Dkfilter

Note: DomainKeys has been obsoleted by DKIM, use OpenDKIM.

Dkfilter is a DomainKeys filter for Postfix.

What is it?

It is digital email signing/verification technology, which included into RFCs and already supported by many mail servers. (For example yahoo, google, etc).

How it works?

Sender signs email with private key.

Receiver gets signed email, request public key from DNS and verify it.

So you can check who actualy sent this email.

See RFC 4870 for more information.

Installation

Install the dkfilter package.

By default, you should add dkfilter user and group. If you do not want to do this, edit /etc/conf.d/dkfilter and change DKFILTER_USER and DKFILTER_GROUP.

Generic configuration

• Generate key:

$ openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out private.key
$ openssl rsa -in private.key -pubout -out public.key

• Adjust /etc/conf.d/dkfilter.
• Add DNS record with your selector (see DKFILTER_SELECTOR in /etc/conf.d/dkfilter, you may choose random name) and key:

server1._domainkey IN TXT "k=rsa; p=MHwwDQYJK ... OprwIDAQAB; t=y"

• Start/enable dkfilter-in.service and dkfilter-out.service.

Postfix integration

Inbound filter

Inbound filter gets connection from port 10025 and output filtered data to port 10026. (Inbound filter does not remove any data, it just adds verification result into mail)

Add following into /etc/postfix/master.cf:

#
# Before-filter SMTP server. Receive mail from the network and
# pass it to the content filter on localhost port 10025.
#
smtp      inet  n       -       n       -       -       smtpd
    -o smtpd_proxy_filter=127.0.0.1:10025
    -o smtpd_client_connection_count_limit=10
#
# After-filter SMTP server. Receive mail from the content filter on
# localhost port 10026.
#
127.0.0.1:10026 inet n  -       n       -        -      smtpd
    -o smtpd_authorized_xforward_hosts=127.0.0.0/8
    -o smtpd_client_restrictions=
    -o smtpd_helo_restrictions=
    -o smtpd_sender_restrictions=
    -o smtpd_recipient_restrictions=permit_mynetworks,reject
    -o smtpd_data_restrictions=
    -o mynetworks=127.0.0.0/8
    -o receive_override_options=no_unknown_recipient_checks

Outbound filter

Outbound filter gets connection from port 10027 and output signed data to port 10028.

Add following into /etc/postfix/master.cf:

#
# modify the default submission service to specify a content filter
# and restrict it to local clients and SASL authenticated clients only
#
submission  inet  n     -       n       -       -       smtpd
    -o smtpd_etrn_restrictions=reject
    -o smtpd_sasl_auth_enable=yes
    -o content_filter=dksign:[127.0.0.1]:10027
    -o receive_override_options=no_address_mappings
    -o smtpd_recipient_restrictions=permit_mynetworks,permit_sasl_authenticated,reject

#
# specify the location of the DomainKeys signing filter
#
dksign    unix  -       -       n       -       10      smtp
    -o smtp_send_xforward_command=yes
    -o smtp_discard_ehlo_keywords=8bitmime

#
# service for accepting messages FROM the DomainKeys signing filter
#
127.0.0.1:10028 inet  n  -      n       -       10      smtpd
    -o content_filter=
    -o receive_override_options=no_unknown_recipient_checks,no_header_body_checks
    -o smtpd_helo_restrictions=
    -o smtpd_client_restrictions=
    -o smtpd_sender_restrictions=
    -o smtpd_recipient_restrictions=permit_mynetworks,reject
    -o mynetworks=127.0.0.0/8
    -o smtpd_authorized_xforward_hosts=127.0.0.0/8