Network Working Group A. Hubert
Request for Comments: 5452 Netherlabs Computer Consulting BV.
Updates: 2181 R. van Mook
Category: Standards Track Equinix
January 2009 Measures for Making DNS More Resilient against Forged Answers
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (c) 2009 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 to this document.
The current Internet climate poses serious threats to the Domain Name
System. In the interim period before the DNS protocol can be secured
more fully, measures can already be taken to harden the DNS to make
'spoofing' a recursing nameserver many orders of magnitude harder.
Even a cryptographically secured DNS benefits from having the ability
to discard bogus responses quickly, as this potentially saves large
amounts of computation.
By describing certain behavior that has previously not been
standardized, this document sets out how to make the DNS more
resilient against accepting incorrect responses. This document
updates RFC 2181.
This document describes several common problems in DNS
implementations, which, although previously recognized, remain
largely unsolved. Besides briefly recapping these problems, this
document contains rules that, if implemented, make complying
resolvers vastly more resistant to the attacks described. The goal
is to make the existing DNS as secure as possible within the current
The words below are aimed at authors of resolvers: it is up to
operators to decide which nameserver implementation to use, or which
options to enable. Operational constraints may override the security
concerns described below. However, implementations are expected to
allow an operator to enable functionality described in this document.
Almost every transaction on the Internet involves the Domain Name
System, which is described in [RFC1034], [RFC1035], and beyond.
Additionally, it has recently become possible to acquire Secure
Socket Layer/Transport Layer Security (SSL/TLS) certificates with no
other confirmation of identity than the ability to respond to a
verification email sent via SMTP ([RFC5321]) -- which generally uses
DNS for its routing.
In other words, any party that (temporarily) controls the Domain Name
System is in a position to reroute most kinds of Internet
transactions, including the verification steps in acquiring an SSL/
TLS certificate for a domain. This in turn means that even
transactions protected by SSL/TLS could be diverted.
It is entirely conceivable that such rerouted traffic could be used
to the disadvantage of Internet users.
These and other developments have made the security and
trustworthiness of DNS of renewed importance. Although the DNS
community is working hard on finalizing and implementing a
cryptographically enhanced DNS protocol, steps should be taken to
make sure that the existing use of DNS is as secure as possible
within the bounds of the relevant standards.
It should be noted that the most commonly used resolvers currently do
not perform as well as possible in this respect, making this document
of urgent importance.
A thorough analysis of risks facing DNS can be found in [RFC3833].
This document expands on some of the risks mentioned in RFC 3833,
especially those outlined in the sections on "ID Guessing and Query
Prediction" and "Name Chaining". Furthermore, it emphasizes a number
of existing rules and guidelines embodied in the relevant DNS
protocol specifications. The following also specifies new
requirements to make sure the Domain Name System can be relied upon
until a more secure protocol has been standardized and deployed.
It should be noted that even when all measures suggested below are
implemented, protocol users are not protected against third parties
with the ability to observe, modify, or inject packets in the traffic
of a resolver.
For protocol extensions that offer protection against these
scenarios, see [RFC4033] and beyond.
2. Requirements and Definitions
This document uses the following definitions:
Client: typically a 'stub-resolver' on an end-user's computer.
Resolver: a nameserver performing recursive service for clients,
also known as a caching server, or a full service resolver
([RFC1123], Section 22.214.171.124).
Stub resolver: a very limited resolver on a client computer, that
leaves the recursing work to a full resolver.
Query: a question sent out by a resolver, typically in a UDP
Response: the answer sent back by an authoritative nameserver,
typically in a UDP packet.
Third party: any entity other than the resolver or the intended
recipient of a question. The third party may have access to an
arbitrary authoritative nameserver, but has no access to packets
transmitted by the resolver or authoritative server.
Attacker: malicious third party.
Spoof: the activity of attempting to subvert the DNS process by
getting a chosen answer accepted.
Authentic response: the correct answer that comes from the right
Target domain name: domain for which the attacker wishes to spoof
in an answer
Fake data: response chosen by the attacker.
2.2. Key Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Description of DNS Spoofing
When certain steps are taken, it is feasible to "spoof" the current
deployed majority of resolvers with carefully crafted and timed DNS
packets. Once spoofed, a caching server will repeat the data it
wrongfully accepted, and make its clients contact the wrong, and
possibly malicious, servers.
To understand how this process works it is important to know what
makes a resolver accept a response.
