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<h1>Postes, télégraphes et téléphones, next steps</h1>
<ul class="tags-list"><li><a href="/tags.html#tag-SMTP">SMTP</a></li><li><a href="/tags.html#tag-emails">emails</a></li><li><a href="/tags.html#tag-mailing-lists">mailing-lists</a></li></ul><p>As you know from <a href="https://blog.robur.coop/articles/finances.html">our article on Robur's
finances</a>, we've just received
<a href="https://nlnet.nl/project/PTT">funding for our email project</a>. This project
started when I was doing my internship in Cambridge and it's great to see that
it's been able to evolve over time and remain functional. This article will
introduce you to the latest changes to <a href="https://github.com/mirage/ptt">our PTT
project</a> and how far we've got towards providing
an OCaml mailing list service.</p>
<h2 id="a-git-repository-or-a-simple-block-device-as-a-database"><a class="anchor" aria-hidden="true" href="#a-git-repository-or-a-simple-block-device-as-a-database"></a>A Git repository or a simple block device as a database?</h2>
<p>One issue that came up quickly in our latest experiments with our SMTP stack was
the database of users with an email address. Since we had decided to ‘break
down’ the various stages of an email submission to offer simple unikernels, we
ended up having to deploy 4 unikernels to have a service that worked.</p>
<ul>
<li>a unikernel for authentication</li>
<li>a unikernel DKIM-signing the incoming email</li>
<li>one unikernel as primary DNS server</li>
<li>one unikernel sending the signed email to its real destination</li>
</ul>
<p>And we're only talking here about the submission of an email, the reception
concerns another ‘pipe’.</p>
<p>The problem with such an architecture is that some unikernels need to have the
same data: the users. In this case, the first unikernel needs to know the user's
password in order to verify authentication. The final unikernel needs to know
the real destinations of the users.</p>
<p>Let's take the example of two users: foo@robur.coop and bar@robur.coop. The
first points to hannes@foo.org and the second to reynir@example.com.</p>
<p>If Hannes wants to send a message to bar@robur.coop under the identity of
foo@robur.coop, he will need to authenticate himself to our first unikernel.
This first unikernel must therefore:</p>
<ol>
<li>check that the user <code>foo</code> exists</li>
<li>the hashed password used by Hannes is the same as the one in the database</li>
</ol>
<p>Next, the email will be signed by our second unikernel. It will then forward the
email to the last unikernel, which will do the actual translation of the
recipients and DNS resolution. In other words:</p>
<ol>
<li>it will see that one (the only) recipient is bar@robur.coop</li>
<li>check that bar@robur.coop exists and obtain its real address</li>
<li>it will obtain reynir@example.com and perform DNS resolution on
<code>example.com</code> to find out the email server for this domain</li>
<li>finally send the email signed by foo@robur.coop to reynir@example.com!</li>
</ol>
<p>So the first and last unikernels need to have the same information about our
users. One for the passwords, the second for the real email addresses.</p>
<p>But as you know, we're talking about unikernels that exist independently of each
other. What's more, they can't share files and the possibility of them sharing
block-devices remains an open question (and a complex one where parallel access
may be involved). In short, the only way to ‘synchronise’ these unikernels in
relation to common data is with a Git repository.</p>
<p><a href="https://github.com/robur-coop/git-kv">Git</a> has the advantage of being widely used for our unikernels
(<a href="https://github.com/robur-coop/dns-primary-git/">primary-git</a>, <a href="https://github.com/dinosaure/pasteur">pasteur</a>, <a href="https://github.com/robur-coop/unipi">unipi</a> and
<a href="https://github.com/dinosaure/contruno">contruno</a>). The advantage is that you can track changes, modify
files and notify the unikernel to update itself (using nsupdate, a simple ping
or an http request to the unikernel).</p>
<p>The problem is that this requires certain skills. Even if it's ‘simple’ to set
up a Git server and then deploy our unikernels, we can restructure our
architecture and simplify the deployment of an SMTP stack!</p>
<h2 id="elit-and-oneffs"><a class="anchor" aria-hidden="true" href="#elit-and-oneffs"></a>Elit and OneFFS</h2>
<p>We have therefore decided to merge the email exchange service and email
submission into a unikernel so that this is the only user information requester.</p>
<p>So we decided to use <a href="https://github.com/robur-coop/oneffs">OneFFS</a> as the file system for our database,
which will be a plain JSON file. This is perhaps one of the advantages of
MirageOS, which is that you can decide exactly what you need to implement
specific objectives.</p>
<p>In this case, those with experience of Postfix, LDAP or MariaDB could confirm
that configuring an email service should be ‘simpler’ than implementing a
multitude of pipes between different applications and authentication methods.</p>
<p>The JSON file is therefore very simple and so is the creation of an OneFFS
image:</p>
<pre><code class="language-sh">$ cat &gt;database.json&lt;&lt;EOF
&gt; [ { &quot;name&quot;: &quot;din&quot;
&gt; , &quot;password&quot;: &quot;xxxxxx&quot;
&gt; , &quot;mailboxes&quot;: [ &quot;romain.calascibetta@gmail.com&quot; ] } ]
&gt; EOF
$ opam install oneffs
$ oneffs create -i database.json -o database.img
</code></pre>
<p>All you have to do is register this image as a block with <a href="https://github.com/robur-coop/albatross">albatross</a> and launch
our Elit unikernel with this block-device.</p>
<pre><code class="language-sh">$ albatross-client create-block --data=database.img database 1024
$ albatross-client create --net=service:br0 --block=database:database \
elit elit.hvt \
--arg=...
</code></pre>
<p>At this stage, and if we add our unikernel signing incoming emails, we have more
or less the same thing as what I've described in <a href="https://blog.osau.re/articles/smtp_1.html">my previous articles</a> on
<a href="https://blog.osau.re/articles/smtp_2.html">deploying</a> an <a href="https://blog.osau.re/articles/smtp_3.html">email service</a>.</p>
<h2 id="multiplex-receiving--sending-emails"><a class="anchor" aria-hidden="true" href="#multiplex-receiving--sending-emails"></a>Multiplex receiving &amp; sending emails</h2>
<p>The PTT project is a toolkit for implementing SMTP servers. It gives developers
the choice of implementing their logic as they see fit:</p>
<ul>
<li>sign an email</li>
<li>resolve destinations according to a database</li>
<li>check SPF information</li>
<li>annotate the email as spam or not</li>
<li>etc.</li>
</ul>
<p>Previously, PTT was split into 2 parts:</p>
<ol>
<li>management of incoming clients/emails</li>
<li>the logic to be applied to incoming emails and their delivery</li>
</ol>
<p>The second point was becoming increasingly complex, however, and errors in
sending emails are legion (DMARC non-alignment, the email is too big for the
destination, the destination doesn't exist, etc.). All the more so since, up to
now, PTT could only report these errors via the logs...</p>
<p>Hannes immediately mentioned the possibility of separating the logic of the
unikernel from the delivery. This will allow us to deal with temporary failures
(greylisting) as well. So a fundamental change was made:</p>
<ul>
<li>improve the <a href="https://github.com/mirage/colombe">sendmail</a> and <code>sendmail-lwt</code> packages (as well as proposing
<code>sendmail-miou</code>!) when sending or submitting an email</li>
<li>improve PTT so that there are now 3 distinct jobs: receiving, what to do with
incoming emails and sending emails</li>
</ul>
<p><img src="../images/smtp.jpg" alt="SMTP" ></p>
<p>This finally allows us to describe a clearer error management policy that is
independent of what we want to do with incoming emails. At this stage, we can
look for the <code>Return-Path</code> in emails that we haven't managed to send and notify
the senders!</p>
<p>All this is still in the experimental stage and practical cases are needed to
