Sender Policy Framework: Encyclopedia II - Sender Policy Framework - Implementation

Sender Policy Framework - Implementation

Implementing SPF has two parts:

• Domains must identify the machines authorized to send e-mail on their behalf. Domains do this by adding an additional record to their existing DNS information.
• Receivers must request and use SPF information. Receivers request this information using ordinary DNS requests (which are typically cached to enhance performance). Receivers must then correctly interpret the SPF information. SPF does not specify how the SPF results should be interpreted or acted upon.

Thus, the key issue in SPF is the specification for the new DNS information that domains set and receivers use. The exact specifications may change, but as of writing, the records are laid out like this (in typical DNS-syntax):

example.org. IN TXT "v=spf1 mx ptr -all"

"v=" defines the version of SPF used, the following words provides the methods to use to determine if a domain is eligible to send mail. The "mx" and "ptr" specify the systems permitted to send messages for the given domain. The "-all" at the end specifies that, if the previous methods did not match, reject the message as a forgery.

The following methods are defined:

• A - If the domain name has an A record corresponding to the sender's address, it will match. (That is, the mail comes directly from the domain name.)
• MX - If the domain name has an MX record resolving to the sender's address, it will match. (That is, the mail comes from one of the domain's mail servers)
• PTR - If the sender reverse-resolves to a name ending in the domain, and that name resolves to the sender's IP address, match.
• IP4 - If the sender is in a given IPv4 range, match.
• IP6 - If the sender is in a given IPv6 range, match.
• EXISTS - If the given domain resolves, match (no matter the address it resolves to). This is typically used along with the SPF macro language to perform more complex matches.

Adapted from the Wikipedia article "Implementation", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki