What are valid characters for a DNS Zone file and how can I sanitize user input?

Question

I'm working on an interface to allow our clients to update their DNS on their own.

I have 2 questions:

1. What constitutes valid a valid host and target records? (A, CNAME, MX, TXT) i.e. if the user enters ........ for the host and target the DNS server won't like that.
2. Is there a regex I can use to sanitize user input?

BTW it is BIND9 DNS and C# web app.

Thanks,

Kyle

Answer 1

Domain name labels can technically contain any octet value, but usually they only contain alphanumerics and the hyphen and underscore characters.

This comes from recommendations in section 2.3.1 of RFC 1035:

The labels must follow the rules for ARPANET host names. They must start with a letter, end with a letter or digit, and have as interior characters only letters, digits, and hyphen. There are also some restrictions on the length. Labels must be 63 characters or less.

The underscore character is a more recent addition, typically used in the label portion of SRV records.

You could also permit the "." character if you're going to let users create their own subdomains.

The values that are possible are:

• A record - must be a dotted-quad IP address
• CNAME record - must be some other legal label
• MX record - 16-bit integer priority field, and a legal hostname. NB: some people put in labels which themselves point only to a CNAME record. This is frowned upon.
• TXT record - anything you like!

Note that in every case, if you do allow any of the characters not in the normal set they would need to be escaped if they're being stored in a BIND format zone file.

Answer 2

Do not forget AAAA, the IPv6 addresses (remember, only two years left before we run out of IPv4 addresses...).

For the legal names, read RFC 1123, section 2.1. domain names can be anything, host names have a much stricter syntax (see RFC 1123).

Answer 3

The answer used to be easy, but not anymore.

You can use almost any Unicode characters, but they should go thru a normalization, and encoding process.

See RFC 3490 (IDNA), RFC 3454 (Stringprep), RFC 3491 (Nameprep), RFC 3492 (Punycode)

Or go with Wikipedia for the big picture (http://en.wikipedia.org/wiki/Internationalized_domain_name).