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.

Generating Keys

You should have:

• At least one email account setup
• Attached a volume for config to persist the generated files to local storage

RSA Key Sizes >= 4096 Bit

Keys of 4096 bits could be denied by some mail servers. According to RFC 6376, keys are preferably between 512 and 2048 bits.

DKIM is currently supported by either OpenDKIM or Rspamd:

OpenDKIMRspamd

OpenDKIM is currently enabled by default.

The command docker exec <CONTAINER NAME> setup config dkim help details supported config options, along with some examples.

Creating a DKIM key

Generate the DKIM files with:

docker exec -ti <CONTAINER NAME> setup config dkim

Your new DKIM key(s) and OpenDKIM config files have been added to /tmp/docker-mailserver/opendkim/.

LDAP accounts need to specify domains explicitly

The command is unable to infer the domains from LDAP user accounts, you must specify them:

setup config dkim domain 'example.com,example.io'

Changing the key size

The private key presently defaults to RSA-4096. To create an RSA 2048-bit key run:

setup config dkim keysize 2048

Restart required

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

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

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).

Creating DKIM Keys

You can simply run

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

which provides you with an overview of what the script can do. Just running

docker exec -ti <CONTAINER NAME> setup config dkim

will execute the helper script with default parameters.

Using Multiple Domains

Unlike the current script for OpenDKIM, the Rspamd script will not create keys for all domains DMS is managing, but only for the one it assumes to be the main domain (derived from DMS' domain name). Moreover, the default dkim_signing.conf configuration file that DMS ships will also only contain one domain. If you have multiple domains, you need to run the command docker exec -ti <CONTAINER NAME> setup config dkim domain <DOMAIN> multiple times to create all the keys for all domains, and then provide a custom dkim_signing.conf (for which an example is shown below).

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 exist, 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

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/supervisor/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. DNS TXT records values that are longer than 255 characters need to be split into multiple parts. This is why the public key has multiple parts wrapped within double-quotes between ( and ).

A DNS web-interface may handle this internally instead, while others may not, but expect the input as a single line_). You'll need to manually format the value as described below.

Your DNS record file (eg: mail.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 dkim-rsa._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/supervisor/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=4; 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 policyd-spf?

As of now, policyd-spf cannot be disabled. This is WIP.

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