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diff --git a/source/blog/2020/email-server-extras/index.md b/source/blog/2020/email-server-extras/index.md new file mode 100644 index 0000000..b19819b --- /dev/null +++ b/source/blog/2020/email-server-extras/index.md @@ -0,0 +1,431 @@ +--- +title: "Setting up an email server in 2020 with OpenSMTPD and Dovecot: extras" +date: 2020-04-28 +layout: "blog" +toc: true +--- + +This sequel to my post +"[Setting up an email server in 2020 with OpenSMTPD and Dovecot](/blog/2020/email-server/)" +gives extra tips and tricks to extend your email setup. +See also the sequel's sequel, +"[Revisiting my email server in 2022](/blog/2022/email-server-revisited/)". + +Last updated on 2022-09-12. + + +## General + +### Multiple domains + +You can generalize your setup to handle multiple domains +with very little effort. In the following, +I'll assume that your two domains are called `foo.com` and `bar.com`. + + +#### DNS records + +There should be MX, SPF, DKIM and DMARC records for both domains, +as explained in the previous guide. Fortunately, these records +can have identical contents for both domains! + +However, it remains essential that the mail server's mailname +and reverse DNS domain name match up exactly, +so you should create MX records with that in mind. +Therefore, if the email server for both domains has `mx1.foo.com` +as reverse DNS name, the MX records should look like this: +```sh +foo.com. MX 42 mx1.foo.com. +bar.com. MX 42 mx1.foo.com. +``` +This is perfectly valid: the only thing that matters is that +what your SMTP server calls itself agrees with what reverse DNS +says that the server is actually called. + + +#### Dovecot + +To make Dovecot aware of multiple domains, +you only need to update the `/etc/dovecot/users` file +to add accounts for both domains. +However, in the original guide, I said to only write `user` +in the file, without the `@foo.com`, for an address `user@foo.com`. +Unsurprisingly, that isn't an option for multiple domains, +so you must put the full address in `/etc/dovecot/users`. + +Then update `/etc/dovecot/dovecot.conf` to reflect that, +by changing `%n` to `%u` in `username_format`: +```sh +userdb { + driver = passwd-file + args = username_format=%u /etc/dovecot/users + override_fields = uid=vmail gid=vmail home=/home/vmail/%d/%n +} +``` +Also note the change in the `home` setting: +the inbox of a user `user@foo.com` will now be stored +in `/home/vmail/foo.com/user`. +That's all you need to change. + + +#### OpenSMTPD + +To inform OpenSMTPD of all the domains, +create a new file `/etc/smtpd/domains`, +and in there put all desired names on their own line: +```sh +foo.com +bar.com +``` +And as I mentioned when discussing the DNS records, +you should check that `/etc/smtpd/mailname` agrees +with your server's reverse DNS. + +Then, in the main configuration file, tell OpenSMTPD to +use the new domains file when deciding whether to accept an message, +by declaring a new table and changing the `match` line for inbound mail: +```sh +table domains "/etc/smtpd/domains" +# ... +match from any for domain <domains> action "RECV" +``` + +#### Rspamd + +The last thing to do is to inform Rspamd of the multiple domains. +It's really easy: simply add multiple domain blocks: +```c +domain { + foo.com { + path = "/path/to/dkim/private.key"; + selector = "hello"; + } +} +domain { + bar.com { + path = "/path/to/dkim/private.key"; + selector = "world"; + } +} +``` + +### Advanced security + +SPF, DKIM and DMARC are email's traditional DNS-based security systems, +but in 2018 the IETF released [RFC 8460](https://tools.ietf.org/html/rfc8460) and [RFC 8461](https://www.rfc-editor.org/rfc/rfc8461.txt), +which respectively define TLSRPT and MTA-STS, +two fancy new systems focused on TLS-encrypted email transport. + +These security mechanisms are pretty new, +so you won't get a huge benefit from enabling them, +but big email providers' draconian spam filters might like it. + + +#### TLSRPT + +TLS reporting, or TLSRPT for short, is very simple: +all it does is provide a contact email address in case +somebody has trouble with the TLS configuration of your SMTP server. + +To enable it for your custom email domain `example.com`, +simply create a DNS TXT record for the `_smtp._tls` subdomain: +```sh +_smtp._tls.example.com. TXT "v=TLSRPTv1; rua=mailto:<contact>" +``` +Where `<contact>` is an email address of your choosing. +That's all! + + +#### MTA-STS + +MTA Strict Transport Security (MTA-STS) tells other servers +that you take TLS encryption of messages very seriously, +so