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# Prevent spoofing using DKIM and SPF

Optional feature

This feature is completely optional. Just skip this page if you don’t care about villains sending emails in your name.

Email sender spoofing means pretending to be in control of someone else’s email address. This is a common problem with phishing. Often scammers send emails with a sender address of `something@paypal.com` and hope that the recipient falls for it and trusts them. SMTP does not care which sender address you use. Most mail service providers enforce that you send emails only using your own email address. But some do not. And bad guys obviously don’t care.

On this page I will help you set up the three common measures to prevent spoofing:

- SPF (Sender Policy Framework)
- DKIM (DomainKeys Identified Mail)
- DMARC (Domain-based Message Authentication, Reporting and Conformance)

## Envelope sender or visible sender?

Section titled “Envelope sender or visible sender?”

Each email that you receive actually contains two different sender addresses. Understanding the difference is important when it comes to DKIM and DMARC.

The envelope sender is used by SMTP for delivery and bounce messages. It is not usually shown to the user. It corresponds to the MAIL FROM command in SMTP. If during the transmission the sending server sent `MAIL FROM:<bounce@example.net>` then this is the envelope sender. It will be added to the header part as `Return-Path: <bounce@example.net>`. If you think of a written letter, then this is the sender which is written onto the envelope. The postman will not look inside the envelope. So if the delivery fails, this is the address that the letter will be sent back to as undeliverable.

The visible sender (this is not an official term) is what the recipient sees in their email software. In an actual email this is the `From: John <john@example.org>` header. If you think of a written letter, then this is the sender as found in the actual letter within the envelope.

## SPF

Section titled “SPF”

SPF defines which servers (IP addresses) are allowed to send emails on behalf of a specific domain.

SPF is essentially a TXT record in your DNS zone that provides machine-readable information for other mail servers. When an email is received, the receiving server can (and usually does) check for an SPF record for your domain to determine which IP addresses are authorized to send emails on its behalf.

Let’s take a look at a typical SPF record:

    "v=spf1 ip4:157.97.194.11 mx ~all"

What it means:

1. this is an SPF record of version 1 of the standard (there is currently no other version)
2. please accept emails from the IP address 157.97.194.11
3. also accept emails from any server that is mentioned in our domain’s MX record (the server(s) that receive email for your domain)
4. any other email should be considered suspicious – proceed with caution

There are websites like EasyDMARC that help you create your SPF string to add to your DNS domain. Keep in mind:

- You must know which mail servers send email from your domain. Do not forget to include mailing list or newsletter services that send in your name.
- Start with “~all” to mark emails as spam that do not meet the criteria. If all goes well switch to “-all” after a few weeks.
- Note that forwarding emails from your domain may break SPF because suddenly the email appears to be coming from an IP address that is not authorized.

Of course SPF not only helps other mail servers. Rspamd on your mail server will check for other domains’ SPF records, too.

## DKIM

Section titled “DKIM”

DKIM can prove that an email has not been tampered with on its journey from the sender to the recipient.

DKIM uses public key cryptography to automatically add a cryptographic signature into the header of each outgoing email. It also uses a TXT record in your domain’s DNS zone to publish the public key.

If you run the sending mail server, you need to:

1. Create a public key and a private key for each domain.
2. Publish the public key through a DNS record of your domain. As only an administrator of the sending domain has access to the DNS zone, this proves the authenticity.
3. Give your mail server the private key so it can sign all emails automatically.

On the other hand the receiving mail server needs to:

1. Check if there is a signature in an incoming email.
2. Fetch the public key from the DNS zone of the sending domain.
3. Make a cryptographic check if the signature matches the public key.

I don’t want to dive deeper into public key cryptography. Take a look at the Wikipedia article for a decent explanation.

Obviously your mail server will both receive and send emails. So if you want to use DKIM (which I recommend) then you need to take care of both parts. But luckily that’s not too hard. The rspamd software on your mail server can take care of both signing outgoing emails and validating incoming signatures.

