Network Working Group D. Thaler
Internet-Draft Microsoft
Intended status: Informational J. Klensin
Expires: November 15, 2010
S. Cheshire
Apple
May 14, 2010
IAB Thoughts on Encodings for Internationalized Domain Names
draft-iab-idn-encoding-02.txt
Abstract
This document explores issues with Internationalized Domain Names
(IDNs) that result from the use of various encoding schemes such as
UTF-8 and the ASCII-Compatible Encoding produced by the Punycode
algorithm. It focuses on the importance of agreeing on a canonical
format and how complicated it ends up being as a result of using
different encodings today.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 15, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
Thaler, et al. Expires November 15, 2010 [Page 1]
Internet-Draft IDN Encodings May 2010
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. APIs . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2. Use of Non-DNS Protocols . . . . . . . . . . . . . . . . . . . 9
3. Use of Non-ASCII in DNS . . . . . . . . . . . . . . . . . . . 10
3.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 14
4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 16
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
8. IAB Members at the time of publication . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
Thaler, et al. Expires November 15, 2010 [Page 2]
Internet-Draft IDN Encodings May 2010
1. Introduction
The goal of this document is to explore what can be learned from some
current difficulties in implementing Internationalized Domain Names
(IDNs).
A domain name consists of a set of labels, conventionally written
separated with dots. An Internationalized Domain Name (IDN) is a
domain name that contains one or more labels that, in turn, contain
one or more non-ASCII characters. Just as with plain ASCII domain
names, each IDN label must be encoded using some mechanism before it
can be transmitted in network packets, stored in memory, stored on
disk, etc. These encodings need to be reversible, but they need not
store domain names the same way humans conventionally write them on
paper. For example, when transmitted over the network in DNS
packets, domain name labels are *not* separated with dots.
IDNA, discussed later in this document, is the standard that defines
the use and coding of internationalized domain names for use on the
public Internet. It is described as "Internationalizing Domain Names
in Applications (IDNA)" and is defined in several documents.
Definitions for the current version and a roadmap of related
documents appears in [IDNA2008-Defs]. An earlier version of IDNA
[RFC3490] is now being phased out. Except where noted, the two
versions are approximately the same with regard to the issues
discussed in this document. However, some explanations appeared in
the earlier documents that did not seem useful when the revision was
created; they are quoted here from the documents in which they
appear. In addition, the terminology of the two version differs
somewhat; this document reflects the terminology of the current
version.
Unicode [Unicode] is a list of characters (including non-spacing
marks that are used to form some other characters), where each
character is assigned an integer value, called a code point. In
simple terms a Unicode string is a string of integer code point
values in the range 0 to 1,114,111 (10FFFF in base 16), which
represent a string of Unicode characters. These integer code points
must be encoded using some mechanism before they can be transmitted
in network packets, stored in memory, stored on disk, etc. Some
common ways of encoding these integer code point values in computer
systems include UTF-8, UTF-16, and UTF-32. In addition to the
material below, those forms and the tradeoffs among them are
discussed in Chapter 2 of The Unicode Standard [Unicode].
UTF-8 [RFC3629] is a mechanism for encoding a Unicode code point in a
variable number of 8-bit octets, where an ASCII code point is
preserved as-is. Those octets encode a string of integer code point
Thaler, et al. Expires November 15, 2010 [Page 3]
Internet-Draft IDN Encodings May 2010
values, which represent a string of Unicode characters.
UTF-16 (formerly UCS-2) is a mechanism for encoding a Unicode code
point in one or two 16-bit integers, described in detail in Sections
3.9 and 3.10 of The Unicode Standard [Unicode]. A UTF-16 string
encodes a string of integer code point values that represent a string
of Unicode characters.
UTF-32 (formerly UCS-4), also described in [Unicode] Sections 3.9 and
3.10, is a mechanism for encoding a Unicode code point in a single
32-bit integer. A UTF-32 string is thus a string of 32-bit integer
code point values, which represent a string of Unicode characters.
Note that UTF-16 results in some all-zero octets when code points
occur early in the Unicode sequence, and UTF-32 always has all-zero
octets.
IDNA specifies validity of a label, such as what characters it can
contain, relationships among them, and so on, in Unicode terms.
Valid labels can take either of two forms, with the appropriate one
determined by particular protocols or by context. One of those
forms, called a U-label, is a direct representation of the Unicode
characters using one of the encoding forms discussed above. This
document discusses UTF-8 strings in many places. While all U-labels
can be represented by UTF-8 strings, not all UTF-8 strings are valid
U-labels (see Section 2.3.2 of [IDNA2008-Defs] for a discussion of
these distinctions). The other, called an A-label, uses a
compressed, ASCII-compatible encoding (an "ACE" in IDNA and other
terminology) produced by an algorithm called Punycode. U-labels and
A-labels are duals of each other: transformations from one to the
other do not lose information. The transformation mechanisms are
specified in [IDNA2008-Protocol].
Punycode [RFC3492] is thus a mechanism for encoding a Unicode string
in an ASCII-compatible encoding, i.e., using only letters, digits,
and hyphens from the ASCII character set. When a Unicode label that
is valid under the IDNA rules (a U-label) is encoded with Punycode
for IDNA purposes, it is prefixed with "xn--"; the result is called
an A-label. The prefix convention assumes that no other DNS labels
(at least no other DNS labels in IDNA-aware applications) are allowed
to start with these four characters. Consequently, when A-label
encoding is assumed, any DNS labels beginning with "xn--" now have a
different meaning (the Punycode encoding of a label containing one or
more non-ASCII characters) or no defined meaning at all (in the case
of labels that are not IDNA-compliant, i.e., are not well-formed
A-labels).
ISO-2022-JP [RFC1468] is a mechanism for encoding a string of ASCII
Thaler, et al. Expires November 15, 2010 [Page 4]
Internet-Draft IDN Encodings May 2010
and Japanese characters, where an ASCII character is preserved as-is.
ISO-2022-JP is stateful: special sequences are used to switch between
character coding tables.
Comparison of Unicode strings is not as easy as comparing for example
ASCII strings. First, there are a multitude of ways of representing
a string of Unicode characters. Second, in many languages and
scripts, the actual definition of "same" is very context-dependent.
Because of this, comparison of two Unicode strings must take into
account how the Unicode strings are encoded. Regardless of the
encoding, however, comparison cannot simply be done by comparing the
encoded Unicode strings byte by byte. The only time that is possible
is when the strings both are mapped into some canonical format and
encoded the same way.
This document focuses on the importance of agreeing on a canonical
format and how complicated it ends up being as a result of using
different encodings today.
Different applications, APIs, and protocols use different encoding
schemes today. Historically, many of them were originally defined to
use only ASCII. Internationalizing Domain Names in Applications
(IDNA) [IDNA2008-Defs] defined a mechanism that required changes to
applications, but in attempt not to change APIs or servers, specified
that the A-label format is to be used in many contexts. In some ways
this could be seen as not changing the existing APIs, in the sense
that the strings being passed to and from the APIs were still
apparently ASCII strings. In other ways it was a very profound
change to the existing APIs, because while those strings were still
syntactically valid ASCII strings, they no longer meant the same
thing as they used to. What looked like a plain ASCII string to one
piece of software or library could be seen by another piece of
software or library (with the application of out-of-band information)
to be in fact an encoding of a Unicode string.
Section 1.3 of the original IDNA specification [RFC3490] states:
The IDNA protocol is contained completely within applications. It
is not a client-server or peer-to-peer protocol: everything is
done inside the application itself. When used with a DNS resolver
library, IDNA is inserted as a "shim" between the application and
the resolver library. When used for writing names into a DNS
zone, IDNA is used just before the name is committed to the zone.
Figure 1 depicts a simplistic architecture that a naive reader might
assume from the paragraph quoted above. (A variant of this same
picture appears in Section 6 of the IDNA specification [RFC3490]
further strengthening this assumption.)
Thaler, et al. Expires November 15, 2010 [Page 5]
Internet-Draft IDN Encodings May 2010
+-----------------------------------------+
|Host |
| +-------------+ |
| | Application | |
| +------+------+ |
| | |
| +----+----+ |
| | DNS | |
| | Resolver| |
| | Library | |
| +----+----+ |
| | |
+-----------------------------------------+
|
_________|_________
/ \
/ \
/ \
| Internet |
\ /
\ /
\___________________/
Simplistic Architecture
Figure 1
There are, however, two problems with this simplistic architecture
that cause it to differ from reality.
First, resolver APIs on Operating Systems (OSs) today (MacOS,
Windows, Linux, etc.) are not DNS-specific. They typically provide a
layer of indirection so that the application can work independent of
the name resolution mechanism, which could be DNS, mDNS
[I-D.cheshire-dnsext-multicastdns], LLMNR [RFC4795], NetBIOS-over-TCP
[RFC1001][RFC1002], etc/hosts file [RFC0952], NIS [NIS], or anything
else. For example, "Basic Socket Interface Extensions for IPv6"
[RFC3493] specifies the getaddrinfo() API and contains many phrases
like "For example, when using the DNS" and "any type of name
resolution service (for example, the DNS)". Importantly, DNS is
mentioned only as an example, and the application has no knowledge as
to whether DNS or some other protocol will be used.
Second, even with the DNS protocol, private name spaces (sometimes
including private uses of the DNS), do not necessarily use the same
character set encoding scheme as the public Internet name space.
We will discuss each of the above issues in subsequent sections. For
Thaler, et al. Expires November 15, 2010 [Page 6]
Internet-Draft IDN Encodings May 2010
reference, Figure 2 depicts a more realistic architecture on typical
hosts today (which don't have IDNA inserted as a shim immediately
above the DNS resolver library). More generally, the host may be
attached to one or more local networks, each of which may or may not
be connected to the public Internet and may or may not have a private
name space.
+-----------------------------------------+
|Host |
| +-------------+ |
| | Application | |
| +------+------+ |
| | |
| +------+------+ |
| | Generic | |
| | Name | |
| | Resolution | |
| | API | |
| +------+------+ |
| | |
| +-----+------+---+--+-------+-----+ |
| | | | | | | |
| +-+-++--+--++--+-++---+---++--+--++-+-+ |
| |DNS||LLMNR||mDNS||NetBIOS||hosts||...| |
| +---++-----++----++-------++-----++---+ |
| |
+-----------------------------------------+
|
______|______
/ \
/ \
/ local \
\ network /
\ /
\_____________/
|
_________|_________
/ \
/ \
/ \
| Internet |
\ /
\ /
\___________________/
Realistic Architecture
Figure 2
Thaler, et al. Expires November 15, 2010 [Page 7]
Internet-Draft IDN Encodings May 2010
1.1. APIs
Section 6.2 of the original IDNA specification [RFC3490] states
(where ToASCII and ToUnicode below refer to conversions using the
Punycode algorithm):
It is expected that new versions of the resolver libraries in the
future will be able to accept domain names in other charsets than
ASCII, and application developers might one day pass not only
domain names in Unicode, but also in local script to a new API for
the resolver libraries in the operating system. Thus the ToASCII
and ToUnicode operations might be performed inside these new
versions of the resolver libraries.
Resolver APIs such as getaddrinfo() and its predecessor
gethostbyname() were defined to accept "char *" arguments, meaning
they accept a string of bytes, terminated with a NULL (0) byte.
Because of the use of a NULL octet as a string terminator, this is
sufficient for ASCII strings (including A-labels) and even
ISO-2022-JP and UTF-8 strings (unless an implementation artificially
precludes them), but not UTF-16 or UTF-32 strings because a NULL
octet could appear in the the middle of strings using these
encodings. Several operating systems historically used in Japan will
accept (and expect) ISO-2022-JP strings in such APIs. Some platforms
used worldwide also have new versions of the APIs (e.g.,
GetAddrInfoW() on Windows) that accept other encoding schemes such as
UTF-16.
It is worth noting that an API using "char *" arguments can
distinguish between conventional ASCII "host name" labels, A-labels,
ISO-2022-JP, and UTF-8 labels in names if the coding is known to be
one of those four. An example method is as follows:
o if the label contains an ESC (0x1B) byte the label is ISO-2022-JP;
otherwise,
o if any byte in the label has the high bit set, the label is UTF-8;
otherwise,
o if the label starts with "xn--" then it is presumed to be an
A-label; otherwise,
o the label is ASCII.
Again this assumes that neither ASCII labels nor UTF-8 strings ever
start with "xn--", and also that UTF-8 strings never contain an ESC
character. Also the above is merely an illustration; UTF-8 can be
detected and distinguished from other 8-bit encodings with good
accuracy [MJD].
It is more difficult or impossible to distinguish the ISO 8859
character sets from each other, because they differ in up to about 90
characters which have exactly the same encodings, and a short string
Thaler, et al. Expires November 15, 2010 [Page 8]
Internet-Draft IDN Encodings May 2010
is very unlikely to contain enough characters to allow a receiver to
deduce the character set. Similarly, it is not possible in general
to distinguish between ISO-2022-JP and any other encoding based on
ISO 2022 code table switching.
Although it is possible (as in the example above) to distinguish some
encodings when not explicitly specified, it is cleaner to have the
encodings specified explicitly, such as specifying UTF-16 for
GetAddrInfoW(), or specifying explicitly which APIs expect UTF-8
strings.
2. Use of Non-DNS Protocols
As noted earlier, typical name resolution libraries are not DNS-
specific. Furthermore, some protocols are defined to use encoding
forms other than IDNA A-labels. For example, mDNS
[I-D.cheshire-dnsext-multicastdns] specifies that UTF-8 be used.
Indeed, the IETF policy on character sets and languages [RFC2277]
states:
Protocols MUST be able to use the UTF-8 charset, which consists of
the ISO 10646 coded character set combined with the UTF-8
character encoding scheme, as defined in [10646] Annex R
(published in Amendment 2), for all text. Protocols MAY specify,
in addition, how to use other charsets or other character encoding
schemes for ISO 10646, such as UTF-16, but lack of an ability to
use UTF-8 is a violation of this policy; such a violation would
need a variance procedure ([BCP9] section 9) with clear and solid
justification in the protocol specification document before being
entered into or advanced upon the standards track. For existing
protocols or protocols that move data from existing datastores,
support of other charsets, or even using a default other than
UTF-8, may be a requirement. This is acceptable, but UTF-8
support MUST be possible.
Applications that convert an IDN to A-label form before calling
getaddrinfo() will result in name resolution failures if the Punycode
name is directly used in such protocols. Having libraries or
protocols to convert from A-labels to the encoding scheme defined by
the protocol (e.g., UTF-8) would require changes to APIs and/or
servers, which IDNA was intended to avoid.
As a result, applications that assume that non-ASCII names are
resolved using the public DNS and blindly convert them to A-labels
without knowledge of what protocol will be selected by the name
resolution library, have problems. Furthermore, name resolution
libraries often try multiple protocols until one succeeds, because
Thaler, et al. Expires November 15, 2010 [Page 9]
Internet-Draft IDN Encodings May 2010
they are defined to use a common name space. For example, the hosts
file, DNS, and NetBIOS-over-TCP are all defined to be able to share a
common syntax (e.g., see ([RFC0952], [RFC1001] section 11.1.1, and
[RFC1034] section 2.1). This means that when an application passes a
name to be resolved, resolution may in fact be attempted using
multiple protocols, each with a potentially different encoding
scheme. For this to work successfully, the name must be converted to
the appropriate encoding scheme only after the choice is made to use
that protocol. In general, this cannot be done by the application
since the choice of protocol is not made by the application.
3. Use of Non-ASCII in DNS
A common misconception is that DNS only supports names that can be
expressed using letters, digits, and hyphens.
This misconception originally stemmed from the definition in 1985 of
an "Internet host name" (and net, gateway, and domain name) for use
in the "hosts" file [RFC0952]. An Internet host name was defined
therein as including only letters, digits, and hyphens, where upper
and lower case letters were to be treated as identical. The DNS
specification [RFC1034] section 3.5 entitled "Preferred name syntax"
then repeated this definition in 1987, saying that this "syntax will
result in fewer problems with many applications that use domain names
(e.g., mail, TELNET)".
The confusion was thus left as to whether the "preferred" name syntax
was a mandatory restriction in DNS, or merely "preferred".
The definition of an Internet host name was updated in 1989
([RFC1123] section 2.1) to allow names starting with a digit (to
support IPv4 addresses in dotted-decimal form). Section 6.1 of
"Requirements for Internet Hosts -- Application and Support"
[RFC1123] discusses the use of DNS (and the hosts file) for resolving
host names to IP addresses and vice versa. This led to confusion as
to whether all names in DNS are "host names", or whether a "host
name" is merely a special case of a DNS name.
By 1997, things had progressed to a state where it was necessary to
clarify these areas of confusion. "Clarifications to the DNS
Specification" [RFC2181] section 11 states:
The DNS itself places only one restriction on the particular
labels that can be used to identify resource records. That one
restriction relates to the length of the label and the full name.
The length of any one label is limited to between 1 and 63 octets.
A full domain name is limited to 255 octets (including the
Thaler, et al. Expires November 15, 2010 [Page 10]
Internet-Draft IDN Encodings May 2010
separators). The zero length full name is defined as representing
the root of the DNS tree, and is typically written and displayed
as ".". Those restrictions aside, any binary string whatever can
be used as the label of any resource record. Similarly, any
binary string can serve as the value of any record that includes a
domain name as some or all of its value (SOA, NS, MX, PTR, CNAME,
and any others that may be added). Implementations of the DNS
protocols must not place any restrictions on the labels that can
be used.
Hence, it clarified that the restriction to letters, digits, and
hyphens does not apply to DNS names in general, nor to records that
include "domain names". Hence the "preferred" name syntax described
in the original DNS specification [RFC1034] is indeed merely
"preferred", not mandatory.
Since there is no restriction even to ASCII, let alone letter-digit-
hyphen use, DNS is in conformance with the IETF requirement to allow
UTF-8 [RFC2277].
Using UTF-16 or UTF-32 encoding, however, would not be ideal for use
in DNS packets or APIs because existing software already uses ASCII,
and UTF-16 and UTF-32 strings can contain all-zero octets that
existing software may interpret as the end of the string. To use
UTF-16 or UTF-32 one would need some way of knowing whether the
string was encoded using ASCII, UTF-16, or UTF-32, and indeed for
UTF-16 or UTF-32 whether it was big-endian or little-endian encoding.
In contrast, UTF-8 works well because any 7-bit ASCII string is also
a UTF-8 string representing the same characters.
If a private name space is defined to use UTF-8 (and not other
encodings such as UTF-16 or UTF-32), there's no need for a mechanism
to know whether a string was encoded using ASCII or UTF-8, because
(for any string that can be represented using ASCII) the
representations are exactly the same. In other words, for any string
that can be represented using ASCII it doesn't matter whether it is
interpreted as ASCII or UTF-8 because both encodings are the same,
and for any string that can't be represented using ASCII, it's
obviously UTF-8. In addition, unlike UTF-16 and UTF-32, ASCII and
UTF-8 are both byte-oriented encodings so the question of big-endian
or little-endian encoding doesn't apply.
While implementations of the DNS protocol must not place any
restrictions on the labels that can be used, applications that use
the DNS are free to impose whatever restrictions they like, and many
have. The above rules permit a domain name label that contains
unusual characters, such as embedded spaces which many applications
would consider a bad idea. For example, the SMTP protocol [RFC5321],
Thaler, et al. Expires November 15, 2010 [Page 11]
Internet-Draft IDN Encodings May 2010
but going back to the original specification in [RFC0821], constrains
the character set usable in email addresses. There is now an effort
underway to permit SMTP to support internationalized email addresses
via an extension.
Shortly after the DNS Clarifications [RFC2181] and IETF character
sets and languages policy [RFC2277] were published, the need for
internationalized names within private name spaces (i.e., within
enterprises) arose. The current (and past, predating IDNA and the
prefixed ACE conventions) practice within enterprises that support
other languages is to put UTF-8 names in their internal DNS servers
in a private name space. For example, "Using the UTF-8 Character Set
in the Domain Name System" [I-D.skwan-utf8-dns-00] was first written
in 1997, and was then widely deployed in Windows. The use of UTF-8
names in DNS was similarly implemented and deployed in MacOS, simply
by virtue of the fact that applications blindly passed UTF-8 strings
to the name resolution APIs, and the name resolution APIs blindly
passed those UTF-8 strings to the DNS servers, and the DNS servers
correctly answered those queries, and from the user's point of view
everything worked properly without any special new code being
written, except that ASCII is matched case-insensitively whereas
UTF-8 is not (although some enterprise DNS servers reportedly attempt
to do case-insensitive matching on UTF-8 within private name spaces).
Within a private name space, and especially in light of the IETF
UTF-8 policy [RFC2277], it was reasonable to assume within a private
name space that binary strings were encoded in UTF-8.
As implied earlier, there are also issues with mapping strings to
some canonical form, independent of the encoding. Such issues are
not discussed in detail in this document. They are discussed to some
extent in, for example, Section 3 of [RFC5198], and are left as
opportunities for elaboration in other documents.
Five years after UTF-8 was already in use in private name spaces in
DNS, the strategy of using a reserved prefix and an ASCII-compatible
Encoding (ACE) was developed for IDNA. That strategy included the
Punycode algorithm, which began to be developed (during the period
from 2002 [I-D.ietf-idn-punycode-00] to 2003 [RFC3492]) for use in
the public DNS name space. One reason the prefixed ACE strategy was
selected for the public DNS name space had to do with concerns about
whether the details of IDNA, including the use of the Punycode
algorithm, were an adequate solution to the problems that were posed.
If either the Punycode algorithm or fundamental aspects of character
handling were wrong, and had to be changed to something incompatible,
it would be possible to switch to a new prefix or adopt another model
entirely. Only the part of the public DNS namespace that starts a
label with "xn--" would be polluted.
Thaler, et al. Expires November 15, 2010 [Page 12]
Internet-Draft IDN Encodings May 2010
Today the algorithm is seen as being about as good as it can
realistically be, so moving to a different encoding (UTF-8 as
suggested in this document) that can be viewed as "native" would not
be as risky as it would have been in 2002.
In any case, the publication of [RFC3492] and the dependencies on it
in [IDNA2008-Protocol] and the earlier [RFC3490] thus resulted in
having to use different encodings for different name spaces (where
UTF-8 for private name spaces was already deployed). Hence,
referring back to Figure 2, a different encoding scheme may be in use
on the Internet vs. a local network.
In general a host may be connected to zero or more networks using
private name spaces, plus potentially the public name space.
Applications that convert a U-label form IDN to an A-label before
calling getaddrinfo() will incur name resolution failures if the name
is actually registered in a private name space in some other encoding
(e.g., UTF-8). Having libraries or protocols convert from A-labels
to the encoding used by a private name space (e.g., UTF-8) would
require changes to APIs and/or servers, which IDNA was intended to
avoid.
Also, a fully-qualified domain name (FQDN) to be resolved may be
obtained directly from an application, or it may be composed by the
DNS resolver itself from a single label obtained from an application
by using a configured suffix search list, and the resulting FQDN may
use multiple encodings in different labels. For more information on
the suffix search list, see section 6 of "Common DNS Implementation
Errors and Suggested Fixes" [RFC1536], the DHCP Domain Search Option
[RFC3397], and section 4 of "DNS Configuration options for DHCPv6"
[RFC3646].
As noted in [RFC1536] section 6, the community has had bad
experiences with "searching" for domain names by trying multiple
variations or appending different suffixes. Such searching can yield
inconsistent results depending on the order in which alternatives are
tried. Nonetheless, the practice is widespread and must be
considered.
The practice of searching for names, whether by the use of a suffix
search list or by searching in different namespaces can yield
inconsistent results. For example, even when a suffix search list is
only used when an application provides a name containing no dots, two
clients with different configured suffix search lists can get
different answers, and the same client could get different answers at
different times if it changes its configuration (e.g., when moving to
another network). A deeper discussion of this topic is outside the
scope of this document.
Thaler, et al. Expires November 15, 2010 [Page 13]
Internet-Draft IDN Encodings May 2010
3.1. Examples
Some examples of cases that can happen in existing implementations
today (where {non-ASCII} below represents some user-entered non-ASCII
string) are:
1. User types in {non-ASCII}.{non-ASCII}.com, and the application
passes it, in the form of a UTF-8 string, to getaddrinfo or
gethostbyname or equivalent.
* The DNS resolver passes the (UTF-8) string unmodified to a DNS
server.
2. User types in {non-ASCII}.{non-ASCII}.com, and the application
passes it to a name resolution API that accepts strings in some
other encoding such as UTF-16, e.g., GetAddrInfoW on Windows.
* The name resolution API decides to pass the string to DNS (and
possibly other protocols).
* The DNS resolver converts the name from UTF-16 to UTF-8 and
passes the query to a DNS server.
3. User types in {non-ASCII}.{non-ASCII}.com, but the application
first converts it to A-label form such that the name that is
passed to name resolution APIs is (say) xn--e1afmkfd.xn--
80akhbyknj4f.com.
* The name resolution API decides to pass the string to DNS (and
possibly other protocols).
* The DNS resolver passes the string unmodified to a DNS server.
* If the name is not found in DNS, the name resolution API
decides to try another protocol, say mDNS.
* The query goes out in mDNS, but since mDNS specified that
names are to be registered in UTF-8, the name isn't found
since it was encoded as an A-label in the query.
4. User types in {non-ASCII}, and the application passes it, in the
form of a UTF-8 string, to getaddrinfo or equivalent.
* The name resolution API decides to pass the string to DNS (and
possibly other protocols).
* The DNS resolver will append suffixes in the suffix search
list, which may contain UTF-8 characters if the local network
uses a private name space.
* Each FQDN in turn will then be sent in a query to a DNS
server, until one succeeds.
5. User types in {non-ASCII}, but the application first converts it
to an A-label, such that the name that is passed to getaddrinfo
or equivalent is (say) xn--e1afmkfd.
* The name resolution API decides to pass the string to DNS (and
possibly other protocols).
* The DNS stub resolver will append suffixes in the suffix
search list, which may contain UTF-8 characters if the local
network uses a private name space, resulting in (say) xn--
e1afmkfd.{non-ASCII}.com
Thaler, et al. Expires November 15, 2010 [Page 14]
Internet-Draft IDN Encodings May 2010
* Each FQDN in turn will then be sent in a query to a DNS
server, until one succeeds.
* Since the private name space in this case uses UTF-8, the
above queries fail, since the A-label version of the name was
not registered in that name space.
6. User types in {non-ASCII1}.{non-ASCII2}.{non-ASCII3}.com, where
{non-ASCII3}.com is a public name space using IDNA and A-labels,
but {non-ASCII2}.{non-ASCII3}.com is a private name space using
UTF-8, which is accessible to the user. The application passes
the name, in the form of a UTF-8 string, to getaddrinfo or
equivalent.
* The name resolution API decides to pass the string to DNS (and
possibly other protocols).
* The DNS resolver tries to locate the authoritative server, but
fails the lookup because it cannot find a server for the UTF-8
encoding of {non-ASCII3}.com, even though it would have access
to the private name space. (To make this work, the private
name space would need to include the UTF-8 encoding of {non-
ASCII3}.com.)
When users use multiple applications, some of which do A-label
conversion prior to passing a name to name resolution APIs, and some
of which do not, odd behavior can result which at best violates the
principle of least surprise, and at worst can result in security
vulnerabilities.
First consider two competing applications, such as web browsers, that
are designed to achieve the same task. If the user types the same
name into each browser, one may successfully resolve the name (and
hence access the desired content) because the encoding scheme was
correct, while the other may fail name resolution because the
encoding scheme was incorrect. Hence the issue can incent users to
switch to another application (which in some cases means switching to
an IDNA application, and in other cases means switching away from an
IDNA application).
Next consider two separate applications where one is designed to be
launched from the other, for example a web browser launching a media
player application when the link to a media file is clicked. If both
types of content (web pages and media files in this example) are
hosted at the same IDN in a private name space, but one application
converts to A-labels before calling name resolution APIs and the
other does not, the user may be able to access a web page, click on
the media file causing the media player to launch and attempt to
retrieve the media file, which will then fail because the IDN
encoding scheme was incorrect. Or even worse, if an attacker was
able to register the same name in the other encoding scheme, may get
the content from the attacker's machine. This is similar to a normal
Thaler, et al. Expires November 15, 2010 [Page 15]
Internet-Draft IDN Encodings May 2010
phishing attack, except that the two names represent exactly the same
Unicode characters.
4. Recommendations
As explained above, using multiple canonical formats, and multiple
encodings in different protocols or even in different places in the
same namespace creates problems. Because of this, and the fact that
both IDNA A-labels and UTF-8 are in use as encoding mechanisms for
domain names today, we recommend the following.
It is inappropriate for an application to convert a name to an
A-label when it does not know whether DNS will be used by the name
resolution library, or whether the name exists in a private name
space that uses UTF-8, or in the global DNS that uses IDNA A-labels.
Instead, conversion to A-label form, UTF-8, or any other encoding,
should be done only by an entity that knows which protocol will be
used (e.g., the DNS resolver, or getaddrinfo upon deciding to pass
the name to DNS), rather than by general applications that call
protocol-independent name resolution APIs. (Of course, it is still
necessary for applications to convert to whatever form those APIs
expect.) Similarly, even when DNS is used, the conversion to
A-labels should be done only by an entity that knows which name space
will be used.
That is, a more intelligent DNS resolver would be more liberal in
what it would accept from an application and be able to query for
both a name in A-label form (e.g., over the Internet) and a UTF-8
name (e.g., over a corporate network with a private name space) in
case the server only recognized one. However, we might also take
into account that the various resolution behaviors discussed earlier
could also occur with record updates (e.g., with Dynamic Update
[RFC2136]), resulting in some names being registered in a local
network's private name space by applications doing conversion to
A-labels, and other names being registered using UTF-8. Hence a name
might have to be queried with both encodings to be sure to succeed
without changes to DNS servers.
Similarly, a more intelligent stub resolver would also be more
liberal in what it would accept from a response as the value of a
record (e.g., PTR) in that it would accept either UTF-8 (U-labels in
the case of IDNA) or A-labels and convert them to whatever encoding
is used by the application APIs to return strings to applications.
Indeed the choice of conversion within the resolver libraries is
consistent with the quote from section 6.2 of the original IDNA
Thaler, et al. Expires November 15, 2010 [Page 16]
Internet-Draft IDN Encodings May 2010
specification [RFC3490] stating that conversion using the Punycode
algorithm (i.e., to A-labels) "might be performed inside these new
versions of the resolver libraries".
That said, some application-layer protocols may be defined to use
A-labels rather than UTF-8 as recommended by the IETF character sets
and languages policy [RFC2277]. In this case, an application may
receive a string containing A-labels and want to pass it to name
resolution APIs. Again the recommendation that a resolver library be
more liberal in what it would accept from an application would mean
that such a name would be accepted and re-encoded as needed, rather
than requiring the application to do so.
Finally, the question remains about what, if anything, a DNS server
should do to handle cases where some existing applications or hosts
do IDNA queries using A-labels within the local network using a
private name space, and other existing applications or hosts send
UTF-8 queries. It is undesirable to store different records for
different encodings of the same name, since this introduces the
possibility for inconsistency between them. Instead, a new DNS
server serving a private name space using UTF-8 could potentially
treat encoding-conversion in the same way as case-insensitive
comparison which a DNS server is already required to do, as long the
DNS server has some way to know what the encoding is. Two encodings
are, in this sense, two representations of the same name, just as two
case-different strings are. However, whereas case comparison of non-
ASCII characters is complicated by ambiguities (as explained in the
IAB's Review and Recommendations for Internationalized Domain Names
[RFC4690]), encoding conversion between A-labels and U-labels is
unambiguous.
5. Acknowledgements
The authors wish to thank Patrik Falstrom, Martin Duerst, and JFC
Morfin for their careful review and helpful suggestions.
6. Security Considerations
Having applications convert names to prefixed ACE format (A-labels)
before calling name resolution can result in security
vulnerabilities. If the name is resolved by protocols or in zones
for which records are registered using other encoding schemes, an
attacker can claim the A-label version of the same name and hence
trick the victim into accessing a different destination. This can be
done for any non-ASCII name, even when there is no possible confusion
due to case, language, or other issues. Other types of confusion
Thaler, et al. Expires November 15, 2010 [Page 17]
Internet-Draft IDN Encodings May 2010
beyond those resulting simply from the choice of encoding scheme are
discussed in "Review and Recommendations for IDNs" [RFC4690].
Designers and users of encodings that represent Unicode strings in
terms of ASCII should also consider whether trademark protection is
an issue, e.g., if one name would be encoded in a way that would be
naturally associated with another organization, such as xn--rfc-
editor.
7. IANA Considerations
[RFC Editor: please remove this section prior to publication.]
This document has no IANA Actions.
8. IAB Members at the time of publication
Bernard Aboba
Marcelo Bagnulo
Ross Callon
Spencer Dawkins
Vijay Gill
Russ Housley
John Klensin
Olaf Kolkman
Danny McPherson
Jon Peterson
Andrei Robachevsky
Dave Thaler
Hannes Tschofenig
9. References
9.1. Normative References
[Unicode] The Unicode Consortium, "The Unicode Standard, Version
5.1.0", 2008.
defined by: The Unicode Standard, Version 5.0, Boston, MA,
Addison-Wesley, 2007, ISBN 0-321-48091-0, as amended by
Unicode 5.1.0
(http://www.unicode.org/versions/Unicode5.1.0/).
Thaler, et al. Expires November 15, 2010 [Page 18]
Internet-Draft IDN Encodings May 2010
9.2. Informative References
[I-D.cheshire-dnsext-multicastdns]
Cheshire, S. and M. Krochmal, "Multicast DNS",
draft-cheshire-dnsext-multicastdns-11 (work in progress),
March 2010.
[I-D.ietf-idn-punycode-00]
Costello, A., "Punycode version 0.3.3",
draft-ietf-idn-punycode-00 (work in progress), July 2002.
[I-D.skwan-utf8-dns-00]
Kwan, S. and J. Gilroy, "Using the UTF-8 Character Set in
the Domain Name System", draft-skwan-utf8-dns-00 (work in
progress), November 1997.
[IDNA2008-Defs]
Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
January 2010, <https://datatracker.ietf.org/drafts/
draft-ietf-idnabis-defs/>.
[IDNA2008-Protocol]
Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", January 2010, <https://
datatracker.ietf.org/drafts/draft-ietf-idnabis-protocol/>.
[MJD] Duerst, M., "The Properties and Promizes of UTF-8", 11th
International Unicode Conference, San Jose ,
September 1997, <http://www.ifi.unizh.ch/mml/mduerst/
papers/PDF/IUC11-UTF-8.pdf>.
[NIS] Sun Microsystems, "System and Network Administration",
March 1990.
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC 821, August 1982.
[RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
host table specification", RFC 952, October 1985.
[RFC1001] NetBIOS Working Group, "Protocol standard for a NetBIOS
service on a TCP/UDP transport: Concepts and methods",
STD 19, RFC 1001, March 1987.
[RFC1002] NetBIOS Working Group, "Protocol standard for a NetBIOS
service on a TCP/UDP transport: Detailed specifications",
STD 19, RFC 1002, March 1987.
Thaler, et al. Expires November 15, 2010 [Page 19]
Internet-Draft IDN Encodings May 2010
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC1468] Murai, J., Crispin, M., and E. van der Poel, "Japanese
Character Encoding for Internet Messages", RFC 1468,
June 1993.
[RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
Miller, "Common DNS Implementation Errors and Suggested
Fixes", RFC 1536, October 1993.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC3397] Aboba, B. and S. Cheshire, "Dynamic Host Configuration
Protocol (DHCP) Domain Search Option", RFC 3397,
November 2002.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6",
RFC 3493, February 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3646] Droms, R., "DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
December 2003.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
Thaler, et al. Expires November 15, 2010 [Page 20]
Internet-Draft IDN Encodings May 2010
Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006.
[RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local
Multicast Name Resolution (LLMNR)", RFC 4795,
January 2007.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
Authors' Addresses
Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
Phone: +1 425 703 8835
Email: dthaler@microsoft.com
John C Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
Phone: +1 617 245 1457
Email: john+ietf@jck.com
Stuart Cheshire
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
Phone: +1 408 974 3207
Email: cheshire@apple.com
Thaler, et al. Expires November 15, 2010 [Page 21]
