I'm finishing up writing what I have learned in the last couple of weeks on setting up a DKIM/DK signing/verifying mail system using Postfix, milters, amavisd-new and SpamAssassin. The following text will be part of the documentation for amavisd-new (text is also available at http://www.ijs.si/software/amavisd/amavisd-new-docs.html ), but may be general enough so I hope it can be of interest to the SpamAssassin community.
Comments, experience, suggestions and further discussion on the topic is most welcome. If considered off-topic, off-list mail would be welcome too. Mark Setting up DKIM and DomainKeys mail signing and verification ============================================================ The goals of DKIM and DomainKeys are: * assurance of sender identities * protection against message tampering. A DKIM draft states the following, which applies to its predecessor DomainKeys as well: DomainKeys Identified Mail (DKIM) defines a mechanism by which email messages can be cryptographically signed, permitting a signing domain to claim responsibility for the introduction of a message into the mail stream. Message recipients can verify the signature by querying the signer's domain directly to retrieve the appropriate public key, and thereby confirm that the message was attested to by a party in possession of the private key for the signing domain. A gentle introduction and deployment guide is available at: http://antispam.yahoo.com/domainkeys. Except for some minor details, it applies to DKIM system as well. With added support in Postfix 2.3 for a milter protocol, it became possible to use with Postfix many of existing milters (mail filters) that were originally developed with sendmail in mind. It was hoped that a widespread use of milters with sendmail offered a fertile ground for software development, producing software of sufficient quality to be able to use it with Postfix. It remains to be seen whether quality of freely available milters comes anywhere close to high standards we are accustomed to with Postfix, but with a bit of luck and reasonable expectations, some of it can be put to good use. Two of such milters are dkim-milter offering support for DomainKeys Identified Mail (DKIM) Signatures, and dk-milter, offering support for Domain-based Email Authentication (DomainKeys). The DomainKeys (DK) is a predecessor of DKIM, as recognized by draft-delany-domainkeys-base-06: The DomainKeys specification was a primary source from which the DomainKeys Identified Mail [DKIM] specification has been derived. The purpose in submitting this document is as an historical reference for deployed implementations written prior to the DKIM specification. At the time of this writing it appears the dkim-milter is more reliable and better maintained than dk-milter, which is slowly fading into oblivion. Similar holds true in the world of Perl modules: there are modules Mail::DomainKeys and Mail::DKIM, both of which can be used by SpamAssassin. Again the Mail::DKIM seems to be of higher quality than an older Mail::DomainKeys. SpamAssassin makes it very easy to use each or both of them (for verification only), just by enabling the already provided plugins. Despite DomainsKeys slowly giving grounds to DKIM, the DomainsKeys is currently still in use by several large players in the Internet world, so this section will describe how to integrate both of them with Postfix and amavisd-new (an after-queue content filter) into a mail system. Mail signing and verification is a two-part job: signing of originating mail (or mail being redistributed) from our domain, and verifying signatures of incoming mail. Both tasks can be done by the same program, or they can be performed by separate entities. Traditionally with sendmail, both tasks are performed by the same milter, which may be easier to maintain, but has certain disadvantages. Verifying signatures should be performed early, before any local mail transformations get a chance of invalidating signature, e.g. by performing MIME conversions to quote-printable, by fixing syntactically invalid mail header, by editing/inserting/removing certain header fields, or by a local mailing list modifying mail text, e.g. by appending footnotes. Signing outgoing mail should be performed late, after mail sanitation, after conversion to 7-bit characters (to avoid later uncontrollable changes by a relaying or receiving MTA), and after adding header fields by a content filter. Similar applies to local mailing lists, which may be rewriting messages, requiring them to be re-signed by the domain hosting a mailing list, just before being sent out. Since SpamAssassin only provides signature verification but not signing, one obvious choice for signing is to use dkim-milter and dk-milter in signing-only mode, invoked by a Postfix smtpd service which is receiving content-checked mail from a content filter such as amavisd-new. As this second-stage smtpd service does not reliably know how a given message came into a mail system and whether it is supposed to be signed or not, a clean solution is to provide two parallel paths through a content filter, one used for mail to be signed (originating mail), the other for all the rest: +------+ |verify| (verify) +--+---+ SpamAssassin ^ ^v incoming: | +----++-----+ MX ----> 25 smtpd ---> 10024 > >--> 10025 smtpd --> submission: | | SASL --> 25 smtpd \ | amavisd | +-> | | mynets-> 25 smtpd ---> 10026 >ORIGINATING>--> 10027 smtpd --> --> 587 smtpd ---> +-----------+ | (convert to 7-bit) v +----+ |sign| +----+ There are other benefits to providing two parallal paths: a content filter may be configured to apply different rules and settings to mail that is known to be originating from our users, compared to the rest. Some suggestions: apply less strict banning rules, enable spam administrator notifications for internally originating spam and viruses, letting SpamAssassin rules be conditionalized based on amavisd-new policy banks loaded, etc. For verification there are two choices: either a SpamAssassin plugin can do it by calling Perl modules, or a milter in verification-only mode can be invoked by the incoming Postfix smtpd service. In the setup described, SpamAssassin sees almost the same message as a milter on the incoming smtpd would (just few header fields including a Received are prepended), so there is no concern that a signature would become invalid. Invoking signature verification by SpamAssassin has an advantage that DKIM-based or DomainKeys-based whitelisting or scoring can be used, but has a disadvantage that possibly not all mail is checked (e.g. large mail may be exempt from spam checks). On the other hand, invoking signature verification by calling a milter from incoming smtpd service can insert a header field reporting a result of verification, which may be used by MUA or by SpamAssassin rules, although currently no such rules are provided ready-made. For the purpose of gathering experience, both methods for signature verification are described. It turns out that one or the other implementation (in C or in Perl) may be buggy, and being able to compare both results of signature verification can help with troubleshooting. Let's begin by starting both milters, each in two instances, one dedicated for signing, the other for verification. For security purposes all milters should be run under a dedicated username, certainly not as root, not as user amavis and not as user postfix: dk, verifying: dk-filter -u dkfilter -b v -m MTA -H \ -l -p inet:[EMAIL PROTECTED] -P /var/run/dk-filter-v.pid dkim, verifying: dkim-filter -u dkfilter -b v -m MTA \ -l -p inet:[EMAIL PROTECTED] -P /var/run/dkim-filter-v.pid dk, signing: dk-filter -u dkfilter -b s -m ORIGINATING \ -H -c nofws \ -d example.com -S myselector -s /var/db/dkim/mykey.pem \ -l -p inet:[EMAIL PROTECTED] -P /var/run/dk-filter-s.pid dkim, signing: dkim-filter -u dkfilter -b s -m ORIGINATING \ -c relaxed/simple -S rsa-sha1 \ -d example.com -s myselector -k /var/db/dkim/mykey.pem \ -l -p inet:[EMAIL PROTECTED] -P /var/run/dkim-filter-s.pid Note that both signing milters may (but need not) use the same signing key and the same selector. Generating a pair of public and private key and publishing a public key and a policy in DNS is described in the documentation of each milter. Now we can tie both verifying milters to a Postfix smtpd service listening for incoming mail: master.cf: smtp inet n - n - 300 smtpd -o milter_default_action=accept -o milter_macro_daemon_name=MTA -o smtpd_milters=inet:127.0.0.1:4442,inet:127.0.0.1:4443 and tie both signing milters to a Postfix smtpd service that is receiving checked mail from amavisd, intended to be signed: master.cf: # mail return from a content filter (non-signing) 10025 inet n - n - - smtpd -o content_filter= ... (other options, mail not to be signed) ... # mail from our users returning from a content filter (DKIM or DK signing) 10027 inet n - n - - smtpd -o content_filter= ... (other options, mail to be signed) ... -o milter_default_action=accept -o milter_macro_daemon_name=ORIGINATING -o smtpd_milters=inet:127.0.0.1:4444,inet:127.0.0.1:4445 Note the order for signing milters: dk-milter is invoked first, and dkim-milter second, so that DK signature will be protected by a later DKIM signature. The reason is that DomainKeys specification does not protect its own signature in its signed data, but the newer DKIM fixes this omission and does protect its own signature. Default content filter is to be amavisd, listening on port 10026 (intended for signing). Locally submited or SASL-authenticated mail will go to a content filter on this default port 10026 and will be signed on its way out. All other mail (incoming) will be diverted to port 10024 for content filtering by a final catchall FILTER, and will never hit the signing milter: main.cf: content_filter = smtp-amavis:[127.0.0.1]:10026 smtpd_recipient_restrictions = ... permit_mynetworks permit_sasl_authenticated reject_unauth_destination ... check_sender_access regexp:/etc/postfix/filter_10024_catchall permit /etc/postfix/filter_10024_catchall: /^/ FILTER smtp-amavis:[127.0.0.1]:10024 In SpamAssassin all that is necessary is to add (or uncomment) lines in any of the .pre files (e.g. in local.pre, or in init.pre and v312.pre): loadplugin Mail::SpamAssassin::Plugin::DomainKeys loadplugin Mail::SpamAssassin::Plugin::DKIM Perl modules Mail::DomainKeys (version 0.86 or better) and Mail::DKIM (0.19 or better) need to be installed. The following SpamAssassin rules (in local.cf) work fairly well, giving verified mail a little bit of advantage and slightly favourize mail from some popular domains, and encourage people to start signing their mail. Possible signed spam can be counterbalanced by other measures (see below). score DK_VERIFIED -1.5 score DK_POLICY_SIGNSOME 0 score DK_POLICY_TESTING 0 score DKIM_VERIFIED -1.5 # DKIM and DK-based whitelisting may be used reliably: score USER_IN_DKIM_WHITELIST -3.0 whitelist_from_dkim [EMAIL PROTECTED] whitelist_from_dk [EMAIL PROTECTED] In amavisd.conf two parallel paths need to be provided, one receiving on port 10024 and forwarding to 10025, the other receiving on port 10026 and forwarding to 10027. $inet_socket_port = [10024,10026]; # listen on two ports The 10024/10025 will use a default policy bank, the other, dedicated to mail needing to be signed, will use a policy bank ORIGINATING: $forward_method = 'smtp:[127.0.0.1]:10025'; # MTA with non-signing service $notify_method = 'smtp:[127.0.0.1]:10027'; # MTA with DKIM signing service # switch policy bank to 'ORIGINATING' for mail received on port 10026: $interface_policy{'10026'} = 'ORIGINATING'; $policy_bank{'ORIGINATING'} = { # mail originating from our users # force MTA to convert mail to 7-bit before DKIM signing # to avoid later conversions which could destroy signature: smtpd_discard_ehlo_keywords => ['8BITMIME'], # # forward to a smtpd service providing DKIM/DomainKeys signing service: forward_method => 'smtp:[127.0.0.1]:10027', # # other special treatment of locally originating mail, e.g.: spam_admin_maps => ["[EMAIL PROTECTED]"], # warn of spam from us banned_filename_maps => ['ALT-RULES'], # more relaxed rules... }; The smtpd_discard_ehlo_keywords=>['8BITMIME'] serves to persuade Postfix to convert mail to 7-bit quoted-printable before submitting it to content filtering and signing (this configuration variable was introduced with version 2.4.3 of amavisd-new). Avoiding 8-bit characters in mail body makes signatures less susceptible to breaking by some relaying or receiving MTA over which we have no control. Note that the same effect (making Postfix convert outgoing mail to 7-bits before DKIM signing) could be achieved by a Postfix setting smtp_discard_ehlo_keywords=8bitmime on a smtp service feeding mail to be signed to amavisd, but this would require setting up two such services, one with the option and one without. While testing how the configured system plays with some mailing lists (such as postfix-users or SpamAssassin users list), one has to keep in mind that amavisd-new caches spam checking results of recently seen message bodies, so a mail going out to a mailing list is not yet signed as it reaches a content filter, but the SpamAssassin verdict is remembered as that point (claiming the message is not signed). When this message with unchanged body comes back from a mailing list, this time signed in the header by our domain, the signature should prove correct, yet the cached result from a minute ago still claims the message is not signed. If this is a problem, one can turn off caching of spam checking results for ham by setting: $spam_check_negative_ttl = 0; ome experience with DKIM and DomainKeys Recent versions of software components must be used to avoid bugs and interoperability problems with earlier versions: * use Postfix 2.3.3 or later (fixing minor 2.3 problems with milter); * amavisd-new 2.4.3 polished some corner issues on modifying mail header on releasing from a quarantine and defanging, and added some goodies affecting DKIM and DomainKeys to facilitate integration; * Mail::DomainKeys 0.86 or later must be used, the last couple of versions squashed several bugs, one at a time. Version 0.84 dropped the use of unreliable Email::Address (which could cause deep recursion in evaluating regular expressions, bringing processing to a halt). Nevertheless, one patch is still needed ( http://www.ijs.si/software/amavisd/Mail-DomainKeys-mark.patch ), hopefully to be included with the next version; * Mail::DKIM 0.19 is very solid, it is the only component where no bugs or design flaws were found during experimenting; in fact, this module helped to discover bugs in other components; * SpamAssassin 3.1.5 added whitelisting based on verified DomainKeys signatures, similar to what was already available for DKIM whitelisting in earlier versions of SA; * both the dkim-milter 0.5.1 and the dk-milter 0.4.1 need a patch as described in the Postfix documentation file MILTER_README. The dkim-milter already supplies a required patch in its bug tracking system under "[1537905] delayed queue ID"; which will be included in the next release; * the dk-milter 0.4.1 seems to be neglected lately and is rather buggy compared to dkim-milter. Nevertheless, with command options as given in the above example, it does its job sufficiently well in a described Postfix + amavisd-new setup, so that it may be deployed for a production use; see its bug tracking system for details; * the dkim-milter 0.4.1 works well with simple/simple or relaxed/simple canononicalization algorithms, while the relaxed/relaxed is to be avoided (and may even be dropped when the specification reaches a RFC stage); Instead of signing mail with dkim-milter, the same can be achieved by using Jason Long's DKIM Proxy, which is a Perl program calling a Perl module Mail::DKIM, i.e. the same modules as used by a SpamAssassin DKIM plugin. As the Mail::DKIM turned out to be a reliable and quite efficient module, this may be a good alternative to dkim-milter (which is also quite good). On the other hand there exist a dkfilter SMTP-proxy by the same author, which calls a Perl module Mail::DomainKays, which in turn is not recommended for processing of entire messages because of its design limitation which requires loading the whole message into memory. The use of Mail::DomainKays from within SpamAssassin does not represent such a problem, as messages checked by SpamAssassin are already limited in size. Mail transformations as performed by some mailing lists are the most challenging problem facing DKIM and DomainKeys deployment (and to other schemes as well). Nevertheless, mailing lists can be configured to either avoid transformations which invalidate mail signatures, or can re-sign fan-out mail. Examples of mailing lists which work very well with DKIM and DomainKeys, preserving existing signatures provided by posters, are the postfix-users ( [EMAIL PROTECTED] ) and the SpamAssassin users list ( users@spamassassin.apache.org ). Example of re-signing mailing lists are Yahoo groups. A representative of another type of mailing lists is currently Mailman, which modifies mail body, but at least it is stripping original signatures and adding a Sender header, so that original signatures do not appear to be broken, they are just missing, and mail may be re-signed. Several big players are already signing mail from their users or employees: Yahoo! (worldwide), gmail.com and google.com, eBay, Earthlink, Cisco, (not to mention porcupine.org :) When signatures are missing on mail from domains which are known to be signing all their mail (yahoo.com, gmail.com), the most common reason is that a sender submitted his mail through some other provider, but supplied his Yahoo or gmail e-mail address in the From header field. Similar to other schemes designed to prevent faking of sending address, the DKIM (and the DomainKeys) encourage mail submission only through a domain which is used in the From address - although there are other possibilities for roving users, especially in the DKIM system. People will need to become aware that their best choice is to submit mail through their home domain to prevent their messages being treated as second-class or appear suspicious. Note that some spam is also being signed by DomainKeys or DKIM lately, which is a good thing -- it indicates the sender owns (or ownz) a domain they are sending mail from. This either shows sender's sincere interest of not hiding behind a faked sender mail address (in which case such mail can be easily filtered if desired), or they are using short-lived temporary domain (perhaps through domain kiting), which can be counteracted by black lists of few-days old freshly registered domains. Adding a small negative spam score to successfully verified mail will encourage people to start signing their mail, benefiting legitimate senders and recipients, while signed spam can be counterbalanced by other measures. Signing and verifying mail is a good mechanism for companies to reliably whitelist mail from their partner companies or frequent clients. Mark
