Routing

Configure traffic routing and NAT from a Windows host (192.168.0.101, eth0) through a Linux VM (192.168.0.181, eth1 bridged interface) to VPN (10.10.10.0/24, tun0).
Enable IP forwarding on Linux VM:
$ sudo sh -c 'echo 1 > /proc/sys/net/ipv4/ip_forward'
Create iptables rules to do the forwarding on Linux VM:
$ sudo iptables -A FORWARD -i tun0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
$ sudo iptables -A FORWARD -i eth1 -o tun0 -j ACCEPT
For the purpose of redirecting NEW connections from Linux tun0 to Windows host I can set socat on a needed port as a quick solution (actually it's not necessary for this routing task):
$ sudo socat TCP-LISTEN:1337,fork TCP:192.168.0.101:1337
Create iptables rules to do NAT on Linux VM:
$ sudo iptables -t nat -A POSTROUTING -s 192.168.0.0/24 -o tun0 -j MASQUERADE
Add a route to Linux VM on Windows host:
Cmd > route add 10.10.10.0 mask 255.255.255.0 192.168.0.181

I shall configure an intermediate OpenVPN server to serve as a jump box (1st hop) to connect to the target lab. It's helpful when the target OpenVPN server (2nd hop) doesn't allow to have multiple connections with the same common name (--duplicate-cn not set), i.e. using the same client's .ovpn profile.
OpenVPN Jump Server Scheme
Quick OpenVPN server installation:
$ sudo apt update
$ wget https://git.io/vpn -O openvpn-install.sh
$ chmod +x openvpn-install.sh
$ sudo bash openvpn-install.sh
Check OpenVPN server status:
$ sudo service [email protected] status
Change server config (/etc/openvpn/server/server.conf):
# Allocate a virtual /30 network
topology net30
# Add as many clients as you need
route 10.8.1.0 255.255.255.0
route 10.8.2.0 255.255.255.0
# Set a directory to look for clients' configs
client-config-dir ccd
Create a directory with clients' configs to push and set static IPs for clients:
$ sudo mkdir /etc/openvpn/server/ccd
$ sudo sh -c 'echo "ifconfig-push 10.8.1.1 10.8.1.2" > /etc/openvpn/server/ccd/kali'
$ sudo sh -c 'echo "ifconfig-push 10.8.1.5 10.8.1.6" > /etc/openvpn/server/ccd/parrot'
$ sudo sh -c 'echo "ifconfig-push 10.8.2.1 10.8.2.2" > /etc/openvpn/server/ccd/ubuntu'
For other clients /30 subnets must be used as well:
[1,2] [5,6] [9,10] [13,14] [17,18] [21,22] [25,26] [29,30] [33,34] [37,38] [41,42] [45,46] [49,50] [53,54] [57,58] [61,62] [65,66] [69,70] [73,74] [77,78] [81,82] [85,86] [89,90] [93,94] [97,98] [101,102] [105,106] [109,110] [113,114] [117,118] [121,122] [125,126] [129,130] [133,134] [137,138] [141,142] [145,146] [149,150] [153,154] [157,158] [161,162] [165,166] [169,170] [173,174] [177,178] [181,182] [185,186] [189,190] [193,194] [197,198] [201,202] [205,206] [209,210] [213,214] [217,218] [221,222] [225,226] [229,230] [233,234] [237,238] [241,242] [245,246] [249,250] [253,254]
Restart OpenVPN server (tun0):
$ sudo service [email protected] restart
Start OpenVPN client (tun1):
$ nohup sudo openvpn --client --config lab.ovpn &
Check interfaces:
$ ifconfig tun0
tun0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST> mtu 1500
inet 10.8.0.1 netmask 255.255.255.255 destination 10.8.0.2
inet6 fe80::ca99:1dec:45c1:5d7a prefixlen 64 scopeid 0x20<link>
inet6 fddd:1194:1194:1194::1 prefixlen 64 scopeid 0x0<global>
unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00 txqueuelen 100 (UNSPEC)
RX packets 5 bytes 420 (420.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 9 bytes 724 (724.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
$ ifconfig tun1
tun1: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST> mtu 1500
inet 10.10.13.37 netmask 255.255.254.0 destination 10.10.13.37
inet6 dead:beef:2::10ef prefixlen 64 scopeid 0x0<global>
inet6 fe80::bbe3:5b14:117e:4b99 prefixlen 64 scopeid 0x20<link>
unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00 txqueuelen 100 (UNSPEC)
RX packets 5 bytes 420 (420.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 10 bytes 800 (800.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
Configure NAT:
# Source NAT to reach resources from tun0 to tun1
$ sudo iptables -t nat -A POSTROUTING -s 10.8.0.0/16 -o tun1 -j SNAT --to-source 10.10.13.37
# Destination NAT to trigger reverse shells from tun1 to tun0 (separate port ranges for separate clients, red arrow on the diagram above)
$ sudo iptables -t nat -A PREROUTING -i tun1 -p tcp --dport 6001:7000 -j DNAT --to-destination 10.8.1.1
$ sudo iptables -t nat -A PREROUTING -i tun1 -p tcp --dport 7001:8000 -j DNAT --to-destination 10.8.1.5
$ sudo iptables -t nat -A PREROUTING -i tun1 -p tcp --dport 8001:9000 -j DNAT --to-destination 10.8.2.1
Make iptables rules persistent:
$ sudo apt install iptables-persistent -y
$ sudo service netfilter-persistent save
Add the following directive to client's .ovpn config to ignore default gateway redirection:
pull-filter ignore "redirect-gateway"
Connect to tun0 as a client (example for the kali client) and manually add a route only for traffic you want to go through VPN:
$ sudo openvpn kali.ovpn
$ sudo ip route add 10.10.10.0/24 via 10.8.1.2 metric 0 dev tun0
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VM as a Router
OpenVPN Jump Server