DKIM is a security measure targeting email spoofing. It is greatly recommended one activates it. See [the Wikipedia page](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 account**.

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):

```BASH
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 4096 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:

```BASH
docker run --rm \
  -v "$(pwd)/config":/tmp/docker-mailserver \
  -ti tvial/docker-mailserver:latest generate-dkim-config 4096
```

For LDAP systems that do not have any directly created user account you can run the following command (since `8.0.0`) to generate the signature by additionally providing the desired domain name (if you have multiple domains use the command multiple times or provide a comma-separated list of domains): 

```BASH
docker run --rm \
  -v "$(pwd)/config":/tmp/docker-mailserver \
  -ti docker-mailserver/docker-mailserver:latest generate-dkim-config <key-size> <domain.tld>[,<domain2.tld>]
```

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.

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

```

<details>
<summary>Configuration using a web interface</summary>

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.

</details>

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](https://addons.mozilla.org/en-US/thunderbird/addon/dkim-verifier): 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.