Introduction to email (pt. 1): Email basics

[update: Seems like the maildir patch for the Alpine mail client does not currently work with the newest version… So we need to get an older version of the ports tree – this is not ideal, but fortunately this is only a test VM where we can do bad things]
[update2: More changes… A necessary patch is no longer available from its previous location]

Email – short for electronic mail – is one of the things of modern life that we all are familiar with… Or are we? We know how to use it and probably have a rough idea of what’s going on when we press the send button. But email is actually a surprisingly complicated topic. You probably won’t notice that until you think about setting up your own mail server for the first time. Once you do however, you’re in for quite some reading.

I thought to write this mail mini series in late 2016 when I had to debug an issue with a customer’s mail system and thus figured that I really, really could use a bit more knowledge when it comes to email. I quickly went on with other tasks, but I’m still interested in the topic – and maybe I’ll find some time for it this time. This series of posts is intended as a summary of things that I found noteworthy about email.

Daemons & Terminology

When it comes to email, various daemons are involved. Daemons? Why of course! Email is native to Unix; it had precursors, yes, but it was on Unix that things really took off. That’s why for understanding email it really helps to understand Unix.

The part of the mail system that’s visible to the user is called Mail User Agent (MUA or just UA) in mail terminology. That’s what most people just call the Email client: Programs like Thunderbird, Sylpheed or Outlook. It can be used to compose messages (that’s the correct term – you’re actually sending messages not emails) and hand them over to an email server or to retrieve messages from there.

The server-side of the mail system consists of multiple daemons. First there’s the Mail Transfer Agent (MTA) that’s responsible for accepting incoming messages (e.g. from the MUA), process it and either send it to a second MTA on a foreign server or to save it locally (if the message’s recipient has an account on this server). To store a message locally, the MTA can pass it to a Mail Delivery Agent (MDA).

It’s common that an MTA accepts email that’s destined for another server and hands it over to that server’s MTA. This is called relaying. To know where it needs to send the message to, it looks at the domain part of the recipients address (xyz@example.com). Then it does a lookup for the MX (Mail eXchange) records in the DNS (Domain Name System) for that zone (DNS terminology; think domain for now.

Protocols

To enable the various daemons to interact with each other they need to follow standards in communicating. These standards come in form of so-called RFCs – if you don’t know what that is, do some quick research right now. No matter what, if it has anything to do with the Internet at all, RFCs are what define the standards that every implementation is expected to follow. There are several protocols which allow for various actions:

The MUA needs to speak SMTP, the Simple Mail Transfer Protocol, if it wants to pass a message to an MTA because that’s what this daemon is expecting. The MTA can use make use of MLTP, the Mail Local Transfer Protocol, to talk to an MDA. And if the MUA is expected to retrieve messages from the mailbox it needs to implement the POP (Post Office Protocol) or IMAP (Internet Mail Access Protocol) protocols so it can communicate with a POP or IMAP daemon which will then read messages stored on the server and send them to the MUA.

Other components

And there’s more when it comes to mail. Today’s internet is well-known as a hostile place. However when email was developed, people on the net were few and shared a common passion: Tech. Nobody meant to do the others any harm. This infantile period is far from what we experience today. But the basic principles of email haven’t changed and in fact cannot be changed easily. So the problem arises how to retroactively secure something that was designed in a carefree way and therefore proved to be inherently insecure?

When you think email today, you automatically have to think encryption, too. Otherwise there might always be someone to eavesdrop on you. When you think email you have to think authorisation. If you don’t protect your mail account somebody may break into it, steal your secrets or abuse it. You have to think spam. How do you avoid being flooded with all those useless messages that want you to buy blue pills or the like? And worse: How do you prevent spammers to use your mail server so you don’t get blacklisted? You have to think viruses, phishing, trojans, etc. Have to think security holes in your applications, newer protocol versions being established, and so on.

You can already see that mail is a rather complex topic that requires you to have a fair bit of knowledge on other topics like e.g. DNS, security and more. Fortunately all the knowledge that you need is somewhere out there. You just need some determination, a lot of free time and a bit of luck to find the relevant pieces. I cannot help you with the former two but I’ll try to provide a source that could help you learn some important things about mail without having to search on for a tutorial (even though of course I cannot cover everything).

So much for a tiny bit of theory. It’s merely meant to be enough so we can start doing something and cover more of our topic along the way.

Building a test system

So let’s build some kind of test system to play around with, shall we? Sure thing. I suggest using FreeBSD – it is an excellent choice of OS to get into the topic. No, not because I’ve come to like it quite a bit and base a lot of what I post on my blog on that system. For many things Linux would be more or less an equally good choice. When it comes to getting into mail however, it isn’t.

If you already know the whole topic quite well, you can install all the needed programs on Linux without any problems. If however you’re just starting out, FreeBSD makes the first steps so much easier as it already comes with a working mail solution by default! Setting up mail server software is a very complex and complicated thing to do and FreeBSD really makes your life so much easier in this regard by removing this obstacle for you.

In a previous tutorial on backups with Bacula I used VirtualBox together with Vagrant and it proved to be a very convenient solution. So I’m going to use it here as well. If you haven’t used VirtualBox or don’t even know what Vagrant is, I’ve written this post for you which explains things very detailed and with pictures. For creating the base box that we need, you can refer to this other post that explains everything step by step. Just make sure that you read this section before you build your base box! Because here’s the customization we need for this VM:

This post assumes version we’re using FreeBSD 11.1 – any version of 11 should be fine, though. If at the time you are reading this 12.x or even newer is out, you may want to check if fundamental things (like finally putting Sendmail to rest and importing a different MTA) have changed in between these versions. Starting with version 11.0, FreeBSD has a new installation dialog screen that let’s you choose some system hardening options. While this is a great idea and I’ve recommended to disable Sendmail in the post about building the base box for Vagrant, it would ruin the mail functionality that we’re going to use here. So make sure to not disable Sendmail for this installation!

