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Internet Engineering Task Force T. Creighton
Internet-Draft C. Griffiths
Intended status: BCP J. Livingood, Ed.
Expires: January 7, 2010 Comcast
R. Weber
Unaffiliated
July 6, 2009
Recommended Configuration and Use of DNS Redirect by Service Providers
draft-livingood-dns-redirect-00
Status of this Memo
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Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
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document authors. All rights reserved.
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Abstract
The objective of this document is to describe the design of so-called
DNS Redirect services deployed today by Internet Service Providers
(ISPs), DNS Application Service Providers (ASPs), and other
organizations providing so-called DNS Redirect services via their
recursive DNS services, as well as to describe the recommended best
current practices regarding such systems.
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Table of Contents
1. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Document Scope . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Major Types of DNS Redirect Services . . . . . . . . . . . . . 7
5.1. Web Error Redirect . . . . . . . . . . . . . . . . . . . . 7
5.2. Malicious Site Protection . . . . . . . . . . . . . . . . 8
5.3. Legally-Mandated DNS Redirect . . . . . . . . . . . . . . 9
5.4. Content-Based Redirect . . . . . . . . . . . . . . . . . . 10
6. Opt-In or Opt-Out Mechanisms . . . . . . . . . . . . . . . . . 11
7. Practices to Avoid . . . . . . . . . . . . . . . . . . . . . . 13
8. Functional Design . . . . . . . . . . . . . . . . . . . . . . 15
8.1. DNS Recursive Resolver . . . . . . . . . . . . . . . . . . 15
8.2. Web Error Landing Server . . . . . . . . . . . . . . . . . 16
8.3. Web Browser Client . . . . . . . . . . . . . . . . . . . . 17
8.4. Domain White List . . . . . . . . . . . . . . . . . . . . 17
8.5. Malicious Domain List . . . . . . . . . . . . . . . . . . 17
8.6. Legally-Mandated DNS Redirect Domain List . . . . . . . . 18
8.7. Content-Based DNS Redirect Domain List . . . . . . . . . . 19
9. Example DNS and HTTP Flows . . . . . . . . . . . . . . . . . . 20
10. DNSSEC Considerations . . . . . . . . . . . . . . . . . . . . 26
11. Security Considerations . . . . . . . . . . . . . . . . . . . 27
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 28
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
15. Normative References . . . . . . . . . . . . . . . . . . . . . 28
Appendix A. Document Change Log . . . . . . . . . . . . . . . . . 29
Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
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1. Requirements Language
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].
2. Introduction
Internet users typically are provided with several IP addresses for
recursive DNS servers, as described in Section 2.3 of [RFC1591], by
their respective ISPs, typically in an automated fashion via DHCP
[RFC2131]. Some other users and organizations choose to use a
different set of IP address for their DNS servers, which are hosted
and managed by another organization, such as a DNS ASP. It is also
the case that a number of users and organizations choose to operate
their own DNS servers, though those use cases are outside of the
scope of this document.
ISPs and DNS ASPs have discovered over time that their users would
benefit via " enhanced " DNS services, which often rely upon
DNS Redirect functionality. These enhanced services, which are
offered on an opt-in or opt-out basis (with the exception of where
legal mandates preclude this), can perform a number of value added
services for users, such as attempting to interpret web address
errors and protecting users from reaching domains or fully qualified
domain names (FQDNs, Section 5.1 of [RFC1035]) that would cause a
user to inadvertently access malware.
There are a number of ways such services should and should not work.
As a result, a document describing the best current practices in this
area is beneficial to the community, and this is the motivation for
this document.
3. Document Scope
This document focuses on the practices of ISPs and DNS ASPs. All
other use cases, such as when an Internet user or organization
chooses to operate their own DNS servers is outside of the scope of
this document.
In addition, there are several ways that such entities can provide
users with these enhanced services, such as web error redirect
services and malicious domain protection services. In addition to
methods which rely primarily upon a recursive DNS server, alternate
methods include (a) interception and replacement of the error by a
web browser client software, (b) interception and replacement of the
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error by a tool bar, plug-in, personal firewall security software or
other web browser client add-on. These alternate methods, which rely
upon various types of client software, are also outside of the scope
of this document.
It is important to note that while these alternate methods are
considered out of scope for this document, this should not be
interpreted as a negative judgment of their suitability or
applicability to the relevant problem space. