OpenBSD Email Service

caesonia (beta)

OpenBSD Email Service

About

a free-email alternative for ~$2.50/mo.

Root your Inbox 📬

Features

• Efficient: configured to run on min. 512MB RAM and 20GB SSD, a KVM (cloud) VPS for around $2.50/mo
• 15GB+ uncompressed Maildir, rivals top free-email providers (grow by upgrading SSD)
• Email messages are gzip compressed, at least 1/3 more space with level 6 default
• Server side full text search (headers and body) can be enabled (to use the extra space)
• Mobile data friendly: IMAPS connections are compressed
• Subaddress (+tag) support, to filter and monitor email addresses
• Virtual domains, aliases, and credentials in files, Berkeley DB, or SQLite3
• Naive Bayes rspamd filtering with supervised learning: the lowest false positive spam detection rates
• Carefree automated Spam/ and Trash/ cleaning service (default: older than 30 days)
• Automated quota management, gently assists when over quota
• Easy backup MX setup: using the same configuration, install in minutes on a different host
• Worry-free automated master/master replication with backup MX, prevents accidental loss of email messages
• Resilient: the backup MX can be used as primary, even when the primary is not down, both perfect replicas
• Flexible: switching roles is easy, making the process of changing VPS hosts a breeze (no downtime)
• DMARC (with DKIM and SPF) email-validation system, to detect and prevent email spoofing
• Daily (spartan) stats, to keep track of things
• Your sieve scripts and managesieve configuration, let's get started

Considerations

By design, email message headers need to be public, for exchanges to happen. The body of the message can be encrypted by the user, if desired. Moreover, there is no way to prevent the host from having access to the virtual machine. Therefore, full disk encryption (at rest) may not be necessary.

Given our low memory requirements, and the single-purpose concept of email service, Roundcube or other web-based IMAP email clients should be on a different VPS.

Antivirus software users (usually) have the service running on their devices. ClamAV can easily be incorporated into this configuration, if affected by the types of malware it protects against, but will require around 1GB additional RAM (or another VPS).

Every email message is important, if properly delivered, for Bayes classification. At least 200 ham and 200 spam messages are required to learn what one considers junk. By default (change to use case), a rspamd score above 50% will send the message to Spam/. Moving messages in and out of Spam/ changes this score. After 95%, the message is flagged as "seen" and can be safely ignored.

Spamd is effective at greylisting and stopping high volume spam, if it becomes a problem. It will be an option when IPv6 is supported, along with bgp-spamd.

System mail is delivered to an alias mapped to a virtual user served by the service. This way, messages are guaranteed to be delivered via encrypted connection. It is not possible for real users to alias, nor mail an external mail address with the default configuration. e.g. puffy@mercury.example.com is wheel, with an alias mapped to (virtual) puffy@example.com, and user (puffy) can be different for each.

Getting started

Install packages:

pkg_add dovecot dovecot-pigeonhole dkimproxy rspamd opensmtpd-extras

Add users:

useradd -m -u 2000 -g =uid -c "Virtual Mail" -d /var/vmail -s /sbin/nologin vmail
useradd -m -u 2001 -g =uid -c "Dsync Replication" -d /home/dsync -s /bin/sh dsync

Cheatsheet

A quick way around

Let's assume we want to change the (default) virtual domain name from example.net to example.org

cd src/
grep -r example.net .

After close inspection

find . -type f -exec sed -i 's|example.net|example.org|g' {} +

Defaults to customize

primary domain name: example.com
virtual domain name: example.com
                     example.net

primary MX host: mercury.example.com
primary MX IPv4: 93.184.216.34
primary MX IPv6: 2606:2800:220:1:248:1893:25c8:1946

backup MX host: hermes.example.com
backup MX IPv4: 200.100.2.200
backup MX IPv6: 2001:1002:2:1::babe

DKIM selector: obsd
external (egress) interface: vio0

wheel user: puffy
replication user: dsync
virtual user: puffy

autoexpunge: autoexpunge\ =\ 30d
quota: storage=15G
full text search: fts
full sync: replication_full_sync_interval\ =\ 1h

