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+---
+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.
+
+
+