### Example

Section titled “Example”

Let’s take a look at such a signature in real life. I have just sent an email from GMail to my personal email account on my own mail server. Google uses DKIM signing so the email got this additional header from Google’s mail servers:

    DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;  d=gmail.com; s=20230601; t=1760017775; x=1760622575; darn=workaround.org;  h=to:subject:message-id:date:from:mime-version:from:to:cc:subject    :date:message-id:reply-to;  bh=HmztzLfHhgZON2788h5cwCX7liAbwFqnFGISA15M18Q=;  b=OKu5D/Ejp7tGy3uxydb7qDccK03OxrdD9vxEKiAXk/y9T5WViCqpg3HrdeB5Izkzdq    MXvQ+SgGVzJmy9ZgV1A930RRqUIoR2wmlr3GDXPDyRfagJMRV/3v9J71lfSduOFE8q7t    AIw4oWBm91QAnC5gW+XS7h4lNmRd4pVILkoe09qkLz+7WrU5Xp/WGIKHRK0ST5+EAR25…

I need Google’s DKIM public key to verify that signature. It is stored in their DNS zone as a TXT record of “20230601._domainkey.google.com”. The “20230601” is the key selector that is mentioned in the signature as “s=20230601”. You can use multiple keys as long as you create the signatures with the matching private key. The “_domainkey” part is the standard subdomain for DKIM keys. So let’s get that TXT record:

Run this on your server

    dig +short 20230601._domainkey.google.com txt

This returns…

    "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA4zd3nfUoLHWFbfoPZzAb8bvjsFIIFsNypweLuPe4M+vAP1YxObFxRnpvLYz7Z+bORKLber5aGmgFF9iaufsH1z0+aw8Qex7uDaafzWoJOM/6lAS5iI0JggZiUkqNpRQLL7H6E7HcvOMC61nJcO4r0PwLDZKwEaCs8gUHiqRn/SS3wqEZX29v/VOUVcI4BjaOz" "OCLaz7V8Bkwmj4Rqq4kaLQQrbfpjas1naScHTAmzULj0Rdp+L1vVyGitm+dd460PcTIG3Pn+FYrgQQo2fvnTcGiFFuMa8cpxgfH3rJztf1YFehLWwJWgeXTriuIyuxUabGdRQu7vh7GrObTsHmIHwIDAQAB"

That is the public key that I can use to verify the signature. An automated check can be done using the `opendkim-testmsg` tool as described later. I can run it and paste the entire email including headers and body into it. If it doesn’t complain with an error message then the signature is correct.

Note

The signature does not certify the domain of the sender. In this example the signature is for `d=gmail.com`. But it could also have been `d=abc.xyz`. A DKIM signature just says:

This email has been sent by a mail server that has the key for the `gmail.com` domain. And it has not been tampered with on its way to you.

So it is not about the authenticity of the person who sent it to you. Of course it makes sense that the signature matches the domain of the sending user. Enforcing that will be a job for DMARC which is explained later on this page.

### Creating a keypair

Section titled “Creating a keypair”

Sounds good? Then let’s implement that for your email domains, too.

If you have skipped the previous section on detecting and filtering out spam emails, then just run these two commands to tell Postfix to send all emails through rspamd:

Run this on your server

    postconf smtpd_milters=inet:127.0.0.1:11332postconf non_smtpd_milters=inet:127.0.0.1:11332

As explained above you need a private key that your mail server will use for signing and a public key that gets added to your DNS zone.

If you put your key file into /var/lib/rspamd/dkim/ using a certain naming scheme, `rspamd` will pick it up automatically:

Run this on your server – but use your own domain here

    # Create the directorymkdir /var/lib/rspamd/dkim
    # Fix permissionschown _rspamd:_rspamd /var/lib/rspamd/dkim
    # Create a new keypair (use your own domain here)cd /var/lib/rspamd/dkimrspamadm dkim_keygen -d example.org -s 2025100901

The selector (-s) I chose is 2025100901 because that’s the day I created it. The first (01) key on 2025-10-09. It doesn’t matter though – you can name it anything you want. If you are lazy and unconcerned you could even use “dkim” as the selector and later save some work by not needing DKIM maps that define which key is supposed to be used for each domain. “dkim” is the default selector if you do not use maps. But some day you may want to add new keys so I recommend you stay flexible and use a custom selector.

