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OpenDKIM is an open source implementation of the DomainKeys Identified Mail (DKIM) sender authentication system.

DKIM is supported by most common mail providers, including Yahoo, Google and

The idea

Basically, DKIM digitally signs all messages from the server to verify that the message actually was sent from the domain in question and is not forged or modified.

  • The sender's mail server signs outgoing email with the private key.
  • When the message arrives, the receiver (or their server) reads the public key from the domain’s TXT records and verifies the signature.

This ensures the message was sent from a server whose private key matches the domain's public key.

See RFC 6376 for more information.


Install the opendkim package.


The main configuration file for the signing service is /etc/opendkim/opendkim.conf.

  • Copy/move the sample configuration file /usr/share/doc/opendkim/opendkim.conf.sample to /etc/opendkim/opendkim.conf and change the following options:
KeyFile                 /path/to/keys/server1.private
Selector                myselector
Socket                  inet:8891@localhost
UserID                  opendkim
  • Socket address is the one specified in /etc/postfix/ This is what /etc/postfix/ should contain:
# For use by dkim milter
smtpd_milters = inet:localhost:8891
non_smtpd_milters = $smtpd_milters
milter_default_action = accept
  • To generate a secret signing key, you need to specify the domain used to send mails and a selector which is used to refer to the key. You may choose anything you like, see the RFC for details, but alpha-numeric strings should be OK:
$ opendkim-genkey -r -s myselector -d
  • Sometimes mails get reformatted on their way (e.g. tab exchanged for spaces), rendering the DKIM signature invalid. To prevent trivial reformatting in header and body destroying trust, there is Canonicalization, a policy stating how strict formatting is to be conserved. Available settings are simple for no reformatting allowed and relaxed for some reformatting allowed. For details see RFC 4871 3.4. These can be set individually for header and body:
Canonicalization        relaxed/simple

This example allows some reformatting of the header but not in the message body. Default settings for openDKIM are simple/simple.

  • Other configuration options are available. Make sure to read the documentation.
  • Enable and start the opendkim.service. Read Daemons for more information.

DNS Record

Add a DNS TXT record with your selector and public key. The correct record is generated with the private key and can be found in myselector.txt in the same location as the private key.


myselector._domainkey   IN	 TXT	"v=DKIM1; p=...................."

There are several other switches available for the record (see RFC 4871 3.6.1), the most interesting might be the t=y which enables testing mode, signaling a checking receiver that the mail must not be treated differently from an unsigned mail, regardless of the state of the signature.

Check that your DNS record has been correctly updated:

host -t TXT

You may also check that your DKIM DNS record is properly formated using one of the DKIM Key checkers available on the web.

Postfix integration

Either add the following lines to


If you plan to integrate DKIM and DMARC you can use the following lines instead (via unix sockets):

non_smtpd_milters = unix:/run/opendkim/opendkim.sock,
smtpd_milters = unix:/run/opendkim/opendkim.sock,

Or change smtpd options in

smtp      inet  n       -       n       -       -       smtpd
  -o smtpd_client_connection_count_limit=10
  -o smtpd_milters=inet:

submission inet n       -       n       -       -       smtpd
  -o smtpd_enforce_tls=no
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_client_restrictions=permit_sasl_authenticated,reject
  -o smtpd_sasl_path=smtpd
  -o cyrus_sasl_config_path=/etc/sasl2
  -o smtpd_milters=inet:

Sendmail integration

Edit the file and add the following line, after the last line starting with FEATURE:

INPUT_MAIL_FILTER(`opendkim', `S=inet:8891@localhost')

Rebuild the file with:

# m4 /etc/mail/ > /etc/mail/

And then restart the sendmail.service. Read Daemons for more details.

Multiple domains

If you are providing mail server service to multiple virtual domains on the same server, you will need to modify the basic configuration as below:

Provide these directives in /etc/opendkim/opendkim.conf:

KeyTable                refile:/etc/opendkim/KeyTable
SigningTable            refile:/etc/opendkim/SigningTable
ExternalIgnoreList      refile:/etc/opendkim/TrustedHosts
InternalHosts           refile:/etc/opendkim/TrustedHosts
Note: You have to create/move your DKIM keys in separate domain folders eg. /etc/opendkim/keys/ One seperate folder for each domain. Otherwise you will receive “dkim: FAILED, invalid (public key: not available)” error message with DKIM email test.

Create the following two files to tell opendkim where to find the correct keys. You can use the same key for all the domains or generate a key for each domain. Make changes to match your settings. Add more lines as needed.


An existent /etc/opendkim/TrustedHosts file tells opendkim who to let use your keys. This is referenced by the ExternalIgnoreList directive in your conf file. Opendkim will ignore this list of hosts when verifying incoming mail.

And, because it is also referenced by the InternalHosts directive, this same list of hosts will be considered “internal,” and opendkim will sign their outgoing mail. Do not forget to change <server_ip> with your server's IP:


Change ownership of all files to opendkim:

# chown -R opendkim:mail /etc/opendkim

Add a DNS TXT record with your selector and public key for each of the domains.

You can now restart opendkim.


The default configuration for the OpenDKIM daemon is less than ideal from a security point of view (all those are minor security issues):

  • The OpenDKIM daemon does not need to run as root at all (the configuration suggested earlier will have OpenDKIM drop root privileges by itself, but systemd can do this too and much earlier).
  • If your mail daemon is on the same host as the OpenDKIM daemon, there is no need for localhost tcp sockets and unix sockets may be used instead, allowing classic user/group access controls.
  • OpenDKIM is using the /tmp folder by default whereas it could use its own folder with additional access restrictions.
Note: This example is for a single domain setup.

