Introduction

DomainKeys Identified Mail (DKIM) is a method for E-mail authentication, allowing a person who receives email to verify that the message actually comes from the domain that it claims to have come from. The need for this type of authentication arises because spam often has forged headers.

DKIM uses public-key cryptography to allow the sender to electronically sign legitimate emails in a way that can be verified by recipients.

DKIM also guards against tampering with mail, offering almost end-to-end integrity from a signing to a verifying Mail transfer agent (MTA).

Read more on Wikipedia

dkim-milter is a milter-based application (dkim-filter) which plugs in to Postfix to provide DomainKeys Identified Mail service for your mail server. dkim-milter is no longer being developed, and it's original author has forked the source and is now developing opendkim. For Lucid and later releases, opendkim is preferred over dkim-filter. The instructions on this page should be the same (just with adjusted package names as needed).

Installation

We assume you already successfully installed Postfix MTA, if not, please read the Postfix dedicated page.

To install opendkim, you need Universe repositories added, if so, use your favorite package manager and install the package.

sudo aptitude install opendkim opendkim-tools

Important: For 12.04 Precise you must install opendkim from the precise backports. Note that backports are enabled only by using /precise-backports on a given package, so this will not affect any other packages you have installed.

sudo aptitude install opendkim/precise-backports
sudo aptitude install opendkim-tools/precise-backports

Configuration

opendkim configuration consists of two files:

/etc/opendkim.conf
/etc/default/opendkim

Use your favorite editor to edit those files. Here's an example of /etc/opendkim.conf file already edited to suit my needs:

# Log to syslog
Syslog                  yes
# Required to use local socket with MTAs that access the socket as a non-
# privileged user (e.g. Postfix)
#UMask                  002
# dkim-milter (2.5.2.dfsg-1ubuntu1) hardy:
# Disable new umask option by default (not needed since Ubuntu default
# uses a TCP socket instead of a Unix socket).

# Attempt to become the specified userid before starting operations.
#UserID                 105 # 'id postfix' in your shell


# Sign for example.com with key in /etc/mail/dkim.key using
# selector '2007' (e.g. 2007._domainkey.example.com)
Domain                  ubuntu.ro
KeyFile                 /etc/mail/dkim.key # See bellow how to generate and set up the key
Selector                mail

# Common settings. See dkim-filter.conf(5) for more information.
AutoRestart             yes
Background              yes
Canonicalization        relaxed/relaxed
DNSTimeout              5
Mode                    sv
SignatureAlgorithm      rsa-sha256
SubDomains              no
#UseASPDiscard          no
#Version                rfc4871
X-Header                no

#InternalHosts          /etc/mail/dkim-InternalHosts.txt
# The contents of /etc/mail/dkim-InternalHosts.txt should be
#   127.0.0.1/8
#   192.168.1.0/24
#   other.internal.host.domain.tld
# You need InternalHosts if you are signing e-mails on a gateway mail server
# for each of the computers on your LAN.


###############################################
# Other (less-standard) configuration options #
###############################################
#
# If enabled, log verification stats here
Statistics              /var/log/dkim-filter/dkim-stats
#
# KeyList is a file containing tuples of key information. Requires
# KeyFile to be unset. Each line of the file should be of the format:
#    sender glob:signing domain:signing key file
# Blank lines and lines beginning with # are ignored. Selector will be
# derived from the key's filename.
#KeyList                /etc/dkim-keys.conf
#
# If enabled, will generate verification failure reports for any messages
# that fail signature verification. These will be sent to the r= address
# in the policy record, if any.
#ReportInfo             yes
#
# If enabled, will issue a Sendmail QUARANTINE for any messages that fail
# signature verification, allowing them to be inspected later.
#Quarantine             yes
#
# If enabled, will check for required headers when processing messages.
# At a minimum, that means From: and Date: will be required. Messages not
# containing the required headers will not be signed or verified, but will
# be passed through
#RequiredHeaders        yes

Actually /etc/opendkim.conf is the most important file. It provides our milter with required information about selector (used for DNS requests and email verifications) and used signing key (the key is used for signing the outgoing emails).

Here's an example of /etc/default/opendkim This file is used to connect the milter to MTA:

# Command-line options specified here will override the contents of
# /etc/dkim-filter.conf. See dkim-filter(8) for a complete list of options.
#DAEMON_OPTS=""
#
# Uncomment to specify an alternate socket
# Note that setting this will override any Socket value in dkim-filter.conf
#SOCKET="local:/var/run/dkim-filter/dkim-filter.sock" # Debian default
#SOCKET="inet:54321" # listen on all interfaces on port 54321
SOCKET="inet:8891@localhost" # Ubuntu default - listen on loopback on port 8891
#SOCKET="inet:12345@192.0.2.1" # listen on 192.0.2.1 on port 12345

In my case, this file needs no additional editing.

Now, to tell the Postfix about the existing milter, and where to connect with it, edit your Postfix main.cf file /etc/postfix/main.cf, and append to its content the following data:

# DKIM
milter_default_action = accept
milter_protocol = 2
smtpd_milters = inet:localhost:8891
non_smtpd_milters = inet:localhost:8891

If you are using already some milter (for example Postfix/DomainKeys), you can add the new one like this:

milter_default_action = accept
milter_protocol = 2
smtpd_milters = inet:localhost:8891,inet:localhost:8892
non_smtpd_milters = inet:localhost:8891,inet:localhost:8892

Key generation for dkim-milter and its setup with DNS

The opendkim-tools package provides a tool, opendkim-genkey for creating your key pairs:

opendkim-genkey -t -s mail -d ubuntu.ro

This will generate two files: mail.private which is your private key, and mail.txt which is your DNS record containing your public key.

The -s argument supplies the selector (in our case "mail"), the -d argument supplies the domain, and the -t argument says that we are running DKIM in test mode. This indicates that verifiers shouldn't drop your mail if something's wrong with the signature. Its seems that the majority using DKIM run it in test mode.

Copy your private key in place:

cp mail.private /etc/mail/dkim.key

Now create your DNS record as supplied in mail.txt, which should look like this:

mail._domainkey.ubuntu.ro. IN TXT "v=DKIM1; g=*; k=rsa; p=PpYHdE2tevfEpvL1Tk2dDYv0pF28/f 5MxU83x/0bsn4R4p7waPaz1IbOGs/6bm5QIDAQAB" ; ----- DKIM mail for ubuntu.ro

Startup and testing

Once configuration above was done, the daemon can be started with:

sudo service opendkim start

If it doesn't start, search the logs for problems and see what it requires more:

grep -i dkim /var/log/mail.log

Instead of using sudo service opendkim start you can run dkim-filter directly:

dkim-filter -x /etc/dkim-filter.conf

If you get the error like: dkim-filter: milter socket must be specified Then try manually specifying the socket. Use this to specify local (which does not match /etc/default/dkim-filter above):

dkim-filter -x /etc/dkim-filter.conf -p local

Now restart the Postfix MTA, and check for email signing:

sudo service postfix restart

For testing purposes, I recommend you tools like:

• http://www.sendmail.org/dkim/tools

• or just send an email to autorespond+dkim[at]dk.elandsys.com

Testing results should look like this in Gmail:

Common errors and fixes

Missing signature

If something is not functioning properly (emails are not being signed) look for errors in the log:

grep -i dkim /var/log/mail.log

The following error indicates the filter ran properly however could not match the outgoing email domain with a filter (and thus no signature was generated):

Nov 21 06:59:56 appname dkim-filter[2911]: 81AA7E688: no signature data

Consider changing the domain to a wildcard in /etc/opendkim.conf:

# Sign for example.com with key in /etc/mail/dkim.key using
# selector '2007' (e.g. 2007._domainkey.example.com)
Domain                  *
KeyFile                 /etc/mail/dkim.key
Selector                mail

Using a domain of * will require putting the dkim key into EACH domain's DNS zone file for those domains that send email using this server. The dkim signing will work for your server, but without updating each DNS zone file, the public key will not be found by the recipient mail server.

Multiple signatures

If amavis-new is installed and dkim signs emails multiple times with same domain and selector, is the configuration error likely to be how you feed messages back to postfix from amavis.

Head to /etc/postfix/master.cf look for the section starting with:

127.0.0.1:10025 inet    n       -       -       -       -       smtpd

Find the option -o receive_override_options= and add ,no_milters to the end of that line. A corrected config could look something like this:

127.0.0.1:10025 inet    n       -       -       -       -       smtpd
        -o content_filter=
        -o local_recipient_maps=
        -o relay_recipient_maps=
        -o smtpd_restriction_classes=
        -o smtpd_delay_reject=no
        -o smtpd_client_restrictions=permit_mynetworks,reject
        -o smtpd_helo_restrictions=
        -o smtpd_sender_restrictions=
        -o smtpd_recipient_restrictions=permit_mynetworks,reject
        -o smtpd_data_restrictions=reject_unauth_pipelining
        -o smtpd_end_of_data_restrictions=
        -o mynetworks=127.0.0.0/8
        -o smtpd_error_sleep_time=0
        -o smtpd_soft_error_limit=1001
        -o smtpd_hard_error_limit=1000
        -o smtpd_client_connection_count_limit=0
        -o smtpd_client_connection_rate_limit=0
        -o receive_override_options=no_header_body_checks,no_unknown_recipient_checks,no_milters