Tag Archives: DNS

SPF, DKIM and DMARC in Exchange Online Protection

In the previous two blog posts I’ve explained how to implement Exchange Online Protection as a message hygiene solution for your on-premises Exchange environment, both for inbound as well as outbound mail flow.

In this blog post I’ll go more into detail when configuring Exchange Online Protection

SPF, DKIM and DMARC

When Exchange Online Protection is fully up-and-running you can continue configuring SPF, DKIM and DMARC for message authentication purposes. This will make sure your email domain is much harder to spoof and receiving email systems know that the source of your email is a trusted source.

SPF

SPF has been around for years, and in my previous blog post I already mentioned the SPF record needed for use with Exchange Online Protection. If you want to read more about implementing SPF, please check my SenderID, SPF, DKIM and DMARC in Exchange 2016 – Part I blog post.

When only using Exchange Online Protection your SPF record will look like v=spf1 include:spf.protection.outlook.com -all.

DKIM

The good thing about Exchange Online Protection is that it supports DKIM signing and verification out of the box. The only thing you have to do is enable it in the Exchange Admin Center!

Logon to the Exchange (Online Protection) Admin Center, select protection in the navigation menu and click DKIM in the toolbar. When you click enable, an error message (in yellow) is shown that you need to create the appropriate DNS records first (click to enlarge).

EOP-DKIM

The error message reads (for search engine purposes):
CNAME record does not exist for this config. Please publish the following two CNAME records first.
selector1-domain-com._domainkey.tenantname.onmicrosoft.com
selector2-domain-com._domainkey.tenantname.onmicrosoft.com

The description isn’t that clear when you’re not a DKIM guru, but you need to create the following CNAME records:

Selector1._domainkey.domain.com CNAME selector1-domain-com._domainkey.tenant.onmicrosoft.com
Selector2._domainkey.domain.com CNAME selector2-domain-com._domainkey.tenant.onmicrosoft.com

When the DNS records are created you can use the MXToolbox to check if they are valid:MXToolbox-DKIM

If an email is sent from the on-premises Exchange server via Exchange Online Protection to for example Gmail, you can check the headers. If configured correctly you can see the SPF check passes, you can see the DKIM signature created by EOP and you can see the authentication results as well. It should read spf=pass and dkim=pass under Authentication Results as shown in the following screenshot (click to enlarge):

EOP-DKIM-Signed

For more information regarding DKIM and Exchange, please check my SenderID, SPF, DKIM and DMARC in Exchange 2016 – Part II blogpost.

DMARC

When DKIM and SPF are configured correctly you can create a DMARC record in public DNS. A DMARC policy will tell a receiving mail server what to do with email that does not comply with other settings. For example, if email is coming from a mail server that’s not listed in the SPF record it might well be spoofed. If the DKIM signature is missing it might be spoofed, if the DKIM signature is not valid, the message might be tempered with. If this is the case, you can define a policy that will reject such a message. The DNS record will be like this:

v=DMARC1;p=reject;sp=reject;pct=100;rua=mailto:dmarcreports@exchange2019.nl

The RUA is an email address where DMARC reports are sent to, so it’s a good thing to have such a mailbox on your Exchange server.

When sending an email from the Exchange server via EOP to Gmail when SPF, DKIM and DMARC are configured, you can see information like the following screenshot in the email headers (click to enlarge):

EOP-DKIM-Signed-DMARC

Everything is configured correctly now and spoofing is much harder to achieve for malicious users.

For more information regarding DMARC and Exchange, please check my SenderID, SPF, DKIM and DMARC in Exchange 2016 – Part III blog post.

Summary

In this blogpost I showed you how to configure SPF, DKIM and DMARC in Exchange Online Protection to prevent spoofing by malicious users.
In my next blog I’ll go more into detail about configuring the message hygiene options themselves in Exchange Online Protection.

SenderID, SPF, DKIM and DMARC in Exchange 2016 – Part III

In the previous two blog posts I have discussed SPF and DKIM as a way of validating the authenticity of email messages. SPF is using an SPF record in public DNS where all legitimate outbound SMTP servers for a domain are listed. A receiving SMTP server can check this DNS record to make sure the sending mail server is allowed to send email messages on behalf of the user or his organization.