The following configuration files will fix most of those issues (assuming you are using Postfix) and drop some unnecessary options in the systemd service unit:

BaseDirectory           /var/lib/opendkim
KeyFile                 /etc/opendkim/myselector.private
Selector                myselector
Socket                  local:/run/opendkim/opendkim.sock
Syslog                  Yes
TemporaryDirectory      /run/opendkim
UMask                   002
Description=OpenDKIM daemon

ExecStart=/usr/bin/opendkim -x /etc/opendkim/opendkim.conf


Edit /etc/postfix/ accordingly to make Postfix listen to this unix socket:

smtpd_milters = unix:/run/opendkim/opendkim.sock
non_smtpd_milters = unix:/run/opendkim/opendkim.sock


Error: "milter-reject: END-OF-MESSAGE from localhost"

Most likely the Postfix milter protocol is set wrong in /etc/postfix/

# Postfix ≥ 2.6
milter_protocol = 6
# 2.3 ≤ Postfix ≤ 2.5
milter_protocol = 2

Authentication-Results: "dkim=neutral (bad format)"

Most likely this was caused by the DNS TXT records for given selector split into three or more resource record (RR). The authenticator, when concating the records got wrong record value.

For example, given DNS TXT record for selector "" with the correct, expected value

"v=DKIM1; k=rsa; s=email; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAqrXzI8BMAv3rTYU9FA4F1m2aLyT7JF8qnhTuqWibR/X55ZxoUX8fceXkRbM03tgn+1UWo5mbNN5siLPDlNOKU6fWCmkCbroPXe0vpip72zkFCtYxO4NTQY0kVaKVyFpUbFbxN3oabYTmaty3eE2yQDDAmJeZiVyEE7K7E0vnW9KpiJypFPFoft52Dqr3BTB8197gHPEMXgeP5gYkjJxVEfJZiZVco6p41JUr0CzD2dPun6pSLOO8NCkx3bWNKsL1DA7CR6qX/o2oOsd821N+0tn+8oc6x0rnhetaR0442NAGzxna4jTkUe9jwAK4aU7nKQxqNn/wOw1K2qT7uhsVMwIDAQAB".

The authenticator on the other side receive the record split into three,

$ resolver -t TXT
= options: &{sqtype:TXT sqclass:IN nameserver:udp:// insecure:false qtype:16 qclass:1}
= resolv.conf: &{Domain:localhost Search:[] NameServers:[] NDots:1 Timeout:5 Attempts:2 OptMisc:map[]}
> Lookup at
< From:
> Header: {ID:2136 IsQuery:false Op:0 IsAA:false IsTC:false IsRD:true IsRA:true RCode:0 QDCount:1 ANCount:3 NSCount:0 ARCount:0}
> Question: &{ Type:TXT}
> Status: OK
> Answer #1:
>> Resource record: { Type:16 Class:1 TTL:1822 rdlen:143}
>> RDATA: YkjJxVEfJZiZVco6p41JUr0CzD2dPun6pSLOO8NCkx3bWNKsL1DA7CR6qX/o2oOsd821N+0tn+8oc6x0rnhetaR0442NAGzxna4jTkUe9jwAK4aU7nKQxqNn/wOw1K2qT7uhsVMwIDAQAB
> Answer #2:
>> Resource record: { Type:16 Class:1 TTL:1822 rdlen:253}
>> RDATA: p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAqrXzI8BMAv3rTYU9FA4F1m2aLyT7JF8qnhTuqWibR/X55ZxoUX8fceXkRbM03tgn+1UWo5mbNN5siLPDlNOKU6fWCmkCbroPXe0vpip72zkFCtYxO4NTQY0kVaKVyFpUbFbxN3oabYTmaty3eE2yQDDAmJeZiVyEE7K7E0vnW9KpiJypFPFoft52Dqr3BTB8197gHPEMXgeP5g
> Answer #3:
>> Resource record: { Type:16 Class:1 TTL:1822 rdlen:26}
>> RDATA: v=DKIM1; k=rsa; s=email;

When authenticator combine the record the value its got is,

YkjJxVEfJZiZVco6p41JUr0CzD2dPun6pSLOO8NCkx3bWNKsL1DA7CR6qX/o2oOsd821N+0tn+8oc6x0rnhetaR0442NAGzxna4jTkUe9jwAK4aU7nKQxqNn/wOw1K2qT7uhsVMwIDAQABp=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAqrXzI8BMAv3rTYU9FA4F1m2aLyT7JF8qnhTuqWibR/X55ZxoUX8fceXkRbM03tgn+1UWo5mbNN5siLPDlNOKU6fWCmkCbroPXe0vpip72zkFCtYxO4NTQY0kVaKVyFpUbFbxN3oabYTmaty3eE2yQDDAmJeZiVyEE7K7E0vnW9KpiJypFPFoft52Dqr3BTB8197gHPEMXgeP5gv=DKIM1; k=rsa; s=email;

Solution: Generate keys with 1024 bits length (or less than 2048 bits) to make it fit into 255 chars on DNS TXT record.


While you are about to fight spam and increase people's trust in your server, you might want to take a look at Sender Policy Framework, which basically means adding a DNS Record stating which servers are authorized to send email for your domain.

See also