The following sentence in Section 5.3.3 of [RFC1034] presaged the
The resolver should be highly paranoid in its parsing of responses.
It should also check that the response matches the query it sent
using the ID field in the response.
DNS data is to be accepted by a resolver if and only if:
1. The question section of the reply packet is equivalent to that of
a question packet currently waiting for a response.
2. The ID field of the reply packet matches that of the question
3. The response comes from the same network address to which the
question was sent.
4. The response comes in on the same network address, including port
number, from which the question was sent.
In general, the first response matching these four conditions is
If a third party succeeds in meeting the four conditions before the
response from the authentic nameserver does so, it is in a position
to feed a resolver fabricated data. When it does so, we dub it an
"attacker", attempting to spoof in fake data.
All conditions mentioned above can theoretically be met by a third
party, with the difficulty being a function of the resolver
implementation and zone configuration.
4. Detailed Description of Spoofing Scenarios
The previous paragraph discussed a number of requirements an attacker
must match in order to spoof in manipulated (or fake) data. This
section discusses the relative difficulties and how implementation-
defined choices impact the amount of work an attacker has to perform
to meet said difficulties.
Some more details can be found in Section 2.2 of [RFC3833].
4.1. Forcing a Query
Formally, there is no need for a nameserver to perform service except
for its operator, its customers, or more generally its users.
Recently, open recursing nameservers have been used to amplify
Providing full service enables the third party to send the target
resolver a query for the domain name it intends to spoof. On
receiving this query, and not finding the answer in its cache, the
resolver will transmit queries to relevant authoritative nameservers.
This opens up a window of opportunity for getting fake answer data
Queries may however be forced indirectly, for example, by inducing a
mail server to perform DNS lookups.
Some operators restrict access by not recursing for unauthorized IP
addresses, but only respond with data from the cache. This makes
spoofing harder for a third party as it cannot then force the exact
moment a question will be asked. It is still possible however to
determine a time range when this will happen, because nameservers
helpfully publish the decreasing time to live (TTL) of entries in the
cache, which indicate from which absolute time onwards a new query
could be sent to refresh the expired entry.
The time to live of the target domain name's RRSets determines how
often a window of opportunity is available, which implies that a
short TTL makes spoofing far more viable.
Note that the attacker might very well have authorized access to the
target resolver by virtue of being a customer or employee of its
operator. In addition, access may be enabled through the use of
reflectors as outlined in [RFC5358].
4.2. Matching the Question Section
DNS packets, both queries and responses, contain a question section.
Incoming responses should be verified to have a question section that
is equivalent to that of the outgoing query.
4.3. Matching the ID Field
The DNS ID field is 16 bits wide, meaning that if full use is made of
all these bits, and if their contents are truly random, it will
require on average 32768 attempts to guess. Anecdotal evidence
suggests there are implementations utilizing only 14 bits, meaning on
average 8192 attempts will suffice.
Additionally, if the target nameserver can be forced into having
multiple identical queries outstanding, the "Birthday Attack"
phenomenon means that any fake data sent by the attacker is matched
against multiple outstanding queries, significantly raising the
chance of success. Further details in Section 5.
4.4. Matching the Source Address of the Authentic Response
It should be noted that meeting this condition entails being able to
transmit packets on behalf of the address of the authoritative
nameserver. While two Best Current Practice documents ([RFC2827] and
[RFC3013] specifically) direct Internet access providers to prevent
their customers from assuming IP addresses that are not assigned to
them, these recommendations are not universally (nor even widely)
Many zones have two or three authoritative nameservers, which make
matching the source address of the authentic response very likely
with even a naive choice having a double digit success rate.
Most recursing nameservers store relative performance indications of
authoritative nameservers, which may make it easier to predict which
nameserver would originally be queried -- the one most likely to
respond the quickest.
Generally, this condition requires at most two or three attempts
before it is matched.
4.5. Matching the Destination Address and Port of the Authentic
Note that the destination address of the authentic response is the
source address of the original query.
The actual address of a recursing nameserver is generally known; the
port used for asking questions is harder to determine. Most current
resolvers pick an arbitrary port at startup (possibly at random) and
use this for all outgoing queries. In quite a number of cases, the
source port of outgoing questions is fixed at the traditional DNS
assigned server port number of 53.
If the source port of the original query is random, but static, any
authoritative nameserver under observation by the attacker can be
used to determine this port. This means that matching this
conditions often requires no guess work.
