Setting up an outbound DKIM provider

tested on netbsd and slackware

this one pissed me off because of unix permissions on the unix socket — one could also discard DKIM and only rely on SPF — it was finally solved using umask 0007 and adding postfix to the opendkim group — oops it should be the inverse, adding opendkim to the postfix group

Install

See dkim-install.

Da key pair

the selector can be anything, it is just a marker e.g. use month/year to remind yourself how old the key pair will become (and eventually renew it once a year or so)

MONTHYEAR=`date +%b%Y | tr A-Z a-z`
mkdir -p ~/certs/dkim/
cd ~/certs/dkim/
opendkim-genkey -h
opendkim-genkey --selector=$MONTHYEAR --domain=nethence.com
ls -lF $MONTHYEAR.private $MONTHYEAR.txt
cat $MONTHYEAR.txt

Da DNS record

add this record to your zone and check

host -t txt $MONTHYEAR._domainkey.nethence.com

Setup

note the UID vs GID difference here

#vi /usr/pkg/etc/opendkim.conf
vi /etc/opendkim.conf

Domain                  nethence.com
KeyFile                 /root/certs/dkim/MONTHYEAR.private
Selector                MONTHYEAR
Socket                  local:/var/opendkim/dkim-socket
Syslog                  Yes
UMask                   0007
UserID                  opendkim:postfix
PidFile                 /var/opendkim/opendkim.pid
Mode            sv
#SignatureAlgorithm rsa-sha256
SignatureAlgorithm rsa-sha1
AllowSHA1Only       Yes

fix that folder permissions so the socket gets shared AND BECOMES WRITABLE BY POSTFIX

usermod -aG postfix opendkim

#exists on Slackware after install from source
#mkdir /var/opendkim/

#netbsd
#/var/db/opendkim/

chmod 770 /var/opendkim/
chown -R opendkim:postfix /var/opendkim/

Ready to go

make sure the user and group exist

ls -lhF /usr/local/sbin/opendkim
opendkim -V
ls -alF /var/opendkim/
grep postfix /etc/group
grep dkim /etc/group
grep dkim /etc/passwd

start & enable

vi /etc/rc.local

echo dkim...
/usr/local/sbin/opendkim -x /etc/opendkim.conf && echo done || echo FAIL
#-P -l -u

status

ps auxww | grep dkim
cat /var/opendkim/opendkim.pid
#ls -alF /var/db/opendkim/
ls -alF /var/opendkim/

stop

pkill opendkim

Postfix

See the DKIM section from the Postfix guide.

Acceptance

outgoing

Now send a mail an MUA. For some reason, it does not even try to DKIM here when trying to send it locally.

#date | mail -s `uname -n` root

then watch the logs and look for DKIM-Signature header in the resulting message’s source.

incoming

TODO - try to check if wrongly signed incoming messages are refused

Troubleshooting

compilation time issues

#39 Impossible to install OpenDkim : milter not found https://sourceforge.net/p/opendkim/support-requests/39/?limit=25

#9 ./configure –without-milter –disable-filter doesn’t work https://sourceforge.net/p/opendkim/bugs/9/

OpenSSL missing during ./configure. How to fix? https://superuser.com/questions/371901/openssl-missing-during-configure-how-to-fix

shared object

ld: /usr/local/ssl/lib/libcrypto.a(ecp_mont.o): relocation R_X86_64_32 against `.rodata.str1.1' can not be used when making a shared object; recompile with -fPIC
...

==> --disable-shared

unix socket perms

postfix/cleanup[15894]: warning: connect to Milter service unix:/var/opendkim/dkim-socket: Permission denied

==> several issues need to be considered but in short, postfix needs write access to the socket

Postfix - Opendkim - Unable to connect to local socket https://serverfault.com/questions/724584/postfix-opendkim-unable-to-connect-to-local-socket

postfix/smtpd: warning: connect to Milter service unix:/var/run/opendkim/opendkim.sock: No such file or directory https://unix.stackexchange.com/questions/74477/postfix-smtpd-warning-connect-to-milter-service-unix-var-run-opendkim-opendki

oh and the other way around, when opendkim got added to postfix group

postfix/sasl/smtpd[1809]: warning: connect to Milter service unix:/var/opendkim/dkim-socket: Permission denied

==> 770 / 660

Resources

dkim-milter https://sourceforge.net/projects/dkim-milter/

DomainKeys Identified Mail (DKIM) http://dkim.org/

OpenDKIM http://opendkim.org/

DKIM http://silas.net.br/tech/apps/netbsd-mailserver.html#dkim

Postfix before-queue Milter support http://www.postfix.org/MILTER_README.html

Postfix Configuration Parameters http://www.postfix.org/postconf.5.html

Configure DomainKeys (OpenDKIM) with Postfix on CentOS 7 https://www.linuxtechi.com/configure-domainkeys-with-postfix-on-centos-7/

Set Up DKIM (DomainKeys Identified Mail) Working With Postfix On CentOS Using OpenDKIM - Page 2 https://www.howtoforge.com/set-up-dkim-domainkeys-identified-mail-working-with-postfix-on-centos-using-opendkim-p2#-testing-your-setup

Configure SPF and DKIM With Postfix on Debian 8 https://www.linode.com/docs/email/postfix/configure-spf-and-dkim-in-postfix-on-debian-8/

Part 4: How to Set up SPF and DKIM with Postfix on Ubuntu Server https://www.linuxbabe.com/mail-server/setting-up-dkim-and-spf

5 common mistakes to avoid when deploying DMARC https://www.dmarcanalyzer.com/common-mistakes-deploying-dmarc/

https://securityintelligence.com/understanding-the-spf-and-dkim-spam-filtering-mechanisms/

alternatives

Chapter 56 - Support for DKIM (DomainKeys Identified Mail) http://exim.org/exim-html-4.85/doc/html/spec_html/ch-support_for_dkim_domainkeys_identified_mail.html

Mail-DKIM and DKIMproxy http://dkimproxy.sourceforge.net/


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