Merge branch 'master' into terminal

content/blog/_index.md

@@ -0,0 +1,23 @@
++++
+draft = false
+title = 'Blog'
+
+[params]
+  Toc = false
++++
+
+This is my blog.
+
+### Syntax notes
+
+When I write shell commands in code blocks, anything that should be run unprivileged is prefaced with `$`, and anything
+that should be run with root privileges is prefaced with `#`. e.g.
+
+    $ vim _index.md
+
+means to run `vim _index.md` as an unprivileged user.
+
+    # rm -rf --no-preserve-root /
+
+means to run `rm -rf --no-preserve-root /` as the root user, e.g. with `doas rm -rf --no-preserve-root /`.
+

content/blog/mail_server_alpine_postfix_dovecot_tutorial.md

@@ -23,6 +23,35 @@ A mail server is software which can be run on any computer, including yours. You
 already own that is hosting other services, so long as those other services aren't using any of the [mail
 ports](#unblock-your-ports).
 
+## Why run my own mail server?
+
+I'll cut to the chase: the main reason why you'd want to run your own mail server is for related reasons of privacy and
+digital sovereignty. For privacy benefits, as much as you have control over your server, you can protect your email
+from the eyes of prying server admins (given that you yourself are the admin). Even for email providers that market
+themselves around privacy such as Protonmail, rely on trust that Proton are not reading your unencrypted incoming
+email. This is not an issue exclusive to any particular mail provider; if information arrives unencrypted at a server,
+those with access to the server (i.e. administrators) can read that information, simple as. And as nice as it would be
+if everyone used GPG end-to-end encryption for email, the vast majority of emails people receive are not end-to-end
+encrypted, and entirely legible to the mail servers involved. If you're not exchanging <abbr
+title="End-to-End Encrypted">E2EE</abbr> email, you can't mitigate the fact that your exchange is entirely legible to
+the mail server of the person you're corresponding with, but you can at least eliminate your anxieties about the mail
+server you yourself are using.[^server_trust]
+
+Running your own mail server also allows you to implement things your way, with the features you want. For instance,
+[you can run a sieve filter for encrypting all incoming mail with a user's public GPG
+key](https://www.grepular.com/Automatically_Encrypting_all_Incoming_Email); for obvious reasons, public (as in, open to
+public sign-up) mail servers that implement sieve do not allow users to create their own executables for sieve filters.
+
+Hosting your own mail server is not something I would universally recommend to people. While I'm very much against
+"nothing to hide, nothing to fear", a combination of that factor alongside a low state threat model (i.e. there is
+little state interest in you, domestic or foreign), a lack of relevant knowledge, and a lack of interest in managing
+your own server/learning how to, likely make self-hosting email not a reasonable privacy suggestion.
+
+When you host your own mail server, you are responsible for securing the server. If you entrust Google with your email,
+you can at least know that your email is secure, though not private; Google will hire people with the relevant
+knowledge and skills to secure a mail server. If you are not confident in your ability to do this and not interested in
+learning, you may want to find another solution.
+
 # Why this tutorial?
 
 There are many tutorials on the internet about how to set up a mail server. I don't claim that mine is particularly
@@ -55,7 +84,11 @@ The mail server will be composed of the following software:
     </tr>
     <tr>
         <td>Mail delivery agent</td>
-        <td colspan="2">DKIM authentication and signing</td>
+        <td colspan="2">Dovecot</td>
+    </tr>
+    <tr>
+        <td>DKIM authentication and signing</td>
+        <td colspan="2">OpenDKIM</td>
     </tr>
     <tr>
         <td>Spam filter</td>
@@ -79,7 +112,7 @@ in through a standard SMTP/IMAP/POP3 email client, read their emails, and send e
 modular, i.e. you can opt to have e.g. Pigeonhole but not Amavis.
 
 We will end up with a small-scale mail server running on Alpine Linux with one domain, and we will use Unix user
-accounts as mail accounts.
+accounts as mail accounts. We will only set up IMAP, not POP3.
 
 This tutorial was written for Alpine Linux 3.20.3, though will most likely work on other versions too.
 
@@ -112,7 +145,7 @@ choice.
 An MX record denotes that your domain is used to send and receive email, and tells other MTAs the domain name of your
 mail server. We will use `mail.domain.com` for your MX record. For instance, my MX record looks like:
 
-```bindzone
+```dns
 revsuine.xyz.                    14400 IN MX    0 mail.revsuine.xyz
 ```
 
@@ -124,19 +157,19 @@ same as the IP address of `domain.com`, or an A record if the IP address is not
 
 I use a CNAME record because the IP addresses of `mail.revsuine.xyz` and `revsuine.xyz` are the same, so my record is:
 
-```bindzone
+```dns
 mail.revsuine.xyz.               14400 IN CNAME revsuine.xyz
 ```
 
 If you use an A record, your record may look something like
 
-```bindzone
+```dns
 mail.domain.com.                    14400 IN A     ip.address.here
 ```
 
 If you use IPv6, you should also add an AAAA record, e.g.:
 
-```bindzone
+```dns
 mail.domain.com.                    14400 IN AAAA  ip:address:here::
 ```
 
@@ -176,6 +209,57 @@ following TCP ports are open on your firewall:
 | 587  | Email message submission          |
 | 993  | IMAPS (IMAP over TLS)             |
 
+## Obtain a TLS certificate
+
+To enable TLS encryption, you need a certificate. [Let's Encrypt](https://letsencrypt.org/) provides free TLS
+certificates. To get a certificate from them, you can use certbot:
+
+    # apk add certbot
+
+We will need a web server to use certbot. I'm going to use nginx for this guide, because nginx is what I use on my
+server, but [the certbot website](https://certbot.eff.org/) has instructions for a variety of setups. If you don't
+already have an nginx server, install nginx and set it up now.
+
+Install `certbot-nginx` with:
+
+    # apk add certbot-nginx
+
+Add the following to your nginx config (for instance, inside `http {}` in `/etc/nginx/nginx.conf`, or in a dedicated
+virtual host file `/etc/nginx/http.d/mail.domain.com.conf`):
+
+```nginx
+server {
+    listen 80;
+    listen [::]:80;
+    server_name mail.domain.com;
+
+    root /usr/share/nginx/html/;
+
+    location ~ /.well-known/acme-challenge {
+        allow all;
+    }
+}
+```
+
+Replace `mail.domain.com` with the <abbr title="Fully-Qualified Domain Name">FQDN</abbr> of your mail server.
+
+The `root` can be set to any extant directory on your system that you're happy to publish to the web. You can just make
+an empty directory at `/usr/share/nginx/html`, or make this the directory of your website, etc.
+
+Reload or restart nginx for the changes to take effect:
+
+    # rc-service nginx reload
+
+Now run the following command to get your free TLS certificate:
+
+    # certbot certonly -a nginx --staple-ocsp --email your@email.here -d mail.domain.com
+
+If you have several subdomains in your nginx config that you'd like covered by the same certificate, you can omit `-d
+mail.domain.com` and get a certificate covering all the domains in your nginx config. On my server, I have one
+certificate at `/etc/letsencrypt/live/revsuine.xyz/` covering my apex domain and all subdomains. If you go for a
+certificate with only one domain name, e.g. for `mail.domain.com`, it will be at
+`/etc/letsencrypt/live/mail.domain.com/`.
+
 # Postfix
 
