DKIM Milter

DKIM Milter is a milter application that signs and verifies email messages using the DomainKeys Identified Mail (DKIM) protocol. It is meant to be integrated with a milter-capable MTA (mail server) such as Postfix. DKIM is specified in RFC 6376.

DKIM Milter is based on the viadkim library. Therefore, it inherits the approach to DKIM used in that library. Notably, viadkim fully supports internationalised email, including Unicode signing domains in the d= tag (RFC 8616). More practically, it inherits the performance characteristics of viadkim when processing many, or large, messages, and integrates them with the asynchronous-paradigm milter implementation.

DKIM Milter tries to work efficiently. Public key queries are done in parallel. When multiple signatures use the same parameters for calculating the body hash, the hash is calculated only once, and the result is shared among the signatures. Also, when the body hash does not need to be calculated, such as when the signature was already determined to be failing, body processing is skipped entirely. A further example is the handling of large messages: DKIM Milter processes message bodies in chunks of fixed size, meaning that it does not come under pressure even when processing hundreds of messages of one or two megabytes each concurrently; the total memory used for these messages’ bodies at any point in time will never exceed a few megabytes or so (ie, a ceiling relative to the number of messages, not their size).

DKIM Milter can be used as a simple alternative to the OpenDKIM milter. Credit goes to that project, of which I have been a long-time user and which has inspired some choices made here.

Installation

DKIM Milter is a Rust project. It can be built and/or installed using Cargo as usual. For example, use the following command to install the latest version published on crates.io:

cargo install --locked dkim-milter

During building and installation the option --features pre-rfc8301 can be specified to revert cryptographic algorithm and key usage back to before RFC 8301: it enables support for the insecure, historic SHA-1 algorithm, and allows use of RSA key sizes below 1024 bits. Use of this feature is discouraged.

As discussed in the following sections, the default, compiled-in configuration file path is /etc/dkim-milter/dkim-milter.conf. When building DKIM Milter, this default path can be overridden by setting the environment variable DKIM_MILTER_CONFIG_FILE to the desired path.

The minimum supported Rust version is 1.67.0.

Usage

Once installed, DKIM Milter can be started on the command-line as dkim-milter.

Configuration parameters can be set in the default configuration file /etc/dkim-milter/dkim-milter.conf. The mandatory parameter socket must be set in that file.

Invoking dkim-milter starts the milter in the foreground. Send a termination signal to the process or press Control-C to shut the milter down. While the milter is running, send signal SIGHUP to reload the configuration.

DKIM Milter is usually set up as a system service. Use the provided systemd service as a starting point. See the included tutorial document for how to create the system service.

The supported signature algorithms, for both signing and verifying, are rsa-sha256 and ed25519-sha256. By default, the historic signature algorithm rsa-sha1 is not supported, evaluation of such signatures yields a permerror result (RFC 8301; but see feature pre-rfc8301 above).

Configuration

The default configuration file is /etc/dkim-milter/dkim-milter.conf. The included manual page dkim-milter.conf(5) serves as the reference documentation. (You can view the manual page without installing by passing the file’s path to man: man ./dkim-milter.conf.5)

See the included example configuration for what a set of configuration files might look like.

For a hands-on introduction to getting started with DKIM Milter, see the included tutorial document.

Design

The configuration is currently entirely file-based. In the future, other data sources such as an SQL database may be added.

The configuration consists at the minimum of the main configuration file dkim-milter.conf. The main configuration file contains global settings.

The global settings can be overridden for selected inputs through overrides in table-like override files. Overrides can be applied to connecting network addresses, recipients (given in the RCPT TO: SMTP command), and to senders (in the Sender or From headers).

For example, the recipient_overrides parameter can be used to specify configuration overrides for certain message recipients. This allows, for example, to disable use of the l= tag in generated signatures globally, but enable it for certain recipients only.

This design, with main configuration whose parameters can be overridden with some granularity, should be flexible enough to implement many configuration requirements.

Sign–verify decision

For all messages passed to DKIM Milter, the decision whether the message should undergo verification or signing is made in the following way.

If a message comes from a trusted source and is submitted by an originator that matches a configured signing sender, then the message is signed. If a message comes from an untrusted source, it is verified instead. In other words, a message from a trusted source is authorised or eligible for signing; it is not eligible for verification.

A trusted source is either a connection from an IP address in trusted_networks (default: loopback), or, if trust_authenticated_senders is set (default: yes), a sender that has been authenticated.

The originator of a message is taken from the message’s Sender header if present, else from the message’s From header. (Usually, Sender is not present, so the originator will be taken from From; however, if From contains multiple mailboxes, Sender must be included according to RFC 5322, and thus the originator will then be taken from Sender.)

