The frustration of getting falsely flagged as a spammer is not strange to most of the mail server admins. By excluding the possibility of a compromised server, a false flag is usually caused by one of the following:

• the server is an open mail relay
• the sender's or server's IP address is blacklisted
• the server does not have a Fully Qualified Domain Name (FQDN) and a PTR record
• the Sender Policy Framework (SPF) DNS record is missing or it is misconfigured
• the DomainKeys Identified Mail (DKIM) implementation is missing or it's not properly set up

These are some of the basic properties that are being checked by the majority of proprietary and open source spam filters (including SpamAssassin). Passing these tests is extremely important for a well configured mail server.

This tutorial will focus on installing and configuring OpenDKIM]: an open source implementation of the DKIM sender authentication system.

It is assumed that the reader knows how to access the server over SSH, Postfix and Dovecot is already installed and configured (tutorial), the host name and the FQDN are set up (tutorial, tutorial) and the SPF record is in place (tutorial).

About DKIM

DKIM is an Internet Standard that enables a person or organisation to associate a domain name with an email message. This, in effect, serves as a method of claiming responsibility for a message. At its core, DKIM is powered by asymmetric cryptography. The sender's Mail Transfer Agent (MTA) signs every outgoing message with a private key. The recipient retrieves the public key from the sender's DNS records and verifies if the message body and some of the header fields were not altered since the message signing took place.

Install OpenDKIM

Before starting the installation, a system update is recommended:

sudo apt-get update
sudo apt-get dist-upgrade

Install OpenDKIM and it's dependencies:

sudo apt-get install opendkim opendkim-tools

Additional packages will be listed as dependencies, typeyesand pressEnterto continue.

Configure OpenDKIM

A couple of files must be created and edited in order to configure OpenDKIM.

Nano will be used as an editor because it's installed by default on DigitalOcean droplets and it's simple to operate:

• navigate with the arrow keys
• exit without saving changes: pressCTRL + Xand thenN
• exit and save changes: pressCTRL + Xand thenY, and finally pressEnter

Important: replace every instance of example.com with your own domain in all commands and configuration files. Don't forget to save your files after editing.

Let's start with the main configuration file:

sudo nano /etc/opendkim.conf

Append the following lines to the end of the conf file (each parameter is explained below). Optionally, you can choose a custom port number for theSocket. Make sure that it's not used by a different application.

AutoRestart             Yes
AutoRestartRate         10/1h
UMask                   002
Syslog                  yes
SyslogSuccess           Yes
LogWhy                  Yes

Canonicalization        relaxed/simple

ExternalIgnoreList      refile:/etc/opendkim/TrustedHosts
InternalHosts           refile:/etc/opendkim/TrustedHosts
KeyTable                refile:/etc/opendkim/KeyTable
SigningTable            refile:/etc/opendkim/SigningTable

Mode                    sv
PidFile                 /var/run/opendkim/opendkim.pid
SignatureAlgorithm      rsa-sha256

UserID                  opendkim:opendkim

Socket                  inet:12301@localhost

• AutoRestart: auto restart the filter on failures

• AutoRestartRate: specifies the filter's maximum restart rate, if restarts begin to happen faster than this rate, the filter will terminate;10/1h- 10 restarts/hour are allowed at most

• UMask: gives all access permissions to the user group defined byUserIDand allows other users to read and execute files, in this case it will allow the creation and modification of a Pid file.

• Syslog, SyslogSuccess, *LogWhy: these parameters enable detailed logging via calls to syslog

• Canonicalization: defines the canonicalization methods used at message signing, thesimplemethod allows almost no modification while therelaxedone tolerates minor changes such as
  whitespace replacement;relaxed/simple- the message header will be processed with therelaxedalgorithm and the body with thesimpleone

• ExternalIgnoreList: specifies the external hosts that can send mail through the server as one of the signing domains without credentials

• InternalHosts: defines a list of internal hosts whose mail should not be verified but signed instead

• KeyTable: maps key names to signing keys

• SigningTable: lists the signatures to apply to a message based on the address found in theFrom:header field

• Mode: declares operating modes; in this case the milter acts as a signer (s) and a verifier (v)

• PidFile: the path to the Pid file which contains the process identification number

• SignatureAlgorithm: selects the signing algorithm to use when creating signatures

• UserID: the opendkim process runs under this user and group

• Socket: the milter will listen on the socket specified here, Posfix will send messages to opendkim for signing and verification through this socket;12301@localhostdefines a TCP socket that listens onlocalhost, port12301

This simple configuration is meant to allow message signing for one or more domains, to learn about other options please go here.