 Postfix is a [mail transport agent](https://en.wikipedia.org/wiki/Message_transfer_agent) (aka SMTP server). [In its
@@ -228,6 +312,76 @@ one at `/etc/logrotate.d/postfix`:
 }
 ```
 
+Add the following TLS settings, replacing `your.domain.com` with your mail server's FQDN, [or otherwise where the TLS
+certificate we generated would be](#obtain-a-tls-certificate):
+
+```conf
+# Enable TLS encryption when Postfix receives incoming emails
+smtpd_tls_cert_file = /etc/letsencrypt/live/your.domain.com/fullchain.pem
+smtpd_tls_key_file = /etc/letsencrypt/live/your.domain.com/privkey.pem
+smtpd_tls_security_level = may
+smtpd_tls_loglevel = 1
+smtpd_tls_session_cache_database = lmdb:${data_directory}/smtpd_scache
+
+# Enable TLS encryption when Postfix sends outgoing emails
+smtp_tls_security_level = may
+smtp_tls_loglevel = 1
+smtp_tls_session_cache_database = lmdb:${data_directory}/smtp_scache
+
+# Enforce TLSv1.3 or TLSv1.2
+smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
+smtpd_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
+smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
+smtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
+
+# only offer authentication after STARTTLS
+smtpd_tls_auth_only = yes
+
+# disable SSL compression
+tls_ssl_options = NO_COMPRESSION
+
+# Configure the allowed cipher list
+smtpd_tls_mandatory_ciphers = high
+smtp_tls_mandatory_ciphers = high
+smtpd_tls_ciphers = high
+smtpd_tls_mandatory_ciphers = high
+tls_high_cipherlist = ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256
+tls_preempt_cipherlist = yes
+```
+
+The allowed cipher list is from [Mailcow](https://docs.mailcow.email/manual-guides/Postfix/u_e-postfix-harden_ciphers/).
+
+If you're using this as a personal mail server, you may not want to have a mailbox size limit, so you can set:
+
+```conf
+mailbox_size_limit = 0
+```
+
+By default, `mailbox_size_limit` is `51200000`. This number is in bytes. You can similarly set a `message_size_limit`.
+
+Finally, here are some various hardening settings you can add to your `/etc/postfix/main.conf`:
+
+```conf
+# connections rate limit: no of connections allowed per unit
+# `postconf anvil_rate_time_unit` will give the time unit; by default it's
+# 60 seconds, so 600/60=10 connections allowed per second
+smtpd_client_connection_rate_limit = 600
+# messages rate limit, again over same time limit
+smtpd_client_message_rate_limit = 60
+# VRFY command used to check if an email address exists
+# not needed and can be used to find spam recipients
+disable_vrfy_command = yes
+# servers that don't use HELO or EHLO are either not properly configured
+# or sending spam usually
+smtpd_helo_required = yes
+smtpd_delay_reject = yes
+smtpd_helo_restrictions =
+    permit_mynetworks,
+    reject_invalid_helo_hostname,
+    reject_unknown_helo_hostname,
+    permit
+```
+
 ## Send your first email
 
 Have the `postfix` service auto-start upon boot, and start it during this session:
@@ -239,7 +393,7 @@ You can now send an email with the following command:
 
     $ echo "test email" | sendmail user@externaldomain.com
 
-Send this email to your email account with an external server, e.g. a gmail account. Note that Protonmail has quite
+Send this email to your email account with an external server, e.g. a Gmail account. Note that Protonmail has quite
 stringent spam filters and this likely would be rejected by Protonmail, i.e. not even reach your spam folder.
 
 ## Configure email aliases
@@ -264,11 +418,148 @@ so ultimately `revsuine` will get `postmaster`'s mail.
 
 You can continue to populate the aliases file with whatever aliases you want.
 
+## Enable Postfix submission and smtps service
+
+To send emails from email clients, you'll need to enable Postfix's submission service so that Postfix can receive
+emails to send via SMTP. Edit `/etc/postfix/master.cf` and ensure that the following lines are present:
+
+```conf
+submission inet n       -       n       -       -       smtpd
+  -o syslog_name=postfix/submission
+  -o smtpd_tls_security_level=encrypt
+  -o smtpd_sasl_auth_enable=yes
+  -o smtpd_relay_restrictions=permit_sasl_authenticated,reject
+  -o smtpd_recipient_restrictions=permit_mynetworks,permit_sasl_authenticated,reject
+  -o smtp_sasl_type=dovecot
+  -o smtpd_sasl_path=private/auth
+
+smtps     inet  n       -       n       -       -       smtpd
+  -o syslog_name=postfix/smtps
+  -o smtpd_tls_wrappermode=yes
+  -o smtpd_sasl_auth_enable=yes
+  -o smtpd_relay_restrictions=permit_sasl_authenticated,reject
+  -o smtpd_recipient_restrictions=permit_mynetworks,permit_sasl_authenticated,reject
