c 'iptables-save > /etc/iptables/rules.v4'

4. Add an IP Route on all the other nodes that need to access the VPN, to hop over the VPN Gateway node. (If a global routing table exists on the network, this rule can be added there instead.)

   sudo ip route add 192.168.0.0/24 via 10.10.0.15