SOA record RNAME: Do special characters besides dot need backslash?

https://www.rfc-editor.org/rfc/rfc1035#section-8

If the <local-part> contains dots or other special characters, its representation in a master file will require the use of backslash quoting to ensure that the domain name is properly encoded. For example, the mailbox [email protected] would be represented as Action\.domains.ISI.EDU.

However, in practice I've never seen any special character besides the dot need backslash. According to RFC 1035 [email protected] should be encoded as Action\+domains.ISI.EDU but in practice it seems to be always Action+domains.ISI.EDU. Is the plus sign not considered a "special character" or is the dot the only special character that needs backslash?

+ is not a special character in the context of DNS, nowhere.

DNS has confusing terminology:

• domain names are any characters, absolutely no restrictions, only dot has specific meaning as separator between labels
• hostnames are subset of domain names that follow the "letter-digit-hyphen" or LDH constraint; in that subset + can not appear, so it is not special in the sense, there is nothing to do to protect it as it can't just appear at all.

The few characters that need special treatment are scattered around the document, but in short:

• . needs to be escaped, with \
• as such if you want a literal \ somewhere, you will need to escape it too
• then depending on where you are in the file, an end of line, or a quote may need to be escaped because otherwise would mean something else. For this need, and any other character, the default defined is to use \DDD where DDD is the decimal value of the byte (and as such US-ASCII for strings) you want to escape
• $ might need to be escaped depending on where you are in the fine and who consumes it because bind for example, and in part maybe some other software, treat $ORIGIN or $INCLUDE as specific instructions and not as names in the zone.