+  -o smtpd_sasl_type=dovecot
+  -o smtpd_sasl_path=private/auth
+```
+
+They may be commented out, or partially present without some options.
+
+Restart Postfix.
+
+    # rc-service postfix restart
+
+# Dovecot
+
+[Dovecot](https://www.dovecot.org/) is a popular IMAP and POP3 server which we'll be using for our MDA. Let's install
+it:
+
+    # apk add dovecot
+
+Check the Dovecot version with:
+
+    $ dovecot --version
+
+Now let's enable IMAP by editing `/etc/dovecot/dovecot.conf`. Find a `protocols = ` line, or add one, and set it to:
+
+```conf
+protocols = imap
+```
+
+## Configure how to store emails
+
+You probably want to use the Maildir format for storing emails, where each user's mail is stored at `~/Maildir` (this
+can be set to another location if desired).
+
+In `/etc/dovecot/conf.d/10-mail.conf`, set:
+
+```conf
+mail_location = maildir:~/Maildir
+mail_privileged_group = mail
+```
+
+`mail_privileged_group` tells us which group of Unix users can send mail; in this case, it's anyone in the `mail`
+group. You can create the group with:
+
+    # addgroup mail
+    # adduser postfix mail
+    # adduser dovecot mail
+
+We want to ensure that `postfix` and `dovecot` users have the right to access mail.
+
+To change the Maildir directory, e.g. to set it to `~/mail`, you would set the following:
+
+`/etc/dovecot/conf.d/10-mail.conf`:
+
+```conf
+mail_location = maildir:~/mail
+```
+
+`/etc/postfix/main.cf`:
+
+```conf
+home_mailbox = mail/
+```
+
+## Get emails with LMTP
+
+<abbr title="Local Mail Transfer Protocol">LMTP</abbr> is a protocol which can be used for Postfix to pass incoming
+emails to Dovecot. To install it for Dovecot:
+
+    # apk add dovecot-lmtpd
+
+Add `lmtp` to the supported protocols in `/etc/dovecot/dovecot.conf`:
+
+```conf
+protocols = imap lmtp
+```
+
+Now change the LMTP service (or add if it isn't already there) in `/etc/dovecot/conf.d/10-master.conf` to:
+
+```conf
+service lmtp {
+  unix_listener /var/spool/postfix/private/dovecot-lmtp {
+    mode = 0600
+    user = postfix
+    group = postfix
+  }
+```
+
+Postfix needs to be configured to use this socket. Edit `/etc/postfix/main.cf` with the following lines:
+
+```conf
+mailbox_transport = lmtp:unix:private/dovecot-lmtp
+smtputf8_enable = no
+```
+
 <!-- FOOTNOTES: -->
 
+[^server_trust]: This is only true to the extent that your server is not compromised. You could say there's an order of
+    server trust-ability that goes:
+
+    ```
+    VPS < rented dedicated server < server you yourself physically own, store, and manage
+    ```
+
+    There is little that can be done to secure a VM running on a compromised host. Even with full-disk encryption, the
+    host can dump the encryption key from RAM, because the encryption key must be stored in memory whilst a
+    full-disk-encrypted system is booted.
+
+    For a dedicated server you rent, there are at least no concerns about a compromised host, but an attacker with
+    physical access (in this case, the untrusted people you rent the dedicated server from) can attempt evil maid
+    attacks. You are hopefully able to implement mechanisms to detect this, though.
+
+    There are reasons you may want to go with a rented server instead of one you own, though. For instance, if you live
+    in a jurisdiction known for terrible privacy laws such as a [5/14 eyes
+    country](https://restoreprivacy.com/5-eyes-9-eyes-14-eyes/), or if you are a political dissident with domestic
+    state interest in you, you likely want to go offshore for server hosting. Changing the jurisdiction can protect you
+    if the jurisdiction you choose won't work with your national intelligence agencies.
+
 [^postfix_aliases_location]: Your aliases file will most likely be in this location by default, but you can run
 
+    ```
     $ postconf alias_maps
+    ```
 
     to find out where this file should be.
 

layouts/partials/footer.html

@@ -9,6 +9,8 @@ href="https://github.com/panr/hugo-theme-terminal">Theme</a> by <a href="https:/
 licensed under the <a href="https://github.com/panr/hugo-theme-terminal/blob/master/LICENSE.md">MIT licence</a>.</p>
 </footer>
 
+<!-- REQUIRED FOR PRISMJS: -->
+
 {{ $menu := resources.Get "js/menu.js" | js.Build }}
 {{ $prism := resources.Get "js/prism.js" | js.Build }}
 
@@ -16,3 +18,5 @@ licensed under the <a href="https://github.com/panr/hugo-theme-terminal/blob/mas
 
 <script type="text/javascript" src="{{ $bundle.RelPermalink }}"></script>
 
+<!-- /end prismjs stuff -->
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