diff options
Diffstat (limited to 'content/blog/2020/email-server-extras.md')
| -rw-r--r-- | content/blog/2020/email-server-extras.md | 414 |
1 files changed, 414 insertions, 0 deletions
diff --git a/content/blog/2020/email-server-extras.md b/content/blog/2020/email-server-extras.md new file mode 100644 index 0000000..72299c9 --- /dev/null +++ b/content/blog/2020/email-server-extras.md @@ -0,0 +1,414 @@ +--- +title: "Setting up an email server in 2020 with OpenSMTPD and Dovecot: extras" +publishDate: 2020-04-27 +date: 2021-02-22T17:19:49+01:00 +draft: false +--- + +# + +This sequel to my earlier [guide](/blog/2020/email-server/) discusses +extra tips and tricks to extend your email setup. +This page will be updated continuously as I come up with ideas. + +Last updated 2020-04-29. + + +## 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. + + +#### 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) + +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. + + + |