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Since different networks frequently have different restrictions on what ports are allowed to access the internet, it’s useful to have OpenVPN listening on multiple ports for incoming connections. However OpenVPN does not support listening on multiple ports from a single instance of the server. The usual advice is to run separate instances of OpenVPN for each port.

This has some obvious disadvantages - more configurations to manage, more subnets, more firewall rules, etc. Fortunately there’s another way we can do this, by having the router translate the destination port for incoming packets to a different port, commonly called port forwarding.

The Ubiquiti EdgeRouter products offer two different configuration interfaces for setting this up, destination NAT (DNAT) and port forwarding. I’ve chosen to use DNAT, simply because I wanted to play around with the NAT functionality.

Which Ports?

Before starting we need to know which ports we want OpenVPN to listen on. This will vary depending on the situation. I’m going to assume a typical home user who wants to be able to access their home network from various locations and who does not run a public facing web server from their network.

Any network which allows general web access must allow clients to establish connections over TCP ports 80 and 443, which respectively are the standard ports for HTTP and HTTPS. Therefore setting up OpenVPN to listen on these ports will allow access from practically any network. In this example I’ll assume that OpenVPN is listening on TCP port 443 and we want to also forward connections on port 80.

Setting Up DNAT

The instructions here assume that you already have OpenVPN up and running on the EdgeRouter. Please see here for more information about OpenVPN setup. That post also notes a pitfall of running OpenVPN on port 443 and how to handle it.

The following commands set up a DNAT rule to forward incoming connections on port 80 to port 443.

$ configure
# edit service nat rule <rule-number>
# set description "Port forward for OpenVPN"
# set type destination
# set inbound-interface <wan-iface>
# set destination port 80
# set inside-address port 443
# set protocol tcp

Fill in the rule number and the WAN interface. The rule number is only going to be significant if you have other DNAT rules, as rules will be executed sequentially. I found this article from Ubiquiti which states that source/masquerade NAT rules must be numbered 5000 or higher, but I didn’t find anything stating a similar limitation on the number of DNAT rules. It’s probably a good idea to leave some space in case you later need to insert rules before this one, so something like 100 should be fine. You can always change the rule number later by copying the rule to a new one and deleting the original.

After committing these changes it will be possible to connect to OpenVPN on both TCP ports 80 and 443. You might think that the firewall rules need updates to allow incoming TCP connections on port 80, but this isn’t the case. DNAT happens before the firewall, so a rule is only needed for the translated address.

Limitations

There is one significant limitation to this approach - it’s not possible to forward TCP connections to a UDP port, or vice versa, since they are different protocols. If you wish to accept both TCP and UDP connections it is necessary to run two OpenVPN instances. In that case the rule above can be modified to specify tcp_udp for the protocol so that connections using either protocol will be forwarded.

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Seth Forshee


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