forked from robur/blog.robur.coop
55 lines
4.1 KiB
Markdown
55 lines
4.1 KiB
Markdown
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---
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date: 2024-06-26
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title: Testing MirageVPN against OpenVPN™
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description: Some notes about how we test MirageVPN against OpenVPN™
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tags:
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- OCaml
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- MirageOS
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- cryptography
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- security
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- testing
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- vpn
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author:
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name: Reynir Björnsson
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email: reynir@reynir.dk
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link: https://reyn.ir/
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---
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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™.
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During the development we have conducted many manual tests.
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However, this scales poorly and it is easy to forget testing certain cases.
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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.
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Thus we test establishment of the tunnel as well as the tunnel itself.
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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.
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A nice property of this test setup is that it runs with an unmodified OpenVPN binary.
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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.
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Testing against a future version of OpenVPN becomes trivial.
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We do not just test a single part of our implementation but achieve an end-to-end test.
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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.
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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.
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Our client and server can be run without a tun device which otherwise would require elevated privileges.
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Unfortunately, OpenVPN requires privileges to at least configure a tun device.
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Our MirageVPN implementation does IP packet parsing in userspace.
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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.
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We explored unit testing the packet decoding and decryption with our implementation and the C implementation.
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Specifically, we encountered a packet whose message authentication code (MAC) was deemed invalid by the C implementation.
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It helped us discover the MAC computation was correct but the packet encoding was truncated - both implementations agreed that the MAC was bad.
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The test was very tedious to write and would not easily scale to cover a large portion of the code.
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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).
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The end-to-end testing is in addition to our unit tests and fuzz testing; and to our [benchmarking](miragevpn-performance.html) binary.
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Our results are that with 4 configurations we achieve above 75% code coverage in MirageVPN.
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While investigating the code coverage results, we found various pieces of code that were never executed, and we were able to remove them.
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Code that does not exist is bug-free :D
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With these tests in place future maintenance is less daunting as they will help us guard us from breaking the code.
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At the moment we do not exercise the error paths very well in the code.
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This is much less straightforward to test in this manner, and is important future work.
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We plan to develop a client and server that injects faults at various stages of the protocol to test these error paths.
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OpenVPN built with debugging enabled also comes with a `--gremlin` mode that injects faults, and would be interesting to investigate.
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