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DKIM features for Sympa

DKIM has been introduced in Sympa version 6.1.

DKIM is a crytographic signature method designed to prevent spam, phishing etc. As postmaster or listmaster you should consider 1) checking the DKIM status of each incoming message and 2) signing all or a subset of outgoing messages.

Processing of DKIM status of incoming messages is done by the MTA (Message transfert Agent) that delivers emails to the Sympa server Sympa domain (in order to reject some messages). This topic is not addressed in the chapter of the documentation.

The mailing list server can take advantage of incoming DKIM signature in order to measure the trust of the message while evaluating message workflow. This is based on scenario mechanism. An authentication level named dkim can be used within scenario rules to check that an incoming message has a valid DKIM signature (dkim signature status = pass).

In addition, you must consider signature of outgoing messages. Should messages brodcasted by Sympa to list subscribers be signed by your organization? Should all of them be signed? Should a subset of trusted messages be signed? Should service messages (automatic answer, welcome messages etc) be signed ?

Prerequisites

DKIM features in Sympa are based on the Mail::DKIM cpan module ; you should install it first. Check the documentation related to cpan modules installation.

Incoming messages

To make Sympa check the DKIM signature of incoming messages, you need to set the dkim_feature configuration parameter to on. Before doing that you must first update your customized scenario to introduce dkim authentication method, otherwise Sympa may reject messages because they include a valid DKIM signature !. All default scenario starting at version 6.1 already include rules for DKIM, both for command and lists messages.

What kind of changes is required in scenarios?

Turning on the dkim_feature configuration parameter will provide a new authentication level to the scenario engine. Scenario evaluation for incoming messages with a valid DKIM signature (but no S/MIME signature) will be evaluated with authentication method dkim. So rules that use authentication method smtp will not match.

Example:

  is_subscriber([listname],[sender])   smtp      request_auth
  is_subscriber([listname],[sender])   md5,smime do_it

Those 2 rules will not match any messsage with a valid DKIM signature, you must replace them with one of the following:

  is_subscriber([listname],[sender])   smtp,dkim request_auth
  is_subscriber([listname],[sender])   md5,smime do_it

  is_subscriber([listname],[sender])   smtp           request_auth
  is_subscriber([listname],[sender])   dkim,md5,smime do_it

If you choose the second solution, you accept DKIM as a valid authentication mecanism.

If the front MTA adds the Authentication-results header, Sympa can of course check this SMTP header field using standard match() and equal() scenario conditions.

Outgoing messages

You may want to make Sympa sign outgoing messages. Almost every aspects of DKIM signature behavior can be customized via Sympa configuration parameters. Please check the DKIM parameters section for further details. Note that each parameter can also be set for a given virtual robot; and most of them are available as list parameter.

Which messages should be signed

In order to configure Sympa for signing outgoing messages, you have to decide which messages Sympa should sign . This should be decided for four kind of messages:

  • Services messages : these are all messages sent by Sympa itself : welcome messages, answers to mail commands, various notification such as remind message and digest messages;
  • List messages : messages distributed to list members (where the initial From: header is preserved). These messages will fall is one one the following subcategory:
    • authenticated messages (using S/MIME signature, challenge or password);
    • received with a valid DKIM signature;
    • validated by one of the list editors;
    • other messages.

This behavior is controlled by dkim_add_signature_to and dkim_signature_apply_on parameters.

Prerequisites for DKIM signing

Before Sympa is able to DKIM-sign messages, you need to set several related parameters. The most important ones are dkim_private_key_path (private key file location) and dkim_selector. Other parameters related to RFC 4871: dkim_signer_domain, dkim_signer_identity,dkim_header_list.

The private key is a PEM encoded RSA key 1) (a PEM encoded key include base64 encoded informations and starts with —–BEGIN RSA PRIVATE KEY—–.). The public key associated with that private key must be published in a DNS TXT record for entry <selector>._domainkey.<domain> where <selector> is dkim_selector and <domain> is dkim_signer_domain. The signer domain should be the domain of the list ; this is the default, don't change it unless you have strong reason for it.

example with selector = 'lists' and domain 'sympa.org':

  lists._domainkey.sympa.org. IN TXT "v=DKIM1; g=*; k=rsa; t=y; p=MDB34............DB"

In order to generate the public and private keys, you may use openssl or dkim-genkey included in milter_dkim software. There are also online tools to generate them, but those services will generate the private key for you (is it still a private key?).

* http://www.socketlabs.com/services/dkwiz
* http://www.port25.com/support/support_dkwz.php

Summary of parameters

parameter name (sympa.conf or robot.conf context) default overwritten by (list configuration)
dkim_feature off not pertinent
dkim_add_signature_to list,robot not pertinent
dkim_signature_apply_on
1) The private key can't be encrypted with a passphase
manual/dkim.txt · Last modified: 2016/09/12 09:29 (external edit)

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