observe how we should handle errors and how others do.</p>
<h2 id="insights--next-goals"><a class="anchor" aria-hidden="true" href="#insights--next-goals"></a>Insights &amp; Next goals</h2>
<p>We're already starting to have a bit of fun with email and we can start sending
and receiving emails right away.</p>
<p>We're also already seeing hacking attempts on our unikernel:</p>
<ul>
<li>people trying to authenticate themselves without <code>STARTTLS</code> (or with it,
depending on how clever the bot is)</li>
<li>people trying to send emails as non-existent users in our database</li>
<li>we're also seeing content that has nothing to do with SMTP</li>
</ul>
<p>Above all, this shows that, very early on, bots try to usurp the identity linked
to your server (in our case, osau.re) in order to send spam, authenticate
themselves or simply send ‘stuff’ and observe what happens. In this case, for
all the cases mentioned, Elit (and PTT) reacts well: in other words, it simply
cuts off the connection.</p>
<p>We were also able to observe how services such as gmail work. In addition, for
the purposes of a mailing list, email forwarding distorts DMARC verification
(specifically, SPF verification). The case is very simple:</p>
<p>foo@gmail.com tries to reply to robur@osau.re. robur@osau.re is a mailing list
to several addresses (one of them is bar@gmail.com). The unikernel will receive
the email and send it to bar@gmail.com. The problem is the alignment between
the <code>From</code> field (which corresponds to foo@gmail.com) and our osau.re server.
From gmail.com's point of view, there is a misalignment between these two
pieces of information and it therefore refuses to receive the email.</p>
<p>This is where our next objectives come in:</p>
<ul>
<li>finish our DMARC implementation</li>
<li>implement ARC so that our server notifies us that, on our side, the DMARC
check went well and that gmail.com should trust us on this.</li>
</ul>
<p>There is another way of solving the problem, perhaps a little more problematic,
modify the incoming email and in particular the <code>From</code> field. Although this
could be done quite simply with <a href="https://github.com/mirage/mrmime">mrmime</a>, it's better to concentrate on
DMARC and ARC so that we can send our emails as they are and never alter them
(especially as this will invalidate previous DKIM signatures!).</p>
<h2 id="conclusion"><a class="anchor" aria-hidden="true" href="#conclusion"></a>Conclusion</h2>
<p>It's always satisfying to see your projects working ‘more or less’ correctly.
This article will surely be the start of a series on the intricacies of email
and the difficulty of deploying such a service at home.</p>
<p>We hope that this NLnet-funded work will enable us to replace our current email
system with unikernels. We're already past the stage where we can, more or less
(without DMARC checking), send emails to each other, which is a big step!</p>
<p>So follow our work on our blog and if you like what we're producing (which
involves a whole bunch of protocols and formats - much more than just SMTP), you
can make <a href="https://robur.coop/Donate">a donation here</a>!</p>
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Robur's blog - Sponsor us via GitHub
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<meta name="description" content="A new way to sponsor our cooperative">
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<h1>Sponsor us via GitHub</h1>
<ul class="tags-list"><li><a href="/tags.html#tag-cooperative">cooperative</a></li><li><a href="/tags.html#tag-github">github</a></li></ul><p>We're delighted to announce the possibility of helping our cooperative through
the GitHub Sponsors platform. The link is available here:</p>
<p><a href="https://github.com/sponsors/robur-coop">https://github.com/sponsors/robur-coop</a></p>
<p>We would also like to reiterate the possibility of making a donation<sup><a href="#fn-preferable" id="ref-1-fn-preferable" role="doc-noteref" class="fn-label">[1]</a></sup> to our
cooperative via the IBAN of <a href="https://aenderwerk.de/">Änderwerk</a> available here (if you need
