OpenVPN multi purpose single server

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Introduction

This Howto details one way to make a single OpenVPN server go a long way. Using this method access can be provided to a large pool of addresses for general access and then make use of some of the less intuitive features of OpenVPN to provide properly locked down access for various classes of user.

The end result is this:

  • Single OpenVPN server instance listening on port 443/tcp
  • A pool of addresses for general access on a single subnet
  • A series of tiny address ranges (/30) that effectively allocate a static IP address to specific end users that can be easily grouped and firewalled

Setup

OpenVPN server

  • Create the OpenVPN server as normal
  • Set TCP, port 443, and mode tun
  • Set the IPV4 Tunnel Network as something similar to 10.33.249.0/24
  • Do not set IPv4 Local Network(s).

The third octet should be a number far removed from VLAN/subnet numbers, a /24 is enough for most configurations.

This means that all connections will get an address from a global pool but they are useless unless access is allowed from that subnet in the firewall rules for the OpenVPN "interface"

Pick a subnet such as 10.33.250.0/24 which is not in use. This will be broken up into /30 mini subnets - one per client. If those run out, then start on 10.33.251.0/24. Each of these new subnets needs a route in the main OpenVPN server Advanced settings, such as:

route 10.33.250.0 255.255.255.0;

OpenVPN certificate

Create a certificate in the usual way. I suggest setting the common name to first.last or company.first.last.

OpenVPN Client specific overrides

For each client create a Client specific override.

The tunnel networks will be /30s (i.e. One address for the network, one for the pfSense OpenVPN server, one for the client and one for broadcast). So the first one will be 10.33.127.0/30 and the second one will be 10.33.127.4/30 and so on.

  • Set the Common Name to first.last or what ever was used for the certificate
  • Description - set to the Tunnel Network range, to make it easy to spot who has what
  • Tunnel Network = last one allocated + 4 (see above)
  • Advanced - push "route 10.33.x.0 255.255.255.0";

In the route above x is the customer network that this client may access.

Firewall rules

The client specific override forces a static IP onto the client which will be the third address in the range, for example:

10.33.250.8/30
10.33.250.9
10.33.250.10 - this is the static IP address for the client.
10.33.250.11

If there are several clients that access the same VLANs/subnets then put them together in an alias. Now add a rule on the OpenVPN tab of the Firewall rules granting access from the alias to the relevant subnets.

Notes

  • Remember a client in this scheme needs to have a push route and a firewall rule to be be able access resources.
  • It is recommended to allow ICMP everywhere on the OpenVPN firewall rules tab to help debugging.

Why use port 443/tcp ?

Listening on port 443/tcp is optional but can be useful. Many firewalls allow outbound access to destination port 443/tcp (https) or the ability of OpenVPN to go through web proxies may be utilized. There tend to be less problems using port 433. If there is only one external IP address available and need to run a web server on it then this will be impractical, use port 1194 in that case, or see Sharing a Port with OpenVPN and a Web Server. UDP is best for VPNs, but port 443 is used, then use TCP.

This article is part of the How-To series.