If multiple ports are used for sending queries, this enlarges the
effective ID space by a factor equal to the number of ports used.
Less common resolving servers choose a random port per outgoing
query. If this strategy is followed, this port number can be
regarded as an additional ID field, again containing up to 16 bits.
If the maximum ports range is utilized, on average, around 32256
source ports would have to be tried before matching the source port
of the original query, as ports below 1024 may be unavailable for
use, leaving 64512 options.
It is in general safe for DNS to use ports in the range 1024-49152
even though some of these ports are allocated to other protocols.
DNS resolvers will not be able to use any ports that are already in
use. If a DNS resolver uses a port, it will release that port after
a short time and migrate to a different port. Only in the case of a
high-volume resolver is it possible that an application wanting a
particular UDP port suffers a long term block-out.
It should be noted that a firewall will not prevent the matching of
this address, as it will accept answers that (appear to) come from
the correct address, offering no additional security.
4.6. Have the Response Arrive before the Authentic Response
Once any packet has matched the previous four conditions (plus
possible additional conditions), no further responses are generally
This means that the third party has a limited time in which to inject
its spoofed response. For calculations, we will assume a window in
order of at most 100 ms (depending on the network distance to the
authentic authoritative nameserver).
This time period can be far longer if the authentic authoritative
nameservers are (briefly) overloaded by queries, perhaps by the
5. Birthday Attacks
The so-called "birthday paradox" implies that a group of 23 people
suffices to have a more than even chance of having two or more
members of the group share a birthday.
An attacker can benefit from this exact phenomenon if it can force
the target resolver to have multiple equivalent (identical QNAME,
QTYPE, and QCLASS) outstanding queries at any one time to the same
Any packet the attacker sends then has a much higher chance of being
accepted because it only has to match any of the outstanding queries
for that single domain. Compared to the birthday analogy above, of
the group composed of queries and responses, the chance of having any
of these share an ID rises quickly.
As long as small numbers of queries are sent out, the chance of
successfully spoofing a response rises linearly with the number of
outstanding queries for the exact domain and nameserver.
For larger numbers, this effect is less pronounced.
More details are available in US-CERT [vu-457875].
6. Accepting Only In-Domain Records
Responses from authoritative nameservers often contain information
that is not part of the zone for which we deem it authoritative. As
an example, a query for the MX record of a domain might get as its
responses a mail exchanger in another domain, and additionally the IP
address of this mail exchanger.
If accepted uncritically, the resolver stands the chance of accepting
data from an untrusted source. Care must be taken to only accept
data if it is known that the originator is authoritative for the
QNAME or a parent of the QNAME.
One very simple way to achieve this is to only accept data if it is
part of the domain for which the query was intended.
7. Combined Difficulty
Given a known or static destination port, matching ID field, the
source and destination address requires on average in the order of 2
* 2^15 = 65000 packets, assuming a zone has 2 authoritative
If the window of opportunity available is around 100 ms, as assumed
above, an attacker would need to be able to briefly transmit 650000
packets/s to have a 50% chance to get spoofed data accepted on the
A realistic minimal DNS response consists of around 80 bytes,
including IP headers, making the packet rate above correspond to a
respectable burst of 416 Mbit/s.
As of mid-2006, this kind of bandwidth was not common but not scarce
either, especially among those in a position to control many servers.
These numbers change when a window of a full second is assumed,
possibly because the arrival of the authentic response can be
prevented by overloading the bona fide authoritative hosts with decoy
queries. This reduces the needed bandwidth to 42 Mbit/s.
If, in addition, the attacker is granted more than a single chance
and allowed up to 60 minutes of work on a domain with a time to live
of 300 seconds, a meager 4 Mbit/s suffices for a 50% chance at
getting fake data accepted. Once equipped with a longer time,
matching condition 1 mentioned above is straightforward -- any
popular domain will have been queried a number of times within this
hour, and given the short TTL, this would lead to queries to
authoritative nameservers, opening windows of opportunity.
7.1. Symbols Used in Calculation
Assume the following symbols are used:
I: Number distinct IDs available (maximum 65536)
P: Number of ports used (maximum around 64000 as ports under 1024 are
not always available, but often 1)
N: Number of authoritative nameservers for a domain (averages around
F: Number of "fake" packets sent by the attacker
R: Number of packets sent per second by the attacker
W: Window of opportunity, in seconds. Bounded by the response time
of the authoritative servers (often 0.1s)
D: Average number of identical outstanding queries of a resolver
(typically 1, see Section 5)
A: Number of attempts, one for each window of opportunity
The probability of spoofing a resolver is equal to the amount of fake
packets that arrive within the window of opportunity, divided by the
size of the problem space.
When the resolver has 'D' multiple identical outstanding queries,
each fake packet has a proportionally higher chance of matching any
of these queries. This assumption only holds for small values of
In symbols, if the probability of being spoofed is denoted as P_s:
D * F
P_s = ---------
N * P * I
It is more useful to reason not in terms of aggregate packets but to
convert to packet rate, which can easily be converted to bandwidth if
If the window of opportunity length is 'W' and the attacker can send
'R' packets per second, the number of fake packets 'F' that are
candidates to be accepted is:
D * R * W
F = R * W -> P_s = ---------
N * P * I
Finally, to calculate the combined chance 'P_cs' of spoofing over a
chosen time period 'T', it should be realized that the attacker has a
new window of opportunity each time the TTL 'TTL' of the target
domain expires. This means that the number of attempts 'A' is equal
to 'T / TTL'.
To calculate the combined chance of at least one success, the
following formula holds:
(T / TTL)
A ( D * R * W )
P_cs = 1 - ( 1 - P_s ) = 1 - ( 1 - --------- )
( N * P * I )
When common numbers (as listed above) for D, W, N, P, and I are
inserted, this formula reduces to:
(T / TTL)
( R )
P_cs = 1 - ( 1 - ------- )
( 1638400 )
From this formula, it can be seen that, if the nameserver
implementation is unchanged, only raising the TTL offers protection.
Raising N, the number of authoritative nameservers, is not feasible
beyond a small number.
For the degenerate case of a zero-second TTL, a window of opportunity
opens for each query sent, making the effective TTL equal to 'W'
above, the response time of the authoritative server.
This last case also holds for spoofing techniques that do not rely on
TTL expiry, but use repeated and changing queries.
The calculations above indicate the relative ease with which DNS data
can be spoofed. For example, using the formula derived earlier on an
RRSet with a 3600 second TTL, an attacker sending 7000 fake response
packets/s (a rate of 4.5 Mbit/s), stands a 10% chance of spoofing a
record in the first 24 hours, which rises to 50% after a week.
For an RRSet with a TTL of 60 seconds, the 10% level is hit after 24
minutes, 50% after less than 3 hours, 90% after around 9 hours.
For some classes of attacks, the effective TTL is near zero, as noted
Note that the attacks mentioned above can be detected by watchful
server operators - an unexpected incoming stream of 4.5 Mbit/s of
packets might be noticed.
An important assumption however in these calculations is a known or
static destination port of the authentic response.
If that port number is unknown and needs to be guessed as well, the
problem space expands by a factor of 64000, leading the attacker to
need in excess of 285Gb/s to achieve similar success rates.
Such bandwidth is not generally available, nor is it expected to be
so in the foreseeable future.
Note that some firewalls may need reconfiguring if they are currently
set up to only allow outgoing queries from a single DNS source port.
8.1. Repetitive Spoofing Attempts for a Single Domain Name
Techniques are available to use an effectively infinite number of
queries to achieve a desired spoofing goal. In the math above, this
reduces the effective TTL to 0.
If such techniques are employed, using the same 7000 packets/s rate
mentioned above, and using 1 source port, the spoofing chance rises
to 50% within 7 seconds.
If 64000 ports are used, as recommended in this document, using the
same query rate, the 50% level is reached after around 116 hours.
9. Forgery Countermeasures
9.1. Query Matching Rules
A resolver implementation MUST match responses to all of the
following attributes of the query:
o Source address against query destination address
o Destination address against query source address
o Destination port against query source port
o Query ID
o Query name
o Query class and type
before applying DNS trustworthiness rules (see Section 5.4.1 of
A mismatch and the response MUST be considered invalid.
9.2. Extending the Q-ID Space by Using Ports and Addresses
Resolver implementations MUST:
o Use an unpredictable source port for outgoing queries from the
range of available ports (53, or 1024 and above) that is as large
as possible and practicable;
o Use multiple different source ports simultaneously in case of
multiple outstanding queries;
o Use an unpredictable query ID for outgoing queries, utilizing the
full range available (0-65535).
Resolvers that have multiple IP addresses SHOULD use them in an
unpredictable manner for outgoing queries.
Resolver implementations SHOULD provide means to avoid usage of
Resolvers SHOULD favor authoritative nameservers with which a trust
relation has been established; stub-resolvers SHOULD be able to use
Transaction Signature (TSIG) ([RFC2845]) or IPsec ([RFC4301]) when
communicating with their recursive resolver.
In case a cryptographic verification of response validity is
available (TSIG, SIG(0)), resolver implementations MAY waive above
rules, and rely on this guarantee instead.
Proper unpredictability can be achieved by employing a high quality
(pseudo-)random generator, as described in [RFC4086].
9.2.1. Justification and Discussion
Since an attacker can force a full DNS resolver to send queries to
the attacker's own nameservers, any constant or sequential state held
by such a resolver can be measured, and it must not be trivially easy
to reverse engineer the resolver's internal state in a way that
allows low-cost, high-accuracy prediction of future state.
A full DNS resolver with only one or a small number of upstream-
facing endpoints is effectively using constants for IP source address
and UDP port number, and these are very predictable by potential
attackers, and must therefore be avoided.
A full DNS resolver that uses a simple increment to get its next DNS
query ID is likewise very predictable and so very spoofable.
Finally, weak random number generators have been shown to expose
their internal state, such that an attacker who witnesses several
sequential "random" values can easily predict the next ones. A
crypto-strength random number generator is one whose output cannot be
predicted no matter how many successive values are witnessed.
9.3. Spoof Detection and Countermeasure
If a resolver detects that an attempt is being made to spoof it,
perhaps by discovering that many packets fail the criteria as
outlined above, it MAY abandon the UDP query and re-issue it over
TCP. TCP, by the nature of its use of sequence numbers, is far more
resilient against forgery by third parties.
10. Security Considerations
This document provides clarification of the DNS specification to
decrease the probability that DNS responses can be successfully
forged. Recommendations found above should be considered
complementary to possible cryptographical enhancements of the domain
name system, which protect against a larger class of attacks.
This document recommends the use of UDP source port number
randomization to extend the effective DNS transaction ID beyond the
available 16 bits.
A resolver that does not implement the recommendations outlined above
can easily be forced to accept spoofed responses, which in turn are
passed on to client computers -- misdirecting (user) traffic to
possibly malicious entities.
This document directly impacts the security of the Domain Name
System, implementers are urged to follow its recommendations.
Most security considerations can be found in Sections 4 and 5, while
proposed countermeasures are described in Section 9.
For brevity's sake, in lieu of repeating the security considerations
references, the reader is referred to these sections.
Nothing in this document specifies specific algorithms for operators
to use; it does specify algorithms implementations SHOULD or MUST
It should be noted that the effects of source port randomization may
be dramatically reduced by NAT devices that either serialize or limit
in volume the UDP source ports used by the querying resolver.
DNS recursive servers sitting behind at NAT or a statefull firewall
may consume all available NAT translation entries/ports when
operating under high query load. Port randomization will cause
translation entries to be consumed faster than with fixed query port.
To avoid this, NAT boxes and statefull firewalls can/should purge
outgoing DNS query translation entries 10-17 seconds after the last
outgoing query on that mapping was sent. [RFC4787]-compliant devices
need to treat UDP messages with port 53 differently than most other
To minimize the potential that port/state exhaustion attacks can be
staged from the outside, it is recommended that services that
generate a number of DNS queries for each connection should be rate
limited. This applies in particular to email servers.
Source port randomization in DNS was first implemented and possibly
invented by Dan J. Bernstein.
Although any mistakes remain our own, the authors gratefully
acknowledge the help and contributions of:
12.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP
Source Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D., and B.
Wellington, "Secret Key Transaction Authentication for
DNS (TSIG)", RFC 2845, May 2000.
[RFC3013] Killalea, T., "Recommended Internet Service Provider
Security Services and Procedures", BCP 46, RFC 3013,
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086,
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
12.2. Informative References
[RFC1123] Braden, R., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123, October 1989.
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the
Domain Name System (DNS)", RFC 3833, August 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive
Nameservers in Reflector Attacks", BCP 140, RFC 5358,
[vu-457875] United States CERT, "Various DNS service implementations
generate multiple simultaneous queries for the same
resource record", VU 457875, November 2002.