OpenDKIM

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DomainKeys Identified Mail (DKIM) is a digital email signing/verification technology, which is already supported by some common mail providers (for example yahoo, google, etc).

The idea

Basically DKIM means digitally signing all messages on the server to verify the message actually was sent from the domain in question and is not spam or pishing (and has not been modified).

  • The sender's mail server signs outgoing email with the private key.
  • When the message arrives, the receiver (or his server) requests the public key from the domain's DNS and verifies the signature.

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

For more info see RFC 6376

Installation

Install the package opendkim from the Official repositories.

Configuration

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

  • Copy/move the sample configuration file /etc/opendkim/opendkim.conf.sample to /etc/opendkim/opendkim.conf and 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
  • 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 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 [1]. 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 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.

Example:

myselector._domainkey   IN	 TXT	"v=DKIM1; k=rsa; s=email; p=...................."

There are several other switches available for the record (see RFC4871), 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 myselector._domainkey.example.com

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 main.cf:

 non_smtpd_milters=inet:127.0.0.1:8891
 smtpd_milters=inet:127.0.0.1:8891

Or change smtpd options in master.cf:

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 sendmail.mc file and add the following line, after the last line starting with FEATURE:

/etc/mail/sendmail.mc

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

Rebuild the sendmail.cf file with:

# m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf

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

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.

/etc/opendkim/KeyTable
 
myselector._domainkey.example1.com example1.com:myselector:/etc/opendkim/myselector.private
myselector._domainkey.example2.com example2.com:myselector:/etc/opendkim/myselector.private
...
/etc/opendkim/SigningTable
*@example1.com myselector._domainkey.example1.com
*@example2.com myselector._domainkey.example2.com
...

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. Example: /etc/opendkim/TrustedHosts

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 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.

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:

/etc/tmpfiles.d/opendkim.conf
D /run/opendkim 0750 opendkim postfix
/etc/opendkim/opendkim.conf
BaseDirectory           /var/lib/opendkim
Domain                  example.com
KeyFile                 /etc/opendkim/myselector.private
Selector                myselector
Socket                  local:/run/opendkim/opendkim.sock
Syslog                  Yes
TemporaryDirectory      /run/opendkim
UMask                   002
/etc/systemd/system/opendkim.service
[Unit]
Description=OpenDKIM daemon
After=network.target remote-fs.target nss-lookup.target

[Service]
Type=forking
User=opendkim
Group=postfix
ExecStart=/usr/bin/opendkim -x /etc/opendkim/opendkim.conf

[Install]
WantedBy=multi-user.target

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

/etc/postfix/main.cf
smtpd_milters = unix:/run/opendkim/opendkim.sock
non_smtpd_milters = unix:/run/opendkim/opendkim.sock

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.