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"Replacing OpenDKIM with dkimpy-milter"
by Colin Cogle
Published Thursday, August 29, 2019.
Updated September 2, 2019 with new build instructions.
For almost a decade, DKIM (DomainKeys Identified Mail) has been one of the most important anti-spam and anti-spoofing technologies available. In that little window between someone sending an email and it going out to the Internet, a domain's email server sticks a little hidden signature in each message that says that the sender address is real and the message hasn't been modified. Receiving email servers can verify the signature to prove this.
However, DKIM's age was becoming apparent. Its predecessor, DomainKeys, which dated back as far as 2004, allowed RSA keys as small as 512 bits, and signatures with SHA-1. Naturally, for compatibility reasons, both of these glaring weaknesses made it into the final DKIM specification in 2011 (as RFC 6376, though using either weakness was discouraged). 1024-bit RSA keys were already considered weak at the time, but some people who tried to use 2048-bit (or larger) RSA keys found out that the resulting TXT record was too big for their registrar's DNS zone editor.
In 2017, the IETF created a working group to modernize DKIM, and they did (released as RFC 8301): 512-bit RSA keys were finally banned, and likewise, SHA-1 signatures were declared "historic" and "MUST NOT be used for signing or verifying." 4096-bit RSA keys were also permitted, but if you're one of the unlucky ones who can't fit a 2048-bit key into your DNS zone, this doesn't help you at all.
More importantly, a new key type was added to DKIM shortly thereafter, in RFC 8463: Curve25519. The main advantage is that elliptic curve algorithms like this can provide the same or better security as RSA, while using shorter keys; for example, a 256-bit EC key is as strong as a 3072-bit RSA key, so everything is faster and uses less power.
To support the old while phasing in the new, emails will be dual-signed with RSA and Ed25519, until RSA eventually falls out of style. Verifiers only need to verify one signature, so old ones will check the RSAone, and newer verifiers will check the Ed25519 one.
Since 2010, OpenDKIM has been one of the most popular DKIM signers and verifiers in the Linux world. However, it's laid dormant since 2015, with bug reports and feature requests being largely ignored. I decided it was time to make the switch to a new app that is being actively developed, dkimpy-milter.
Due to a bug in the pip version of this, I recommend building directly from source. Clone the Git repository to your server:
git clone -b master https://git.launchpad.net/dkimpy-milter cd dkimpy-milter sudo python3 ./setup.py install --record=/dev/null --single-version-externally-managed
It installed under /usr/local, but that's fine. Then, I used the code from their README to create a user account for it:
sudo adduser --system --no-create-home --quiet --disabled-login --disabled-password --shell /bin/false --group --home /run/dkimpy-milter dkimpy-milter
And now, set it to start up with your server:
sudo systemctl daemon-reload sudo systemctl enable dkimpy-milter sudo systemctl start dkimpy-milter sudo systemctl status dkimpy-milter
(If you don't like systemd, I believe dkimpy-milter includes a classic SysV init script.)
Stop, check for errors, and continue.
I was reading that dkimpy-milter was developed to be a drop-in replacement for OpenDKIM. So, I copied opendkim.conf over dkimpy-milter.conf and tried to start the service. I was greeted with plenty of warnings about unknown or unsupported options. Well, it looks like dkimpy-milter is still a work in progress on that front. I removed all of the "offending" lines.
I also adjusted permissions inside Postfix's chroot to make sure that dkimpy-milter could put its socket there.
The creators of dkimpy-milter stuck to OpenDKIM syntax for their configuration file. Instead of overloading existing options like "KeyTable" and "SelectorTable", there are new ones, such as "KeyTableEd25519" and "SelectorTableEd25519", that behave identically.
I have multiple domains with multiple selectors, so I went ahead and created new tables, started dkimpy-milter, and… I got an error saying none of those were valid options.
It turns out that OpenDKIM-style tables will be supported in the next version. So, for now, it's back to the drawing board.
I decided to create two keys for all of my domains to share, one RSA key and one Ed25519 key. Go to whatever directory you keep your keys in, and create and secure some new keys:
dknewkey -k rsa boring-old-selector dknewkey -k ed25519 cool-new-selector chown dkimpy-milter:dkimpy-milter boring-old-selector.key chown dkimpy-milter:dkimpy-milter cool-new-selector.key chmod 440 boring-old-selector.key chmod 440 cool-new-selector.key
You'll get two files, a .dns file with your DKIM DNS record -- go deploy those now (preferably with DNSSEC) -- and a .key file that contains your secret, that should only be readable by dkimpy-milter.
Make your configuration file look something like mine.
