Hi,
The openvpn server configuration will depends on the configuration of your prospective vpn client and its service/program which you would like to have an access to (through forwarded port).

Let’s go through an example which may give you some tips on how to configure port forwarding. Let’s assume that our vpn client is a linux machine, and we would like to ssh it from the internet, addressing your openvpn/pihole droplet.

1. The things to do on prospective vpn client.

We need to find out what IP address and port sshd (ssh daemon) listens on.

sudo netstat -tulpn | grep -i -e sshd -e address

Output
Proto     Local Address   Foreign Address     State       PID/Program name
tcp       0 0.0.0.0:22       0.0.0.0:*        LISTEN         1548/sshd       

Local Address 0.0.0.0:22 indicates that sshd listens on port 22 on each network interface with IP address assigned to it. So, we are lucky because sshd service will be immediately available on virtual vpn interface when it is up. It would be a bit more complicated if sshd listened on particular network interface only. It would demand changing sshd configuration or changing our approach to openvpn configuration.

The port that sshd listens on has to be open in firewall’s settings on vpn client. Let’s assume we use iptables to configure firewall on our vpn client. So, we are adding an appropriate rule:

sudo iptables --table filter --insert INPUT 1 --protocol tcp --dport 22 --jump ACCEPT

We need to save iptables ruleset to a file, to recreate firewall settings after rebooting. For example:

sudo sh -c "iptables-save > /etc/iptables.rules"

2. Things to do on openvpn/pihole droplet.
We have to provide a static IP address to our vpn client to configure port forwarding efficiently. We will do that with config file put in openvpn client configuration directory (ccd) on droplet. First, we need to enable ccd configuration in openvpn server settings. Open configuration file /etc/openvpn/server/server.conf in your favourite editor, and add a directive that specify a location of openvpn client config directory:

client-config-dir /etc/openvpn/ccd

Then, create this directory:

mkdir /etc/openvpn/ccd

We are going to assign particular IP address to a vpn client due to its certificate common name (CN). So, we need to find out what is CN of a certificate of our prospective vpn client. A certificate is placed in VPN Client configuration file that has been created during deploying openvpn/pihole droplet. It is located in root’s home directory /root/client.ovpn on the droplet. You will copy it to your vpn client device, and use it to establish vpn connection to your droplet. But now, open this file in your fovourite editor and copy certificate content from it (it is a section between the lines BEGIN CERTIFICATE and END CERTIFICATE):

<cert>
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
</cert>

Paste copied content into online decoder, e.g. you can try here. It will reveal certificate’s CN (common name). In our case, CN will be client.
To configure static IP address for our vpn client, open a new file /etc/openvpn/ccd/client in your text editor, add below content to it, and then save the file. Note that a filename is CN from the certificate.

ifconfig-push 10.8.0.6 255.255.255.0

Thanks to that, openvpn server will assign IP 10.8.0.6 to a client that uses certificate with CN=client. The IP address and mask (subnet) correspond to the settings in /etc/openvpn/server/server.conf. There are some directives in config file, defining network topology (subnet) and particular subnet:

/etc/openvpn/server/server.conf
...
topology subnet
server 10.8.0.0 255.255.255.0
...

Finally, we have a static IP configured for our vpn client. Now, we can redirect a traffic to its particular port (port forwarding). We cannot forward port TCP 22 because we ssh openvpn/pihole droplet through this port. So, let’s forward any other one, e.g. TCP 1818. First, we have to open it in droplet’s firewall settings. There are some iptables rules predefined on openvpn/pihole droplet. To list ruleset of INPUT chain, run below command.

iptables --table filter --list INPUT --line-numbers

Output
Chain INPUT (policy ACCEPT)
num  target     prot opt source               destination         
1    ACCEPT     all  --  anywhere             anywhere             ctstate RELATED,ESTABLISHED
2    ACCEPT     udp  --  anywhere             anywhere             udp dpt:openvpn
3    DROP       udp  --  anywhere             anywhere            
4    DROP       tcp  --  anywhere             anywhere             tcp dpt:ssh recent: UPDATE seconds: 60 hit_count: 6 name: SSH side: source mask: 255.255.255.255
5    ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:ssh recent: SET name: SSH side: source mask: 255.255.255.255
6    DROP       tcp  --  anywhere             anywhere            

The last rule in this chain DROPs all TCP packets that do not match previous rules. So, we need to insert our rule before that one:

iptables --table filter --insert INPUT 6 --protocol tcp --dport 1818 --jump ACCEPT

The traffic to our client has to be forwarded, so let’s check how FORWARD chain looks like:

iptables --table filter --list FORWARD --line-numbers

Output
Chain FORWARD (policy ACCEPT)
num  target     prot opt source               destination         
1    ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
2    ACCEPT     all  --  10.8.0.0/24          anywhere            
3    DROP       all  --  anywhere             anywhere  

We need to allow forwarding to our vpn client but again, we need to insert our rule before the rule dropping all packets (no. 3).

iptables --table filter --insert FORWARD 3 --protocol tcp --destination 10.8.0.6 --jump ACCEPT

Now, we are going to add a rule to redirect input traffic from port 1818 to our client’s IP (10.8.0.6), to port 22 (ssh).

iptables --table nat --append PREROUTING --protocol tcp --dport 1818  --jump DNAT --to-destination 10.8.0.6:22

The ssh traffic to our vpn client is redirected now, thanks to DNAT rule (Destination Network Address Translation) added to PREROUTING chain in nat table. But we need to configure traffic from our vpn client as well. To ensure that the remote station, sshing our vpn client, gets correct response, we need to modify the packets outgoing from our vpn client. The source address of our vpn client has to be replaced with public IP address of openvpn/pihole droplet, as it is the gateway/server that remote workstation actually communicate with. Such kind of address manipulation is called SNAT (Source Network Address Translation or masquerade), and it is done in POSTROUTING chain of nat table. Let’s take a look at that chain:

iptables --table nat --list POSTROUTING --line-numbers

Output
Chain POSTROUTING (policy ACCEPT)
num  target     prot opt source               destination         
1    SNAT       all  --  10.8.0.0/24         !10.8.0.0/24          to:167.166.165.164

There is an appropriate rule already preconfigured. It has been created during openvpn/pihole droplet deployment. This rule replaces source address in packets outdoing from the subnet 10.8.0.0/24 to destinations not belonging to this subnet. A new source address, that is put into packets, is our openvpn/pihole droplet’s public IP (in this example: 167.166.165.164).

It seems that iptables configuration is done. To preserve iptables settings, and load them automatically during system booting we need to save them. There is an iptables ruleset file on the droplet that we can use for that purpose. To save iptables ruleset, run below command.

sh -c "iptables-save > /etc/iptables/rules.v4"

Now, you can restart droplet and test if iptables settings are automatically applied.

shutdown -r now

iptables --table filter --list --line-numbers

iptables --table nat --list --line-numbers

If everythig is OK, you can try to connect your vpn client to openvpn/pihole droplet, using configuration file copied to vpn client from droplet root’s home directory (/root/client.ovpn). Then, test if port forwarding works.

Let me know how if it helps, pls.