Selecting hardening options

Ports work

Also don’t install Bacula as described in the customization section for the Vagrant base box – we’re not going to use it here. Instead do the following (if you’re not using a c-shell variant, leave out the “env” for the second command):

# svnlite co svn://svn.freebsd.org/ports/tags/RELEASE_11_0_0 /usr/ports
# env ASSUME_ALWAYS_YES=1 pkg bootstrap
# sed -i.bak -e 's|pkg$|pkg register|' /usr/share/mk/bsd.own.mk 

This downloads an old version of the ports tree which holds a version of Alpine that we’re going to use. It also bootstraps the package manager and makes a change so that it will work with the old tree. Now we need to prepare Alpine; unfortunately one required file is no longer available from the original site. I chose to mirror it since it took me some time to find the right one…

# mkdir -p /usr/ports/distfiles/alpine-2.20
# cd /usr/ports/distfiles/alpine-2.20
# fetch http://elderlinux.org/files/maildir.patch.gz
# cd /usr/ports/mail/alpine
# make makesum

Now we should be good to configure the port:

# make config-recursive

Be sure to check MAILDIR here as we’re going to need it later. Unfortunately this option is not built into Alpine would we install it as a package. So we have to resort to ports. While we’re at it, we can disable IPv6 (which we certainly don’t need), mouse support, NLS and check NOSPELL since we do not require spelling correction in this tutorial either. Configure pico-alpine accordingly. For all the other packages you can generally turn off NLS and DOCS. Then fetch the source for all packages recursively:

# make fetch-recursive

When it’s done change into OpenSMTPD’s port directory and configure it:

# cd /usr/ports/mail/opensmtpd
make config-recursive

Here you want to check the MAILERCONF option. The rest is fine. You don’t need to build the EXAMPLES for m4 and again can generally deselect NLS. Then fetch the sources:

# make fetch-recursive

Then configure dovecot2 (you can keep the default options this time) and fetch the distfiles for it:

# cd /usr/ports/mail/dovecot2
# make config fetch

Then follow the rest of the base box build process.

Preparation

You have your base box built and imported? Good. Create your Vagrant directories if you haven’t and a Vagrantfile in there:

% mkdir -p ~/vagrant/mail-vm
% cd ~/vagrant/mail-vm
% vi Vagrantfile

Put the following lines into it (assuming that your box has the name fbsd11.1-template):

Vagrant.configure("2") do |config|
config.vm.box = "fbsd11.1-template"
config.vm.network "private_network", ip: "192.168.0.10"
#config.ssh.username = "root"
config.vm.synced_folder ".", "/vagrant", disabled: true
end

Now fire up the VM, ssh into it and switch to root. Then edit rc.conf:

% vagrant up
% vagrant ssh
% sudo su -
# vi /etc/rc.conf

Change the line that defines the hostname to:

hostname="mail-vm.local"

Save the file and exit the editor. Let’s do a bad thing next: Let’s configure the VM so that we can SSH into it directly as root. If this just sent a shiver down your spine then you’re having the right feeling. You’re not supposed to do this like… ever! But now and then you feel like doing forbidden things, right? And now’s the perfect opportunity since this is just a private, non internet facing VM. Just remember to never do this on a production machine! Ok, ready? First we need to copy Vagrant’s public key and allow it for root:

# mkdir .ssh
# touch .ssh/authorized_keys
# chmod 0700 .ssh
# chmod 0600 .ssh/authorized_keys
# cp /home/vagrant/.ssh/authorized_keys /root/.ssh/authorized_keys

Now edit the SSH daemon’s config:

# vi /etc/ssh/sshd_config

Look for the line #PermitRootLogin no, remove the comment sign and change it to yes. Save and exit the editor. Then power down the VM:

# shutdown -p now

When you’re back at your host machine, edit the Vagrantfile:

% vi Vagrantfile

Add

config.ssh.username = "root"

to the configuration. Save and exit.

Test the change you just made by starting the VM and ssh’ing into it:

% vagrant up
% vagrant ssh

If everything is correct, you should have entered the VM as root now. Power down the VM again:

# shutdown -p now

Intermission

We’ve now prepared our test environment for trying out all things mail. Use vagrant to create a snapshot now to save your progress so that we can pick up right there next time. But before we do so, we need to make sure that the VM is already powered down:

% vagrant status

If it says running (virtualbox) then wait a moment and issue the same command again. It’s important that the status is poweroff (virtualbox). If you snapshot the VM while it’s powering down, your machine will always power down if you restore the previous state! As soon as it’s off, snapshot it:

% vagrant snapshot save mail1

You now have an idea of what the whole topic email is all about. And you have a test system to try out things and get familiar with mail from the very fundamentals to – perhaps – a fully working mail server. I hope that this was interesting for you and that you’ll follow the next part, too, where we’ll explore sending mail from the console and discuss using a text mode MUA.

Advertisements

4 thoughts on “Introduction to email (pt. 1): Email basics

  1. Pingback: In Other BSDs for 2018/03/31 – DragonFly BSD Digest

  2. Pingback: In Other BSDs for 2018/03/31 – FreshBSD.com

  3. Pingback: Email basics (1) | 0ddn1x: tricks with *nix

  4. Pingback: Email intro (1) | 0ddn1x: tricks with *nix

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

w

Connecting to %s