The short answer is that the DKIM tag v= indicates the version of the DKIM specification being used. This tag is a mandatory component of a DKIM signature header, and its presence is essential for the receiving mail server to correctly interpret and verify the signature.
Every DKIM signature found in an email's headers must start with this version tag. According to the official specification that defines DKIM, RFC 6376, this tag is required.
At present, there is only one version of the DKIM standard in widespread use, so you will almost always see this tag with the value v=DKIM1. While other tags in a DKIM signature can appear in different orders, the v= tag is consistently the first one.
The purpose of the version tag
The primary purpose of the v= tag is to provide forward compatibility. By including a version number, the creators of the DKIM standard ensured that the protocol could be updated in the future without invalidating existing implementations. If a new version, say DKIM2, were ever introduced, email servers could identify it and process it using a different set of rules, while still correctly handling older v=DKIM1 signatures.
While v= is the tag you'll find in the email header's DKIM-Signature, it's worth noting that the DKIM record itself, which is stored in your DNS, also technically has a version tag. However, its value is often assumed to be v=DKIM1 and it is sometimes omitted for brevity, as it's the only version in use.
Other tags in a DKIM signature
The v= tag is just one piece of the puzzle. A full DKIM signature contains several other critical pieces of information:
- a=: The algorithm used to create the signature, such as rsa-sha256.
- d=: The domain that signed the message. This should match the domain in the email's 'From' address.
- s=: The 'selector', which tells the receiving server where to find the public key in the domain's DNS records.
- h=: The list of headers from the email that were included in the cryptographic signature.
- bh=: The hash of the email's body.
In summary, the v=DKIM1 tag is the starting point for any DKIM signature. It sets the context for all other tags, ensuring that mail servers around the world can consistently and accurately verify the authenticity of your emails.
What DKIM tag indicates the algorithm used for signing?
What DKIM tag specifies the time the signature was created?
What DKIM tag is used to specify the public key?
What DKIM tag indicates the expiration time of the signature?
What DKIM tag indicates the service type?
What DKIM tag indicates the signature itself?
