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206 lines
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Robur's blog - Postes, télégraphes et téléphones, next steps
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<meta name="description" content="An update of our email stack">
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<h1>blog.robur.coop</h1>
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<blockquote>
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The <strong>Robur</strong> cooperative blog.
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</blockquote>
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<main><a href="/index.html">Back to index</a>
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<article>
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<h1>Postes, télégraphes et téléphones, next steps</h1>
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<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
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finances</a>, we've just received
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<a href="https://nlnet.nl/project/PTT">funding for our email project</a>. This project
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started when I was doing my internship in Cambridge and it's great to see that
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it's been able to evolve over time and remain functional. This article will
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introduce you to the latest changes to <a href="https://github.com/mirage/ptt">our PTT
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project</a> and how far we've got towards providing
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an OCaml mailing list service.</p>
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<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>
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<p>One issue that came up quickly in our latest experiments with our SMTP stack was
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the database of users with an email address. Since we had decided to ‘break
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down’ the various stages of an email submission to offer simple unikernels, we
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ended up having to deploy 4 unikernels to have a service that worked.</p>
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<ul>
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<li>a unikernel for authentication</li>
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<li>a unikernel DKIM-signing the incoming email</li>
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<li>one unikernel as primary DNS server</li>
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<li>one unikernel sending the signed email to its real destination</li>
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</ul>
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<p>And we're only talking here about the submission of an email, the reception
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concerns another ‘pipe’.</p>
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<p>The problem with such an architecture is that some unikernels need to have the
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same data: the users. In this case, the first unikernel needs to know the user's
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password in order to verify authentication. The final unikernel needs to know
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the real destinations of the users.</p>
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<p>Let's take the example of two users: foo@robur.coop and bar@robur.coop. The
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first points to hannes@foo.org and the second to reynir@example.com.</p>
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<p>If Hannes wants to send a message to bar@robur.coop under the identity of
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foo@robur.coop, he will need to authenticate himself to our first unikernel.
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This first unikernel must therefore:</p>
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<ol>
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<li>check that the user <code>foo</code> exists</li>
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<li>the hashed password used by Hannes is the same as the one in the database</li>
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</ol>
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<p>Next, the email will be signed by our second unikernel. It will then forward the
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email to the last unikernel, which will do the actual translation of the
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recipients and DNS resolution. In other words:</p>
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<ol>
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<li>it will see that one (the only) recipient is bar@robur.coop</li>
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<li>check that bar@robur.coop exists and obtain its real address</li>
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<li>it will obtain reynir@example.com and perform DNS resolution on
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<code>example.com</code> to find out the email server for this domain</li>
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<li>finally send the email signed by foo@robur.coop to reynir@example.com!</li>
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</ol>
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<p>So the first and last unikernels need to have the same information about our
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users. One for the passwords, the second for the real email addresses.</p>
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<p>But as you know, we're talking about unikernels that exist independently of each
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other. What's more, they can't share files and the possibility of them sharing
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block-devices remains an open question (and a complex one where parallel access
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may be involved). In short, the only way to ‘synchronise’ these unikernels in
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relation to common data is with a Git repository.</p>
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<p><a href="https://github.com/robur-coop/git-kv">Git</a> has the advantage of being widely used for our unikernels
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(<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
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<a href="https://github.com/dinosaure/contruno">contruno</a>). The advantage is that you can track changes, modify
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files and notify the unikernel to update itself (using nsupdate, a simple ping
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or an http request to the unikernel).</p>
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<p>The problem is that this requires certain skills. Even if it's ‘simple’ to set
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up a Git server and then deploy our unikernels, we can restructure our
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architecture and simplify the deployment of an SMTP stack!</p>
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<h2 id="elit-and-oneffs"><a class="anchor" aria-hidden="true" href="#elit-and-oneffs"></a>Elit and OneFFS</h2>
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<p>We have therefore decided to merge the email exchange service and email
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submission into a unikernel so that this is the only user information requester.</p>
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<p>So we decided to use <a href="https://github.com/robur-coop/oneffs">OneFFS</a> as the file system for our database,
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which will be a plain JSON file. This is perhaps one of the advantages of
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MirageOS, which is that you can decide exactly what you need to implement
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specific objectives.</p>
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<p>In this case, those with experience of Postfix, LDAP or MariaDB could confirm
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that configuring an email service should be ‘simpler’ than implementing a
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multitude of pipes between different applications and authentication methods.</p>
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<p>The JSON file is therefore very simple and so is the creation of an OneFFS
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image:</p>
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<pre><code class="language-sh">$ cat >database.json<<EOF
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> [ { "name": "din"
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> , "password": "xxxxxx"
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> , "mailboxes": [ "romain.calascibetta@gmail.com" ] } ]
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> EOF
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$ opam install oneffs
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$ oneffs create -i database.json -o database.img
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</code></pre>
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<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
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our Elit unikernel with this block-device.</p>
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<pre><code class="language-sh">$ albatross-client create-block --data=database.img database 1024
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$ albatross-client create --net=service:br0 --block=database:database \
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elit elit.hvt \
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--arg=...
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</code></pre>
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<p>At this stage, and if we add our unikernel signing incoming emails, we have more
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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
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<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>
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<h2 id="multiplex-receiving--sending-emails"><a class="anchor" aria-hidden="true" href="#multiplex-receiving--sending-emails"></a>Multiplex receiving & sending emails</h2>
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<p>The PTT project is a toolkit for implementing SMTP servers. It gives developers
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the choice of implementing their logic as they see fit:</p>
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<ul>
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<li>sign an email</li>
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<li>resolve destinations according to a database</li>
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<li>check SPF information</li>
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<li>annotate the email as spam or not</li>
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<li>etc.</li>
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</ul>
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<p>Previously, PTT was split into 2 parts:</p>
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<ol>
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<li>management of incoming clients/emails</li>
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<li>the logic to be applied to incoming emails and their delivery</li>
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</ol>
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<p>The second point was becoming increasingly complex, however, and errors in
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sending emails are legion (DMARC non-alignment, the email is too big for the
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destination, the destination doesn't exist, etc.). All the more so since, up to
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now, PTT could only report these errors via the logs...</p>
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<p>Hannes immediately mentioned the possibility of separating the logic of the
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unikernel from the delivery. This will allow us to deal with temporary failures
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(greylisting) as well. So a fundamental change was made:</p>
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<ul>
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<li>improve the <a href="https://github.com/mirage/colombe">sendmail</a> and <code>sendmail-lwt</code> packages (as well as proposing
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<code>sendmail-miou</code>!) when sending or submitting an email</li>
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<li>improve PTT so that there are now 3 distinct jobs: receiving, what to do with
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incoming emails and sending emails</li>
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</ul>
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<p><img src="../images/smtp.jpg" alt="SMTP" ></p>
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<p>This finally allows us to describe a clearer error management policy that is
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independent of what we want to do with incoming emails. At this stage, we can
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look for the <code>Return-Path</code> in emails that we haven't managed to send and notify
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the senders!</p>
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<p>All this is still in the experimental stage and practical cases are needed to
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observe how we should handle errors and how others do.</p>
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<h2 id="insights--next-goals"><a class="anchor" aria-hidden="true" href="#insights--next-goals"></a>Insights & Next goals</h2>
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<p>We're already starting to have a bit of fun with email and we can start sending
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and receiving emails right away.</p>
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<p>We're also already seeing hacking attempts on our unikernel:</p>
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<ul>
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<li>people trying to authenticate themselves without <code>STARTTLS</code> (or with it,
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depending on how clever the bot is)</li>
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<li>people trying to send emails as non-existent users in our database</li>
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<li>we're also seeing content that has nothing to do with SMTP</li>
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</ul>
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<p>Above all, this shows that, very early on, bots try to usurp the identity linked
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to your server (in our case, osau.re) in order to send spam, authenticate
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themselves or simply send ‘stuff’ and observe what happens. In this case, for
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all the cases mentioned, Elit (and PTT) reacts well: in other words, it simply
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cuts off the connection.</p>
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<p>We were also able to observe how services such as gmail work. In addition, for
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the purposes of a mailing list, email forwarding distorts DMARC verification
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(specifically, SPF verification). The case is very simple:</p>
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<p>foo@gmail.com tries to reply to robur@osau.re. robur@osau.re is a mailing list
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to several addresses (one of them is bar@gmail.com). The unikernel will receive
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the email and send it to bar@gmail.com. The problem is the alignment between
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the <code>From</code> field (which corresponds to foo@gmail.com) and our osau.re server.
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From gmail.com's point of view, there is a misalignment between these two
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pieces of information and it therefore refuses to receive the email.</p>
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<p>This is where our next objectives come in:</p>
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<ul>
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<li>finish our DMARC implementation</li>
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<li>implement ARC so that our server notifies us that, on our side, the DMARC
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check went well and that gmail.com should trust us on this.</li>
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</ul>
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<p>There is another way of solving the problem, perhaps a little more problematic,
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modify the incoming email and in particular the <code>From</code> field. Although this
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could be done quite simply with <a href="https://github.com/mirage/mrmime">mrmime</a>, it's better to concentrate on
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DMARC and ARC so that we can send our emails as they are and never alter them
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(especially as this will invalidate previous DKIM signatures!).</p>
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<h2 id="conclusion"><a class="anchor" aria-hidden="true" href="#conclusion"></a>Conclusion</h2>
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<p>It's always satisfying to see your projects working ‘more or less’ correctly.
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This article will surely be the start of a series on the intricacies of email
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and the difficulty of deploying such a service at home.</p>
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<p>We hope that this NLnet-funded work will enable us to replace our current email
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system with unikernels. We're already past the stage where we can, more or less
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(without DMARC checking), send emails to each other, which is a big step!</p>
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<p>So follow our work on our blog and if you like what we're producing (which
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involves a whole bunch of protocols and formats - much more than just SMTP), you
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can make <a href="https://robur.coop/Donate">a donation here</a>!</p>
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