Build Your Email Server on Ubuntu Part 2: Setting up DKIM and SPF

In the 1st part of our build your own email server on ubuntu tutorial series, we have a basic Postfix email server up and running and can send plain text email, read incoming emails using the command line.

I tested that tutorial again on my Ubuntu 16.04 server and found that although I set the correct MX, A and PTR record, my emails were flagged as spam by Gmail and Outlook mail. So in part 2, we are going to look at how to ensure email delivery to recipient’s inbox by setting up SPF and DKIM records.

What are SPF and DKIM Records?

SPF and DKIM are two types of TXT records in DNS that can help prevent email spoofing and ensure legitimate emails are delivered into the recipient’s inbox instead of spam folder. If your domain is abused by email spoofing, then your emails are likely to landed in recipient’s spam folder if they didn’t add you in address book.

SPF (Sender Policy Framework) record specifies which hosts or IP address are allowed to send emails on behalf of a domain. You should allow only your own email server or your ISP’s server to send emails for your domain.

DKIM (DomainKeys Identified Mail) use a private key to add a signature to emails sent from your domain. Receiving SMTP server verify the signature using the pubic key which is published in your DNS manager.

Create an SPF Record in DNS

In your DNS management interface, create a new TXT record like below.

TXT  @   v=spf1 mx ~all

Explanation:

• TXT indicates this is a TXT record.
• Enter @ in the name field.
• v=spf1 indicates this is a SPF record and the SPF record version is SPF1.
• mx means all hosts listed in the MX records are allowed to send emails for your domain and all other hosts are disallowed.
• ~all indicates that emails from your domain should only come from hosts specified in the SPF record. Emails that are from other hosts will be flagged as forged.

Note that some DNS managers require you to wrap the SPF record with quotes like below.

TXT  @   "v=spf1 mx ~all"

To check if your SPF record is propagated to the public Internet, you can use the dig utility on your Linux machine like below:

dig your-domain.com txt

The txt option tells dig that we only want to query TXT records.

You can also use online SPF validator such as spf.myisp.ch to see which hosts are allowed to send emails for your domain and debug your SPF record if any error occurs. The dmarcian SPF surveyor can help test your SPF record syntax.

Configuring SPF Policy Agent

We also need to tell our Postfix SMTP server to check for SPF record of incoming emails. This doesn’t help ensure outgoing email delivery but help with detecting forged incoming emails.

First install required packages:

sudo apt install postfix-policyd-spf-python

Then edit the Postfix master process configuration file.

sudo nano /etc/postfix/master.cf

Add the following lines at the end of the file.

policyd-spf  unix  -       n       n       -       0       spawn
    user=policyd-spf argv=/usr/bin/policyd-spf

Save and close the file. Next, edit Postfix main configuration file.

sudo nano /etc/postfix/main.cf

Append the following lines at the end of the file.

policyd-spf_time_limit = 3600
smtpd_recipient_restrictions =
   reject_unauth_destination,
   check_policy_service unix:private/policyd-spf

The first line specifies the Postfix policy agent timeout setting. The following lines will add restriction on incoming emails by rejecting unauthorized email and checking SPF record.

Save and close the file. Then restart Postfix.

sudo service postfix restart

or

sudo systemctl restart postfix

Next time, when you receive an email from a domain that has an SPF record, you can see the SPF check results in the raw email header. The following header indicates a successful check against SPF.

Received-SPF: Pass (sender SPF authorized).

You can also send an email from your domain to Gmail or Microsoft Mail to see if SPF check can be passed. It’s worth noting that setting an SPF record alone won’t ensure your email land in recipient’s inbox.

Setting up DKIM

First install OpenDKIM which is an open source implementation of the DKIM sender authentication system.

sudo apt install opendkim opendkim-tools

Then add postfix user to opendkim group.

sudo gpasswd -a postfix opendkim

Edit OpenDKIM main configuration file.

sudo nano /etc/opendkim.conf

Uncomment the following lines. Replace simple with relaxed/simple.

Canonicalization   simple
Mode               sv
SubDomains         no

Then add the following lines below #ADSPAction continue line. If your file doesn’t have #ADSPAction continue line, then just add them below SubDomains  no.

AutoRestart         yes
AutoRestartRate     10/1M
Background          yes
DNSTimeout          5
SignatureAlgorithm  rsa-sha256

Add the following lines at the end of this the file.

#OpenDKIM user
# Remember to add user postfix to group opendkim
UserID             opendkim

# Map domains in From addresses to keys used to sign messages
KeyTable           /etc/opendkim/key.table
SigningTable       refile:/etc/opendkim/signing.table

# Hosts to ignore when verifying signatures
ExternalIgnoreList  /etc/opendkim/trusted.hosts
InternalHosts       /etc/opendkim/trusted.hosts

The final configuration file is as follows:

# This is a basic configuration that can easily be adapted to suit a standard
# installation. For more advanced options, see opendkim.conf(5) and/or
# /usr/share/doc/opendkim/examples/opendkim.conf.sample.

# Log to syslog
Syslog			yes
# Required to use local socket with MTAs that access the socket as a non-
# privileged user (e.g. Postfix)
UMask			002

# Sign for example.com with key in /etc/mail/dkim.key using
# selector '2007' (e.g. 2007._domainkey.example.com)
#Domain			example.com
#KeyFile		/etc/mail/dkim.key
#Selector		2007

# Commonly-used options; the commented-out versions show the defaults.
Canonicalization	relaxed/simple
Mode			sv
SubDomains		no
#ADSPAction             continue
AutoRestart     	yes
AutoRestartRate     	10/1M
Background      	yes
DNSTimeout      	5
SignatureAlgorithm  	rsa-sha256

# Always oversign From (sign using actual From and a null From to prevent
# malicious signatures header fields (From and/or others) between the signer
# and the verifier.  From is oversigned by default in the Debian pacakge
# because it is often the identity key used by reputation systems and thus
# somewhat security sensitive.
OversignHeaders		From

# List domains to use for RFC 6541 DKIM Authorized Third-Party Signatures
# (ATPS) (experimental)

#ATPSDomains		example.com

#OpenDKIM user
# Remember to add user postfix to group opendkim
UserID                 opendkim

# Map domains in From addresses to keys used to sign messages
KeyTable            /etc/opendkim/key.table
SigningTable        refile:/etc/opendkim/signing.table

# Hosts to ignore when verifying signatures
ExternalIgnoreList  /etc/opendkim/trusted.hosts
InternalHosts       /etc/opendkim/trusted.hosts

Save and close the file.

Create Signing table, key table and trusted hosts file

Create a directory structure for OpenDKIM

sudo mkdir /etc/opendkim

sudo mkdir /etc/opendkim/keys

Change owner from root to opendkim and make sure only opendkim user can read and write to the keys directory.

sudo chown -R opendkim:opendkim /etc/opendkim

sudo chmod go-rw /etc/opendkim/keys

Create the signing table.

sudo nano /etc/opendkim/signing.table

Add this line to the file. Replace your-domain.com with your real domain.

*@your-domain.com default._domainkey.your-domain

Then create the key table.

sudo nano /etc/opendkim/key.table

Add the following line. Replace your-domain with your actual domain.

default._domainkey.your-domain    your-domain.com:default:/etc/opendkim/keys/your-domain.com/default.private

Save and close the file.

Configure Trusted Hosts

Create the file.

sudo nano /etc/opendkim/trusted.hosts

Add the following lines to the newly created file.

127.0.0.1
localhost

*.your-domain.com

The above means that messages coming from the above IP addresses and domains will be trusted and signed.

Generate Private/Public Keypair

Since DKIM is used to sign outgoing messages and verify incoming messages, we need to generate a private key for signing and a public key for remote verifier. Public key will be published in DNS.

Create a separate folder for the domain.

sudo mkdir /etc/opendkim/keys/your-domain.com

Generate keys using opendkim-genkey tool.

sudo opendkim-genkey -b 2048 -d your-domain.com -D /etc/opendkim/keys/your-domain.com -s default -v

The above command will create 2048 bits keys. -d (domain) specifies the domain. -D (directory) specifies the directory where the keys will be stored and we use default as the selector (-s), also known as the name. Once the command is executed, the private key will be default.private and default.txt will be the TXT record that contains public key.

Make opendkim as the owner of the private key.

sudo chown opendkim:opendkim /etc/opendkim/keys/your-domain.com/default.private

Add Public Key in DNS Records

Display the public key

sudo cat /etc/opendkim/keys/your-domain.com/default.txt

The string after the p parameter is the public key.

In you DNS manager, create a TXT record, enter default._domainkey in the name field. Then copy everything in the parentheses and paste into the value field. Delete all double quotes and white spaces.

Test your configuration

Enter the following command on Ubuntu 16.04 server to test your key.

sudo opendkim-testkey -d your-domain.com -s default -vvv

If everything is OK, you will see

key OK

Connect Postfix to OpenDKIM

Create a directory to hold the OpenDKIM socket file and only allow opendkim user and postfix group to access it.

sudo mkdir /var/spool/postfix/opendkim

sudo chown opendkim:postfix /var/spool/postfix/opendkim

Then edit the socket configuration file.

sudo nano /etc/default/opendkim

Uncomment the first SOCKET line and replace it with the following line.

SOCKET="local:/var/spool/postfix/opendkim/opendkim.sock"

Edit Postfix main configuration file.

sudo nano /etc/postfix/main.cf

Add the following lines after smtpd_recipient_restriction section.

# Milter configuration
# OpenDKIM
milter_default_action = accept
milter_protocol = 2
smtpd_milters = local:/opendkim/opendkim.sock
non_smtpd_milters = local:/opendkim/opendkim.sock

Save and close the file. Then restart opendkim and postfix service.

sudo service opendkim restart

sudo service postfix restart

or

sudo systemctl restart opendkim

sudo systemctl restart postfix

DKIM Check

You can send a test email to [email protected] and get a free email authentication report.

echo "test email" | sendmail [email protected]

Here’s the report I got from port25.com

==========================================================
Summary of Results
==========================================================
SPF check:          pass
DomainKeys check:   neutral
DKIM check:         pass
Sender-ID check:    pass
SpamAssassin check: ham

You can see that my email passed both SPF and DKIM check. DomainKeys is the forerunner to DKIM and it is obsolete now. You don’t need to worry about it. ham, which means the email is not spam, is a terminology used by Apache SpamAssassin.

You can also send a test email to your Gmail account to see if DKIM test is passed.

echo "test email" | sendmail your-account@gmail.com

If you click the show original button in Gmail, you should see

dkim=pass

in the authentication-results header.

In my test, the email landed in my Gmail inbox. However, it’s stilled labeled as spam in my outlook.com email although both SPF and DKIM are passed.

If your message is not signed and DKIM check failed, you may want to check postfix log (/var/log/mail.log) to see what’s wrong in your configuration.

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