Layout

Disklabel: var/www/htdocs/mercury.example.com/disklabel.min

Permissions are described in usr/local/bin/hier.sh

Ansible: ansible-role-mailserver

Prerequisites

A DNS name server (from a registrar, a free service, VPS host, or self-hosted) is required, which allows editing the following record types: A, AAAA, MX, CAA, TXT, SSHFP

Forward-confirmed reverse DNS (FCrDNS)

Each MX subdomain has record types A, and AAAA with the VPS IPv4, and IPv6:

mercury.example.com.	86400	IN	A	93.184.216.34
mercury.example.com.	86400	IN	AAAA	2606:2800:220:1:248:1893:25c8:1946

Each IPv4 and IPv6 has record type PTR with the MX subdomain (reverse DNS configured on VPS host):

...6				IN	PTR 	mercury.example.com.

Verify:

dig +short mercury.example.com a
> 93.184.216.34
dig +short -x 93.184.216.34
> mercury.example.com.

dig +short mercury.example.com aaaa
> 2606:2800:220:1:248:1893:25c8:1946
dig +short -x 2606:2800:220:1:248:1893:25c8:1946
> mercury.example.com.

MX

Each domain has first priority MX record "mercury.example.com"

Each domain has second priority MX record "hermes.example.com"

example.com.	86400	IN	MX	10 mercury.example.com.
example.com.	86400	IN	MX	20 hermes.example.com.

CAA

Primary domain name's CAA record sets "letsencrypt.org" as the only CA allowed to issue certificates:

example.com.	86400	IN	CAA	128 issue "letsencrypt.org"
example.com.	86400	IN	CAA	128 issuewild ";"

SSHFP

Each MX subdomain needs their hosts's SSHFP records:

ssh-keygen -r mercury.example.com

mercury.example.com.	86400	IN	SSHFP	1 1 2...
mercury.example.com.	86400	IN	SSHFP	1 2 5...
mercury.example.com.	86400	IN	SSHFP	2 1 a...
mercury.example.com.	86400	IN	SSHFP	2 2 c...
mercury.example.com.	86400	IN	SSHFP	3 1 6...
mercury.example.com.	86400	IN	SSHFP	3 2 8...
mercury.example.com.	86400	IN	SSHFP	4 1 7...
mercury.example.com.	86400	IN	SSHFP	4 2 a...

Sender Policy Framework (SFP)

Each domain and subdomain needs a TXT record with SPF data:

example.com.		86400	IN	TXT	"v=spf1 mx mx:example.com -all"
mercury.example.com.	86400	IN	TXT	"v=spf1 a mx ip4:93.184.216.34 ip6:2606:2800:220:1:248:1893:25c8:194 -all"
hermes.example.com.	86400	IN	TXT	"v=spf1 a mx ip4:200.100.2.200 ip6:2001:1002:2:1::/64 -all"
www.example.com.	86400	IN	TXT	"v=spf1 -all"

Domain Keys Identified Mail (DKIM)

Generate a private and public key:

mkdir -p /etc/ssl/dkim/private
chmod 750 /etc/ssl/dkim/private

Some web-interfaces only allow TXT record with max 1024 bits key:

openssl genrsa -out /etc/ssl/dkim/private/private.key 1024
openssl rsa -in /etc/ssl/dkim/private/private.key -pubout -out /etc/ssl/dkim/public.key
chgrp -R _dkimproxy /etc/ssl/dkim/private
chmod 440 /etc/ssl/dkim/private/private.key

Add public key in TXT record:

obsd._domainkey.example.com.	86400	IN	TXT	"v=DKIM1; k=rsa; p=M..."

Domain-based Message Authentication, Reporting & Conformance (DMARC)

Each domain name needs a TXT record for subdomain "_dmarc" with DMARC data:

_dmarc.example.com.	86400	IN	TXT	v=DMARC1\;p=reject\;pct=100\;rua=mailto:dmarcreports\@example.com

Support

Via issues and #openbsd:matrix.org

Contribute

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