DKIM is about signing and verifying header information in email messages. A sending mail server can digitally sign messages, using a private key that’s only available to the sending mail server. The receiving mail server checks the public key in DNS to verify the signed information in the email message. Since the private key is only available to the sending organization’s mail servers, the receiving mail server knows that it’s a legitimate mail server, and thus a legitimate email message.

As a reminder, my test environment is configured as follows:

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There’s an Exchange 2016 CU2 Mailbox server hosting several Mailboxes, and there’s an Exchange 2016 CU2 Edge Transport server. Using Edge Synchronization all inbound and outbound SMTP traffic is handled by the Edge Transport server.

In the previous two blog posts an SPF record was created and implemented, and DKIM including a DKIM signing module on the Edge Transport server was implemented and functioning correctly.

This last blog in a series of three discusses DMARC, which is built on top of SPF and DKIM. Continue reading SenderID, SPF, DKIM and DMARC in Exchange 2016 – Part III

SenderID, SPF, DKIM and DMARC in Exchange 2016 – Part II

In the previous blogpost I have been discussing how SPF works and how it uses public DNS to validate the authenticity of the sending SMTP servers. When SPF is implemented correctly a receiving mail server can validate is the sending mail server is allowed to send email on behalf of the sender or his organization.

In this blogpost I will discuss DKIM signing as an additional (and more complicated, and more difficult to spoof) step in email validation.

As a quick reminder, here’s how my lab environment looks like:

image

There’s an Exchange 2016 CU2 Mailbox server hosting several Mailboxes, and there’s an Exchange 2016 CU2 Edge Transport server. An Edge synchronization will make sure that all inbound and outbound SMTP traffic is handled by the Edge Transport server.

In my previous blogpost an SPF record was created and implemented with the following value:

v=spf1 a:smtphost.exchangelabs.nl ~all

so receiving mail servers can validate that my Edge Transport server is allowed to send email on my behalf, and when mail is originating from another mail server it might well be a spoofed message.

But for now let’s continue with DKIM. Continue reading SenderID, SPF, DKIM and DMARC in Exchange 2016 – Part II

SenderID, SPF, DKIM and DMARC in Exchange 2016 – Part I

SenderID has been used in Exchange as a means for anti-spam for quite some time, as far as I can remember this was first used in Exchange 2010. Related to SenderID is SPF (Sender Policy Framework). SPF looks like SenderID functionality, but it differs in the way how it checks email messages.

Both use public DNS records with TXT records where information is stored regarding the sending SMTP server, and this information is used by the receiving (Exchange) server to validate if the sending server is allowed to send email on behalf of the sender.

Getting more popular for fighting spam are DKIM (DomainKeys Identified Mail) and DMARC (Domain-based Message Authentication, Reporting & Conformance). Just like SenderID and SPF, these solutions use public DNS for additional information as well, but since encryption is used most Exchange admin have some doubts about the complexity of DKIM and DMARC.

In the upcoming blogpost I’ll discuss SPF, DKIM and DMARC as implemented in my lab environment which looks like this:

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There’s an Exchange 2016 CU2 Mailbox server hosting several Mailboxes. The server is accessible via webmail.exchangelabs.nl and autodiscover.exchangelabs.nl (same IP address, behind a Kemp LM3600 load balancer) and configured with a Digicert UC certificate.

In addition to this there’s an Exchange 2016 CU2 Edge Transport server with FQDN smtphost.exchangelabs.nl. Besides the regular A and MX record, the IP address is also configured in Reverse DNS. The Edge Transport server is also behind a Kemp LM3600 load balancer, and it has a Digicert SSL Certificate with the same domain name. There’s an Edge Synchronization configured between the Mailbox server and the Edge Transport server, and all inbound and outbound mail is handled by the Edge Transport server. Continue reading SenderID, SPF, DKIM and DMARC in Exchange 2016 – Part I

Change SMTP mail flow in hybrid scenario

After building a hybrid Exchange environment as outlined in a couple of previous blog posts we have an Exchange 2013/2016 environment where some Mailboxes exist on-premises and some Mailboxes exist in Exchange Online. Autodiscover is still pointing to the on-premises environment, and so are the MX records. Inbound SMTP mail flow from the Internet is still accessing the on-premises Exchange 2016 Edge Transport servers before being delivered to the intended recipients.

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Figure 1. The Exchange hybrid environment with Mailboxes on-premises and in Exchange online.

Continue reading Change SMTP mail flow in hybrid scenario