REPUTE M. Kucherawy
Internet-Draft May 5, 2013
Intended status: Informational
Expires: November 6, 2013
Operational Considerations Regarding Reputation Services
draft-ietf-repute-considerations-01
Abstract
The use of reputation systems is has become a common tool in many
applications that seek to apply collected intelligence about traffic
sources. Often this is done because it is common or even expected
operator practice. It is therefore important to be aware of a number
of considerations for both operators and consumers of the data. This
document includes a collection of the best advice available regarding
providers and consumers of reputation data, based on experience to
date. Much of this is based on experience with email reputation
systems, but the concepts are generally applicable.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Reputation Clients . . . . . . . . . . . . . . . . . . . . . . 4
5. Reputation Service Providers . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
8. Informative References . . . . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
Reputation services involve collecting feedback from the community
about sources of Internet traffic and aggregating that feedback into
a rating of some kind. Common examples include feedback about
traffic associated with specific email addresses, URIs or parts of
URIs, IP addresses, etc. The specific collection, analysis, and
rating methods vary from one service to the next and one problem
domain to the next, but several operational concepts appear to be
common to all of these.
The promise of the protection that reputation services offers can be
enticing, and many users and operators alike typically engage those
services merely because it is expected of them. A critical notion,
however, is that doing so explicitly involves a third party in the
flow of data those parties receive. This is often taken for granted,
with potentially disastrous results.
This document highlights this and other considerations in providing
and consuming reputation data services.
2. Background
The community has historically focused on identifying sources that
misbehave, i.e., that earn negative reputations. The purpose here is
to identify and filter traffic from bad actors. This grew out of
operational need. As the Internet grew, so did the occurence of
problematic traffic, especially in email. The pragmatics of email
(i.e., the fact that the total IP address space is more constrained
than the total email address space) drove the focus on using IP
addresses as the focus of reputation, in addition to the fact that IP
addresses have a degree of validation (via the TCP/IP infrastructure)
where email addresses have had none.
A specific example of a reputation service in common use in the email
space is the DNS blacklist [DNSBL]. This is a method of querying a
database as to whether a source of incoming [SMTP] email traffic
should be allowed to relay email, based on previous observations and
feedback. The method uses the IP address of the source as the basis
for a query to the database using the Domain Name System [DNS] as the
interface. [DNSBL] includes several points in its Security
Considerations document that are repeated and further developed here.
However, regardless of the identifier used as the identifier for a
reputation, bad actors can evade detection or the effects of their
observed behavior by changing identifiers (e.g., move to a new IP
address, register a new domain name, use a sub-domain). This makes
the problem space effectively boundless, especially as IPv6 rolls
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out.
3. Evolution
More modern thinking is evolving toward the identification of good
actors rather than bad actors, and giving them preferential
treatment. This drastically reduces the problem space: There are
vastly more IP addresses and email addresses used by bad actors to
generate problematic traffic than are used by good actors to generate
desirable traffic.
Moreover, good actors tend to be represented by stable names and
addresses, allowing users to rely on these to identify and give
preferential treatment to their traffic. Good actors have no need to
hop around to different addresses, and already work to keep their
traffic clean.
This notion has only been tried to date using manually edited
whitelists, but has shown promising results on that scale.
4. Reputation Clients
Operators that choose to make use of reputation services to influence
content allowed to pass into or through their infrastructures need to
understand that they are granting a third party (the reputation
service provider, or RSP) the ability to affect incoming traffic, for
better or worse. Of course, this is the whole point of engaging an
RSP when everything is working properly, but a number of issues are
worthy of consideration before establishing such a relationship.
Some cases have occurred where an RSP made the unilateral decision to
terminate its service. To encourage its clients to stop issuing
queries, it began reporting a maximally negative reputation about all
subjects, causing rejection of all incoming traffic during the
incident period. Although one would hope such incidents to be rare,
automated means to detect such unfortunate returns (malicious or
otherwise) and take remedial should be considered.
RSPs will be the subject of attacks once it is understood that sucess
in doing so will allow malicious content to evade detection and
filtering. Users of RSPs need to be aware of possible interruptions
in service availability or quality.
Similarly, some actors will try to "game" the service, which is to
say that such actors will attempt to determine patterns of behavior
that result in the reporting of favorable reputations, and in doing
so, acquire artifically inflated reputations. One could reasonably
assume that a reputation service is inherently fragile. For
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operational clients, this should prompt balanced and comparative,
rather than unilateral, use of the service.
