DKIM, DMARC & SPF

Cloudflare has written an article about DKIM, DMARC and SPF that we highly recommend you to read to get acquainted with the topic.

Rspamd vs Individual validators

With v12.0.0, Rspamd was integrated into DMS. It can perform validations for DKIM, DMARC and SPF as part of the spam-score-calculation for an email. DMS provides individual alternatives for each validation that can be used instead of deferring to Rspamd:

• DKIM: opendkim is used as a milter (like Rspamd)
• DMARC: opendmarc is used as a milter (like Rspamd)
• SPF: policyd-spf is used in Postfix's smtpd_recipient_restrictions

In a future release Rspamd will become the default for these validations, with a deprecation notice issued prior to the removal of the above alternatives.

We encourage everyone to prefer Rspamd via ENABLE_RSPAMD=1.

DNS Caches & Propagation

While modern DNS providers are quick, it may take minutes or even hours for new DNS records to become available / propagate.

DKIM

What is DKIM

DomainKeys Identified Mail (DKIM) is an email authentication method designed to detect forged sender addresses in email (email spoofing), a technique often used in phishing and email spam.

Source

When DKIM is enabled:

1. Inbound mail will verify any included DKIM signatures
2. Outbound mail is signed (when you're sending domain has a configured DKIM key)

DKIM requires a public/private key pair to enable signing (via private key) your outgoing mail, while the receiving end must query DNS to verify (via public key) that the signature is trustworthy.

Verification expiry

Unlike your TLS certificate, your DKIM keypair does not have a fixed expiry associated to it.

Instead, an expiry may be included in your DKIM signature for each mail sent, where a receiver will refuse to validate the signature for an email after that expiry date. This is an added precaution to mitigate malicious activity like "DKIM replay attacks", where an already delivered email from a third-party with a trustworthy DKIM signature is leveraged by a spammer when sending mail to an MTA which verifies the DKIM signature successfully, enabling the spammer to bypass spam protections.

Unlike a TLS handshake where you are authenticating trust with future communications, with DKIM once the mail has been received and trust of the signature has been verified, the value of verifying the signature again at a later date is less meaningful since the signature was to ensure no tampering had occurred during delivery through the network.

DKIM key rotation

You can rotate your DKIM keypair by switching to a new DKIM selector (and DNS updates), while the previous key and selector remains valid for verification until the last mail signed with that key reaches it's expiry.

DMS does not provide any automation or support for key rotation, nor is it likely to provide a notable security benefit to the typical small scale DMS deployment.

Generating Keys

You'll need to repeat this process if you add any new domains.

You should have:

• At least one [email account setup][docs-accounts-add]
• Attached a volume for config to persist the generated files to local storage

Creating DKIM Keys

DKIM keys can be generated with good defaults by running:

docker exec -it <CONTAINER NAME> setup config dkim

If you need to generate your keys with different settings, check the help output for supported config options and examples:

docker exec -it <CONTAINER NAME> setup config dkim help

As described by the help output, you may need to use the domain option explicitly when you're using LDAP or Rspamd.

Changing the key size

The keypair generated for using with DKIM presently defaults to RSA-2048. This is a good size but you can lower the security to 1024-bit, or increase it to 4096-bit (discouraged as that is excessive).

To generate a key with different size (for RSA 1024-bit) run:

setup config dkim keysize 1024

RSA Key Sizes >= 4096 Bit

According to RFC 8301, keys are preferably between 1024 and 2048 bits. Keys of size 4096-bit or larger may not be compatible to all systems your mail is intended for.

You should not need a key length beyond 2048-bit. If 2048-bit does not meet your security needs, you may want to instead consider adopting key rotation or switching from RSA to ECC keys for DKIM.

You may need to specify mail domains explicitly

Required when using LDAP and Rspamd.

setup config dkim will generate DKIM keys for what is assumed as the primary mail domain (derived from the FQDN assigned to DMS, minus any subdomain).

When the DMS FQDN is mail.example.com or example.com, by default this command will generate DKIM keys for example.com as the primary domain for your users mail accounts (eg: hello@example.com).

The DKIM generation does not have support to query LDAP for additional mail domains it should know about. If the primary mail domain is not sufficient, then you must explicitly specify any extra domains via the domain option:

# ENABLE_OPENDKIM=1 (default):
setup config dkim domain 'example.com,another-example.com'

# ENABLE_RSPAMD=1 + ENABLE_OPENDKIM=0:
setup config dkim domain example.com
setup config dkim domain another-example.com

OpenDKIM with ACCOUNT_PROVISIONER=FILE

When DMS uses this configuration, it will by default also detect mail domains (from accounts added via setup email add), generating additional DKIM keys.

DKIM is currently supported by either OpenDKIM or Rspamd:

OpenDKIMRspamd

OpenDKIM is currently enabled by default.

After running setup config dkim, your new DKIM key files (and OpenDKIM config) have been added to /tmp/docker-mailserver/opendkim/.

Restart required

After restarting DMS, outgoing mail will now be signed with your new DKIM key(s)

Requires opt-in via ENABLE_RSPAMD=1 (and disable the default OpenDKIM: ENABLE_OPENDKIM=0).

Rspamd provides DKIM support through two separate modules:

1. Verifying DKIM signatures from inbound mail is enabled by default.
2. Signing outbound mail with your DKIM key needs additional setup (key + dns + config).

Using Multiple Domains

If you have multiple domains, you need to:

• Create a key wth docker exec -it <CONTAINER NAME> setup config dkim domain <DOMAIN> for each domain DMS should sign outgoing mail for.
• Provide a custom dkim_signing.conf (for which an example is shown below), as the default config only supports one domain.

About the Helper Script

The script will persist the keys in /tmp/docker-mailserver/rspamd/dkim/. Hence, if you are already using the default volume mounts, the keys are persisted in a volume. The script also restarts Rspamd directly, so changes take effect without restarting DMS.

The script provides you with log messages along the way of creating keys. In case you want to read the complete log, use -v (verbose) or -vv (very verbose).

---

In case you have not already provided a default DKIM signing configuration, the script will create one and write it to /etc/rspamd/override.d/dkim_signing.conf. If this file already exists, it will not be overwritten.

When you're already using the rspamd/override.d/ directory, the file is created inside your volume and therefore persisted correctly. If you are not using rspamd/override.d/, you will need to persist the file yourself (otherwise it is lost on container restart).

An example of what a default configuration file for DKIM signing looks like can be found by expanding the example below.

DKIM Signing Module Configuration Examples

A simple configuration could look like this:

# documentation: https://rspamd.com/doc/modules/dkim_signing.html

enabled = true;

sign_authenticated = true;
sign_local = true;

use_domain = "header";
use_redis = false; # don't change unless Redis also provides the DKIM keys
use_esld = true;
check_pubkey = true; # you want to use this in the beginning

selector = "mail";
# The path location is searched for a DKIM key with these variables:
# - `$domain` is sourced from the MIME mail message `From` header
# - `$selector` is configured for `mail` (as a default fallback)
path = "/tmp/docker-mailserver/dkim/keys/$domain/$selector.private";

# domain specific configurations can be provided below:
domain {
    example.com {
        path = "/tmp/docker-mailserver/rspamd/dkim/mail.private";
        selector = "mail";
    }
}

As shown next:

• You can add more domains into the domain { ... } section (in the following example: example.com and example.org).
• A domain can also be configured with multiple selectors and keys within a selectors [ ... ] array (in the following example, this is done for example.org).

# ...

domain {
    example.com {
        path = /tmp/docker-mailserver/rspamd/example.com/ed25519.private";
        selector = "dkim-ed25519";
    }
    example.org {
        selectors [
            {
                path = "/tmp/docker-mailserver/rspamd/dkim/example.org/rsa.private";
                selector = "dkim-rsa";
            },
            {
                path = "/tmp/docker-mailserver/rspamd/dkim/example.org/ed25519.private";
                selector = "dkim-ed25519";
            }
        ]
    }
}

Support for DKIM Keys using ED25519

This modern elliptic curve is supported by Rspamd, but support by third-parties for verifying Ed25519 DKIM signatures is unreliable.

If you sign your mail with this key type, you should include RSA as a fallback, like shown in the above example.