The output will contain you both the private key…

    -----BEGIN PRIVATE KEY-----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-----END PRIVATE KEY-----

…and the public key – in the format of a DNS record:

    2025100901._domainkey IN TXT ( "v=DKIM1; k=rsa;"  "p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC4cnrz6IZ3rnERkC14KrWo14Q4Dq/ZL6F+YoEbgoQvo+xcb8IeErlKYTCi8p2mwL3FFBfedQYFQeD11EKMWnu3BJI1gEzUor/hKtMPgChK3QvgRTQ0k5O+BmQwQYZa6McrnZO0DAmdp4YjgP62NS8qWg0MwYkUlfM43BwvBNmeZQIDAQAB" ) ;

### Adding the public key to a DNS record

Section titled “Adding the public key to a DNS record”

Before you start signing your emails you must make sure that the public key is properly present in your DNS zone for the domain you are sending emails from. Otherwise the recipient will be unable to verify the signature and may incorrectly assume that the email was spoofed.

Take a look at the TXT record. It will look something like this:

    2019122401._domainkey IN TXT v=DKIM1; k=rsa;

If you are running your own DNS server you should be able to copy this entire file and put it into your DNS zone. However if your internet provider offers you just a web interface to manage your domains then create a new TXT record with a host name of “2019122401._domainkey” in your domain and put the string within the double-quotes into it as the value. In my example:

    2025100901._domainkey IN TXT ( "v=DKIM1; k=rsa;"  "p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC4cnrz6IZ3rnERkC14KrWo14Q4Dq/ZL6F+YoEbgoQvo+xcb8IeErlKYTCi8p2mwL3FFBfedQYFQeD11EKMWnu3BJI1gEzUor/hKtMPgChK3QvgRTQ0k5O+BmQwQYZa6McrnZO0DAmdp4YjgP62NS8qWg0MwYkUlfM43BwvBNmeZQIDAQAB" ) ;

Be aware that the string you got contains two strings ”…” + ”…” that must be merged into one to work. If you are running your own name server with `bind` then you can just take that record as is. Other software or your favorite DNS provider may expect a different syntax. In the end the value of the TXT record needs to be one long string. You can use dig to verify that:

Run this on your server

    dig +short 2025100901._domainkey.example.org txt

If you get the TXT entry like as follows then you are ready to enable DKIM signing in rspamd for that domain:

    "p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC4cnrz6IZ3rnERkC14KrWo14Q4Dq/ZL6F+YoEbgoQvo+xcb8IeErlKYTCi8p2mwL3FFBfedQYFQeD11EKMWnu3BJI1gEzUor/hKtMPgChK3QvgRTQ0k5O+BmQwQYZa6McrnZO0DAmdp4YjgP62NS8qWg0MwYkUlfM43BwvBNmeZQIDAQAB"

### Adding the private key to rspamd

Section titled “Adding the private key to rspamd”

Take the private key that was created earlier (the multi-line string including “`…BEGIN PRIVATE KEY…`” and “`…END PRIVATE KEY…`”) and put it into a file at the location where rspamd will look for it:

    /var/lib/rspamd/dkim/example.org.2025100901.key

The name of the file has to be DOMAIN + dot + SELECTOR + “.key” like above. If you name the file incorrectly you will get an error in your rspamd.log file like “lua_dkim_sign_handler: cannot load dkim key /var/lib/rspamd/dkim/example.org.dkim.key”.

Make sure that only the `_rspamd` user can read it:

Run this on your server

    chown _rspamd /var/lib/rspamd/dkim/*chmod u=r,go= /var/lib/rspamd/dkim/*

rspamd will automatically pick up the files and does not need to be restarted when you add new keys for further domains.