It is suggested that, when engaging an RSP, an operator should try to
learn the following things about the RSP in order to understand the
exposure potential:
o the RSP's basis for listing or not listing particular subjects;
o if an RSP is paid by its listees, the rate and criteria for
rejection from being listed;
o how the RSP collects data about subjects;
o how many data points are input to the reported reputation;
o whether reputation is based on a reliable identifier;
o how the RSP establishes reliability and authenticity of those
data;
o how data validity is maintained (e.g., on-going monitoring of the
reported data and sources);
o how actively data validity is tracked (e.g., how changes are
detected);
o how disputed reputations are handled;
o how often input data expire;
o whether older information more or less influential than newer;
o whether the reported reputation a scalar, a Boolean value, a
collection of values, or something else;
o when transitioning among RSPs, the differences between them among
these above points; that is, whether a particular score from one
means the same thing from another.
An operator using an RSP would be wise to ensure it has the
capability to effect local overrides for cases where the client
expects to disagree with the reported reputation.
An operator should be able limit the impact of a negative reputation
on content acceptance. For example, rather than rejecting content
outright when a negative reputation is returned, simply subject it to
additional (i.e., more thorough) local analyis before permitting the
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traffic to pass.
A sensible default should apply when the RSP is not available. This
may also be a query to a different RSP known to be less robust than
the primary one.
Recent proposals have focused on tailoring operation to prefer or
emphasize content whose sources have positive reputations. As stated
above, negative reputations are easy to shed, and the universe of
things that will earn and maintain positive reputations is relatively
small. Designing a filtering system that observes these notions is
expected to be more lightweight to operate and harder to game.
One choice is to query and cross-referencing multiple RSPs. This can
help to detect which ones under comparison are reliable, and offsets
the effect of anomalous replies.
5. Reputation Service Providers
Operators intending to provide a reptuation service need to consider
that there are many flavors of clients. There will be clients that
are prepared to make use of a reputation service blindly, while
others will be interested in understanding more fully the nature of
the service being provided. An operator of an RSP should be prepared
to answer as may of the questions identified in Section 4 as
possible, not only because wise clients will ask, but also because
they reflect issues that have arisen over the years, and exploration
of the points they raise will result in a more robust reputation
service.
Obviously, in computing reputations via traffic analysis, some
private algorithms may come into play. For some RSPs, such "secret
sauce" comprises their competitive advantage over others in the same
space. This document is not suggesting that all private algorithms
need to be exposed for a reputation service to be acceptable.
Instead, it is anticipated that enough of the above details need to
be available to ensure consumers (and in some cases, industry or the
general public) that the RSP can be trusted to influence key local
policy decisions.
Reptuations should be based on accurate identifiers, i.e., some
property of the content under analysis that is difficult to falsify.
For example, in the realm of email, the address found in the From:
field of a message is typically not verifiable, while the domain name
found in a validated domain-level signature is. In this case,
constructing a reputation system based on the domain name is more
useful than one based on the From: field.
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The biggest frustration with most RSPs to date has been the absence
of a visible, accessible, and transparent process for remediating the
errant addition of an identifier to a negative reputation list. An
RSP in widespread use is perceived to have enormous power when its
results are used to reject traffic outright; when a "bad" entry is
added referencing a good actor, it can have destructive effects, so
an effective mechanism to fix such problems needs to exist.
To accommodate clients with varying sensitivities, it is advisable
for the query mechanism used to access the RSP to provide the ability
to request details in the returned result about how the result was
reached, allowing the client to decide if the result should be
applied. For example, it shoudl be possible for the reply to
contain:
o the result itself;
o the number of data points used to compute the result;
o the age range of the data;
o source diversity of the input data;
o currency of the result (i.e., when it was computed);
o basis of the result (i.e., which identifier was used).
The systems and algorithms used by the RSP to compute the reported
reputation will need to be hardened as much as practicable against
gaming or other forms of data poisoning. Larger source diversities
are harder to overcome with poisoned input, but are expensive to
build in terms of both infrastructure and time.
Systems focused on assigning positive reputations rather than negtive
ones are promising since positive reputations, if made difficult to
earn, put a large cost on bad actors, which may be enough to dissuade
them entirely.
6. Security Considerations
Several points are raised above that can be described as threats to
the delivery of valid user data. This document highlights and
discusses those matters, but introduces no new security issues.
7. IANA Considerations
This memo contains no actions for IANA.
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[RFC Editor: Please remove this section prior to publication.]
8. Informative References
[DNS] Mockapetris, P., "Domain Names -- Concepts and Facilities",
RFC 1034, November 1987.
[DNSBL] Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
February 2010.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
Appendix A. Acknowledgements
The author wishes to acknowledge the following for their review and
constructive criticism of this proposal: Chris Barton, Vincent
Schonau
Author's Address
Murray S. Kucherawy
EMail: superuser@gmail.com
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