Let Rspamd Check Your Keys

When check_pubkey = true; is set, Rspamd will query the DNS record for each DKIM selector, verifying each public key matches the private key configured.

If there is a mismatch, a warning will be emitted to the Rspamd log /var/log/mail/rspamd.log.

DNS Record

When mail signed with your DKIM key is sent from your mail server, the receiver needs to check a DNS TXT record to verify the DKIM signature is trustworthy.

Configuring DNS - DKIM record

When you generated your key in the previous step, the DNS data was saved into a file <selector>.txt (default: mail.txt). Use this content to update your DNS via Web Interface or directly edit your DNS Zone file:

Web InterfaceDNS Zone file

Create a new record:

Field	Value
Type	TXT
Name	<selector>._domainkey (default: mail._domainkey)
TTL	Use the default (otherwise 3600 seconds is appropriate)
Data	File content within ( ... ) (formatted as advised below)

When using Rspamd, the helper script has already provided you with the contents (the "Data" field) of the DNS record you need to create - you can just copy-paste this text.

<selector>.txt is already formatted as a snippet for adding to your DNS Zone file.

Just copy/paste the file contents into your existing DNS zone. The TXT value has been split into separate strings every 255 characters for compatibility.

<selector>.txt - Formatting the TXT record value correctly

This file was generated for use within a DNS zone file. The file name uses the DKIM selector it was generated with (default DKIM selector is mail, which creates mail.txt_).

For your DNS setup, DKIM support needs to create a TXT record to store the public key for mail clients to use. TXT records with values that are longer than 255 characters need to be split into multiple parts. This is why the generated <selector>.txt file (containing your public key for use with DKIM) has multiple value parts wrapped within double-quotes between ( and ).

A DNS web-interface may handle this separation internally instead, and could expect the value provided all as a single line instead of split. When that is required, you'll need to manually format the value as described below.

Your generated DNS record file (<selector>.txt) should look similar to this:

mail._domainkey IN TXT ( "v=DKIM1; k=rsa; "
"p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAqQMMqhb1S52Rg7VFS3EC6JQIMxNDdiBmOKZvY5fiVtD3Z+yd9ZV+V8e4IARVoMXWcJWSR6xkloitzfrRtJRwOYvmrcgugOalkmM0V4Gy/2aXeamuiBuUc4esDQEI3egmtAsHcVY1XCoYfs+9VqoHEq3vdr3UQ8zP/l+FP5UfcaJFCK/ZllqcO2P1GjIDVSHLdPpRHbMP/tU1a9mNZ"
"5QMZBJ/JuJK/s+2bp8gpxKn8rh1akSQjlynlV9NI+7J3CC7CUf3bGvoXIrb37C/lpJehS39KNtcGdaRufKauSfqx/7SxA0zyZC+r13f7ASbMaQFzm+/RRusTqozY/p/MsWx8QIDAQAB"
) ;

Take the content between ( ... ), and combine all the quote wrapped content and remove the double-quotes including the white-space between them. That is your TXT record value, the above example would become this:

v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAqQMMqhb1S52Rg7VFS3EC6JQIMxNDdiBmOKZvY5fiVtD3Z+yd9ZV+V8e4IARVoMXWcJWSR6xkloitzfrRtJRwOYvmrcgugOalkmM0V4Gy/2aXeamuiBuUc4esDQEI3egmtAsHcVY1XCoYfs+9VqoHEq3vdr3UQ8zP/l+FP5UfcaJFCK/ZllqcO2P1GjIDVSHLdPpRHbMP/tU1a9mNZ5QMZBJ/JuJK/s+2bp8gpxKn8rh1akSQjlynlV9NI+7J3CC7CUf3bGvoXIrb37C/lpJehS39KNtcGdaRufKauSfqx/7SxA0zyZC+r13f7ASbMaQFzm+/RRusTqozY/p/MsWx8QIDAQAB

To test that your new DKIM record is correct, query it with the dig command. The TXT value response should be a single line split into multiple parts wrapped in double-quotes:

$ dig +short TXT mail._domainkey.example.com
"v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAqQMMqhb1S52Rg7VFS3EC6JQIMxNDdiBmOKZvY5fiVtD3Z+yd9ZV+V8e4IARVoMXWcJWSR6xkloitzfrRtJRwOYvmrcgugOalkmM0V4Gy/2aXeamuiBuUc4esDQEI3egmtAsHcVY1XCoYfs+9VqoHEq3vdr3UQ8zP/l+FP5UfcaJFCK/ZllqcO2P1GjIDVSHLdPpRHbMP/tU1a9mNZ5QMZBJ/JuJK/s+2bp8gpxKn8rh1akSQjlynlV9NI+7J3CC7CUf3bGvoXIrb37C/lpJehS39" "KNtcGdaRufKauSfqx/7SxA0zyZC+r13f7ASbMaQFzm+/RRusTqozY/p/MsWx8QIDAQAB"

Troubleshooting

MxToolbox has a DKIM Verifier that you can use to check your DKIM DNS record(s).

When using Rspamd, we recommend you turn on check_pubkey = true; in dkim_signing.conf. Rspamd will then check whether your private key matches your public key, and you can check possible mismatches by looking at /var/log/mail/rspamd.log.

DMARC

With DMS, DMARC is pre-configured out of the box. You may disable extra and excessive DMARC checks when using Rspamd via ENABLE_OPENDMARC=0.

The only thing you need to do in order to enable DMARC on a "DNS-level" is to add new TXT. In contrast to DKIM, DMARC DNS entries do not require any keys, but merely setting the configuration values. You can either handcraft the entry by yourself or use one of available generators (like this one).

Typically something like this should be good to start with:

_dmarc.example.com. IN TXT "v=DMARC1; p=none; sp=none; fo=0; adkim=r; aspf=r; pct=100; rf=afrf; ri=86400; rua=mailto:dmarc.report@example.com; ruf=mailto:dmarc.report@example.com"

Or a bit more strict policies (mind p=quarantine and sp=quarantine):

_dmarc.example.com. IN TXT "v=DMARC1; p=quarantine; sp=quarantine; fo=0; adkim=r; aspf=r; pct=100; rf=afrf; ri=86400; rua=mailto:dmarc.report@example.com; ruf=mailto:dmarc.report@example.com"

The DMARC status may not be displayed instantly due to delays in DNS (caches). Dmarcian has a few tools you can use to verify your DNS records.

SPF

What is SPF

Sender Policy Framework (SPF) is a simple email-validation system designed to detect email spoofing by providing a mechanism to allow receiving mail exchangers to check that incoming mail from a domain comes from a host authorized by that domain's administrators.

Source

Disabling the default SPF service policy-spf

Set ENABLE_POLICYD_SPF=0 to opt-out of the default SPF service. Advised when Rspamd is configured to handle SPF instead.

Adding an SPF Record

To add a SPF record in your DNS, insert the following line in your DNS zone:

example.com. IN TXT "v=spf1 mx ~all"

This enables the Softfail mode for SPF. You could first add this SPF record with a very low TTL. SoftFail is a good setting for getting started and testing, as it lets all email through, with spams tagged as such in the mailbox.

After verification, you might want to change your SPF record to v=spf1 mx -all so as to enforce the HardFail policy. See http://www.open-spf.org/SPF_Record_Syntax for more details about SPF policies.

In any case, increment the SPF record's TTL to its final value.

Backup MX & Secondary MX for policyd-spf

For whitelisting an IP Address from the SPF test, you can create a config file (see policyd-spf.conf) and mount that file into /etc/postfix-policyd-spf-python/policyd-spf.conf.

Example: Create and edit a policyd-spf.conf file at docker-data/dms/config/postfix-policyd-spf.conf:

debugLevel = 1
#0(only errors)-4(complete data received)

skip_addresses = 127.0.0.0/8,::ffff:127.0.0.0/104,::1

# Preferably use IP-Addresses for whitelist lookups:
Whitelist = 192.168.0.0/31,192.168.1.0/30
# Domain_Whitelist = mx1.not-example.com,mx2.not-example.com

Then add this line to compose.yaml:

volumes:
  - ./docker-data/dms/config/postfix-policyd-spf.conf:/etc/postfix-policyd-spf-python/policyd-spf.conf