Network Working Group D. Atkins
draft-ietf-dnsext-dns-threats-03.txt IHTFP Consulting
R. Austein
Grunchweather Associates
June 2003
Threat Analysis Of The Domain Name System
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Abstract
Although the DNS Security Extensions (DNSSEC) have been under
development for most of the last decade, the IETF has never written
down the specific set of threats against which DNSSEC is designed to
protect. Among other drawbacks, this cart-before-the-horse situation
has made it difficult to determine whether DNSSEC meets its design
goals, since its design goals are not well specified. This note
attempts to document some of the known threats to the DNS, and, in
doing so, attempts to measure to what extent (if any) DNSSEC is a
useful tool in defending against these threats.
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1. Introduction
The earliest organized work on DNSSEC within the IETF was an open
design team meeting organized by members of the DNS working group in
November 1993 at the 28th IETF meeting in Houston. The broad
outlines of DNSSEC as we know it today are already clear in Jim
Galvin's summary of the results of that meeting [Galvin93]:
- While some participants in the meeting were interested in
protecting against disclosure of DNS data to unauthorized parties,
the design team made an explicit decision that "DNS data is
`public'", and ruled all threats of data disclosure explicitly out
of scope for DNSSEC.
- While some participants in the meeting were interested in
authentication of DNS clients and servers as a basis for access
control, this work was also ruled out of scope for DNSSEC per se.
DNS Transaction Signatures (TSIG) were eventually developed as a
separate mechanism to address threats of unauthorized access to
DNS's zone transfer and dynamic update mechanisms.
- Backwards compatibility and co-existence with "insecure DNS" was
listed as an explicit requirement.
- The resulting list of desired security services was
1) data integrity, and
2) data origin authentication.
- The design team noted that a digital signature mechanism would
support the desired services.
While a number of detail decisions were yet to be made (and in some
cases remade after implementation experience) over the subsequent
decade, the basic model and design goals have remained fixed.
Nowhere, however, does any of the DNSSEC work attempt to specify in
any detail the sorts of attacks against which DNSSEC is intended to
protect, or the reasons behind the list of desired security services
that came out of the Houston meeting. For that, we have to go back
to a paper originally written by Steve Bellovin in 1990 but not
published until 1995, for reasons that Bellovin explained in the
paper's epilogue [Bellovin95].
While it may seem a bit strange to publish the threat analysis a
decade after starting work on the protocol designed to defend against
it, that is nevertheless what this note attempts to do. Better late
than never.
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This note assumes that the reader is familiar with both the DNS and
with DNSSEC, and does not attempt to provide a tutorial on either.
For purposes of discussion, this note uses the term "DNSSEC" to refer
to the core hierarchical public key and signature mechanism specified
in the DNSSEC documents, and refers to TKEY and TSIG as separate
mechanisms, even though TKEY and TSIG are also part of the larger
problem of "securing DNS" and thus are often considered part of the
overall set of "DNS security extensions". This is an arbitrary
distinction that in part reflects the way in which the protocol has
evolved (introduction of a putatively simpler transaction model for
certain operations), and perhaps should be changed in a future
revision of this note.
2. Known Threats
There are several distinct classes of threats to the DNS, most of
which are DNS-related instances of more general problems, but a few
of which are specific to peculiarities of the DNS protocol.
2.1. 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.
To further complicate things, the DNS query the attacker intercepts
may just be a means to an end for the attacker: the attacker might
even chose to return the correct result in the answer section of a
reply message while using other parts of the message to set the stage
for something more complicated, for example, a name-based attack (see
below).
While it certainly would be possible to sign DNS messages using TSIG
or IPsec, or even to encrypt them using IPsec, this would not be a
very good solution. First, this approach would impose a fairly high
processing cost per DNS message, as well as a very high cost
associated with establishing and maintaining bilateral trust
relationships between all the parties that might be involved in
resolving any particular query. For heavily used name servers (such
as the servers for the root zone), this cost would almost certainly
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be prohibitively high. Even more important, however, is that the
underlying trust model in such a design would be wrong, since at best
it would only provide a hop-by-hop integrity check on DNS messages
and would not provide any sort of end-to-end integrity check between
the producer of DNS data (the zone administrator) and the consumer of
DNS data (the application that triggered the query).
By contrast, DNSSEC (when used properly) does provide an end-to-end
data integrity check, and is thus a much better solution for this
class of problems during basic DNS lookup operations.
TSIG does have its place in corners of the DNS protocol where there's
a specific trust relationship between a particular client and a
particular server, such as zone transfer, dynamic update, or a
resolver (stub or otherwise) that is not going to check all the
DNSSEC signatures itself.
Note that DNSSEC does not provide any protection against modification
of the DNS message header, so any properly paranoid resolver must:
- Perform all of the DNSSEC signature checking on its own,
- Use TSIG (or some equivalent mechanism) to insure the integrity of
its communication with whatever name servers it chooses to trust,
or
- Resign itself to the possibility of being attacked via packet
interception (and via other techniques discussed below).
2.2. 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. This is not a particularly large range, and
is not proof against a brute force search; furthermore, in practice
both the client UDP port and the ID can often be predicted from
previous traffic, and it is not uncommon for the client port to be a
known fixed value as well (due to firewalls or other restrictions),
thus frequently reducing the search space to a range smaller than
2**16.
By itself, ID guessing is not enough to allow an attacker to inject
bogus data, but combined with knowledge (or guesses) about QNAMEs and
QTYPEs for which a resolver might be querying, this leaves the
resolver only weakly defended against injection of bogus responses.
Since this attack relies on predicting a resolver's behavior, it's
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most likely to be successful when the victim is in a known state,
whether because the victim rebooted recently, or because the victim's
behavior has been influenced by some other action by the attacker, or
because the victim is responding (in a predictable way) to some third
party action known to the attacker.
This attack is both more and less difficult for the attacker than the
simple interception attack described above: more difficult, because
the attack only works when the attacker guesses correctly; less
difficult, because the attacker doesn't need to be on a transit or
shared network.
In most other respects, this attack is similar to a packet
interception attack. A resolver that checks DNSSEC signatures will
be able to detect the forged response; resolvers that do not
themselves perform DNSSEC signature checking should use TSIG or some
equivalent mechanism to insure the integrity of their communication
with a recursing name server that does perform DNSSEC signature
checking.
2.3. Name Games
Perhaps the most interesting class of DNS-specific threats are the
name-based attacks. There are several variations within this class,
sometimes called "cache poisoning" or "fake authority" attacks. What
all of these attacks have in common is that they all involve DNS RRs
whose RDATA portion (right hand side) includes a DNS name. Any such
RR is, at least in principle, a hook that lets an attacker feed bad
data into a victim's cache, thus potentially subverting subsequent
decisions based on DNS names.
The worst examples in this class of RRs are CNAME, NS, and DNAME RRs,
because they can redirect a victim's query to a location of the
attacker's choosing. RRs like MX and SRV are somewhat less
dangerous, but in principle they can also be used to trigger further
lookups at a location of the attacker's choosing.
The general form of a name-based attack is something like this:
- Victim issues a query, perhaps at the instigation of the attacker
or some third party; in some cases the query itself may be
unrelated to the name under attack (that is, the attacker is just
using this query as a means to inject false information about some
other name).
- Attacker injects response, whether via packet interception, query
guessing, or by being a legitimate name server that's involved at
some point in the process of answering the query that the victim
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issued.
- Attacker's response includes one or more RRs with DNS names in
their RDATA; depending on which particular form this attack takes,
the object may be to inject false data associated with those names
into the victim's cache via the Additional section of this
response, or may be to redirect the next stage of the query to a
server of the attacker's choosing (in order to inject more complex
lies into the victim's cache than will fit easily into a single
response, or in order to place the lies in the Authority or Answer
section of a response where they will have a better chance of
sneaking past a resolver's defenses).
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.
This class of attack is particularly insidious given that it's quite
easy for an attacker to provoke a victim into querying for a
particular name of the attacker's choosing, for example, by embedding
a link to a 1x1-pixel "web bug" in a piece of Text/HTML mail to the
victim.
DNSSEC should provide a good defense against most (all?) variations
on this class of attack. By checking signatures, a resolver can
determine whether the data associated with a name really was inserted
by the delegated authority for that portion of the DNS name space
(more precisely, a resolver can determine whether the entity that
injected the data had access to an allegedly secret key whose
corresponding public key appears at an expected location in the DNS
name space with an expected chain of parental signatures that start
with a public key of which the resolver has prior knowledge).
DNSSEC signatures do not cover glue records, so there's still a
possibility of a name-based attack involving glue, but it should be
possible to detect the attack by temporarily accepting the glue in
order to fetch the signed authoritative version of the same data,
then checking the signatures on the authoritative version.
