"ftp://ftp.iana.org/assignments/enum-services/".
3.2.1.2. Change Control
Change control of Enumservices stay with the IETF via the RFC
publication process. Especially, Enumservice registrations may not
be deleted; Enumservices which are no longer believed appropriate for
use can be declared OBSOLETE by publication of a new RFC and a change
to their "intended use" field; such Enumservice will be clearly
marked in the lists published by IANA.
3.2.2. Registration Template
Enumservice Type:
Enumservice Subtype(s):
URI Scheme(s):
Functional Specification:
Security considerations:
Intended usage: (One of COMMON, LIMITED USE or OBSOLETE)
Author:
Any other information that the author deems interesting:
Note: In the case where a particular field has no value, that field
is left completely blank, especially in the case where a given type
has no subtypes.
4. Examples
The examples below use theoretical services that contain Enumservices
which might not make sense, but that are still used for educational
purposes. For example, the protocol used is in some cases exactly
the same string as the URI scheme. That was the specification in RFC
2916, but this ’default’ specification of an Enumservice is no longer
allowed. All Enumservices need to be registered explicitly by the
procedure specified in section Section 3.
4.1. Example
$ORIGIN 3.8.0.0.6.9.2.3.6.1.4.4.e164.arpa.
NAPTR 10 100 "u" "E2U+sip" "!^.*$!sip:info@example.com!" .
NAPTR 10 101 "u" "E2U+h323" "!^.*$!h323:info@example.com!" .
NAPTR 10 102 "u" "E2U+msg" "!^.*$!mailto:info@example.com!" .
This describes that the domain 3.8.0.0.6.9.2.3.6.1.4.4.e164.arpa. is
preferably contacted by SIP, secondly via H.323 for voice, and
thirdly by SMTP for messaging. Note that the tokens "sip", "h323",
and "msg" are Types registered with IANA, and they have no implicit
connection with the protocols or URI schemes with the same names.
In all cases, the next step in the resolution process is to use the
resolution mechanism for each of the protocols, (specified by the URI
schemes sip, h323 and mailto) to know what node to contact for each.
5. IANA Considerations
RFC 2916 (which this document replaces) requested IANA to delegate
the E164.ARPA domain following instructions to be provided by the
IAB. The domain was delegated according to those instructions.
Names within this zone are to be delegated to parties according to
the ITU-T Recommendation E.164. The names allocated should be
hierarchic in accordance with ITU-T Recommendation E.164, and the
codes should be assigned in accordance with that Recommendation.
IAB is to coordinate with ITU-T TSB if the technical contact for the
domain e164.arpa is to change, as ITU-T TSB has an operational
working relationship with this technical contact which needs to be
reestablished.
Delegations in the zone e164.arpa (not delegations in delegated
domains of e164.arpa) should be done after Expert Review, and the
IESG will appoint a designated expert.
IANA has created a registry for Enumservices as specified in Section
3. Whenever a new Enumservice is registered by the RFC process in
the IETF, IANA is at the time of publication of the RFC to register
the Enumservice and add a pointer to the RFC itself.
6. Security Considerations
6.1. DNS Security
As ENUM uses DNS, which in its current form is an insecure protocol,
there is no mechanism for ensuring that the data one gets back is
authentic. As ENUM is deployed on the global Internet, it is
expected to be a popular target for various kind of attacks, and
attacking the underlying DNS infrastructure is one way of attacking
the ENUM service itself.
There are multiple types of attacks that can happen against DNS that
ENUM implementations should be aware of. The following threats are
taken from Threat Analysis Of The Domain Name System [10]:
Packet Interception
Some of the simplest threats against DNS are various forms of
packet interception: monkey-in-the-middle attacks, eavesdropping
on requests combined with spoofed responses that beat the real
response back to the resolver, and so forth. In any of these
scenarios, the attacker can simply tell either party (usually the
resolver) whatever it wants that party to believe. While packet
interception attacks are far from unique to DNS, DNS’s usual
behavior of sending an entire query or response in a single
unsigned, unencrypted UDP packet makes these attacks particularly
easy for any bad guy with the ability to intercept packets on a
shared or transit network.
ID Guessing and Query Prediction
Since the ID field in the DNS header is only a 16-bit field and
the server UDP port associated with DNS is a well-known value,
there are only 2**32 possible combinations of ID and client UDP
port for a given client and server. Thus it is possible for a
reasonable brute force attack to allow an attacker to masquerade
as a trusted server. In most respects, this attack is similar to
a packet interception attack except that it does not require the
attacker to be on a transit or shared network.
Name-based Attacks
Name-based attacks use the actual DNS caching behavior as a tool
to insert bad data into a victim’s cache, thus potentially
subverting subsequent decisions based on DNS names. Most examples
occur with CNAME, NS and DNAME Resource Records as they redirect a
victim’s query to another location. The common thread in all of
these attacks is that response messages allow the attacker to
introduce arbitrary DNS names of the attacker’s choosing and
provide further information that the attacker claims is associated
with those names; unless the victim has better knowledge of the
data associated with those names, the victim is going to have a
hard time defending against this class of attacks.
