OpenVPN-compatible library purely in OCaml

MirageVPN creates secure point-to-point or site-to-site connections in routed or bridged configurations and remote access facilities. It uses TLS to establish a (mutually) authenticated connection, over which material to derive the symmetric keys for packet encryption is exchanged.

The goal of this project is to provide:

• A pure library implementing the protocol logic, and the OpenVPN config file format to enable interoperabilty and a smooth transition for existing deployments.
• A MirageOS unikernel that acts as an OpenVPN-compatible client and server.

Our goal is not to implement the complete protocol, but rather a small useful subset with modern crypto and the latest key exchange methods, without deprecated or redundant features (embodying the philosophy of nqsb-tls). An initial draft of the network setup is depicted in the diagram below:

Since OpenVPN is not detailed in a protocol specificaton specified, apart from comments in the header files, we have written a specification document in Markdown, still work in progress:

• openvpn.md

Our OpenVPN configuration parser can be tested with an OpenVPN configuration file:

• ./_build/default/app/openvpn_config_parser.exe my.openvpn.conf

Unix client miragevpn_client_lwt

Included in this repository is a unix program that will connect to an OpenVPN server, open a tun interface, and tunnel packets between the two.

Unix client on Linux

There are two ways to open tun interfaces:

1. Using a dynamically allocated interface (dev tun). In order to dynamically allocate a tun interface, the process will need privileges to do so. Either by running the client as root or with the CAP_NET_ADMIN privilege. You would then add dev tun to your configuration file.
2. Using a preallocated interface (dev tunX) This is the recommend configuration. To allocate such an interface for tun5 you can use this command:

   sudo ip tuntap add mode tun user MYUSERNAME name tun5

   You would then add dev tun5 to your configuration file.

dune build

# Bestowing the binary with CAP_NET_ADMIN if using dynamic tun allocation:
sudo setcap cap_net_admin=ep ./_build/default/app/miragevpn_client_lwt.exe

./_build/default/app/miragevpn_client_lwt.exe -v MY-CONFIG-FILE.CONF

OpenVPN-compatible config parser

Our goal has been to implement a usable subset (as found in various real-world configuration files available to us during the implementation phase).

As far as possible we have strived to derive a representation that does not permit ambiguity or conflicting options to be present in the parsed config. Consult the type 'a k declaration in openvpn_config.mli for more information.

This does not mean that conflicting options cannot be accepted from an on-disk configuration file, but rather that such conflicts are explicitly handled in the parser code (specifically in the resolve_conflict function).

A notable difference from OpenVPN configuration parser is that we treat relative paths in a configuration file to be relative to the configuration file location, and not relative to the current working directory. OpenVPN supports a --cd argument, which we do not.

You can check compatibility with your configuration file by executing

dune build
./_build/default/app/openvpn_config_parser.exe MY-CONFIG-FILE.CONF

Discrepancies between MirageVPN and OpenVPN

The "verify-x509-name name" in OpenVPN checks by default only the commonName of the subject in the X.509 certificate. MirageVPN validates the provided host against the set of hostnames in the certificate, namely the union of the commonName and the DNS entries in the SubjectAlternativeName extension.

When using a PKCS#12 file the certificates in it are not used to authenticate the remote. OpenVPN will use the certificates if (and only if) no "ca" option is specified. If it is desired to use the certificates from the PKCS#12 file to authenticate the remote the certificates can be added with the "ca" option by extracting the certificates with e.g. openssl pkcs12.

For tls clients (as opposed to static key clients) we only support data channel AEAD ciphers. This means the --auth option is ignored for data channel authentication for tls clients. For --tls-auth it is still used to choose the hmac used for the control channel while for --tls-crypt and --tls-crypt-v2 the hmac is hardcoded (as per OpenVPN™).

Funding

This project was funded in 2019 for six months by the German federal ministry for education and research via the Prototypefund - the amount was 47500 EUR.

In 2023, we received funding from European Union in the Next Generation Internet project (NGI assure, via NLnet. The scope was updating to the current protocol version (tls-crypt-v2 etc.), a QubesOS client, a server implementation, and more documentation. The amount was 49500 EUR.