Setup DomainKeys Identified Mail (DKIM) in Postfix

DomainKeys Identified Mail (DKIM) is a method for email authentication that allows an organization to take responsibility for a message it has sent in a way that can be validated by a recipient. The technique is based on public-key cryptography: Responsibility is claimed by the signer by adding a digital signature to a message’s header, the DKIM-Signature header field. The verifier recovers the signer’s public key using the DNS, and then verifies that the signed parts have not been altered.

1. Install DKIM filter

apt-get install dkim-filter

2. Create a key for each domain verified

mkdir -p /etc/dkim/keys/domain1.com
cd /etc/dkim/keys/domain1.com
dkim-genkey -r -d domain1.com

Replace domain1.com with the domain that this mail server should authenticate using DKIM

3. Add a line for each domain to dkim-keys.conf file (pico /etc/dkim-keys.conf)

[email protected]:domain1.com:/etc/dkim/keys/domain1.com/default.private

Replace domain1.com with the domain that this mail server should authenticate using DKIM

4. Add a TXT record to the DNS for the domain being authenticated using DKIM. The record is automatically created and stored in /etc/dkim/keys/domain1.com/default.txt. You just need to add it to the DNS server. (cat /etc/dkim/keys/domain1.com/default.txt)

Here’s a sample output:

default._domainkey IN TXT "v=DKIM1; g=*; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC8IQNYlS+8jyrbAxNsghsPrWYgOQQWI0Ab4e9MToZYLTBGI41V3Zet5Awrt19nMIUlTpuE+/YVnjP/pu3fgeYkoe6NUzp+oEcWAioQXBmx0njigac7iJ/I0naTP1xTrDacnwsTp/F+lMwGgjiHpaJA7iBmL0AfYMXlTBo5pFog2QIDAQAB" ; ----- DKIM default for domain1.com

Repeat steps 2, 3 and 4 for every domain that this server should authenticate using DKIM.

5. Uncomment line 37 to make DKIM filter use the dkim-keys.conf file to look up domains available (pico /etc/dkim-filter.conf)

KeyList /etc/dkim-keys.conf

6. Add a inet socket that Postfix can communicate with (pico /etc/default/dkim-filter)

SOCKET="inet:[email protected]"

7. Restart DKIM filter

/etc/init.d/dkim-filter restart

8. Configure Postfix to query DKIM filter using the socket created earlier. Add these lines to main.cf (pico /etc/postfix/main.cf)

milter_default_action = accept
milter_protocol = 2
smtpd_milters = inet:localhost:8891
non_smtpd_milters = inet:localhost:8891

9. Reload Postfix config

postfix reload

11 comments

  1. In step 2 at end you should rename default.private to default. So the key refers correctly to default._domainkey.domain.com instead of default.private._domainkey.domain.com when the DKIM test is performed

  2. Does it work for ZIMBRA too.

    I tried but it failed to start.

    I am running postfix from zimbra installation.

    Please let me know if i’m doing anything wrong.

    My error log is given below.

    Restarting DKIM Filter: No /usr/sbin/dkim-filter found running; none killed.
    dkim-filter: /etc/dkim/keys/americanmutualloans.com/default.: open(): No such file or directory

    dkim-filter: /etc/dkim-filter.conf: key load from /etc/dkim-kets/conf failed

    Starting for DKIM verification only
    dkim-filter: /etc/dkim/keys/americanmutualloans.com/default.:open(): No such file or directory

    dkim-filter: /etc/dkim-filter.conf: ket load from /etc/dkim-keys.conf failed.

    I followed Cruz instructions to not use default.private.

    ANy help is highly appreciated .

    Thanks

  3. In my previous comment it was indeed

    dkim-filter: /etc/dkim-filter.conf: key load from /etc/dkim-keys/conf failed.

    Just a typographical error while writing here…

    Any help please come to my rescue.

    Thanks……..

  4. actually with debian squeeze the txt record should be:

    default.private._domainkey.example.com. 3 IN TXT “v=DKIM1; g=*; k=rsa; p=…”

  5. Thanks for that tuto. I had a problem with my first domain in the dkim-keys.conf file and guess what, the file was an UTF-8 type with a BOM instead of plain Asci…
    Jeeez, I almost have no more hair !

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