Configuring for DKIM Signing

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Article Information
This article applies to the following ZCS versions. ZCS 8.0

Zimbra Server with DKIM Signing

DomainKeys Identified Mail (DKIM) lets an organization take responsibility for a message that is in transit. The organization is a handler of the message, either as its originator or as an intermediary. Their reputation is the basis for evaluating whether to trust the message for further handling, such as delivery. Technically DKIM provides a method for validating a domain name identity that is associated with a message through cryptographic authentication

Configuring ZCS for DKIM signing

Starting with Zimbra 8.0, the ability to add DKIM signing to outgoing mail is available. Signing is done at the domain level. Setting up signing consists of two steps:

1. Running zmdkimkeyutil to generate the DKIM keys and selector. The generated data is stored in the LDAP server as part of the domain LDAP entry.
2. Updating the DNS server with the public DNS entry

The zmdkimkeyutil utility

The zmdkimkeyutil script allows you to do the following:

1. Add DKIM data to a domain that does not currently have DKIM enabled
2. Update DKIM data for a domain that already has DKIM enabled
3. Query the DKIM data for a domain
4. Remove the DKIM data for a domain

The domain "example.com" will be used throughout this wiki. Substitute it with your domain.

Adding DKIM data to a domain with no existing DKIM configuration

 /opt/zimbra/libexec/zmdkimkeyutil -a -d example.com

After the data is generated, the public DNS record data that must be added for the domain to your DNS server will be displayed:

 zimbra@example.com:~$ /opt/zimbra/libexec/zmdkimkeyutil -a -d example.com
 DKIM Data added to LDAP for domain example.com with selector 0E9F184A-9577-11E1-AD0E-2A2FBBAC6BCB
 Public key to enter into DNS:
 0E9F184A-9577-11E1-AD0E-2A2FBBAC6BCB._domainkey IN TXT "v=DKIM1;=rsa;
 p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDY5CBg15nZ2vYnRmrNub6Jn6ghQ2DXQbQgOJ/E5IGziUYEuE2OnxkBm1h3jived21uHjpNy0naOZjLj0xLyyjclVy1chrhSbsGAhe8HLXUsdXyfRvNTq8NWLsUnMEsoomtJCJ
 /6LYWYU1whOQ9oKZVAwWHSovAWZpByqNMZmFg7QIDAQAB" ; ----- DKIM 0E9F184A-9577-11E1-AD0E-2A2FBBAC6BCB for example.com

Updating DKIM data for a domain

 /opt/zimbra/libexec/zmdkimkeyutil -u -d example.com

Whenthe DKIM keys are updated, the DNS server will need to be reloaded with the new TXT record. It is advised to leave the previous TXT record in DNS for a period of time to allow verification of emails that were signed with the previous key to continue to succeed.

Removing DKIM data for a domain

 /opt/zimbra/libexec/zmdkimkeyutil -r -d example.com

This command deletes the DKIM data from LDAP. New emails will no longer be signed for the domain. The DNS TXT record should remain for a period of time to allow verification of emails signed with this key.

Retrieving the stored DKIM data for a domain

 /opt/zimbra/libexec/zmdkimkeyutil -q -d example.com

This command will output all the stored DKIM information, specifically

 DKIM Domain
 DKIM Selector
 DKIM Private Key
 DKIM Public Signature
 DKIM Identity

Updating DNS

1. The public key DNS record should appear as a TXT resource record at:

SELECTOR._domainkey.DOMAIN
The Selector is the first portion of the output from zmdkimkeyutil In the above example, it is 0E9F184A-9577-11E1-AD0E-2A2FBBAC6BCB

1. Once you have added the record to your nameserver, reload DNS.
2. Verify that the DNS server is returning the DNS record.

 dig -t txt SELECTOR._domainkey.DOMAIN NAMESERVER
 Example:
 dig -t txt 0E9F184A-9577-11E1-AD0E-2A2FBBAC6BCB._domainkey.example.com ns.example.com

1. If the key is retrieved correctly, then use /opt/zimbra/opendkim/bin/opendkim-testkey to verify that the public key matches the private key.

 /opt/zimbra/opendkim/bin/opendkim-testkey -d example.com -s 0E9F184A-9577-11E1-AD0E-2A2FBBAC6BCB -x /opt/zimbra/conf/opendkim.conf

Revoking a DKIM key in DNS

If it becomes necessary to revoke a DKIM signing key, this can be easily done in DNS by using an empty "p=" tag in the TXT record.