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Swift: Deconstruct SPF an Outline and Background

basman profile image Adam.S Originally published at bas-man.github.io ・2 min read

What is SPF

If you have not followed my rust series on deconstructing SPF you can check it out here.

Here is some back ground on SPF.

What is a qualifier

A qualifier changes the way that a mechanism is interpreted, they are

  • + Pass
  • - Fail
  • ~ SoftFail
  • ? Neutral
    • Neutral is the default if no qualifier is matched.

What is a mechanism

An SPF mechanism can be one of several types

  • Include
  • A
  • Mx
  • Ip4
  • Ip6
  • All
  • Ptr
  • Exists
  • Redirect (Technially this is a Modifier, but I will be using it as a Mechanism)

I will be attempting to just learn some of the basics of the Swift language. So I will be looking at things like:

  • enum
  • struct
  • Optional variables
  • Perhaps more if I find that I need them.

Swift: The building blocks

Qualifiers

So I need to have a way to handle qualifiers

enum Qualifier: String {
    case Pass = "+"
    case Fail = "-"
    case SoftFail = "~"
    case Neutral = "?"
    case None = ""
}
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This enumeration will manage my qualifiers. I have added a None for the moment since in many cases, there is no qualifier provided, and I think I might want to be able to reproduce the mechanism in the future.

Mechanism

To handle these I will create a second enumeration

enum MechanismKind {
    case Redirect, Include, A, MX, Ip4, Ip6, All;
}
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Although Redirect is a modifier, I am listing it here as a mechanism. I am also omitting ptr and exists. These may or may not be handled.

A mechanism is made up of a qualifier and some mechanism value. It also needs to have a kind. So a struct will probably work here.

struct Mechanism {
    var kind: MechanismKind;
    var qualifier: Qualifier;
    var mechanism: String;
}
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Examples

Let's say we have an SPF record of the following type:

"v=spf1 a ip4:x.x.x.x ~all"
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We want to have a struct that will represent A and another that represents Ip4

These are conceptual examples only.

A
struct {
  kind => MechanismKind.A
  qualifier => Qualifers.None
  mechanism => ""
}
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Ip4
struct {
  kind => MechanismKind.Ip4
  qualifier => None
  mechanism => "x.x.x.x"
}
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Seeing this I can see that there will be some good cases to make some variables Optional

SPF

This struct will probably look something like what follows.

struct SPF {
    var redirect: Mechanism?;
    var is_redirect: Bool = false;
    var include: Mechanism?;
    var a: Mechanism?;
    var mx: Mechanism?;
    var all: Mechanism?;
}
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This is merely a rough outline. Most will need to be a list of records.
You might notice the use if the ? at the end of Mechanism. This denotes that the variable is optional and hence can have a nil value. That meaning it can be absent.

I refer you to the Swift Documentation

In the next article we will start to develop some code focused on the struct for a Mechanism

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