RFC 6763 Non-Compliance: Instance Name Not Escaped in ServiceInfo::new()

[thibaut-pascal]

mDNS Instance Name Escaping Issue

The mdns-sd library does not escape special characters (dots . and backslashes \) in the Instance Name parameter when constructing Service Instance Names, violating RFC 6763 Section 4.3 requirements.

RFC 6763 Requirements

According to RFC 6763 Section 4.1.1, the DNS Instance name can contain almost any characters, including dots.

4.1.1.  Instance Names

   The <Instance> portion of the Service Instance Name is a user-
   friendly name consisting of arbitrary Net-Unicode text [RFC5198].  It
   MUST NOT contain ASCII control characters (byte values 0x00-0x1F and
   0x7F) [RFC20] but otherwise is allowed to contain any characters,
   without restriction, including spaces, uppercase, lowercase,
   punctuation -- including dots -- accented characters, non-Roman text,
   and anything else that may be represented using Net-Unicode.

According to RFC 6763 Section 4.3:

4.3.  Internal Handling of Names

   If client software takes the <Instance>, <Service>, and <Domain>
   portions of a Service Instance Name and internally concatenates them
   together into a single string, then because the <Instance> portion is
   allowed to contain any characters, including dots, appropriate
   precautions MUST be taken to ensure that DNS label boundaries are
   properly preserved.  Client software can do this in a variety of
   ways, such as character escaping.

   This document RECOMMENDS that if concatenating the three portions of
   a Service Instance Name, any dots in the <Instance> portion be
   escaped following the customary DNS convention for text files: by
   preceding literal dots with a backslash (so "." becomes "\.").
   Likewise, any backslashes in the <Instance> portion should also be
   escaped by preceding them with a backslash (so "\" becomes "\\").
   Having done this, the three components of the name may be safely
   concatenated.  The backslash-escaping allows literal dots in the name
   (escaped) to be distinguished from label-separator dots (not
   escaped), and the resulting concatenated string may be safely passed
   to standard DNS APIs like res_query(), which will interpret the
   backslash-escaped string as intended.

This document specifies that if concatenating the three portions of a Service Instance Name, any dots in the portion must be escaped following the customary DNS convention for text files: by preceding literal dots with a backslash (so "." becomes "\."). Likewise, any backslashes in the portion should also be escaped by preceding them with a backslash (so "\" becomes "\\").

Current Library Behavior

In mdns-sd/src/service_info.rs, the ServiceInfo::new() function performs direct string concatenation without escaping:

let fullname = format!("{my_name}.{ty_domain}");

This creates incorrect DNS label boundaries when the Instance Name contains dots or backslashes.

In reality, it is unlikely that anyone would insert dots into the DNS instance name, but this could be an improvement to the library in order to comply with this specification.

There is also no sanitizer on the hostname (RFC 952/1123), which is quite strict. Could it be that this library is responsible for ensuring that all strings comply with the specifications?

[keepsimple1]

In reality, it is unlikely that anyone would insert dots into the DNS instance name, but this could be an improvement to the library in order to comply with this specification.

Agreed. I will come back to this one after working on a couple of other enhancements. If you'd like to create PR for this, just let me know and it'll be great!

There is also no sanitizer on the hostname (RFC 952/1123), which is quite strict. Could it be that this library is responsible for ensuring that all strings comply with the specifications?

Did you mean the hostname inside SRV records (and A/AAAA records), or host_name passed in ServiceInfo::new()?

[thibaut-pascal]

I created a PR #420 to escape dots in the mDNS instance name.

Did you mean the hostname inside SRV records (and A/AAAA records), or host_name passed in ServiceInfo::new()?

Yes I mean the hostname inside the SRV record.
host_name in ServiceInfo::new() is actually called "instance name" in the specification, this can be bit confusing.