DMARC and DKIM

Microsoft’s latest security newsletter included the fact that more than 90% of Fortune 500 companies have not fully implemented DMARC. Wow — that’s something I do at home! Worse still, the Fortune 500 company for which I work is in that 90% … a fact I hope to rectify this week. SPF is just some DNS entries that indicate the source IPs that are expected to be sending email from your domain. Lots of SPF record generators online.

DKIM is a little more involved, but it’s a lot easier now that packages for DKIM are available on Linux distro repositories. You still *can* build it from source, but it’s easier to install the OpenDKIM package.

Once the package is installed, generate the key(s) to be used with your domain(s).

cd /etc/opendkim/keys/
openssl genrsa -out dkim.private 2048
openssl rsa -in dkim.private -out dkim.public -pubout -outform PEM
# secure private key file
chown opendkim:opendkim dkim.private
chmod go-r dkim.private

Decide on the selector you are using — I use ‘mail’ as my selector. At work, I use ‘2017Q3Key’ — this allows us to change to a new key without in-transit mail being impacted. Old mail was sent with the 2017Q2 selector and *that* public key is in DNS. New mail comes across with 2017Q3 and uses the new DNS record to verify. I do *not* share these keys – anyone else sending mail from our domain needs to generate their own key (or I make one for them), use their own unique selector, and I will create the DNS records for their selector. When marketing engages a third party to send e-mails on our behalf, we have a 2017VendorName selector too.

Edit /etc/opendkim.conf. The socket line is not necessary – I just tend away from default ports as a habit. Since it’s bound to localhost, not such a big deal.

Mode sv
Socket   inet:8895@localhost
Selector mail
KeyFile /etc/opendkim/keys/dkim.private
KeyTable /etc/opendkim/KeyTable
SigningTable refile:/etc/opendkim/SigningTable
InternalHosts refile:/etc/opendkim/TrustedHosts

There’s a config option to “SendReports” — it’s a boolean that indicates if you want your system to send failure reports when the sender indicates they want such reports and provide a reporting address. Especially for testing purposes, I recommend indicating your domain wants reports — it is helpful in case you’ve got something configured not quite right and are failing delivery on some messages. As such, configure my installation to send reports. It’s additional overhead in cases where verification fails; I don’t see all that many failures, and it isn’t a lot of extra load. Since I know my installation will send detailed failure information, I can use my domain when testing new implementations.

Once you have the base configuration set, edit /etc/opendkim/SigningTable and add your domain(s) and the appropriate selector

*@rushworth.us mail._domainkey.rushworth.us
*@lisa.rushworth.us mail._domainkey.lisa.rushworth.us
*@scott.rushworth.us mail._domainkey.scott.rushworth.us
*@anya.rushworth.us mail._domainkey.anya.rushworth.us

Edit /etc/opendkim/KeyTable and map each selector from the SigningTable to a key file

mail._domainkey.rushworth.us rushworth.us:default:/etc/opendkim/keys/dkim.private
mail._domainkey.lisa.rushworth.us lisa.rushworth.us:default:/etc/opendkim/keys/lisa.dkim.private
mail._domainkey.scott.rushworth.us scott.rushworth.us:default:/etc/opendkim/keys/scott.dkim.private
mail._domainkey.anya.rushworth.us anya.rushworth.us:default:/etc/opendkim/keys/anya.dkim.private

Edit /etc/opendkim/TrustedHosts and add the internal IPs that relay your domain’s mail through the server (IP addresses or subnets)

Create DNS TXT records – the part after p= is the content of the public key file for that selector. When you are first setting up DKIM, use t=y (yes, we are just testing this). Once you confirm everything is functional, you can change to y=n (nope, really pay attention to our DKIM signature and policy). The policy is an individual preference. I use ‘all’ (all mail from my domain will be signed) and “o=-” (again all mail from my domain will be signed). You can use “o=~” (some mail from my domain is signed, some isn’t … who knows) and “dkim=unknown” (again, some is signed). You can use “dkim=discardable” (don’t just consider the message as more likely to be spam if it is not signed … you can outright drop the message). As a business, I don’t use this *just in case*. Something crazy happens – the dkim service falls over, your key gets mangled – and receiving parties can start dropping your messages. Using “dkim=all” means they are more apt to quarantine them as spam, but someone can go and get the messages. And hopefully notice something odd is happening.

mail._domainkey.domain.tld  TXT k=rsa;t=y;p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAzTnpc7tHfyH1zgT3Jx/JHmGSz8WCy1jvzu5QsYvDBmimKEHRY4Kz4mya5bOYsDQuJ/sz+BJo6xDwsUXCuyEkykIlgqP+7E9oK2EcW0dZms87SGmNEnNBN5iTe0pdzk1lXx2js3QdOWswO+cmA9F1Z8OzSR+2u79huugPFBHl79zFvOEHbigrmeHEfo0KHWpeNomf/xKx+wyYr1n3R5gS+28CeC3abSyKgmaYYRLoZsjrCLbEM0m2YPJRKd1ZGOObBMa4PZWj7pT07ISEjoNnXQ27BtcL/QjKKeLkbJ0UGEOSdPEJKuEpAUvYU9lA5hbtzrqiwdlPxWYocDVPrcqAHwIDAQAB
_adsp._domiankey.domain.tld TXT dkim=all
_domainkey.rushworth.us TXT t=y;o=-;r=dkim@lisa.rushworth.us
_ssp._domainkey.rushworth.us TXT t=y;dkim=all

 

Edit /etc/mail/sendmail.mc (using the port defined in /etc/opendkim.conf

INPUT_MAIL_FILTER(`dkim-filter’, `S=inet:8895@localhost’)

Make your sendmail.mc to sendmail.cf and verify that you’ve got the dkim-filter line

Xdkim-filter, S=inet:8895@localhost

Start opendkim, then restart sendmail. Now test it — inbound mail should have *their* DKIM signatures verified, outbound mail should be signed with the appropriate key.

Once you have verified your DKIM is functioning properly — well, first of all you can update your DNS records to remove testing mode. Then create your DMARC record:

_dmarc.rushworth.us     v=DMARC1; p=quarantine; sp=quarantine; rua=mailto:dmarc-rua@lisa.rushworth.us!10m; ruf=mailto:dmarc-ruf@lisa.rushworth.us!10m; rf=afrf; pct=100; ri=172800
 Again, you don’t need to use quarantine — ‘reject’ would recommend mail be dropped or ‘none’ recommends no action (good for testing). The rua (aggregate reporting email address) and ruf (address to recieve failing samples for analysis) should be in your domain.
You could add either/both “adkim=s” or “aspf=s” to indicate your DKIM or SPF adhere to strict standards. I use relaxed (default, do not need to specify it in the TXT record).
If you want the reports delivered to an address outside of your domain, that domain needs to publish a DNS record authorizing receipt of the reports:
     rushworth.us._report._dmarc.lisa.rushworth.us     v=DMARC1
Share this...
Share on FacebookPin on PinterestTweet about this on TwitterShare on LinkedIn

Leave a Reply

Your email address will not be published. Required fields are marked *