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:
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).
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.
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, type yes
and press Enter
to continue.
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:
CTRL + X
and then N
CTRL + X
and then Y
, and finally press Enter
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 the Socket
. 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 by UserID
and 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, the simple
method allows almost no modification while the relaxed
one tolerates minor changes such as
whitespace replacement; relaxed/simple
- the message header will be processed with the relaxed
algorithm and the body with the simple
one
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 the From:
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@localhost
defines a TCP socket that listens on localhost
, port 12301
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 in opendkim.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 both ExternalIgnoreList
and InternalHosts
, messages originating from these hosts, domains and IP addresses will be trusted and signed.
Because our main configuration file declares TrustedHosts
as a regular expression file (refile
), we can use wildcard patters, *.example.com
means 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 example mail
is 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
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
-s
specifies the selector and -d
the domain, this command will create two files, mail.private
is our private key and mail.txt
contains the public key.
Change the owner of the private key to opendkim
:
sudo chown opendkim:opendkim mail.private
Open mail.txt
:
sudo nano -$ mail.txt
The public key is defined under the p
parameter. 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"
Please note that the DNS changes may take a couple of hours to propagate.
Restart Postfix and OpenDKIM:
sudo service postfix restart
sudo service opendkim restart
Congratulations! You have successfully configured DKIM for your mail server!
The configuration can be tested by sending an empty email to check-auth@verifier.port25.com
and a reply will be received. If everything works correctly you should see DKIM check: pass
under Summary of Results
.
==========================================================
Summary of Results
==========================================================
SPF check: pass
DomainKeys check: neutral
DKIM check: pass
Sender-ID check: pass
SpamAssassin check: ham
Alternatively, you can send a message to a Gmail address that you control, view the received email’s headers in your Gmail inbox, dkim=pass
should be present in the Authentication-Results
header field.
Authentication-Results: mx.google.com;
spf=pass (google.com: domain of contact@example.com designates --- as permitted sender) smtp.mail=contact@example.com;
dkim=pass header.i=@example.com;
<div class=“author”>Submitted by: P. Sebastian</a></div>
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If Domainkey is neutural… Check your postfix version with command below
If Postfix version higher than 2.6, set “milter_protocol” value 6 instead of 2.
And it is more easier to spam test from http://www.mail-tester.com/
Hope this helps anyone who has same issue with me.
I spent quite freaking several hours trying to make this thing work, DKIM test services all kept saying no such key, key not found bla bla bla.
Solution is: DO NOT add mail._domainkey.example.com TXT record, instead DO it this way - mail._domainkey
Yes, just mail._domainkey as record name.
That was it.
I am getting a Your DKIM signature is not valid error when testing my email with mail-tester Any ideas why?
Restarting OpenDKIM: No /usr/sbin/opendkim found running; none killed. opendkim: /etc/opendkim.conf: refile:/etc/opendkim/SigningTable: dkimf_db_open(): No such file or directory opendkim.
I am getting above error on sudo service opendkim restart. Please help!!!
in the digital ocean DNS records its wrong its not
mail._domainkey.example.com.
it’s
mail._domainkey.
I was gettin this error:
Result: permerror (key "mail._domainkey.mydomain.com" doesn't exist)
thank you for the tutorial, worked like a charm! :) anyway the check-auth@verifier.port25.com is still failing, but on gmail I got
Theres also a nice website to test the email: http://www.mail-tester.com/
Works like a charm! The only thing that’s really annoying is those example.com URLs… it would be so easy to add a simple JS (based on jQuery or whatever) that would give us the option of auto-replacing ALL occurrences of “example.com” on this page with whatever domain name we need to set and gone is the tedious step of “copy into text editor - edit - copy again”. I could volunteer to write such a thing if you want.
If my mail server is sub.domain.com, is it still safe to replace all example.com with sub.domain.com in the tutorial?
Even after following the full article my mails were not getting signed.
The Fix was that in the TrustedHosts file remove the * It should be like .domain.com
thanks for the excellent tutorial.
use this check after running through this tutorial. ignore the key: not secure.