### Enabling DKIM maps in rspamd

Section titled “Enabling DKIM maps in rspamd”

As explained above it is advised to use DKIM maps. It’s nothing fancy. Just a simple file defining which selector you want to use for signing for a certain domain. rspamd will assume that your selector is always `dkim` unless specified otherwise in a map. Always use a specific selector so that you can later add new keys to DNS without getting into trouble.

Using maps is simple. First we need to change the selector_map setting of the dkim_signing module. Create a new rspamd configuration file:

Run this on your server

    # Tell rspamd to look up selectors in our mapping filecat > /etc/rspamd/local.d/dkim_signing.conf << 'EOF'path = "/var/lib/rspamd/dkim/$domain.$selector.key";selector_map = "/etc/rspamd/dkim_selectors.map";allow_username_mismatch = true;EOF
    # Restart rspamdsystemctl restart rspamd

The configuration is simple. rspamd will look for the domain-to-key mapping in the `dkim_selectors.map` file. Create that file for your own domain and selector:

Run this on your server

    # Add the mapping of domain/selector (use your own domain here)echo example.org 2025100901 > /etc/rspamd/dkim_selectors.map

That’s all. rspamd now knows that whenever it sees an outgoing email from `anyone@example.org` it will get the DKIM private key from /var/lib/rspamd/dkim/example.org.2025100901.key and use it to sign the email.

The `allow_username_mismatch` setting makes rspamd sign an email even if the authenticated user (during relaying) does not match the actual sender address. Sounds complicated? Well, the use case is aliases. Imagine that `john@example.com` has an alias address of `pizza@example.org`. If John logs in as `john@example.org` but chooses to set his sender address to `pizza@example.org` then rspamd will (by default) not add a DKIM signature due to a mismatch. Setting `allow_username_mismatch` overrides this behavior and John’s email will be signed even if he sends on behalf of `pizza@example.org`. (Thanks, DBL, for the hint.)

### Send a test email

Section titled “Send a test email”

You could either just send an email from your mail client (or Roundcube) through your mail server to another email address. Or you could use swaks. Use your own addresses and password of course:

Run this on your server

    swaks --server localhost:587 \    --from some.address@example.org \    --to some.external.address@example.net \    -tls \    --auth-user some.address@example.org \    --auth-password your_secreet_password

If you take a look at the received email it should have a DKIM header now like:

    DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.org; s=2019122401; t=1519400838h=from:subject:date:message-id:to:mime-version:content-type:content-transfer-encoding;bh=kpYel1IlDvqXEUc0SyIpXbMte3XpQOCXHl+zTyHQvGc=; b=NEUyWUoeKE…

To verify the signature install the opendkim-tools package, copy the entire test email (including headers and body), run opendkim-testmsg in your shell and paste the email (finish with CTRL-D).

If you get no output then the signature verified correctly. But if you get something like “opendkim-testmsg: dkim_eom(): Unable to verify” then double-check your DNS record.

You can also use websites like dkimvalidator.com or mail-tester.com to verify that your signatures are working well.

Click here if you do not get a DKIM signature…

Enable logging:

Run this on your server

    cat > /etc/rspamd/local.d/logging.inc << 'EOF'debug_modules = ["dkim", "dkim_signing"];EOF
    systemctl restart rspamd
    tail -f /var/log/rspamd/rspamd.log

Look for any errors. And check that the correct key from `/var/lib/rspamd/dkim` is chosen and used. This is what it should look like:

    2025-10-19 10:23:40 #1806716(normal) <ec2f11>; dkim_signing; lua_dkim_tools.lua:185: user is authenticated2025-10-19 10:23:40 #1806716(normal) <ec2f11>; dkim_signing; lua_dkim_tools.lua:407: use domain(header) for signature: example.org2025-10-19 10:23:40 #1806716(normal) <ec2f11>; dkim_signing; lua_dkim_tools.lua:427: final DKIM domain: example.org2025-10-19 10:23:40 #1806716(normal) <ec2f11>; dkim_signing; lua_dkim_tools.lua:48: add selector "2025100901" using selector_map2025-10-19 10:23:40 #1806716(normal) <ec2f11>; dkim_signing; lua_dkim_tools.lua:53: set key to "/var/lib/rspamd/dkim/$domain.$selector.key" using default path2025-10-19 10:23:40 #1806716(normal) <ec2f11>; dkim_signing; lua_dkim_tools.lua:53: set domain to "example.org" using dkim_domain2025-10-19 10:23:40 #1806716(normal) <ec2f11>; dkim_signing; dkim_signing.lua:130: using key "/var/lib/rspamd/dkim/example.org.2025100901.key", use selector "2025100901" for domain "example.org"

One caveat is the handling of subdomains. `example.org` is a second-level domain. But you may want to use `service.example.org` for your domain. That’s totally fine. But rspamd will strip it down to `example.org` and use the DKIM key for `example.org`. If that’s not what you want, then add this line to `/etc/rspamd/local.d/dkim_signing.conf`:

    use_esld = false;

eSLD is a rarely used acronym that stands for effective second-level domain. In layman’s terms: the part of a domain that only consists of the rightmost two parts. For `mails.hawkeye.example.com` the eSLD is `example.com`.

## DMARC

Section titled “DMARC”

### Example

Section titled “Example”

Let’s take an example header of a spoofed mail:

    Return-Path: <noreply@evil-spammer.com>DKIM-Signature: d=evil-spammer.comFrom: <service@paypal.com>

In this example a bad guy just sent you this email. The SPF check for the `evil-spammer.com` domain was successful. (You can’t see that from these headers though.) He even added a DKIM signature from his domain. So everything looks right. But in your mail program you see `service@paypal.com`. So nobody checks that the envelop sender and the visible sender “align”.

### How it works

Section titled “How it works”

That’s what DMARC is supposed to solve. It works like this:

- (A) SPF
  - Run a normal SPF check (on the envelope sender’s domain as usual)
  - Make sure that the domain of this envelope sender aligns with (matches) the domain of the visible sender
- (B) DKIM
  - Look for a valid DKIM signature (also of the envelope sender’s domain)
  - Make sure that the domain of the envelope sender aligns with (matches) the domain of the visible sender
- Make sure that at least one of (A) or (B) are true

There are also two flavors of that alignment:

- strict means that the domains need to match exactly
- relaxed means that it can be a subdomain

You can find more examples about the alignment in the RFC 7489 in Appendix B.1.

### Reporting

Section titled “Reporting”

Another feature of DMARC is reporting. You may want to know whether your emails had delivery problems. DMARC has two tags for that purpose:

- ruf stands for “Reporting URI for Failure data”. The email address you use in the “ruf” tag will get machine-readable information on every mail that failed the DMARC test. If the receiving mail server supports DMARC, it may automatically generate and send that email to this address. However most internet providers just ignore “ruf” because it contains sensible information on email addresses.
- rua stands for “Reporting URI for Aggregated data”. The email address you use in the “rua” tag will get statistical data usually once a day. If you want to know if your emails deliver properly, you can use this.

There are commercial offers that allow you to point your “rua” record to their service and show you human-readable information. Or you can use the tools `dmarc-cat` or `dmarcts-report-parser` that can be installed as Debian packages on your system.

### Creating a record

Section titled “Creating a record”

If you want to use DMARC (and I suggest that you do), just add a DNS record to your domain. These web sites help you craft a valid record:

- http://www.kitterman.com/dmarc/assistant.html
- https://dmarcian.com/dmarc-inspector/
- https://www.unlocktheinbox.com/dmarcwizard/
- https://www.agari.com/resources/tools/dmarc

Please note that the DMARC record is a TXT record like `_dmarc.example.com`. That `_dmarc` host name is important.