These have value types explicitly defined in the RFC (https://tools.ietf.org/html/rfc8659#section-4.2).

Perhaps instead of having tag and value fields for CAA RRs, we could just have value whose type is an enum like

pub enum Value { Issue(DomainName), IssueWild(DomainName), IoDef(url::Url), Unknown(Tag, Vec<u8>), }

?