they should avoid sending you unencrypted email, +and should only accept certain certificates from your side. + +Compared to the previously discussed DNS-based security extensions, +MTA-STS is a bit more work to set up, +because you'll also need an HTTP web server. + +The DNS part is still pretty simple: +create yet another DNS TXT record, +this time for the subdomain `_mta-sts`: +```sh +_mta-sts.example.com. TXT "v=STSv1; id=<id>" +``` +The `<id>` should identify the version of your policy, +so other servers can quickly see if something changed. +I recommend using today's date. + +For the next part, I'll assume that you already have +a web server running on a server with the IP address `1.2.3.4`. +I use [nginx](https://nginx.org/) for this, running +on the same server as OpenSMTPD and Dovecot, +but you don't have to do the same. + +Create an A record which binds your server +to the subdomain `mta-sts` (without underscore): +```sh +mta-sts.example.com. A 1.2.3.4 +``` +Set your web server to serve the file +`https://mta-sts.example.com/.well-known/mta-sts.txt` +(we'll discuss that file in a moment). +Note that this policy file **must** be served over HTTPS, +so you need a valid TLS certificate for that domain. + +The contents of the `mta-sts.txt` policy file are as follows, +where `mx1.example.com` and `mx2.example.com` are the hosts +mentioned in `example.com`'s DNS MX records: +```sh +version: STSv1 +mode: enforce +mx: mx1.example.com +mx: mx2.example.com +max_age: <age> +``` +All MX servers must be mentioned this way. +If you're feeling cautious, you may want to set +`mode` to `testing` in the beginning. +This policy is valid for `<age>` seconds, +which is recommended to be several weeks, +but to start with, I suggest using 86400 seconds (one day). +Finally, ensure that this file has CRLF Windows-style line endings. + +To correctly pass an MTA-STS test, the TLS certificate +presented by e.g. `mx1.example.com` should be valid for `mx1.exaple.com`. +To achieve this without needing to manage too many certificates, +you can specify multiple domains when requesting a certificate, +or you can use a wildcard domain (`*.example.com`). +Note, however, that MTA-STS testing tools don't like +the latter option, so I recommend the former. + +Once you're done, check your work by using either +[ESMTP](https://esmtp.email/tools/mta-sts/)'s or [Ayke](https://aykevl.nl/apps/mta-sts/)'s +online MTA-STS validation tools, +ignoring any warnings about DNSSEC or DANE. +If all is good, great! + +Even if you did everything correctly, +these tools will warn you that you're not using DNSSEC/DANE. +It might then be tempting to set that up for even more security, +but I recommend against that for private servers: take a look at [this](https://dane.sys4.de/common_mistakes). + + + +## OpenSMTPD + +### Client certificates (in addition to passwords) + +You can configure OpenSMTPD to request a client certificate +for sending emails, as a second factor for authentication. + +UPDATE: When I wrote this two years ago, it worked, +but now it doesn't anymore, and I can't figure out why. +It seems OpenSMTPD always rejects the client certificates for being self-signed, +even if they can manually be verified for our CA using the `openssl` tool. +I'm leaving this tutorial here for anyone who's interested, +but it's unlikely I'll fix it anytime soon. + + +#### Certificates + +We need to start with some cryptography to create and verify certificates. +I recommend that you do all of this on your trusted *client* device, +and only copy the necessary files to the server later. + +DISCLAIMER: +All the keys and certificates that we'll generate in this section are +for **private use** only, to handle a small number of trusted clients. +I'm not a cryptography expert, so you should **not** listen to me +for large-scale systems that may involve untrusted devices. + +The first step is to set up a private Certificate Authority (CA), +which issues the client certificates and can be used to verify them. +Start by generating an RSA private key, +which you should store in a safe place and not share with anyone: +```sh +$ openssl genrsa -out mailca.key 2048 +``` +Extract a public certificate from this key as follows. +Because we're lazy, we give it a lifetime of 36500 days: +```sh +$ openssl req -new -x509 -days 36500 -key mailca.key -out mailca.crt +``` +When running this command, OpenSSL will ask you some questions +about who this certificate is intended for. +Since this is for personal use, your answers don't matter, +so just use the defaults. +Some fields (I