dkim

A DKIM signing library in ruby.

Installation

sudo gem install dkim

Usage

Calling Dkim.sign on a string representing an email message returns the message with a DKIM signature inserted.

For example

mail = <<eos
To: [email protected]
From: [email protected]
Subject: hi

Howdy
eos

Dkim.sign(mail)

# =>
# To: [email protected]
# From: [email protected]
# Subject: hi
# DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com; q=dns/txt; s=mail; t=1305917829;
#   bh=qZxwTnSM1ywsrq0Ag9UhQSOtVIG+sW5zDkB+hPbuX08=; h=from:subject:to;
#   b=0mKnNOkxFGiww63Zu4t46J7eZc3Uak3I9km3IH2Le3XcnSNtWJgxiwBX26IZ5yzcT
#   VwJzcCnPKCScIJMQ7yfbfXmNsKVIOV6eSUqu1YvJ1fgzlSAXuDEMNFTjoto5rrdA+
#   BgX849hEY/bWHDl1JJgNpiwtpl4t0Q7M4BVJUd7Lo=
# 
# Howdy

Necessary configuration

A private key, a domain, and a selector need to be specified in order to sign messages.

These can be specified globally

Dkim::domain      = 'example.com'
Dkim::selector    = 'mail'
Dkim::private_key = open('private.pem').read

Options can be overridden per message.

Dkim.sign(mail, :selector => 'mail2', :private_key => open('private2.pem').read)

Additional configuration

The following is the default configuration

Dkim::signable_headers        = Dkim::DefaultHeaders # Sign only the specified headers
Dkim::signing_algorithm       = 'rsa-sha256' # can be rsa-sha1 or rsa-sha256 (default)
Dkim::header_canonicalization = 'relaxed'    # Can be simple or relaxed (default)
Dkim::body_canonicalization   = 'relaxed'    # Can be simple or relaxed (default)

The defaults should fit most users needs; however, certain use cases will need them to be customized.

For example, for sending mesages through amazon SES, certain headers should not be signed

Dkim::signable_headers = Dkim::DefaultHeaders - %w{Message-Id Resent-Message-ID Date Return-Path Bounces-To}

rfc4871 states that signers SHOULD sign using rsa-sha256. For this reason, dkim will not use rsa-sha1 as a fallback if the openssl library does not support sha256. If you wish to override this behaviour and use whichever algorithm is available you can use this snippet (not recommended).

Dkim::signing_algorithm = defined?(OpenSSL::Digest::SHA256) ? 'rsa-sha256' : 'rsa-sha1'

Usage With Rails

Dkim contains Dkim::Interceptor which can be used to sign all mail delivered by the mail gem or rails 3, which uses mail. For rails, create an initializer (for example config/initializers/dkim.rb) with the following template.

# Configure dkim globally (see above)
Dkim::domain      = 'example.com'
Dkim::selector    = 'mail'
Dkim::private_key = open('private.pem').read

# This will sign all ActionMailer deliveries
ActionMailer::Base.register_interceptor('Dkim::Interceptor')

Example executable

The library includes a dkimsign.rb executable suitable for testing the library or performing simple signatures.

dkimsign.rb DOMAIN SELECTOR KEYFILE [MAILFILE]

If MAILFILE is not specified dkimsign.rb will read the mail message from standard in.

Limitations

Strictly a DKIM signing library. No support for signature verification. (none planned)
No support for the older Yahoo! DomainKeys standard (RFC 4870) (none planned)
No support for specifying DKIM identity i= (planned)
No support for body length l= (planned)
No support for signature expiration x= (planned)
No support for copied header fields z= (not immediately planned)

Resources

RFC 4871
Inspired by perl's Mail-DKIM

Copyright

(The MIT License)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION