docker-mailserver/docs/content/config/best-practices/dkim.md

DKIM is a security measure targeting email spoofing. It is greatly recommended one activates it.

See https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail for more details on DKIM.

Enabling DKIM signature

To enable DKIM signature, you must have created at least one email accounts.

Once its done, just run the following command to generate the signature (here show from inside the directory of docker-compose.yml in order to use a volume on ./config):

docker run --rm \
  -v "$(pwd)/config":/tmp/docker-mailserver \
  -ti tvial/docker-mailserver:latest generate-dkim-config

The default keysize when generating the signature is 2048 bits for now. If you need to change it (e.g. your DNS-Provider limits the size), then provide the size as the first parameter of the command:

docker run --rm \
  -v "$(pwd)/config":/tmp/docker-mailserver \
  -ti tvial/docker-mailserver:latest generate-dkim-config 2048

Now the keys are generated, you can configure your DNS server with DKIM signature, simply by adding a TXT record.

If you have direct access to your DNS zone file, then it's only a matter of pasting the content of config/opendkim/keys/domain.tld/mail.txt in your domain.tld.hosts zone.

; OpenDKIM
mail._domainkey	IN	TXT	( "v=DKIM1; k=rsa; "
	  "p=AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN" )  ; ----- DKIM key mail for domain.tld

Configuration using a web interface

1. Generate a new record of the type TXT.
2. Paste mail._domainkey the Name txt field.
3. In the Target or Value field fill in v=DKIM1; k=rsa; p=AZERTYUGHJKLMWX....
4. In TTL (time to live): Time span in seconds. How long the DNS server should cache the TXT record.
5. Save.

After generating DKIM keys, you should restart the mail server. DNS edits may take a few minutes to hours to propagate.

Note: Sometimes the key in config/opendkim/keys/domain.tld/mail.txt can be on multiple lines. If so then you need to concatenate the values in the TXT record:

; OpenDKIM
mail._domainkey	IN	TXT	( "v=DKIM1; k=rsa; "
	  "p=AZERTYUIOPQSDF..."
	  "asdfQWERTYUIOPQSDF..." )  ; ----- DKIM key mail for domain.tld

the target (or value) field must then have all the parts together: v=DKIM1; k=rsa; p=AZERTYUIOPQSDF...asdfQWERTYUIOPQSDF...

Verify-only

If you want DKIM to only verify incoming emails, the following version of /etc/opendkim.conf may be useful (right now there is no easy mechanism for installing it other than forking the repo):

# This is a simple config file verifying messages only

#LogWhy                 yes
Syslog                  yes
SyslogSuccess           yes

Socket                  inet:12301@localhost
PidFile                 /var/run/opendkim/opendkim.pid

ReportAddress           postmaster@my-domain.com
SendReports             yes

Mode                    v

Debugging

Tools

• DKIM-verifer: A add-on for the mail client Thunderbird.

DKIM TXT Record

You can debug your TXT records with the dig tool.

dig TXT mail._domainkey.domain.tld

Output:

; <<>> DiG 9.10.3-P4-Debian <<>> TXT mail._domainkey.domain.tld
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 39669
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;mail._domainkey.domain.tld. IN	TXT

;; ANSWER SECTION:
mail._domainkey.domain.tld. 3600 IN TXT	"v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCxBSjG6RnWAdU3oOlqsdf2WC0FOUmU8uHVrzxPLW2R3yRBPGLrGO1++yy3tv6kMieWZwEBHVOdefM6uQOQsZ4brahu9lhG8sFLPX4MaKYN/NR6RK4gdjrZu+MYSdfk3THgSbNwIDAQAB"

;; Query time: 50 msec
;; SERVER: 127.0.1.1#53(127.0.1.1)
;; WHEN: Wed Sep 07 18:22:57 CEST 2016
;; MSG SIZE  rcvd: 310

Switch off DKIM

Simply remove the DKIM key by recreating (not just relaunching) the mailserver container.