think only *Country Name* and *Organization Name*) +cannot be empty, but the others can. + +Moving on to the client, once again generate an RSA private key: +```sh +$ openssl genrsa -out mailclient.key 2048 +``` +From this private key, create a Certificate Signing Request (CSR) +as follows, where you'll be asked the same questions as before: +```sh +$ openssl req -new -key mailclient.key -out mailclient.csr +``` +By feeding this CSR to the CA, we can create a signed client certificate +that can be verified using the CA's public certificate. +```sh +$ openssl x509 -req -in mailclient.csr -out mailclient.crt \ + -days 36499 -CA mailca.crt -CAkey mailca.key +``` +If you want to multiple client certificates, +just repeat the last few steps for each one. + + +#### Server + +OpenSMTPD needs to verify the validity of client certificates +using the CA's public certificate, so you should copy that +to somewhere on the server, e.g. `/etc/smtpd/mailca.crt`, +and declare it to OpenSMTPD by adding this near +the top of `/etc/smtpd/smtpd.conf`: +```sh +ca "mailca" cert "/etc/smtpd/mailca.crt" +``` +Then replace the entire configuration for outbound mail as follows. +Note that this removes SMTPS support, leaving only STARTTLS: +```sh +# Outbound +listen on eth0 port 587 tls-require verify pki "example.com" ca "mailca" auth <passwds> filter "rspamd" +action "SEND" relay srs +match from any auth for any action "SEND" +``` +The magic word here is "`verify`", which tells OpenSMTPD +to ask for a client certificate and to verify it using the given CA. + + +#### Client + +Now you won't be able to send emails if your client doesn't +present its certificate to the server! +Unfortunately, not all mail clients support this; personally +I use [Thunderbird](https://www.thunderbird.net/) with success. +I won't include any client-specific configuration here, +but I will say this: + +For some clients (like Thunderbird), you'll have an easier time +importing your client certificate if you encode it in the +[PKCS #12](https://en.wikipedia.org/wiki/PKCS_12) storage format: +```sh +$ openssl pkcs12 -export -in mailclient.crt -inkey mailclient.key \ + -certfile mailca.crt -out mailclient.pfx +``` +OpenSSL will ask you to set a password, which you'll need to +enter again when importing the certificate into the client. + + + +### ~~Client certificates (instead of passwords)~~ + +UPDATE: Don't do this. +As said above, OpenSMTPD's certificate verification is a mystery, +so for all I know, if you follow the instructions in this subsection, +you might find yourself running an *open* SMTP relay! +That would be bad, because anyone on the Internet +could send emails through your server with zero authentication. +In theory, the client certificates act as authentication, +but, again, the verification process is mysterious, +so I'm just not confident enough to say. + +If you really want to, you can use the client certificates +as a substitute for passwords. This is especially useful +if you set up a catchall inbox in Dovecot, +because this will allow you to send emails +from arbitrary addresses from your domain. + +To do this, follow the same procedure as in the previous section, +but with a slightly different OpenSMTPD configuration: +```sh +listen on eth0 port 587 tls-require verify pki "example.com" ca "mailca" filter "rspamd" tag "VALID" +action "SEND" relay srs +match from any tag "VALID" for any action "SEND" +``` +All incoming connections that present a good certificate +will be tagged as being `VALID`, and their mail will be relayed. + +Unfortunately, we're not quite done yet here, +because Rspamd is now very confused... + + +#### Rspamd + +When OpenSMTPD passes a message through Rspamd, it also includes +some metadata, most notably whether the sender has authenticated +successfully with OpenSMTPD... which is now no longer the case +for submissions, because we've removed the `auth` directive! + +Rspamd therefore starts regarding these outgoing emails +as *incoming* emails, because they don't seem +to come from a trusted user. So instead of signing them with DKIM +and handing them back to OpenSMTPD, it will do a full spam scan. +If they get a high spam score (which is likely for short test emails), +*your* spam filter, running on *your* server, +will be flagging *your* messages as spam! + +The solution is to whitelist your domain(s) in Rspamd, +so it won't scan them. To do this, create a new file +`/etc/rspamd/local.d/settings.conf` with these contents, +where `foo.com` and `bar.com` are the domains to whitelist: +```c +outbound { + priority = high; + from = "@foo.com"; + from = "@bar.com"; + apply { + actions { + add_header = 1000; + } + } +} +``` +Setting `priority` to `high` ensures that Rspamd checks +this rule before doing anything else. +You can add any number of `from` directives; +this rule will be applied if any of them match. +It only sets the threshold for the action `add_header` to `1000`. +That is, if the email doesn't get a spam score of at least 1000 +(the default is 6) Rspamd will not add any spam tags. + +Because Rspamd is still regarding your emails as inbound, +you also need to change the global settings of +the DKIM signer in `/etc/rspamd/local.d/dkim_signing.conf`, +such that they include the following: +```c +sign_inbound = true; +allow_hdrfrom_mismatch = true; +allow_username_mismatch = true; +``` +This tells Rspamd to add DKIM signatures to incoming emails, +which in this case includes yours. +Allowing these mismatches ensures that the messages still get signed, +even if you're sending from an arbitrary address. + + + +## Dovecot + +### Catchall inbox + +In Dovecot, you can create a catch-all inbox that will accept all +emails sent to your domain that don't match anyone in `/etc/dovecot/users`. +Just add another `userdb` block *after* the first: +```sh +userdb { + driver = static + args = uid=vmail gid=vmail home=/var/vmail/catchall allow_all_users=yes +} +``` +The `static` driver means there is no table file: +all configuration is directly within this `userdb` block. +If we don't specify `allow_all_users=yes`, then Dovecot +will check whether users exist using the `passdb` table, +and will conclude that the recipient is invalid. + + + diff --git a/source/blog/2020/email-server/index.md b/source/blog/2020/email-server/index.md new file mode 100644 index 0000000..962e6aa --- /dev/null +++ b/source/blog/2020/email-server/index.md @@ -0,0 +1,729 @@ +--- +title: "Setting up an email server in 2020 with OpenSMTPD and Dovecot" +date: 2020-04-27 +layout: "blog" +toc: true +--- + +So, you want to set up your own email server? In that case, welcome. + +There are many reasons to run a custom email server, +ranging from privacy concerns about providers like Google, +to just wanting to do it for fun and/or learning. +Since you're here, I assume you've already found a reason. + +Beware: this is a messy topic, and the available documentation +is even messier, so it could take a while before you get it to work properly. +I've compiled this guide according to my experiences +in an attempt to make this dark art more accessible, +but your mileage may vary considerably. I hope you find it useful. + +This guide is aimed at people who are comfortable with +the Linux/*BSD command line. + +When you're done (if you get that far), take a look at the sequels +"[Setting up an email server in 2020 with OpenSMTPD and Dovecot: extras](/blog/2020/email-server-extras/)" +and "[Revisiting my email server in 2022](/blog/2022/email-server-revisited/)" +for ideas on how to extend your setup. + +Last updated on 2022-09-12. + + + +## Preparation + +Setting up email is relatively complex compared to e.g. a static website, +because you need to configure not one, but *two* server programs, +*and* you need to shoehorn modern security features into email's Stone-Age design. +I'll start by explaining the general structure of a mail server setup. + + + +### How email works + +The programs involved in the exchange of emails are called [agents](https://en.wikipedia.org/wiki/Email_agent_(infrastructure)). +Officially, there are 5 different types of agent: MUA, MSA, MTA, MDA and MRA. +But fortunately, it's reasonable to treat the MRA and MSA +as being part of the MUA and MTA, respectively. + +The *Mail User Agent* (MUA) is simply the client on your device at home +that you use to send and receive emails, and this guide assumes +you already have a favourite program for this, e.g. [Thunderbird](https://www.thunderbird.net/en-US/). +Nowadays it's fashionable to use a web interface for emails, +but that's also beyond the scope of this guide. + +The *Mail Delivery Agent* (MDA) is a program that watches over +the server's copy of your mailbox: it manages your inbox, +remembers which messages you have or haven't read, +keeps a copy of your drafts, etc. +When you open your mailbox, your MUA will connect to +your server's MDA using the [IMAP](https://en.wikipedia.org/wiki/Internet_Message_Access_Protocol) protocol +(or [POP3](https://en.wikipedia.org/wiki/Post_Office_Protocol), but that one's [obsolete](https://pop2imap.com/)). + +The *Mail Transfer Agent* (MTA) is responsible for +making messages