Connect the milter to Postfix:

sudo nano /etc/default/opendkim

Add the following line, edit the port number only if a custom one is used:

SOCKET="inet:12301@localhost"

Configure postfix to use this milter:

sudo nano /etc/postfix/main.cf

Make sure that these two lines are present in the Postfix config file and are not commented out:

milter_protocol = 2
milter_default_action = accept

It is likely that a filter (SpamAssasin, Clamav etc.) is already used by Postfix; if the following parameters are present, just append the opendkim milter to them (milters are separated by a comma), the port number should be the same as inopendkim.conf:

smtpd_milters = unix:/spamass/spamass.sock, inet:localhost:12301
non_smtpd_milters = unix:/spamass/spamass.sock, inet:localhost:12301

If the parameters are missing, define them as follows:

smtpd_milters = inet:localhost:12301
non_smtpd_milters = inet:localhost:12301

Create a directory structure that will hold the trusted hosts, key tables, signing tables and crypto keys:

sudo mkdir /etc/opendkim
sudo mkdir /etc/opendkim/keys

Specify trusted hosts:

sudo nano /etc/opendkim/TrustedHosts

We will use this file to define bothExternalIgnoreListandInternalHosts, messages originating from these hosts, domains and IP addresses will be trusted and signed.

Because our main configuration file declaresTrustedHostsas a regular expression file (refile), we can use wildcard patters,*.example.commeans that messages coming from example.com's subdomains will be trusted too, not just the ones sent from the root domain.

Customize and add the following lines to the newly created file. Multiple domains can be specified, do not edit the first three lines:

127.0.0.1
localhost
192.168.0.1/24

*.example.com

#*.example.net
#*.example.org

Create a key table:

sudo nano /etc/opendkim/KeyTable

A key table contains each selector/domain pair and the path to their private key. Any alphanumeric string can be used as a selector, in this examplemailis used and it's not necessary to change it.

mail._domainkey.example.com example.com:mail:/etc/opendkim/keys/example.com/mail.private

#mail._domainkey.example.net example.net:mail:/etc/opendkim/keys/example.net/mail.private
#mail._domainkey.example.org example.org:mail:/etc/opendkim/keys/example.org/mail.private

Create a signing table:

sudo nano /etc/opendkim/SigningTable

This file is used for declaring the domains/email addresses and their selectors.

*@example.com mail._domainkey.example.com

#*@example.net mail._domainkey.example.net
#*@example.org mail._domainkey.example.org

Generate the public and private keys

Change to the keys directory:

cd /etc/opendkim/keys

Create a separate folder for the domain to hold the keys:

sudo mkdir example.com
cd example.com

Generate the keys:

sudo opendkim-genkey -s mail -d example.com

-sspecifies the selector and-dthe domain, this command will create two files,mail.privateis our private key andmail.txtcontains the public key.

Change the owner of the private key toopendkim:

sudo chown opendkim:opendkim mail.private

Add the public key to the domain's DNS records

Openmail.txt:

sudo nano -$ mail.txt

The public key is defined under thepparameter. Do not use the example key below, it's only an illustration and will not work on your server.

mail._domainkey IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC5N3lnvvrYgPCRSoqn+awTpE+iGYcKBPpo8HHbcFfCIIV10Hwo4PhCoGZSaKVHOjDm4yefKXhQjM7iKzEPuBatE7O47hAx1CJpNuIdLxhILSbEmbMxJrJAG0HZVn8z6EAoOHZNaPHmK2h4UUrjOG8zA5BHfzJf7tGwI+K619fFUwIDAQAB" ; ----- DKIM key mail for example.com

Copy that key and add a TXT record to your domain's DNS entries:

Name: mail._domainkey.example.com.

Text: "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC5N3lnvvrYgPCRSoqn+awTpE+iGYcKBPpo8HHbcFfCIIV10Hwo4PhCoGZSaKVHOjDm4yefKXhQjM7iKzEPuBatE7O47hAx1CJpNuIdLxhILSbEmbMxJrJAG0HZVn8z6EAoOHZNaPHmK2h4UUrjOG8zA5BHfzJf7tGwI+K619fFUwIDAQAB"



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