caesonia (beta)

OpenBSD Email Service

About

a free-email alternative

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
• Uncensored DNS validating resolver from root nameservers
• OpenPGP Web Key Service with Web Key Directory, automatic key exchange protocol
• MUA Autoconfiguration, for modern clients
• 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 (2000+ for best results). 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. To build IP lists for greylisting, please use spfwalk with spf_fetch.

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. [email protected] is wheel, with an alias mapped to (virtual) [email protected], and user (puffy) can be different for each.

Getting started

See Prerequisites and the Installation Guide for details.

Grab a copy of this repository, and put overrides in "Makefile.local" e.g.:

# Makefile.local

DOMAIN_NAME =   example.com
VHOSTS_NAME =   example.net \
                example.org

PRIMARY =       yes

PRIMARY_HOST =	mercury
PRIMARY_IPv4 =	203.0.113.1
PRIMARY_IPv6 =	2001:0db8::1

BACKUP_HOST =	hermes
BACKUP_IPv4 =	203.0.113.2
BACKUP_IPv6 =	2001:0db8::2

DKIM_SELECTOR =	obsd
EGRESS =	vio0

WHEEL_USER =	puffy
REPLICATION_USER =	dsync
VIRTUAL_USER =	${WHEEL_USER}

AUTOEXPUNGE =	30d
MAIL_QUOTA =	15G
MAX_MESSAGE_SIZE =	35M
FULL_SYNC_INTERVAL =	1h

UPGRADE =	yes

Install

make install

n.b. UPGRADE uses sdiff side-by-side diff (with new on the right side)

Virtual Users

Update virtual users credentials table src/etc/mail/passwd using smtpctl encrypt

smtpctl encrypt
> secret
> $2b$...encrypted...passphrase...
vi src/etc/mail/passwd
> [email protected]:$2b$...encrypted...passphrase...::::::
smtpctl update table passwd

n.b.: user quota limit can be overriden from src/etc/mail/passwd

[email protected]:$2b$...encrypted...passphrase...::::::userdb_quota_rule=*:storage=7G

Review virtual domains aliasing table src/etc/mail/virtual

n.b. see Administration for virtual user and domain management

Backup MX

n.b. Without backup MX, leave BACKUP_HOST empty in "Makefile.local"

Dovecot Replication user "dsync" SSH limited to one "command" restricted in doas.conf to match "dsync_remote_cmd". On primary and backup hosts

su - dsync
ssh-keygen
echo "command=\"doas -u vmail \${SSH_ORIGINAL_COMMAND#*}\" $(cat ~/.ssh/id_rsa.pub)" | \
	ssh [email protected] "cat >> /home/dsync/.ssh/authorized_keys"
exit

Alternatively, OpenSSH certificates allow fine-grained control to local users and hosts. The force-command is passed to ssh-keygen as certificate option (-O) instead of using "authorized_keys".

Update /home/dsync, on primary and backup hosts

chown -R root:dsync /home/dsync
chmod 750 /home/dsync/.ssh
chmod 640 /home/dsync/.ssh/{authorized_keys,id_*.pub,known_hosts}
chmod 400 /home/dsync/.ssh/{id_ecdsa,id_ed25519,id_rsa}
chown dsync /home/dsync/.ssh/{id_ecdsa,id_ed25519,id_rsa}

Update /root/.ssh/known_hosts on primary and backup hosts

ssh -4 -i/home/dsync/.ssh/id_rsa -ldsync hermes.example.com "exit"
ssh -6 -i/home/dsync/.ssh/id_rsa -ldsync hermes.example.com "exit"

Client Configuration

n.b.: MUA auto-configuration via Autoconfiguration and SRV Records for Locating Email Services

• IMAP server: mercury.example.com (or hermes.example.com)

  • Security: TLS
  • Port: 993
  • Username: [email protected]
  • Password: ********
  • Autodetect IMAP namespace ☑️
  • Use compression ☑️
  • Poll when connecting for push ☑️

