Dkfilter : Proxy SMTP (signatures mails) made by Yahoo
Contents
1 Introduction
DomainKeys is an anti-spam software application in development at Yahoo that uses a form of public key cryptography to authenticate the sender's domain. dkfilter is an SMTP-proxy designed for Postfix. It implements DomainKeys message signing and verification. It comprises two separate filters, an "outbound" filter for signing outgoing email on port 587, and an "inbound" filter for verifying signatures of incoming email on port 25. This document is to describe step by step how to install dkfilter for postfix to deploy domainkeys signing and verification.
+------+
|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|
+----+2 Installation
2.1 Postfix
Install postfix for your domain to send and receive mails with this doc :
Installation et configuration de Postfix et Courrier
2.2 CPAN Perl modules
Dkfilter is written in Perl. It requires the following Perl Modules from CPAN archive.
- Crypt::OpenSSL::RSA
- Mail::Address
- MIME::Base64
- Net::DNS
- Test::More
- Text::Wrap
- Mail::DomainKeys
Following commands would help :
perl -MCPAN -e'CPAN::Shell->install("Crypt::OpenSSL::RSA")'
perl -MCPAN -e'CPAN::Shell->install("Mail::Address")'
perl -MCPAN -e'CPAN::Shell->install("MIME::Base64")'
perl -MCPAN -e'CPAN::Shell->install("Net::DNS")'
perl -MCPAN -e'CPAN::Shell->install("Test::More")'
perl -MCPAN -e'CPAN::Shell->install("Text::Wrap")'
perl -MCPAN -e'CPAN::Shell->install("Email::Address")'
perl -MCPAN -e'CPAN::Shell->install("Mail::DomainKeys")'Note :' Also resolve any dependent Perl Module required in installing the above Perl modules.
2.3 Dkfilter
The following steps are recommended for installing dkfilter.
2.3.1 Download
Download dkfilter from following URL :
wget http://jason.long.name/dkfilter/dkfilter-0.11.tar.gz
2.3.2 Installation
Installing dkfilter :
tar xvf dkfilter-0.11.tar.gz cd dkfilter-0.11 ./configure --prefix=/usr/local/dkfilter make install useradd dkfilter
The filter scripts will be installed in /usr/local/dkfilter/bin and the Perl module files will be in /usr/local/dkfilter/lib.
3 Configuration
3.1 Inbound Filter
We need to make relevant changes inside Postfix configuration files to check incoming mails for the signature.
vi /etc/postfix/master.cf
# Dkfilter Inbound Part1
smtp inet n - n - - smtpd
-o smtpd_proxy_filter=127.0.0.1:10029
-o smtpd_client_connection_count_limit=10
### Amavis filter ###
amavis unix - - n - 2 smtp
-o smtp_data_done_timeout=1200
-o disable_dns_lookups=yes
127.0.0.1:10025 inet n - n - - smtpd
-o content_filter=
-o local_recipient_maps=
-o relay_recipient_maps=
-o smtpd_restriction_classes=
-o smtpd_client_restrictions=
-o smtpd_helo_restrictions=
-o smtpd_sender_restrictions=
-o smtpd_recipient_restrictions=permit_mynetworks,reject
-o mynetworks=127.0.0.0/8
-o strict_rfc821_envelopes=yes
### End of Amavis Filter ###
# Dkfilter Inbound Part2
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
Insert above lines in the last of the files. Here we define that mail will received after smtp for verification on 127.0.0.1 with port 10026. You can define your own desired IP address on which you want to listen for signature checking.
3.2 Outbound filter
The outbound filter needs access to the private key used for signing messages. In addition, in needs to know the name of the key selector being used, and what domain it should sign messages for. This information is specified with command-line arguments to dkfilter.out.
3.2.1 Key pair
- Generate a private/public key pair and publish the public key in DNS :
cd /usr/local/dkfilter openssl genrsa -out private.key 1024 openssl rsa -in private.key -pubout -out public.key
This creates the files private.key and public.key in the current directory, containing the private key and public key. Make sure private.key is not world-readable, but still readable by the dkfilter user.