Signing senders are senders (domains or email addresses) for which a signing key and signing configuration have been set up. They are configured in the table referenced by parameter signing_senders.

The operating mode (sign-only, verify-only, or automatic per the above procedure) can also be configured with the mode parameter.

Signing senders

Signing configuration is set up through two configuration parameters pointing to table-like files. These parameters are signing_senders and signing_keys.

signing_senders = /path/to/signing_senders_file
signing_keys = /path/to/signing_keys_file

The main idea for configuring signing is the signing senders table (parameter signing_senders). This table links sender email addresses to a concrete signing configuration:

# Sender expression   Domain        Selector   Signing key name
example.org           example.org   sel1       key1
.example.org          example.org   sel2       key2

The sender expression example.org matches senders with that domain (me@example.org). The sender expression .example.org matches both senders with that domain and also subdomains (me@subdomain.example.org). Caution: Every matching sender expression results in an additional DKIM signature for the message. In above example, messages from me@example.org are signed with two keys because both sender expressions match the address. (Multiple signatures are primarily useful for double-signing with both an Ed25519 and an RSA key.)

The keys named in the fourth column in the signing senders table are listed in the signing keys table (parameter signing_keys):

# Key name   Key source
key1         </path/to/signing_key1_pem_file
key2         </path/to/signing_key2_pem_file

The key source must currently always be a file path prefixed with <, pointing to a PKCS#8 PEM file. The signing key type (RSA or Ed25519) is detected automatically.

Additional per-signature (ie, per sender expression match) configuration overrides can be specified in the optional fifth column in the signing_senders file.

Some additional features are briefly mentioned in the remainder of this section. In the domain column, a single dot . copies the domain from the matching sender address. The two entries in the following listing are equivalent, both generate tag d=example.com:

# Sender expression   Domain        ...
example.com           example.com   ...
example.com           .             ...

The signing senders table is also where the signing identity, that is the i= tag in the generated signature is configured: By default, signatures do not include the signing identity; use of the @ character in the domain column enables the signing identity.

# Sender expression   Domain/Identity   ...

example.com           @example.com      ...
example.com           @.                ...
# both => d=example.com, i=@example.com

# Double dot separates i= subdomain from d= domain.
mail.example.com      @mail..example.com
# => d=example.com, i=@mail.example.com

# The local-part before the @ may be included literally.
example.com           user@example.com
# => d=example.com, i=user@example.com

# A dot before the @ copies the sender’s local-part into the i= tag.
example.com           .@example.com
# => d=example.com, i=user@example.com

Key setup

Currently no utilities are provided for key management. However, we can use the openssl utility from OpenSSL 3 to provide keys manually. The following tutorial uses exclusively that tool to do all key setup.

For signing, DKIM Milter reads signing keys (private keys) from files in PKCS#8 PEM format. This format can be recognised by its beginning line -----BEGIN PRIVATE KEY-----.

First, generate an RSA 2048-bit or an Ed25519 private key file private.pem with the following commands, respectively:

openssl genpkey -algorithm RSA -out private.pem
openssl genpkey -algorithm ED25519 -out private.pem

The corresponding public key for each signing key must be published in DNS in a special TXT record at domain <selector>._domainkey.<domain>.

The minimal format for the TXT record is as follows, where <key_type> must be either rsa or ed25519 for the respective key type, and <key_data> must be the properly encoded public key data as explained in the following paragraph:

v=DKIM1; k=<key_type>; p=<key_data>

If the key to publish in DNS is of type RSA, use the following command: Extract the public key from the RSA private key as the final Base64-encoded <key_data> value:

openssl pkey -in private.pem -pubout -outform DER |
  openssl base64 -A

If the key to publish in DNS is of type Ed25519, use the following command: First extract the public key from the Ed25519 private key, and then extract and produce the final Base64-encoded <key_data> value:

openssl pkey -in private.pem -pubout |
  openssl asn1parse -offset 12 -noout -out /dev/stdout |
  openssl base64 -A

For example, here is a key record produced in this manner looked up in DNS with the dig utility. Notice selector ed25519.2022 and domain gluet.ch:

dig +short ed25519.2022._domainkey.gluet.ch txt

"v=DKIM1; k=ed25519; p=7mOZGVMZF55bgonwHLfOzwlU+UAat5//VJEugD3fyz0="

(The Ed25519 key record above fits in a single text string. The much larger RSA key record is usually spread over several text strings. How such large TXT records need to be set up depends on DNS software and/or DNS provider.)

Licence

Copyright © 2022–2023 David Bürgin

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see https://www.gnu.org/licenses/.