arrive at the right destination. +When you send an email, your MUA will pass it on to your server's MTA, +which will in turn pass it on to the recipient's mail server. +Likewise, when someone sends *you* an email from another server, +the MTA will receive it and hand it over to the MDA so you can read it later. +In both cases the MTA speaks the [SMTP](https://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol) protocol. + +In this guide our MDA will be [Dovecot](https://dovecot.org/), +which is a very popular choice for that role. +As for the MTA, there exist several options, +the most popular being [Postfix](http://www.postfix.org/) and [Exim](https://exim.org/). +However, this guide uses the newer, lesser-known [OpenSMTPD](https://opensmtpd.org/), +which in my experience is *much* easier to set up: +Postfix and Exim have complex configurations and +are geared towards large-scale email providers, +whereas OpenSMTPD is more beginner-friendly. + + + +### Security + +The base email system is horribly insecure on its own, +so we still need to duct-tape on some security features. +In this context, "security" has two meanings: +spam protecion and privacy protection (encryption). + +Spam protection also means two things here: +defending yourself against spammers, and +preventing that *your* emails get flagged as spam. +The former is optional, but the latter is not: +big providers such as Google and Microsoft +use infamously strict spam filters, +and if they decide that your server is a spammer, +there's almost nothing you can do about it. +Spam protection techniques will be discussed +in more detail over the course of this guide. + +Privacy protection is important in the 21st century: +you don't want a random router in the Internet to read all your emails, +which may contain sensitive information such as +private conversations and account password reset links. +You should therefore try to make sure that emails are +transported over an encrypted channel. +To do this, you have two options for encryption: +*mandatory* and *opportunistic* encryption. + +Mandatory encryption is only practical for client-server +communication (not server-server), and is provided by IMAPS and SMTPS, +which wrap the IMAP and SMTP protocols in [TLS](https://en.wikipedia.org/wiki/Transport_Layer_Security), +in the same way that [HTTPS](https://en.wikipedia.org/wiki/HTTPS) does for [HTTP](https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol). + +For server-server communication, the only option is +opportunistic encryption in the form of [STARTTLS](https://en.wikipedia.org/wiki/STARTTLS), +where communication is only encrypted if both parties agree +after a short unencrypted discussion. +That last part is vulnerable to [MitM](https://en.wikipedia.org/wiki/Man-in-the-middle_attack) attacks, +where anyone along the path of the email servers' discussion +can alter the exchange to block the use of encryption, +which sometimes actually [happens](https://www.eff.org/deeplinks/2014/11/starttls-downgrade-attacks) in practice. + +The only way to make sure that STARTTLS is used in that case +is to refuse any exchange unless the servers agree to use encryption. +Unfortunately, that's a risky approach that I can't recommend, +because not all servers support encryption (unbelievable, right?). +For example, I've received airline booking confirmations, +full of personal details, and made with billion-dollar companies, +sent across the Internet without any protection. + +This guide includes intructions to enable encryption, +but assumes that you already have a TLS certificate for that. +If not, find a guide to get one from [Let's Encrypt](https://letsencrypt.org/) (it's free!), +and remember that you'll need to renew it every few months. +Using a self-signed certificate *may* work, but I don't recommend it. + +In the rest of this guide I'll assume that +you have a public full-chain TLS certificate at `/etc/ssl/certs/example.com.pem`, +and a private encryption key at `/etc/ssl/private/example.com.pem`. + + + +### Server + +Obviously, you'll need a server to run the MTA and MDA on. +You can host your own at home, but the more reliable option is +to rent one in a data center ([VPS](https://en.wikipedia.org/wiki/Virtual_private_server)). +This guide was written with a Linux server in mind, +but in theory it should also work on the BSDs +([OpenBSD](https://www.openbsd.org/), [FreeBSD](https://www.freebsd.org/), +[NetBSD](https://www.netbsd.org/), etc.) with minimal adaptation. + +The server must be online 24/7, you must have root SSH access, +it must have