TODO
DKIM Milter is a milter application that signs or 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. For example, it is lenient with regard to encoding problems actually occurring in header values: a header value containing invalid UTF-8 bytes is not a problem, but processed transparently as a byte string, etc.
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 happy user and which has inspired some choices made here.
TODO
DKIM Milter must be built or run from source code for now (initial development; working, but alpha quality).
A source checkout of viadkim in a sibling directory is also required.
During building and installation the option --features sha1 can be specified
to enable support for the insecure, historic SHA-1 hash algorithm. Use of this
feature is strongly discouraged.
TODO
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.
DKIM Milter is usually set up as a system service. Use the provided systemd service as a starting point.
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 sha1 above).
TODO
The default configuration file is /etc/dkim-milter/dkim-milter.conf. See the
included example configuration for how to configure the milter; documentation is
not complete yet.
TODO
The configuration is currently entirely file-based.
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 certain inputs through override
files. 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.
Overrides can be applied to recipients (given in the RCPT TO: SMTP command),
and to senders (in the Sender or From headers).
This design, with main config whose parameters can be overridden with some granularity, should be flexible enough to implement many configuration requirements.
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.
socket = inet:localhost:3000
signing_senders = /path/to/signing_senders_file
signing_keys = /path/to/signing_keys_file
authserv_id = mail.example.com
The senders for which messages should be signed instead of verified are in the
file configured in signing_senders:
```
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. (Multiple signatures are primarily useful for double-signing with both
an Ed25519 and an RSA key.)
The keys named in the fourth column in signing_senders are listed in the file
configured in signing_keys:
```
key1 key1pemfile key2 key2pemfile ```
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.
Currently no utilities are provided for key management. However, the openssl
utility from OpenSSL 3 can do everything for us. 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.)
TODO
For an extended explanation of key generation with the OpenSSL 3 command-line
utility, see README-keys.md.
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.
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