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* Add multiple string support to SPF optimizer Notes: * This implements [RFC 4408][rfc] for the SPF optimizer. Allowing for more SPF records to fit within the 10 lookups by using multiple strings. * By default the max size of the TXT remains at 255. Meaning users will still only get a single 255 length string unless they modify `txtMaxSize` and opt into this feature. * The general recommendation when using multiple strings for TXT records is to keep the size within a single UDP packet. It seems like the maximum size for this depends on a bunch of factors that are sometimes outside of your control. A similar tool has a [formula for estimating the maximum allowed size][formula]. However I felt giving a user configurable size would fit with the current configuration style that dnscontrol has. Similar to how dnscontrol recommends only flattening a record if absolutely needed, I can see this length being increased by only enough to get you within 10 lookups. [rfc]: https://tools.ietf.org/html/rfc4408#section-3.1.3 [formula]: https://github.com/oasys/mkspf/blob/master/Overhead.md * Add a nice comment for the Chunks function