a static IP address, and TCP network port 25 must be open. +Especially check that last one: you may need to explicitly ask +your home ISP or the server provider to enable port 25, +because they often close it to prevent spam. +You can usually do this from their web interface. + +You also must have a domain name, which I'll call `example.com`. +This will be necessary for basically everything: +DNS records, TLS certificate, MTA network configuration, etc. +If you don't have one yet, you can choose between many registrars +to rent one from. Personally I use and can recommend [Gandi](https://www.gandi.net/). + +Note that it's a **bad** idea to use a domain like `foo.bar.com`, +where you control the `foo` part but *not* the `bar` part: +in that case, a spammer in control of `qux.bar.com` +could negatively affect *your* reputation +in the eyes of other email providers. + +Lastly, when setting up an email server, you also have the choice +between using to *system* users or *virtual* users. +With system users, if an email arrives for `john@example.com`, +then the MTA and MDA will expect that there exists +a `john` Unix user on the server to deliver it to. +With virtual users, you have much more flexibility, so that's what we'll use. +All email will be managed under a single Unix user/group called `vmail`. +Create it as follows: +```sh +# GNU CoreUtils: +$ groupadd vmail +$ useradd -g vmail vmail +# BusyBox: +$ addgroup vmail +$ adduser -D -G vmail vmail +# *BSD: +$ no clue, but it should be similar +``` + + +## DNS records + +Now we must set up all the necessary DNS records, which +is usually possible from the domain registrar's web interface. +It may take a while for your changes to propagate over the Internet, +so I recommend doing this section now and the rest tomorrow. + +Firstly, you should already have an A and/or AAAA record +to associate your domain `example.com` with the server's IP address. +For email it is **essential** that you also have [reverse DNS](https://en.wikipedia.org/wiki/Reverse_DNS_lookup) +set up correctly. If you're renting your server remotely, +you can often do this from the provider's configuration tool, +otherwise, you should create a PTR-type DNS record, +although that's beyond the scope of this guide. + +Once you're done, I recommend testing your DNS records +using the [MX Lookup](https://mxtoolbox.com/MXLookup.aspx) online tool. + + + +### MX + +To inform the rest of the Internet that your server is an email server, +create an MX (Mail eXchanger) DNS record for your domain. +Note the dot at the end of the domain name: +```sh +example.com. MX 42 example.com. +``` +When a message is sent to an email address ending in `@example.com`, +the sender will query DNS for any MX records for `example.com`. +There it will find a domain name (in this case `example.com` again), +for which it will look up the IP address using an A/AAAA record. +The domain name in the record must **not** have an associated CNAME record; +it must be directly translatable to an IP address. + +You may have multiple MX records, containing different domain names, +each with a preference number (`42` in the example above). +The sender will try MX records with *lower* numbers first, +and if that server is unavailable, it will try a higher number. +If you have multiple mail servers (which is a good idea), +you can thus declare those as follows: +```sh +example.com. MX 13 mx1.example.com. +example.com. MX 42 mx2.example.com. +``` +Here, a server sending an email to your domain `example.com` +will try to send it to the IP address of `mx1.example.com` first, +and if that fails, it will move on to `mx2.example.com`. +If both `mx1` and `mx2` have the same number, then the sender +will randomly choose one, which is useful for load balancing, +although that's probably overkill for a private server. + + + +### SPF + +The [Sender Policy Framework](https://en.wikipedia.org/wiki/Sender_Policy_Framework) (SPF), +is a feature which helps prevent spammers from impersonating +your server in an attempt to get around blacklists. +This security feature is **required** nowadays: +if you don't use it, you'll probably get flagged as spam. + +SPF works by specifying which IP addresses are authorized +to send emails from your domain name. +You must publish this information in a +TXT-type DNS record (**not** SPF-type, which also exists!) with the following contents: +```sh +example.com. TXT "v=spf1 mx -all" +``` +Everything after the version `v=spf1` is a list of *verification mechanisms* +for a spam filter to try out in the given order. +The `-all` at the end says to reject your email +if all of the previous mechanisms fail. +See the [SPF spec](https://tools.ietf.org/html/rfc7208) for details. + +I recommend only using the `mx` mechanism, which tells the verifier +to look at the A/AAAA addresses of the domains in your MX records. +This allows you to add, remove, or change your servers +without needing to update this record. + + + +### DKIM + +Then we have [DomainKeys Identified Mail](https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail) (DKIM), +which is a more comprehensive form of anti-impersonation, +and, like SPF, is practically **mandatory** in the modern era. + +It adds a cryptographic signature to all emails from your server, +which the receiver's spam filter will verify using the email's contents, +and a public key that you need to publish in a DNS record. +Again, you should implement *both* SPF and DKIM, despite their overlap. + +To set up DKIM, create an RSA keypair, using the `openssl` utility: +```sh +$ openssl genrsa -out /path/to/dkim/private.key 2048 +$ openssl rsa -in /path/to/dkim/private.key -out /path/to/dkim/public.key +``` +The minimum size is 1024 bits, but I recommend 2048 bits. +Bigger is better, but because DNS is involved you can't stretch it +to 4096 bits without causing discomfort to [some](https://serverfault.com/questions/747185/dkim-can-i-use-a-rsa-key-larger-than-2048bit-i-e-4096) servers. +And I think it goes without saying that you should keep the private key private. + +Importantly, the DKIM DNS record **cannot** be +attached directly to your domain `example.com`; +instead, it should belong to a subdomain of the form +`<selector>._domainkey.example.com`, +where `<selector>` is an alphanumeric string you can choose (e.g. today's date), +just remember your choice for later when configuring the DKIM signer. +And if you change your key, keep the old record around +for a while so old emails can still be verified. + +Your DKIM policy must be published in a TXT record +as follows, where `<pubkey>` is the public RSA key `MI...AB` +stored in `/path/to/dkim/public.key`, with the newlines removed: +```sh +<selector>._domainkey.example.com. TXT "v=DKIM1; t=s; h=sha256; p=<pubkey>" +``` +Here, `v=DKIM1` is the version and must be the first tag. +The flag `t=s` enables strict mode, as recommended by the [DKIM spec](https://tools.ietf.org/html/rfc6376), +meaning that emails sent from subdomains immediately fail verification. +The optional tag `h=sha256` blocks the use of the old SHA1 algorithm. + + + +### DMARC + +Lastly, we have [Domain-based Message Authentication, Reporting and Conformance](https://en.wikipedia.org/wiki/DMARC) (DMARC), +which is *technically* optional, but *highly* recommended, +because it will make you look more legitimate in the eyes of Google and Microsoft. +It can modify the behaviour of SPF and DKIM, +and also provides advice about what a receiver should do +if one of your emails fails verification. + +To enable it, create yet another TXT record, which, +similarly to DKIM, **must** belong to the subdomain `_dmarc.example.com`, +and give it the following contents, +where `<admin>` is an email address of your choosing, +which may or may not belong to your domain: +```sh +_dmarc.example.com. TXT "v=DMARC1; p=reject; sp=reject; pct=100; aspf=s; adkim=s; fo=1; ruf=mailto:<admin>" +``` +The version tag `v=DMARC1` must come first, +followed by `p=` and `sp=`, which control what to do to unverified messages +coming from the main domain and subdomains, respectively. +Here, `reject` means that delivery should be refused, +`none` asks to let it through anyway, and `quarantine` tells +the filter to take a closer look or to put it in a spam folder. +The percentage `pct=100` says how many of your emails to apply the policy to. +Next, `aspf=s` and `adkim=s` enable strict mode for SPF and DKIM, +which blocks subdomains from passing. +Finally, `fo=1` asks for a forensic report if verification fails, +and `ruf=` gives an address to send it to. +If in doubt, see the [DMARC spec](https://tools.ietf.org/html/rfc7489). + + + +## MDA: Dovecot + +[Dovecot](https://www.dovecot.org/) is a very popular IMAP server, +focused on being lightweight, simple, and secure, +and has extensive and up-to-date documentation. +It's very flexible and scalable, and keeps up well +with the lastest security best-practices. + +If you installed Dovecot via a package manager, +you'll probably have lots of configuration files +in the `/etc/dovecot` directory. +I want you to delete all of them. Yes, `rm -rf` that crap. +Dovecot is simple to configure, and