• SMTP server: mercury.example.com (or hermes.example.com)

  • Security: TLS
  • Port: 465
  • Require sign-in ☑️
  • Username: [email protected]
  • Authentication: Normal password
  • Password: ********

• SMTP server: mercury.example.com (or hermes.example.com)

  • Security: STARTTLS
  • Port: 587
  • Require sign-in ☑️
  • Username: [email protected]
  • Authentication: Normal password
  • Password: ********

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, CNAME, SRV, CAA, SSHFP, TXT

n.b. see example zone

DNSSEC is recommended

Forward-confirmed reverse DNS (FCrDNS)

Primary domain has record types A, and AAAA for each MX subdomain with the relays' IPv4, and IPv6

mercury.example.com	86400	IN	A	203.0.113.1
mercury.example.com	86400	IN	AAAA	2001:0db8::1

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
> 203.0.113.1
dig +short -x 203.0.113.1
> mercury.example.com.

dig +short mercury.example.com aaaa
> 2001:0db8::1
dig +short -x 2001:0db8::1
> mercury.example.com.

Mail eXchanger (MX)

Primary and virtual domains have identical records type MX with priority and relay hostnames

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

Mozilla Autoconfiguration

Primary and virtual domains have identical records type CNAME for autoconfig subdomain pointing to Autoconfiguration server

autoconfig.example.com	86400	IN	CNAME	mercury.example.com

OpenPGP Web Key Directory (WKD)

Primary and virtual domains have identical records type CNAME for wkd subdomain pointing to Web Key Server

wkd.example.com		86400	IN	CNAME	mercury.example.com

SRV Records for OpenPGP Web Key Directory

Primary domain has record type SRV with primary WKD subdomain

_openpgpkey._tcp.example.com	86400	IN	SRV	0 0 443	wkd.example.com

Each virtual domain has record type SRV with virtual WKD subdomain

_openpgpkey._tcp.example.net	86400	IN	SRV	0 0 443	wkd.example.net

SRV Records for Locating Email Services

Primary and virtual domains have identical records type SRV for simple MUA auto-configuration

_submission._tcp.example.com	86400	IN	SRV	0 1 465 mercury.example.com
_submission._tcp.example.com	86400	IN	SRV	5 1 587	mercury.example.com
_imaps._tcp.example.com		86400	IN	SRV	0 1 993	mercury.example.com
_imap._tcp.example.com		86400	IN	SRV	0 0 0   .
_pop3s._tcp.example.com		86400	IN	SRV	0 0 0   .
_pop3._tcp.example.com		86400	IN	SRV	0 0 0	.

Certification Authority Authorization (CAA)

Primary and virtual domains have identical records type CAA with 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 ";"

Secure Shell Fingerprint (SSHFP)

Print the SSHFP fingerprint resource record on each hostname

ssh-keygen -r mercury.example.com

Primary domain has records type SSHFP for each MX subdomain with a fingerprint of the server's public key

mercury.example.com	86400	IN	SSHFP	1 1 2...
mercury.example.com	86400	IN	SSHFP	1 2 5...
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 (SPF)

Primary and virtual domains have identical records type TXT with primary domain SPF data

example.com		86400	IN	TXT	"v=spf1 mx:example.com -all"

Primary domain has record type TXT for each MX subdomain, to return receipt notifications (DSNs and MDNs), with relays' IP SPF data

mercury.example.com	86400	IN	TXT	"v=spf1 a -all"
hermes.example.com	86400	IN	TXT	"v=spf1 a -all"

Domain Keys Identified Mail (DKIM)

Generate a private and public key

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

Signers SHOULD use RSA keys of at least 2048 bits. -- https://tools.ietf.org/html/rfc8301#section-3.2

(umask 337; openssl genrsa -out /etc/ssl/dkim/private/private.key 2048)
openssl rsa -in /etc/ssl/dkim/private/private.key -pubout -out /etc/ssl/dkim/public.key
chown -R _rspamd:_dkimproxy /etc/ssl/dkim/private

Widely used DNS configuration software places a practical limit on key sizes, because the software only handles a single 256-octet string in a TXT record, and RSA keys significantly longer than 1024 bits don't fit in 256 octets. -- https://tools.ietf.org/html/rfc8301#section-1

Multiple strings in a single DNS record are useful in constructing records that would exceed the 255-byte maximum length of a string within a single TXT RR record. -- https://tools.ietf.org/html/rfc4408#section-3.1.3

Generate the TXT-DATA field for DKIM TXT record

echo "v=DKIM1; k=rsa; p=$(sed -e '1d' -e '$d' /etc/ssl/dkim/public.key | tr -d '\n')"

Alternatively

(umask 337; rspamadm dkim_keygen -d example.com -s 'obsd' -k /etc/ssl/dkim/private/private.key -b 2048 -t rsa) > /etc/ssl/dkim/public.key

Primary and virtual domains have identical records type TXT for selector._domainkey subdomain with DKIM public key

obsd._domainkey.example.com	86400	IN	TXT	( "v=DKIM1; k=rsa; "
	"p=abcd"
	"ef"
) ;

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

Primary and virtual domains have record type TXT for _dmarc subdomain with DMARC policy data

_dmarc.example.com	86400	IN	TXT	"v=DMARC1; p=reject; pct=100; rua=mailto:[email protected]"

SMTP MTA Strict Transport Security (MTA-STS)

Primary domain has record type TXT for _mta-sts subdomain with MTA-STS reporting data

_mta-sts.example.com	86400	IN	TXT	"v=STSv1; id=20190515085700Z;"

Each virtual domain has record type CNAME for _mta-sts subdomain pointing to MTA-STS TXT record

_mta-sts.example.net	86400	IN	CNAME	_mta-sts.example.com

Primary and virtual domains have identical records type CNAME for mta-sts subdomain pointing to MTA-STS policy

mta-sts.example.com	86400	IN	CNAME	mercury.example.com

SMTP TLS Reporting (SMTP TLSRPT)

Primary and virtual domains have records type TXT for _smtp._tls subdomain with TLS reporting data

_smtp._tls.example.com	86400	IN	TXT	"v=TLSRPTv1;rua=mailto:[email protected]"

SMTP Security via Opportunistic DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS)

requires DNSSEC and manual server key rotation using the procedure form rfc7671

Print each relay's TLSA resource record

pkg_add ldns-utils
ldns-dane create mercury.example.com 443 3 1 1
ldns-dane create hermes.example.com 443 3 1 1

Primary domain has records type TLSA for each tlsa._dane MX subdomain

tlsa._dane.mercury.example.com	86400	IN	TLSA	3 1 1 e3b0c44298fc1c14...
tlsa._dane.hermes.example.com	86400	IN	TLSA	3 1 1 f2c0d55309cf2d25...

Primary domain has records type CNAME for each _service._tcp MX subdomain pointing to TLSA RR

_993._tcp.mercury.example.com	86400	IN	CNAME	tlsa._dane.mercury.example.com
_587._tcp.mercury.example.com	86400	IN	CNAME	tlsa._dane.mercury.example.com
_443._tcp.mercury.example.com	86400	IN	CNAME	tlsa._dane.mercury.example.com
_25._tcp.mercury.example.com	86400	IN	CNAME	tlsa._dane.mercury.example.com

_993._tcp.hermes.example.com	86400	IN	CNAME	tlsa._dane.hermes.example.com
_587._tcp.hermes.example.com	86400	IN	CNAME	tlsa._dane.hermes.example.com
_443._tcp.hermes.example.com	86400	IN	CNAME	tlsa._dane.hermes.example.com
_25._tcp.hermes.example.com	86400	IN	CNAME	tlsa._dane.hermes.example.com

Support

Issues

Social

#caesonia:matrix.org (deadish)

#[email protected]

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Contributions welcome, fork

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