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DKIM

Revision as of 13:37, 14 September 2020 by AA-Andrew (talk | contribs) (What is DKIM?)


Support staff can help in adding DKIM signing for domains we host.

Important:

As with SPF, if you use DKIM/DMARC then you will also want to make sure that all email that you send from your domain is sent through email servers that will sign your messages - ie, all your outbound email should be sent out through smtp.aa.net.uk.

When using DKIM or SPF, you need to be careful that all you other devices, servers or services that send email from your domain all send through a server that will sign your messages.

What is DKIM?

This is just a brief overview of DKIM, you can read more about DKIM on other place or RFC6376

Also see SPF Record

DKIM is one of many methods which tries to protect email senders and recipients from spoofing/spam and to proove that an email hasn't been tampered with.

A domain owner can add DNS records which will publish a public key in a specially formatted TXT record. The email server they use for sending email (eg smtp.aa.net.uk) holds the matching private key, will sign messages and add this signature to the headers of the email. The record can either be a TXT record or a CNAME pointing to a KEY in you email provider's DNS.

Other mail servers which receive the email can then inspect the DNS record and the headers to check if it all matches up. If it does match up, then that means that the email has been sent through a server that is controlled by the domain owner. This gives some level of confidence that the email was actually sent by the owner of the domain and not spoofed.

Optionally, further DNS records (DMARC) can be added which declare to email servers receiving the message what should be done it it fails the DKIM test - eg, do nothing, block or quarantine.

Even if messages are just signed, then this can help them not to be marked as spam by the recipient. This can help prevent messages that our customers send to gmail/outlook being put in to spam folders.

Signing by smtp.aa.net.uk

Customers who meet the following criteria can have their messages signed by our smtp.aa.net.uk email relay:

  • We provide DNS services for the Domain
  • The three CNAME records below are added (via the control pages)
  • You use smtp.aa.net.uk to send out email

The DNS records required on the customer domain are as follows;

aaisp1._domainkey  CNAME   aaisp1.dkim.aa.net.uk
aaisp2._domainkey  CNAME   aaisp2.dkim.aa.net.uk
aaisp3._domainkey  CNAME   aaisp3.dkim.aa.net.uk

We have three selectors which enable us to roll over the keys easily.

What if I send email via other mail servers

If you use smtp.aa.net.uk and other email relays, then you will need to set up domainkey records for those mail servers too. They will use a different Selector and a different Key.

Testing the _domainkey DNS records

dig +short aaisp1._domainkey.YOURDOMAIN TXT
dig +short aaisp2._domainkey.YOURDOMAIN TXT
dig +short aaisp3._domainkey.YOURDOMAIN TXT

eg,:

dig +short aaisp1._domainkey.testing.me.uk TXT

It should give an answer starting v=DKIM1; if no answer, then it isn't working...

Also try an online tool such as https://mxtoolbox.com/dkim.aspx (there are many others!)

Testing if signing is working

There are various ways to test if signing is happening.

Send yourself an email If you send yourself an email, look at the header and you should see a DKIM-Signature: header which will include the signature along with s=aaisp1 which is the 'Selector we use'.

Send to a gmail account and check the DKIM report If you have access to a gmail account then send an email there, then in the little 3-dot menu in the email click 'Show Original'. You will then see the raw message but also information about SPF/DKIM/DMARC.

Contents

DMARC

This is just a brief overview of DKIM, you can read more about DKIM on other place or RFC7489

Adding a DMARC DNS record is optional, but can be beneficial.

If you add DMARC records then you also need to make sure that all the email you send is being sent through smtp relays which will sign your messages. ie, all your email for your domain would want to be sent through smtp.net.uk

There are various online generators for creating a dmarc record, but the contents of the record will cover the following basic settings:

  • 'v' - The version - usually DMARC1
  • 'p' - What to happen if dmarc fails, eg none, quarantine, reject
  • 'rua' - an email address (or multiple comma separated addresses) of where you'd like to XML receive machine readable statistic reports
  • 'ruf' - an email address (or multiple comma separated addresses) where you'd line to receive individual 'forensic' reports, these are XML machine readable copes of email that failed

There are more options available beyond the ones above.

The rua and ruf will be used by some of the bigger email providers - eg gmail, outlook, yahoo.

We'd suggest initially setting up reporting but no rejecting so that you can see how it's working out.

An example dmarc record which would send you reports but not tell other email severs to block if a message fails DKIM would look like this:

Host: _dmarc
Type: TXT
Value: v=DMARC1; p=none; rua=mailto:YOU@YOURDOMAIN.COM; ruf=mailto:YOU@YOURDOMAIN.COM; fo=1

Once you are satisified that DKIM is working and all the email you send is being properly then you can change the record so that p is set to quarantine or reject.