Redirect traffic from a OpenVPN server to an OpenVPN client running in the same device

Question

I have a raspberry box running an OpenVPN server that I'm using as a way to connect to use my Pihole installation when outside my home network. This is working without issues.

                     |          HOME NETWORK/Pihole  |
(Remote client)     --   (Open VPN Server )        ---         Internet

Separately I have a remote VPN service that I can connect to via an OpenVPN client from the Pi device. That I tested successfully as well.

                    |          HOME NETWORK/Pihole  |
                           (Open VPN Client )       -            VPN provider

I would like to be combine both services such that the final installation would look like this

                 |          HOME NETWORK /Pihole           |
(Remote client) -- (Open VPN Server) -- (Open VPN Client) --- Remote VPN service

I have tried solutions like the one described in this similar question (Wireguard server with active OpenVPN client) with negative similar results: Once you activate the Open VPN client (tun1) the server (tun0) becomes unreachable.

Here's what the routing table looks like when both interfaces tun[01] are up

0.0.0.0/1 via 10.50.11.5 dev tun1
default via 192.168.1.1 dev wlan0 src 192.168.1.164 metric 303
10.8.0.0/24 dev tun0 proto kernel scope link src 10.8.0.1
10.50.11.1 via 10.50.11.5 dev tun1
10.50.11.5 dev tun1 proto kernel scope link src 10.50.11.6
128.0.0.0/1 via 10.50.11.5 dev tun1
192.168.1.0/24 dev wlan0 proto kernel scope link src 192.168.1.164 metric 303
199.116.115.135 via 192.168.1.1 dev wlan0

Here's the current output of my openvpn server when I try to connect.

pi@raspberrypi:/etc/openvpn $ sudo more openvpn-status.log
OpenVPN CLIENT LIST
Updated,Sat Jun 29 23:30:41 2019
Common Name,Real Address,Bytes Received,Bytes Sent,Connected Since
UNDEF,x.x.x.x:39159,860,442,Sat Jun 29 23:30:02 2019

This is what the error log looks like

Sat Jun 29 23:34:47 2019 x.x.x.x:28162 TLS Error: incoming packet authentication failed from [AF_INET]172.58.87.22:28162
Sat Jun 29 23:34:48 2019 x.x.x.x:28162 Authenticate/Decrypt packet error: bad packet ID (may be a replay): [ #1 / time = (1561869285) Sat Jun 29 23:34:45 2019 ] -- see the man page entry for --no-replay and --replay-window for more
info or silence this warning with --mute-replay-warnings
Sat Jun 29 23:34:48 2019 x.x.x.x:28162 TLS Error: incoming packet authentication failed from [AF_INET]x.x.x.x:28162

Apparently the client connection is getting corrupted?

I also tried the solution in OpenVPN Client and Server on same machine - Server doesn't allow connections when client is connected which I discovered after posting this question, to no avail. This would appear to be directly related to my issue but it's still not working.

Answer 1

iptables -I POSTROUTING -t nat -s 10.8.0.0/24 -o tun1 -j MASQUERADE

You will need this rule to route the outgoing traffic from the openvpn server (10.8.0.0/24) through your openvpn client connection (tun1)

To reach your openvpn server from outside your home network you have to use these rules from this answer:

ip rule add from 192.168.1.164 lookup 10           # Pi server
ip route add default via 192.168.1.1 table 10    # LAN router

Where 192.168.1.164 is your Pi-IP.
An alternative to only allow vpn traffic would be:

iptables -A PREROUTING -t mangle -p udp --dport 1149 -j MARK --set-mark 1
ip rule add fwmark 1 table 10
ip route add default via 192.168.1.1 dev wlan0 dev table 10