Betrayal By A Trusted Server
Another variation on the packet interception attack is the trusted
server that turns out not to be so trustworthy, whether by
accident or by intent. Many client machines are only configured
with stub resolvers, and use trusted servers to perform all of
their DNS queries on their behalf. In many cases the trusted
server is furnished by the user’s ISP and advertised to the client
via DHCP or PPP options. Besides accidental betrayal of this
trust relationship (via server bugs, successful server break-ins,
etc), the server itself may be configured to give back answers
that are not what the user would expect (whether in an honest
attempt to help the user or to further some other goal such as
furthering a business partnership between the ISP and some third
party).
Denial of Service
As with any network service (or, indeed, almost any service of any
kind in any domain of discourse), DNS is vulnerable to denial of
service attacks. DNS servers are also at risk of being used as
denial of service amplifiers, since DNS response packets tend to
be significantly longer than DNS query packets.
Authenticated Denial of Domain Names
The existence of RR types whose absence causes an action other
than immediate failure (such as missing MX and SRV RRs, which fail
over to A RRs) constitutes a real threat. In the specific case of
ENUM, even the immediate failure of a missing RR can be considered
a problem as a method for changing call routing policy.
Because of these threats, a deployed ENUM service SHOULD include
mechanisms which ameliorate these threats. Most of these threats can
be solved by verifying the authenticity of the data via mechanisms
such as DNSSEC [8] once it is deployed. Others, such and Denial Of
Service attacks, cannot be solved by data authentication. It is
important to remember that these threats include not only the NAPTR
lookups themselves, but also the various records needed for the
services to be useful (for example NS, MX, SRV and A records).
Even if DNSSEC is deployed, a service that uses ENUM for address
translation should not blindly trust that the peer is the intended
party as all kind of attacks against DNS can not be protected against
with DNSSEC. A service should always authenticate the peers as part
of the setup process for the service itself and never blindly trust
any kind of addressing mechanism.
Finally, as an ENUM service will be implementing some type of
security mechanism, software which implements ENUM MUST be prepared
to receive DNSSEC and other standardized DNS security responses,
including large responses, EDNS0 signaling, unknown RRs, etc.
6.2. Caching Security
The caching in DNS can make the propagation time for a change take
the same amount of time as the time to live for the NAPTR records in
the zone that is changed. The use of this in an environment where
IP-addresses are for hire (for example, when using DHCP [9]) must
therefore be done very carefully.
6.3. Call Routing Security
There are a number of countries (and other numbering environments) in
which there are multiple providers of call routing and number/name-
translation services. In these areas, any system that permits users,
or putative agents for users, to change routing or supplier
information may provide incentives for changes that are actually
unauthorized (and, in some cases, for denial of legitimate change
requests). Such environments should be designed with adequate
mechanisms for identification and authentication of those requesting
changes and for authorization of those changes.
6.4. URI Resolution Security
A large amount of Security Issues have to do with the resolution
process itself, and use of the URIs produced by the DDDS mechanism.
Those have to be specified in the registration of the Enumservice
used, as specified in Section 3.1.3.
7. Acknowledgements
Support and ideas leading to RFC 2916 have come from people at
Ericsson, Bjorn Larsson and the group which implemented this scheme
in their lab to see that it worked. Input has also arrived from
ITU-T SG2, Working Party 1/2 (Numbering, Routing, Global Mobility and
Enumservice Definition), the ENUM working group in the IETF, John
Klensin and Leif Sunnegardh.
This update of RFC 2916 is created with specific input from: Randy
Bush, David Conrad, Richard Hill, Jon Peterson, Jim Reid, Joakim
Stralmark, Robert Walter and James Yu.
8. Changes since RFC 2916
Part from clarifications in the text in this document, the major
changes are two:
The document uses an explicit DDDS algorithm, and not only NAPTR
resource records in an "ad-hoc" mode. In reality this doesn’t imply
any changes in deployed base of applications, as the algorithm used
for ENUM resolution is exactly the same.
The format of the service field has changed. The old format was of
the form "example+E2U", while the new format is "E2U+example".
Reason for this change have to with the added subtypes in the
enumservice, the ability to support more than one enumservice per
NAPTR RR, and a general agreement in the IETF that the main selector
between different NAPTR with the same owner (E2U in this case) should
be first.
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Three: The Domain Name System (DNS) Database", RFC 3403, October
2002.
[3] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Four: The Uniform Resource Identifiers (URI) Resolution
Application", RFC 3404, October 2002.
[4] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
[5] ITU-T, "The International Public Telecommunication Number Plan",
Recommendation E.164, May 1997.
[6] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
One: The Comprehensive DDDS", RFC 3401, October 2002.
[7] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Two: The Algorithm", RFC 3402, October 2002.
9.2. Informative References
[8] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[9] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[10] Atkins, D. and R. Austein, "Threat Analysis Of The Domain Name
System", Work in Progress, April 2004.
10. Authors’ Addresses
Patrik Faltstrom
Cisco Systems Inc
Ledasa
273 71 Lovestad
Sweden
EMail: paf@cisco.com
URI: http://www.cisco.com
Michael Mealling
VeriSign
21345 Ridgetop Circle
Sterling, VA 20166
US
Email: michael@verisignlabs.com
URI: http://www.verisignlabs.com
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