Measures for making DNS more resilient against forged answers
draft-ietf-dnsext-forgery-resilience-02.txt

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Abstract

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 answers quickly, as this potentially saves large amounts of computation.

By describing certain behaviour that has previously not been standardised, this document sets out how to make the DNS more resilient against accepting incorrect answers. This document updates RFC 1034.

Table of Contents

1.  Requirements and definitions
    1.1.  Definitions
    1.2.  Key words
2.  Introduction
3.  Description of DNS spoofing
4.  Details
    4.1.  Matching the question
    4.2.  Matching the ID field
    4.3.  Matching the source address of the authentic answer
    4.4.  Matching the destination address of the authentic answer
    4.5.  Have the answer arrive before the authentic answer
5.  Birthday attacks
6.  Accepting only in-zone answers
7.  Combined difficulty
    7.1.  Symbols used in calculation
    7.2.  Calculation
8.  Discussion
9.  Countermeasures
10.  Security Considerations
11.  Acknowledgements
12.  Normative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Requirements and definitions

1.1.  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

Stub resolver: a very limited Resolver on a client computer, that leaves most of the recursing work to a full resolver

Question: a question sent out by a resolver, typically in a UDP packet

Answer: the answer sent back by an authoritative nameserver, typically in a UDP packet

Third party: any host other than the resolver or the intended recipient of a question. The third party may have access to a random 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 answer: the answer that would be accepted if no third party interferes

Target domain: domain for which the attacker wishes to spoof in an answer

Fake data: answer chosen by the attacker

1.2.  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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  Introduction

This document describes several common problems in DNS implementations which, although previously recognized, remain largely unsolved. Besides briefly recapping these problems, this RFC contains rules that, if implemented, make complying resolvers vastly more resistant to the attacks described.

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] (Mockapetris, P., “Domain names - concepts and facilities,” November 1987.), [RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.) and beyond.

Additionally, it has recently become possible to acquire SSL/TLS certificates with no other confirmation of identity than the ability to respond to a verification email sent via SMTP ([RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.)) - 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 finalising 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 resolver currently does 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] (Atkins, D. and R. Austein, “Threat Analysis of the Domain Name System (DNS),” August 2004.).

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 emphasises a number of existing rules and guidelines embodied in the relevant STDs and RFCs. The following also specifies new requirements to make sure the Domain Name System can be relied upon until a more secure protocol has been standardised 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 intercept, change or inject packets sent to the resolver.

For protocol extensions under development that offer protection against these scenarios, see [RFC4033] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.) and beyond.

3.  Description of DNS spoofing

When certain steps are taken it is feasible to 'spoof' the current deployed majority of caching 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 (and more specifically a caching server) accept an answer.

The following sentence in section 5.3.3 of [RFC1034] (Mockapetris, P., “Domain names - concepts and facilities,” November 1987.) presaged the present problem:

  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 accepted by a resolver if and only if:

1. The question section of the reply packet is identical to that of a question packet currently waiting for an answer
2. The ID field of the reply packet matches that of the question packet
3. The answer comes from the same network address the question was sent to
4. The answer comes in on the same network address, including port number, as the question was sent from
5. It is the first answer to match the previous four conditions.

If a third party succeeds in meeting the first four conditions before the answer from the authentic answer 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, with the difficulty being a function of the resolver implementation and zone configuration.

4.  Details

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] (Atkins, D. and R. Austein, “Threat Analysis of the Domain Name System (DNS),” August 2004.).

4.1.  Matching the question

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 denial of service attacks.

In spite of this, many resolvers perform at least partial service for the whole world. This is partially out of lack of concern, and is reminiscent of the open relay SMTP service the net enjoyed up to the early 1990s. Some access providers may serve so many subnets that it is hard to enumerate these all in the DNS configuration.

Providing full service enables the third party to send the target resolver a question for the domain name it intends to spoof. On receiving this question, and not finding the answer in its cache, the resolver will transmit questions to relevant authoritative nameservers. This opens up a window of opportunity for getting fake answer data accepted.

Some operators restrict access by not recursing for unauthorised 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 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' 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 authorised access to the target resolver by virtue of being a customer or employee of its operator.

4.2.  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 utilising only 14 bits, meaning on average 8192 attempts will suffice.

Additionally, if the target nameserver can be forced into having multiple identical questions outstanding, the 'Birthday Attack' phenomenon means that any fake data sent by the attacker is matched against multiple outstanding questions, significantly raising the chance of success. Further details in Section 5 (Birthday attacks).

4.3.  Matching the source address of the authentic answer

Most domains have two or three authoritative nameservers, which make matching the source address of the authentic answer 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.

It should be noted that meeting this condition entails being able to transmit packets on behalf of the address of the authoritative nameserver. While several important documents ([RFC2827] (Ferguson, P. and D. Senie, “Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing,” May 2000.) and [RFC3013] (Killalea, T., “Recommended Internet Service Provider Security Services and Procedures,” November 2000.) 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) implemented.

4.4.  Matching the destination address of the authentic answer

Note that the destination address of the authentic answer is the source address of the original question.

The actual address of a recursing nameserver is generally known; the port used for asking questions is harder to determine. Most current resolvers pick a random port at startup and use this for all outgoing questions. 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 question 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 questions, this enlarges the effective address space by a factor equal to the number of ports used.

Less common resolving servers choose a random port per outgoing question. 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 32128 source ports would have to be tried before matching the source port of the original question as ports below 1024 may be unavailable for use, leaving 64512 options.

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.

4.5.  Have the answer arrive before the authentic answer

Once any packet has matched the previous four conditions, no further answers should be accepted.

This means that the third party has a limited time in which to inject its spoofed answer, typically in the order of at most 100ms.

This time period can be far longer if the authentic authoritative nameservers are (briefly) overloaded by queries, perhaps by the attacker.

5.  Birthday attacks

A curious mathematical phenomenon means that a group of 22 people suffices to have a more than even chance at having two or more members of the group share a birthday.

An attacker can benefit from this phenomenon if it can force the target resolver to have multiple outstanding questions at any one time for the same domain to the same authoritative server.

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 questions and answers, the chance of having any of these share an ID rises quickly.

As long as small numbers of questions are sent out, the chance of successfully spoofing an answer rises linearly with the number of outstanding questions for the exact domain and nameserver.

For larger numbers this effect is less pronounced.

More details are available in US-CERT [vu‑457875] (United States CERT, “Various DNS service implementations generate multiple simultaneous queries for the same resource record,” November 2002.).

6.  Accepting only in-zone answers

Answers 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 answer 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 that data.

One very simple way to achieve this is to only accept data if it is part of the domain we asked the question for.

7.  Combined difficulty

Given a known or static destination port, matching ID field, source and destination address requires on average in the order of 2 * 2^15 = 65000 packets, assuming a domain has 2 authoritative nameservers.

If the window of opportunity available is around 100ms, 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 first attempt.

A realistic minimal DNS answer consists of around 80 bytes, including IP headers, making the packet rate above correspond to a respectable burst of 416Mb/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 answer can be prevented by overloading the bonafide authoritative hosts with decoy questions. This reduces the needed bandwidth to 42 Mb/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 meagre 4Mb/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 questions 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 2.5)

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 questions of a resolver (typically 1, see Section 5 (Birthday attacks))

A: Number of attempts, one for each window of opportunity

7.2.  Calculation

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 questions, each fake packet has a proportionally higher chance of matching any of these questions. This assumption only holds for small values of 'D'.

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 needed.

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 realised 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 question asked, making the effective TTL equal to 'W' above, the response time of the authoritative server.

8.  Discussion

The calculations above indicate the relative ease with which DNS data can be spoofed. For example, using the formula derived earlier on a domain with a 3600 second TTL, an attacker sending 7000 fake answer packets/s (a rate of 4.5Mb/s), stands a 10% chance of spoofing a record in the first 24 hours, which rises to 50% after a week.

For a domain 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.

Note that the attacks mentioned above can be detected by watchful server operators - an unexpected incoming stream of 4.5mbit/s of packets might be noticed.

An important assumption however in these calculations is a known or static destination port of the authentic answer.

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 expected to be so in the foreseeable future.

Note that some firewalls may need reconfiguring if they are currently setup to only allow outgoing queries from a single DNS source port.

9.  Countermeasures

Given the above, a resolver implementation MUST match answers to the following attributes of the question:

• Remote address
• Local address
• Query port
• Query ID
• Question (name and type)

before applying DNS trustworthiness rules.

Additionally, resolver implementations MUST have the ability to:

• Use an unpredictable source port for outgoing queries from a range (53, or > 1024) of ports that is as large as possible;
• Use multiple different source ports simultaneously in case of multiple outstanding questions;
• Use an unpredictable query ID for outgoing queries, utilizing the full range available (0-65535);
• Assure that its choices of port and ID cannot be predicted by an attacker having knowledge of its (pseudo-)random generator.

If a resolver sends out multiple equivalent questions to any authoritative server, before receiving an answer, all MUST have identical ID, source address and source port.

Resolvers SHOULD favour authoritative nameservers with which a trust relation has been established; Stub-resolvers SHOULD be able to use TSIG ([RFC2845] (Vixie, P., Gudmundsson, O., Eastlake, D., and B. Wellington, “Secret Key Transaction Authentication for DNS (TSIG),” May 2000.)) or IPSEC ([RFC2401] (Kent, S. and R. Atkinson, “Security Architecture for the Internet Protocol,” November 1998.)) when communicating with their recursive resolver.

10.  Security Considerations

This document provides clarification of the DNS specification to decrease the probability that DNS answers can be successfully forged. Recommendations found above should be considered complementary to possible cryptographical enhancements of the domain name system.

A resolver that does not implement the recommendations outlined above can easily be forced to accept spoofed answers, 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 Section 5 (Birthday attacks), while proposed countermeasures are described in Section 9 (Countermeasures).

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 support.

The above notwithstanding, it should be noted that using a low Time To Live for DNS records raises the chances of an attacker spoofing a resolver.

11.  Acknowledgements

Source port randomisation 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:

Stephane Bortzmeyer,

Alfred Hoenes,

Sean Leach,

Norbert Sendetzky

12. Normative References

Authors' Addresses

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