OpenDKIM
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 Outlook.com.
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.
Configuration
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.sampleto/etc/opendkim/opendkim.confand change the following options:
/etc/opendkim/opendkim.conf
Domain example.com KeyFile /path/to/keys/server1.private Selector myselector Socket inet:8891@localhost UserID opendkim
- Socket address is the one specified in
/etc/postfix/main.cf. This is what/etc/postfix/main.cfshould 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, specify the domain used to send mails and a selector which is used to refer to the key. The selector may be any value. See the RFC for details, but alpha-numeric strings should be OK:
$ opendkim-genkey -r -s myselector -d example.com
- 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:
/etc/opendkim/opendkim.conf
... 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 the 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.
Example:
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 the DNS record has been correctly updated:
host -t TXT myselector._domainkey.example.com
You may also check that the 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 main.cf:
non_smtpd_milters=inet:127.0.0.1:8891 smtpd_milters=inet:127.0.0.1:8891
To integrate DKIM and DMARC use unix sockets instead, set the UMask to give the group write access to the socket, and add the postfix user to the opendkim group:
UMask 002
non_smtpd_milters = unix:/run/opendkim/opendkim.sock,
unix:/run/opendmarc/opendmarc.sock
smtpd_milters = unix:/run/opendkim/opendkim.sock,
unix:/run/opendmarc/opendmarc.sock
Or change smtpd options in :
smtp inet n - n - - smtpd -o smtpd_client_connection_count_limit=10 -o smtpd_milters=inet:127.0.0.1:8891 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:127.0.0.1:8891
Sendmail integration
Edit the file and add the following line, after the last line starting with :
Rebuild the file with:
# m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
And then restart the sendmail.service. Read Daemons for more details.
Multiple domains
To provide mail server service to multiple virtual domains on the same server, 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
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. In the following example, and share the same key.
/etc/opendkim/KeyTable
myselector._domainkey.example1.com example1.com:myselector:/etc/opendkim/keys/domain1.com/myselector.private myselector._domainkey.example2.com example2.com:myselector:/etc/opendkim/keys/domain2.com/myselector.private myselector._domainkey.example3.com example3.com:myselector:/etc/opendkim/keys/domain1.com/myselector.private ...
An extant file tells opendkim who may use the keys. This is referenced by the directive in the confgiuration file. Opendkim ignores this list of hosts when verifying incoming mail.
Because it is also referenced by the directive, this same list of hosts is be considered "internal," and opendkim signs their outgoing mail. Remember to change to the correct server IP address:
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.
Security
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
rootat all (the configuration suggested earlier will have OpenDKIM droprootprivileges 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 folder by default whereas it could use its own folder with additional access restrictions.
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:
Edit /etc/postfix/main.cf accordingly to make Postfix listen to this unix socket:
Troubleshooting
Error: "milter-reject: END-OF-MESSAGE from localhost"
Most likely the Postfix milter protocol is set wrong in /etc/postfix/main.cf:
# Postfix ≥ 2.6 milter_protocol = 6 # 2.3 ≤ Postfix ≤ 2.5 milter_protocol = 2
Authentication-Results: "dkim=neutral (bad format) header.i=@example.com"
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 "default._domainkey.example.com" 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,
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.
Notes
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.