Intro to Email Security with SPF DKIM and DMARC

4 February 2024 by Justin

Phishing and e-mail in general continue to be the most targeted attack vector according to Cloudflare’s 2023 Phishing Report. With e-mail service being the live blood of communication for most organizations, the security of such a service is paramount.

Hardening e-mail infrastructure and services is one piece to the puzzle, and most services provide integrated malware and spam prevention capabilities that add a layer of security to e-mail. However, another mechanism exists to provide additional security to e-mail services in the form of a number of DNS implementations, specifically, SPF, DKIM and DMARC.

SPF, DKIM, and DMARC are DNS implementations that provide security and reporting functionality to your e-mail delivery and receiving services. Let’s discuss each implementation further.

SPF

Sender Policy Framework (SPF), is a mechanism of identifying what servers are authorised and/or not authorised to send e-mail on behalf of a domain name. SPF also provides a mechanism to determine how strict to enforce the policy, allowing for some flexibility in implementation.

SPF Records

SPF exists in the form of a DNS TXT record. Here is an example:

v=SPF1 ip4:123.123.123.123 ip4:123.123.123.123 include:ses.amazon.com ~all

SPF Tags

The all Tag

The all tag always tests as positive and is used with a qualifier to determine how SPF evaluates the message. For example, the use of the - or fail qualifier, -all, would return a failed SPF check unless the sending server exists exactly in the SPF record, and quarantine the message. Using the ~ or softfail qualifier, ~all, would still return a failed SPF check, however, the message will not be quarantined.

SPF Qualifiers

The following qualifiers exist in the SPF standard:

+ – pass
- – fail
~ – softfail
? – neutral

DKIM

DomainKeys Identified Mail (DKIM) Signatures is a domain-level authentication framework that uses public-key cryptography in order to verify the contents and source of a message. DKIM uses cryptographic signatures to protect integrity of messages, as well as a irrefutable evidence of the source of the message. As DKIM uses DNS records, there is no need for a Certificate Authority (CA) to check the certificate against. A receiving server can simply query the sender domain to validate the necessary keys.

DKIM Records

The DKIM Selector is a DNS TXT record that indicators where a recipient server can locate the domains public key. The public key will is used to create a hash of the received e-mail, and compared to the hash written by the senders server. If the hashes match, we know that the e-mail contents were not changed in transit, and because the public key worked to create the proper has, we also know that the e-mail could have only come from that server.

Here is the DKIM record for roguesecurity.ca at selector1

v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAxAUDUvo/w6sBFGGC/hd8aNO0VNbZhGyvbOuHkG/dmlC+coDRH05zvWQgAbO1QSSatxb37d6AZ1sot+UeRbCpIVFBNvabPf/3QtYTj5gM3yl2k6FPK56uXTFNGZooA+WFYSKltYkaKnV1dNH7B0nlbV9Zh1NMcXLXsYb2idmi3RPkT12hxgeoaqdE1G6WfDstOd9B02kxxWr1gzuPGVmzLRCu8v170xEtvwoTghHcJSxmZT2zvauzTGuhkC9h65aPmebVPUiPipXw3ISmLd4YibzpbC/wGoVVS/McDZBSkUxFGcnM97eZF6ZLiS371TmCz4PqRc8iAmKJGLBRunpdnQIDAQAB;

DKIM Tags

DMARC

Domain-based Message Authentication, Reporting, and Conformance (DMARC) is a mechanism for providing domain-level policies and preferences for message validation, disposition and reporting. DKIM provides domain owners with a way of assigning a preferred method of handling authentication failures to reduce the amount of successfully delivered spoofed e-mail.

DKIM can also be configured to provide domain-level reporting on authentication and the disposition of received e-mail.

DMARC Records

DMARC records, like SPF and DMARC records, are DNS TXT entries that are defined using various mechanisms. Here is the DMARC record for roguesecurity.ca at _dmarc.

v=DMARC1;p=none;ruf=mailto:dnsreports@roguesecurity.ca;rua=mailto:dnsreports@roguesecurity.ca;sp=none

DMARC Tags