2.4. Betrayal By 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
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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).
This problem is particularly acute for frequent travelers who carry
their own equipment and expect it to work in much the same way no
matter which network it's plugged into at any given moment (and no
matter what brand of middle boxes a particular hotel chain might have
installed when adding network drops in every guest room...).
From the protocol standpoint, the only difference between this sort
of betrayal and a packet interception attack is that in this case the
client has voluntarily sent its request to the attacker. The defense
against this is the same as with a packet interception attack: the
resolver must either check DNSSEC signatures itself or use TSIG (or
equivalent) to authenticate the server that it has chosen to trust.
Note that use of TSIG does not by itself guarantee that a name server
is at all trustworthy: all TSIG can do is help a resolver protect its
communication with a name server that it has already decided to trust
for other reasons. Protecting a resolver's communication with a
server that's giving out bogus answers is not particularly useful.
Also note that if the stub resolver does not trust the name server
that is doing work on its behalf and wants to check the DNSSEC
signatures itself, the resolver really does need to have independent
knowledge of the DNSSEC public key(s) it needs to perform the check
(usually the public key for the root zone, but in some cases
knowledge of additional keys may also be appropriate).
It is difficult to escape the conclusion that a properly paranoid
resolver must always perform its own signature checking, and that
this rule even applies to stub resolvers.
2.5. 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. DNSSEC does not help this, and may in fact make the
problem worse for resolvers that check signatures, since checking
signatures both increases the processing cost per DNS message and in
some cases can also increase the number of messages needed to answer
a query. TSIG (and similar mechanisms) have equivalent problems.
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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. Unsurprisingly, DNSSEC doesn't help
here either.
2.6. Authenticated Denial of Domain Names
Much discussion has taken place over the question of authenticated
denial of domain names. The particular question is whether there is
a requirement for authenticating the non-existence of a name. The
issue is whether the resolver should be able to detect when an
attacker removes RRs from a response.
General paranoia aside, 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.
Arguably, in some cases, even the immediate failure of a missing RR
might be considered a problem. The question remains: how serious is
this threat? Clearly the threat does exist; general paranoia says
that some day it'll be on the front page of some major newspaper,
even if we cannot conceive of a plausible scenario involving this
attack today. This implies that some mitigation of this risk is
required.
Note that it's necessary to prove the non-existance of applicable
wildcard RRs as part of the authenticated denial mechanism, and that,
in a zone that is more than one label deep, such a proof may require
proving the non-existance of multiple discrete sets of wildcard RRs.
2.7. Wildcards
Much discussion has taken place over whether and how to provide data
integrity and data origin authentication for "wildcard" DNS names.
Conceptually, RRs with wildcard names are patterns for synthesizing
RRs on the fly according to the matching rules described in section
4.3.2 of RFC 1034. While the rules that control the behavior of
wildcard names have a few quirks that can make them a trap for the
unwary zone administrator, it's clear that a number of sites make
heavy use of wildcard RRs, particularly wildcard MX RRs.
In order to provide the desired services for wildcard RRs, we need to
prove two things:
- We need to prove the existance of the wildcard RR itself (that is,
we need to prove that the synthesis rule exists), and
- We need to prove the non-existance of any RRs which, if they
existed, would make the wildcard RR irrelevant according to the
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synthesis rules the way in which wildcards are used (that is, we
need to prove that the synthesis rule is applicable).
Note that this makes the wildcard proof mechanism dependent upon the
authenticated denial mechanism described in the previous section.
DNSSEC does include mechanisms by which it is possible to furnish
wildcard proofs along the lines described above.
3. Weaknesses of DNSSEC
DNSSEC has some problems of its own:
- DNSSEC is complex to implement, and includes some nasty edge cases
at the zone cuts that require very careful coding. Testbed
experience to date suggests that trivial zone configuration errors
or expired keys can cause serious problems for a DNSSEC-aware
resolver, and that the current protocol's error reporting
capabilities may leave something to be desired.
- DNSSEC significantly increases the size of DNS response packets;
among other issues, this makes DNSSEC-aware DNS servers even more
effective as denial of service amplifiers.
- DNSSEC answer validation increases the resolver's work load, since
a DNSSEC-aware resolver will need to perform signature validation
and in some cases will also need to issue further queries. This
increased workload will also increase the time it takes to get an
answer back to the original DNS client, which will almost certainly
trigger both timeouts and re-queries. (Arguably, many current DNS
clients are already too impatient even before taking the further
delays that DNSSEC will impose into account, but that's a separate
topic for another document....)
- Like DNS itself, DNSSEC's trust model is almost totally
hierarchical. While DNSSEC does allow resolvers to have special
additional knowledge of public keys beyond those for the root, in
the general case the root key is the one that matters. Thus any
compromise in any of the zones between the root and a particular
target name can damage DNSSEC's ability to protect the integrity of
data owned by that target name. This is not really a change, since
insecure DNS has essentially the same problem, but it's not good
either.
- Key rollover at the root is really hard. Work to date has not even
come close to adequately specifying how the root key rolls over, or
even how it's configured in the first place.
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- DNSSEC creates a requirement of loose time synchronization between
the resolver and the host creating the DNSSEC signatures. Prior to
DNSSEC, all time-related actions in DNS could be performed by a
machine that only knew about "elapsed" or "relative" time. Because
the validity period of a DNSSEC signature is based on "absolute"
time, a resolver must have the same concept of absolute time in
order to determine whether the signature is within its validity
period or has expired. An attacker that can change a resolver's
opinion of the current absolute time can fool the resolver using
expired signatures. An attacker that can change the zone signer's
opinion of the current absolute time can fool the zone signer into
generating signatures whose validity period does not match what the
signer intended.
- The mechanism for wildcard proofs in DNSSEC is fairly painful. At
various times there have been questions as to whether the proof
mechanism is completely airtight and whether it would be worthwhile
to optimize the wildcard proof mechanism for the common case in
which wildcards do not exist, but the main problem is just the
inherent complexity of the wildcard mechanism itself. This
complexity probably makes the code for generating and checking
wildcard proofs somewhat fragile, but since the alternative of
giving up wildcards entirely is not practical due to widespread
use, we are going to have to live with wildcards, and the question
just becomes one of whether or not the proposed optimizations would
make DNSSEC's wildcard proof mechanisms more or less fragile.
4. Topics for Future Work
This section lists a few subjects not covered above which probably
need additional study, additional mechanisms, or both.
4.1. Interactions With Other Protocols
The above discussion has concentrated exclusively on attacks within
the boundaries of the DNS protocol itself, since those are the
problems against (some of) which DNSSEC was intended to protect.
There are, however, other potential problems at the boundaries where
DNS interacts with other protocols.
4.2. Securing DNS Dynamic Update
DNS dynamic update opens a number of potential problems when combined
with DNSSEC. Dynamic update of a non-secure zone can use TSIG to
authenticate the updating client to the server. While TSIG does not
scale very well (it requires manual configuration of shared keys
between the DNS name server and each TSIG client), it works well in a
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limited or closed environment such as a DHCP server updating a local
DNS name server.
Major issues arise when trying to use dynamic update on a secure
zone. TSIG can similarly be used in a limited fashion to
authenticate the client to the server, but TSIG only protects DNS
transactions, not the actual data, and the TSIG is not inserted into
the DNS zone, so resolvers cannot use the TSIG as a way of verifying
the changes to the zone. This means that either:
a) The updating client must have access to a zone-signing key in
order to sign the update before sending it to the server, or
b) The DNS name server must have access to an online zone-signing key
in order to sign the update.
In either case, a zone-signing key must be available to create signed
RRsets to place in the updated zone. The fact that this key must be
online (or at least available) is a potential security risk.
Dynamic update also requires an update to the SERIAL field of the
zone's SOA RR. In theory, this could also be handled via either of
the above options, but in practice (a) would almost certainly be
extremely fragile, so (b) is the only workable mechanism.
There are other threats in terms of describing the policy of who can
make what changes to which RRsets in the zone. The current access
control scheme in Secure Dynamic Update is fairly limited. There is
no way to give find-grained access to updating DNS zone information
to multiple entities, each of whom may require different kinds of
access. For example, Alice may need to be able to add new nodes to
the zone or change existing nodes, but not remove them; Bob may need
to be able to remove zones but not add them; Carol may need to be
able to add, remove, or modify nodes, but only A records.
Scaling properties of the key management problem here are a
particular concern that needs more study.
4.3. Securing DNS Zone Replication
As discussed in previous sections, DNSSEC per se attempts to provide
data integrity and data origin authentication services on top of the
normal DNS query protocol. Using the terminology discussed in [SEC-
CONS], DNSSEC provides "object security" for the normal DNS query
protocol. For purposes of replicating entire DNS zones, however,
DNSSEC does not provide object security, because zones include
unsigned NS RRs and glue at delegation points. Use of TSIG to
protect zone transfer (AXFR or IXFR) operations provides "channel
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security", but still does not provide object security for complete
zones, so the trust relationships involved in zone transfer are still
very much a hop-by-hop matter of name server operators trusting other
name server operators, rather than an end-to-end matter of name
server operators trusting zone administrators.
Zone object security was not an explicit design goal of DNSSEC, so
failure to provide this service should not be a surprise.
Nevertheless, there are some zone replication scenarios for which
this would be a very useful additional service, so this seems like a
useful area for future work. In theory it should not be difficult to
zone object security as a backwards compatible enhancement to the
existing DNSSEC model, but the DNSEXT WG has not yet discussed either
the desirability of or the requirements for such an enhancement.
5. Conclusion
Based on the above analysis, the DNSSEC extensions do appear to solve
a set of problems that do need to be solved, and are worth deploying.
Security Considerations
This entire document is about security considerations of the DNS.
The authors believe that deploying DNSSEC will help to address some,
but not all, of the known threats to the DNS.
IANA Considerations
None.
Acknowledgments
This note is based both previous published works by others and on a
number of discussions both public and private over a period of many
years, but particular thanks go to Jaap Akkerhuis, Steve Bellovin,
Dan Bernstein, Randy Bush, Olafur Gudmundsson, Rip Loomis, Allison
Mankin, Paul Mockapetris, Mans Nilsson, Paul Vixie, Xunhua Wang, and
any other members of the DNS, DNSSEC, DNSIND, and DNSEXT working
groups whose names and contributions the authors have forgotten, none
of whom are responsible for what the authors did with their ideas.
Normative References
[DNS-CONCEPTS] Mockapetris, P., "Domain names - concepts and
facilities", RFC 1034, November 1987.
[DNS-IMPLEMENTATION] Mockapetris, P., "Domain names - implementation
and specification", RFC 1035, November 1987.
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[HOST-REQUIREMENTS] Braden, R., Editor, "Requirements for Internet
Hosts - Application and Support", RFC 1123, October 1989.
[DNS-CLARIFY] Elz, R., and R. Bush, "Clarifications to the DNS
Specification" RFC 2181, July 1997.
[NCACHE] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)"
RFC 2308, March 1998.
[DNSSEC] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[EDNS0] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
August 1999.
[TSIG] Vixie, P., Gudmundsson, O., Eastlake, D., and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)" RFC 2845,
May 2000.
[TKEY] Eastlake, D., "Secret Key Establishment for DNS (TKEY RR)" RFC
2930, September 2000.
[SECURE-UPDATE] Wellington, B., "Secure Domain Name System (DNS)
Dynamic Update" RFC 3007, November 2000.
[SIGNING-AUTHORITY] Wellington, B., "Domain Name System Security
(DNSSEC) Signing Authority" RFC 3008, November 2000.
[DNSSEC-ZONE-STATUS] Lewis, E., "DNS Security Extension Clarification
on Zone Status" RFC 3090, March 2001.
Informative References
[Bellovin95] Bellovin, S., "Using the Domain Name System for System
Break-Ins", Proceedings of the Fifth Usenix Unix Security
Symposium, June 1995.
[Galvin93] Design team meeting summary message posted to dns-
security@tis.com mailing list by Jim Galvin on 19 November 1993.
[Schuba93] Schuba, C., "Addressing Weaknesses in the Domain Name
System Protocol", Master's thesis, Purdue University Department
of Computer Sciences, August 1993.
[Vixie95] Vixie, P, "DNS and BIND Security Issues", Proceedings of
the Fifth Usenix Unix Security Symposium, June 1995.
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[SEC-CONS] Rescorla, E., Korver, B., and the Internet Architecture
Board, "Guidelines for Writing RFC Text on Security
Considerations", work in progress (draft-iab-sec-cons-03.txt),
January 2003.
Author's addresses:
Derek Atkins
IHTFP Consulting, Inc.
6 Farragut Ave
Somerville, MA 02144
USA
Email: derek@ihtfp.com
Rob Austein
Grunchweather Associates
Email: sra@hactrn.net
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Atkins & Austein Expires 29 December 2003 [Page 14]
draft-ietf-dnsext-dns-threats-03.txt June 2003
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Atkins & Austein Expires 29 December 2003 [Page 15]