a tax-deductible donation receipt, please use <a href="https://aenderwerk.de/donate">this form</a>).</p>
<pre><code>Account holder: Änderwerk gGmbH
Subject: robur
IBAN: DE46 4306 0967 1289 8604 00
BIC: GENODEM1GLS
Bank: GLS Gemeinschaftsbank, Christstrasse 9, 44789 Bochum, Germany
</code></pre>
<p>More generally, you can refer to our <a href="https://blog.robur.coop/articles/finances.html">article</a> which explains our
funding since the creation of Robur and we would like to point out that,
despite our funding, part of our work remains unfunded: in particular with
regard to the maintenance of certain software as well as certain services made
available to our users.</p>
<p>We would therefore be delighted if users of our software and services could
finance our work according to their means. GitHub in particular offers an
easy-to-use platform for funding us (even if, in all transparency, it takes a
certain amount from each transaction).</p>
<section role="doc-endnotes"><ol>
<li id="fn-preferable">
<p>In fact, this method is preferable to us as this means it will go directly to us instead of through GitHub and Stripe who will take a small cut of the donation in fees.</p>
<span><a href="#ref-1-fn-preferable" role="doc-backlink" class="fn-label">↩︎︎</a></span></li></ol></section>
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Robur's blog - GPTar (update)
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<meta name="description" content="libarchive vs hybrid GUID partition table and GNU tar volume header">
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<h1>GPTar (update)</h1>
<ul class="tags-list"><li><a href="/tags.html#tag-OCaml">OCaml</a></li><li><a href="/tags.html#tag-gpt">gpt</a></li><li><a href="/tags.html#tag-tar">tar</a></li><li><a href="/tags.html#tag-mbr">mbr</a></li><li><a href="/tags.html#tag-persistent storage">persistent storage</a></li></ul><p>In a <a href="gptar.html">previous post</a> I describe how I craft a hybrid GUID partition table (GPT) and tar archive by exploiting that there are disjoint areas of a 512 byte <em>block</em> that are important to tar headers and <em>protective</em> master boot records used in GPT respectively.
I recommend reading it first if you haven't already for context.</p>
<p>After writing the above post I read an excellent and fun <em>and totally normal</em> article by Emily on how <a href="https://uni.horse/executable-tarballs.html">she created <strong>executable</strong> tar archives</a>.
Therein I learned a clever hack:
GNU tar has a tar extension for <em>volume headers</em>.
These are essentially labels for your tape archives when you're forced to split an archive across multiple tapes.
They can (seemingly) hold any text as label including shell scripts.
What's more is GNU tar and bsdtar <strong>does not</strong> extract these as files!
This is excellent, because I don't actually want to extract or list the GPT header when using GNU tar or bsdtar.
This prompted me to <a href="https://github.com/reynir/gptar/pull/1">use a different link indicator</a>.</p>
<p>This worked pretty great.
Listing the archive using GNU tar I still get <code>GPTAR</code>, but with verbose listing it's displayed as a <code>--Volume Header--</code>:</p>
<pre><code class="language-shell">$ tar -tvf disk.img
Vr-------- 0/0 16896 1970-01-01 01:00 GPTAR--Volume Header--
-rw-r--r-- 0/0 14 1970-01-01 01:00 test.txt
</code></pre>
<p>And more importantly the <code>GPTAR</code> entry is ignored when extracting:</p>
<pre><code class="language-shell">$ mkdir tmp
$ cd tmp/
$ tar -xf ../disk.img
$ ls
test.txt
</code></pre>
<h2 id="bsd-tar--libarchive"><a class="anchor" aria-hidden="true" href="#bsd-tar--libarchive"></a>BSD tar / libarchive</h2>
<p>Unfortunately, this broke bsdtar!</p>
<pre><code class="language-shell">$ bsdtar -tf disk.img
bsdtar: Damaged tar archive
bsdtar: Error exit delayed from previous errors.
</code></pre>
<p>This is annoying because we run FreeBSD on the host for <a href="https://opam.robur.coop">opam.robur.coop</a>, our instance of <a href="https://git.robur.coop/robur/opam-mirror/">opam-mirror</a>.