doesn't care where +you put its settings, so having all that chaos in `/etc/dovecot` +just makes things unnecessarily confusing. + + + +### Network + +Create a new configuration file `/etc/dovecot/dovecot.conf`, +and start by filling in the details of your TLS certificate, +making clear that unencrypted connections are unacceptable: +```sh +ssl = required +ssl_cert = </etc/ssl/certs/example.com.pem +ssl_key = </etc/ssl/private/example.com.key + +ssl_min_protocol = TLSv1.2 +ssl_prefer_server_ciphers = yes + +disable_plaintext_auth = yes +``` +The final `disable_plaintext_auth` option tells Dovecot +to reject any passwords that were sent unencrypted. +This means it must be [hashed](https://en.wikipedia.org/wiki/Cryptographic_hash_function) +or sent over an encrypted connection, or both. + +Next, tell Dovecot which protocols to use +and where to expect them as follows: +```sh +protocols = lmtp imap + +service lmtp { + unix_listener lmtp { + user = vmail + group = vmail + } +} + +service imap-login { + inet_listener imap { + port = 143 + } + inet_listener imaps { + port = 993 + } +} +``` +LMTP is the Local Mail Transport Protocol, which is basically SMTP +but for exchanges within a single server or over a trusted network. +When an email is received, Dovecot will start a child process +under the `vmail` user/group to deliver the message to its recipient. + +Since we set `ssl = required` earlier, clients will only get their mail +if the STARTTLS handshake was successful during the IMAP exchange, +or if they connect via IMAPS to force the use of encryption. +You can therefore optionally remove one of the two +`inet_listener`s according to your preferences. + + + +### Users + +Next, we need to inform Dovecot which email addresses it should handle, +and what to do with their messages. Create a file `/etc/dovecot/users` for this, +which describes a user on each line in a similar format as `/etc/passwd`: +```sh +user:password:uid:gid::homedir +``` +In this guide, we're using the `vmail` user for all accounts, +so leave the `uid`, `gid`, and `homedir` fields blank. +We'll be storing all emails in `vmail`'s home directory. +The `user` field should be the email address excluding the `@example.com` +(in fact, you *can* include it, but this guides assumes a small-scale +server managing only one domain, so we exclude it). +Create the password hash to put in the `password` field as follows: +```sh +# If your server is fast and has lots of RAM: +$ doveadm pw -s ARGON2ID-CRYPT +# If you're using a potato: +$ doveadm pw -s SHA512-CRYPT +``` +After you've entered your password, simply copy-paste the entire +hash string outputted by the program into the `password` field. + +Now, Dovecot needs a file describing user accounts on two separate occasions: +* To check whether a client logging into the IMAP server + is valid and has given the right password. + This is handled by the `passdb` block(s) in the configuration. +* To know which email addresses the server is responsible for. + This is given by the `userdb` block(s) in the configuration. + +These functions aren't necessarily fulfilled by the same `users` file: +you can map multiple email addresses to one acccount, +or multiple accounts to one email address. +For simplicity, though, we'll use the `users` file for both: +```sh +passdb { + driver = passwd-file + args = scheme=ARGON2ID-CRYPT username_format=%n /etc/dovecot/users + #args = scheme=SHA512-CRYPT username_format=%n /etc/dovecot/users +} + +userdb { + driver = passwd-file + args = username_format=%n /etc/dovecot/users + override_fields = uid=vmail gid=vmail home=/home/vmail/%n +} +``` +The `driver` option sets the kind of table Dovecot should expect. +We tell it to use a file in the `passwd`-like format described above, +but other possibilities include e.g. an SQL database. +The options available in `args` depend on the chosen `driver`. + +In the `passdb` block, the hashing algorithm is given by `scheme`, +while `username_format=%n` says that the `users` file +only contains `name` out of `name@example.com`. + +In the `userdb` block, we force the use of `vmail:vmail` +for all accounts, and tell Dovecot to put their data in `/home/vmail/%n`, +where `%n` means the address up until the `@`, so +e.g. mail to `name@example.com` is stored in `/home/vmail/name`. + +Now that Dovecot knows where to store messages, +we just need to specify in what format to store them: +```sh +mail_location = maildir:~/Maildir +``` +The two standard mailbox formats to choose from are `maildir` and `mbox`. +I highly recommend `maildir`; it's more mode |