- Pick a selector name... e.g. m1
3.2.2 Bind
- Put the public-key data in DNS, in your domain, using the selector name you picked. Copy the contents of the public.key file and remove the PEM header and footer, and paste it in dns zone file by creating a TXT entry, like this:
_deimos.fr TXT "t=y; o=-;" m1._deimos.fr TXT "g=; k=rsa; p=MIGxm...MeQIDAQAB;"
where m1 is the name of the selector chosen in the last step and the p= parameter contains the public-key as one long string of characters.
3.2.3 Postfix
Finally, configure Postfix to filter outgoing, authorized messages only through the dkfilter.out service on port 10027. In the following example, messages sent via SMTP on port 587 (the submission port) will go through an After-Queue content filter that signs messages with DomainKeys.
vi /etc/postfix/master.cf
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
dksign unix - - n - 10 smtp
-o smtp_send_xforward_command=yes
-o smtp_discard_ehlo_keywords=8bitmime
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
Execute postfix reload for Postfix to respond to changes in /etc/postfix/master.cf :
postfix reload
4 Start / Stop script
This is the startup/shutdown script :
#!/bin/sh
#
# Copyright (c) 2005 Messiah College.
# Modified by Pierre Mavro
DKFILTERUSER=dkfilter
DKFILTERGROUP=dkfilter
DKFILTERDIR=/usr/local/dkfilter
HOSTNAME=`hostname -f`
DOMAIN=`hostname -d`
DKFILTER_IN_ARGS="--hostname=$HOSTNAME 127.0.0.1:10029 127.0.0.1:10026"
DKFILTER_OUT_ARGS="--keyfile=$DKFILTERDIR/private.key --selector=postfix --domain=$DOMAIN --method=nofws --headers 127.0.0.1:10027 127.0.0.1:10028"
DKFILTER_IN_BIN="$DKFILTERDIR/bin/dkfilter.in"
DKFILTER_OUT_BIN="$DKFILTERDIR/bin/dkfilter.out"
PIDDKFILTER_IN="/var/run/dkfilter.in"
PIDDKFILTER_OUT="/var/run/dkfilter.out"
case "$1" in
start)
echo -n "Starting inbound DomainKeys-filter (dkfilter.in)..."
start-stop-daemon --start -q -p$PIDDKFILTER_IN -u $DKFILTERUSER -g $DKFILTERGROUP -x `$DKFILTER_IN_BIN $DKFILTER_IN_ARGS` &
RETVAL=$?
if [ $RETVAL -eq 0 ]; then
echo done.
else
echo failed.
exit $RETVAL
fi
echo -n "Starting outbound DomainKeys-filter (dkfilter.out)..."
start-stop-daemon --start -q -p $PIDDKFILTER_OUT -u $DKFILTERUSER -g $DKFILTERGROUP -x `$DKFILTER_OUT_BIN $DKFILTER_OUT_ARGS` &
RETVAL=$?
if [ $RETVAL -eq 0 ]; then
echo done.
else
echo failed.
exit $RETVAL
fi
;;
stop)
echo -n "Shutting down inbound DomainKeys-filter (dkfilter.in)..."
start-stop-daemon --stop -p $PIDDKFILTER_IN
RETVAL=$?
if [ $RETVAL -eq 0 ]; then
echo done.
else
echo failed.
fi
echo -n "Shutting down outbound DomainKeys-filter (dkfilter.out)..."
start-stop-daemon --stop -p $PIDDKFILTER_OUT
RETVAL=$?
if [ $RETVAL -eq 0 ]; then
echo done.
else
echo failed.
exit $RETVAL
fi
;;
restart)
$0 stop
$0 start
;;
*)
echo "Usage: $0 {start|stop|restart}"
exit 1
;;
esac
Copy that script in /etc/rc.d/init.d (Red Hat System) or /etc/init.d (Debian System) and edit it as per your requirement.
Then add the good rights :
chmod 755 /etc/init.d/dkfilter
You can also this command on debian :
update-rc.d dkfilter defaults