# List all of your domains. Domain colincogle.name, myotherdomain.com, another.site # Provide your RSA key information. KeyFile /etc/dkimkeys/boring-old-selector.key Selector boring-old-selector # Provide your Ed25519 key information. KeyFileEd25519 /etc/dkimkeys/cool-new-selector.key SelectorEd25519 cool-new-selector ##### POSTFIX CONFIGURATION ##### # Copy whatever setup you had with OpenDKIM. UserID dkimpy-milter:postfix UMask 007 PidFile /var/run/dkimpy-milter/dkimpy-milter.pid Socket local:/var/spool/postfix/dkimpy-milter.sock # Make dkimpy-milter sign as well as verify, so # Postfix can also check incoming DKIM signatures. Mode sv # This Postfix variable will tell dkimpy-milter what to do. MacroList daemon_name|ORIGINATING MacroListVerify daemon_name|VERIFYING
Restart dkimpy-milter. It still won't do anything, though, until we have our MTA send it some messages.
Edit your master.cf file, and look for your SMTP and Submission services. Obviously, we'll change the socket name. However, we need to add a macro name that will tell dkimpy-milter whether it will sign a message or verify one. This is new behavior in dkimpy-milter compared to OpenDKIM. Fortunately, Postfix already has this functionality via milter_macro_name:
Changes to your /etc/postfix/master.cf file:
smtp inet n - - - - smtpd - -o smtpd_milters=sock:/opendmarc.sock + -o smtpd_milters=sock:/dkimpy-milter.sock -o milter_macro_daemon_name=VERIFYING submission inet n - - - - smtpd - -o smtpd_milters=sock:/opendmarc.sock + -o smtpd_milters=sock:/dkimpy-milter.sock -o milter_macro_daemon_name=ORIGINATING
You should check your main.cf file to see if you've referenced OpenDKIM in there. If so, go ahead and change it accordingly. When you're all done, run a `postfix reload`.
Send some email and read the source, or hop over to my favorite DKIM testing site and send it an email.
When it arrives, you should see two separate signatures, one RSA and one Ed25519.
DKIM Information: DKIM Signature Message contains this DKIM Signature: DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/simple; d=colincogle.name; i=@colincogle.name; q=dns/txt; s=colin-ed25519; t=1567049647; h=from : content-type : mime-version : subject : message-id : date : to : from; bh=zrU0Ey8SLpTcRJGyugNEgYdAi2nzWHakc1f42rljNfg=; b=7PieJLlpiMRV0DYkyMeTZWavJIRhjAF2irj2qRzjlQflkpluUvr3t97NDkjBhMq6JSH+Nz/DX1o2wVhQluJxAg== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=colincogle.name; i=@colincogle.name; q=dns/txt; s=colin-rsa; t=1567049647; h=from : content-type : mime-version : subject : message-id : date : to : from; bh=zrU0Ey8SLpTcRJGyugNEgYdAi2nzWHakc1f42rljNfg=; b=wnrPT8C+JVBUVdoTC3YKQM6CRlH6E0PAyqaUi7PBqCJfQuUpTWkaog6zNe0dxKF4AvY0YPXBGlGiGIp1jyizIIbA6hqdQ4IhzFuQ4JXB9k2vVTd5tqeSLUOIhWCt8kiry2ShQmIi0tczBuVBLxq59ovfmQKSwZVg3rhUBDfsYtbydUESlcoOkn9fpT6727HcMbGsfzf67itycfm8w/hbjt7esWd5VMJ33wg8NYvte+mI/WpqNapwV8Reg5kQJ9FD3/vBOtKd8kM+DnzOciuqUZX0akQ2EwnujBEgHOVMqTKnYZUHT2Q1Pli31mSaXaAVxI2oVWIjIw82XfIEG5jK6Q== Signature Information: v= Version: 1 a= Algorithm: ed25519-sha256 c= Method: relaxed/simple d= Domain: colincogle.name s= Selector: colin-ed25519 q= Protocol: dns/txt bh= zrU0Ey8SLpTcRJGyugNEgYdAi2nzWHakc1f42rljNfg= h= Signed Headers: from : content-type : mime-version : subject : message-id : date : to : from b= Data: 7PieJLlpiMRV0DYkyMeTZWavJIRhjAF2irj2qRzjlQflkpluUvr3t97NDkjBhMq6JSH+Nz/DX1o2wVhQluJxAg== Public Key DNS Lookup Building DNS Query for colin-ed25519._domainkey.colincogle.name Retrieved this publickey from DNS: v=DKIM1; k=ed25519; s=email; t=s; p=csY5YoFbP8dojeDjEIQwFmb88vdA8l6Ip7fESx39wNc= Validating Signature result = pass
Look! A dual-signed email! And this Ed25519-aware validator checked the newer of the two signatures.
Congratulations! You're now helping to move the Internet forward, one tiny step at a time. Hopefully your dual signatures will make another postmaster somewhere Google to find out what you're doing, and why.
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"A Kick in the Glass" by Colin Cogle is licensed under Creative Commons Attribution-ShareAlike 4.0 International (CC-BY-SA).
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