re-add old miragevpn testing article

articles/2024-08-21-OpenVPN-and-MirageVPN.md

@@ -236,7 +236,7 @@ This work was funded by [the EU NGI Assure Fund through NLnet](https://nlnet.nl/
 In my opinion, this shows that funding one open source project can have a positive impact on other open source projects, too.
 
 [robur]: https://robur.coop/
-[miragevpn-server]: https://blog.robur.coop/articles/miragevpn-server.html
+[miragevpn-server]: miragevpn-server.html
 [contact]: https://reyn.ir/contact.html
 
 [^openvpn-tls]: This is not always the case. It is possible to use static shared secret keys, but it is mostly considered deprecated.

articles/miragevpn-testing.md

@@ -0,0 +1,54 @@
+---
+date: 2024-06-26
+title: Testing MirageVPN against OpenVPN™
+description: Some notes about how we test MirageVPN against OpenVPN™
+tags:
+  - OCaml
+  - MirageOS
+  - cryptography
+  - security
+  - testing
+  - vpn
+author:
+  name: Reynir Björnsson
+  email: reynir@reynir.dk
+  link: https://reyn.ir/
+---
+
+As our last milestone for the [EU NGI Assure](https://www.assure.ngi.eu/) funded MirageVPN project (for now) we have been working on testing MirageVPN, our OpenVPN™-compatible VPN implementation against the upstream OpenVPN™.
+During the development we have conducted many manual tests.
+However, this scales poorly and it is easy to forget testing certain cases.
+Therefore, we designed and implemented interoperability testing, driving the C implementation on the one side, and our OCaml implementation on the other side. The input for such a test is a configuration file that both implementations can use.
+Thus we test establishment of the tunnel as well as the tunnel itself.
+
+While conducting the tests, our instrumented binaries expose code coverage information. We use that to guide ourselves which other configurations are worth testing. Our goal is to achieve a high code coverage rate while using a small amount of different configurations. These interoperability tests are running fast enough, so they are executed on each commit by CI.
+
+A nice property of this test setup is that it runs with an unmodified OpenVPN binary.
+This means we can use an off-the-shelf OpenVPN binary from the package repository and does not entail further maintenance of an OpenVPN fork.
+Testing against a future version of OpenVPN becomes trivial.
+We do not just test a single part of our implementation but achieve an end-to-end test.
+The same configuration files are used for both our implementation and the C implementation, and each configuration is used twice, once our implementation acts as the client, once as the server.
+
+We added a flag to our client and our [recently finished server](miragevpn-server) applications, `--test`, which make them to exit once a tunnel is established and an ICMP echo request from the client has been replied to by the server.
+Our client and server can be run without a tun device which otherwise would require elevated privileges.
+Unfortunately, OpenVPN requires privileges to at least configure a tun device.
+Our MirageVPN implementation does IP packet parsing in userspace.
+We test our protocol implementation, not the entire unikernel - but the unikernel code is a tiny layer on top of the purely functional protocol implementation.
+
+We explored unit testing the packet decoding and decryption with our implementation and the C implementation.
+Specifically, we encountered a packet whose message authentication code (MAC) was deemed invalid by the C implementation.
+It helped us discover the MAC computation was correct but the packet encoding was truncated - both implementations agreed that the MAC was bad.
+The test was very tedious to write and would not easily scale to cover a large portion of the code.
+If of interest, take a look into our [modifications to OpenVPN](https://github.com/reynir/openvpn/tree/badmac-test) and [modifications to MirageVPN](https://github.com/robur-coop/miragevpn/tree/badmac-test).
+
+The end-to-end testing is in addition to our unit tests and fuzz testing; and to our [benchmarking](miragevpn-performance.html) binary.
+
+Our results are that with 4 configurations we achieve above 75% code coverage in MirageVPN.
+While investigating the code coverage results, we found various pieces of code that were never executed, and we were able to remove them.
+Code that does not exist is bug-free :D
+With these tests in place future maintenance is less daunting as they will help us guard us from breaking the code.
+
+At the moment we do not exercise the error paths very well in the code.
+This is much less straightforward to test in this manner, and is important future work.
+We plan to develop a client and server that injects faults at various stages of the protocol to test these error paths.
+OpenVPN built with debugging enabled also comes with a `--gremlin` mode that injects faults, and would be interesting to investigate.