This Autumn we updated <a href="https://git.robur.coop/robur/opam-mirror/">opam-mirror</a> to use the hybrid GPT+tar GPTar <em>tartition table</em><sup><a href="#fn-tartition" id="ref-1-fn-tartition" role="doc-noteref" class="fn-label">[1]</a></sup> instead of hard coded or boot parameter specified disk offsets for the different partitions - which was extremely brittle!
So we were no longer able to inspect the contents of the tar partition from the host!
Unacceptable!
So I started to dig into libarchive where bsdtar comes from.
To my surprise, after building bsdtar from the git clone of the source code it ran perfectly fine!</p>
<pre><code class="language-shell">$ ./bsdtar -tf ../gptar/disk.img
test.txt
</code></pre>
<p>I eventually figure out <a href="https://github.com/libarchive/libarchive/pull/2127">this change</a> fixed it for me.
I got in touch with Emily to let her know that bsdtar recently fixed this (ab)use of GNU volume headers.
Her reply was basically &quot;as of when I wrote the article, I was pretty sure bsdtar ignored it.&quot;
And indeed it did.
Examining the diff further revealed that it ignored the GNU volume header - just not &quot;correctly&quot; when the GNU volume header was abused to carry file content as I did:</p>
<pre><code class="language-diff"> /*
* Interpret 'V' GNU tar volume header.
*/
static int
header_volume(struct archive_read *a, struct tar *tar,
struct archive_entry *entry, const void *h, size_t *unconsumed)
{
- (void)h;
+ const struct archive_entry_header_ustar *header;
+ int64_t size, to_consume;
+
+ (void)a; /* UNUSED */
+ (void)tar; /* UNUSED */
+ (void)entry; /* UNUSED */
- /* Just skip this and read the next header. */
- return (tar_read_header(a, tar, entry, unconsumed));
+ header = (const struct archive_entry_header_ustar *)h;
+ size = tar_atol(header-&gt;size, sizeof(header-&gt;size));
+ to_consume = ((size + 511) &amp; ~511);
+ *unconsumed += to_consume;
+ return (ARCHIVE_OK);
}
</code></pre>
<p>So thanks to the above change we can expect a release of libarchive supporting further flavors of abuse of GNU volume headers!
🥳</p>
<section role="doc-endnotes"><ol>
<li id="fn-tartition">
<p>Emily came up with the much better term &quot;tartition table&quot; than what I had come up with - &quot;GPTar&quot;.</p>
<span><a href="#ref-1-fn-tartition" role="doc-backlink" class="fn-label">↩︎︎</a></span></li></ol></section>
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Robur's blog - Testing MirageVPN against OpenVPN™
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<h1>Testing MirageVPN against OpenVPN™</h1>
<ul class="tags-list"><li><a href="/tags.html#tag-OCaml">OCaml</a></li><li><a href="/tags.html#tag-MirageOS">MirageOS</a></li><li><a href="/tags.html#tag-cryptography">cryptography</a></li><li><a href="/tags.html#tag-security">security</a></li><li><a href="/tags.html#tag-testing">testing</a></li><li><a href="/tags.html#tag-vpn">vpn</a></li></ul><p>As our last milestone for the <a href="https://www.assure.ngi.eu/">EU NGI Assure</a> 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.</p>
<p>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.</p>
<p>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.</p>
<p>We added a flag to our client and our <a href="miragevpn-server">recently finished server</a> applications, <code>--test</code>, 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.</p>
<p>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 <a href="https://github.com/reynir/openvpn/tree/badmac-test">modifications to OpenVPN</a> and <a href="https://github.com/robur-coop/miragevpn/tree/badmac-test">modifications to MirageVPN</a>.</p>
<p>The end-to-end testing is in addition to our unit tests and fuzz testing; and to our <a href="miragevpn-performance.html">benchmarking</a> binary.</p>
<p>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.</p>
<p>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 <code>--gremlin</code> mode that injects faults, and would be interesting to investigate.</p>
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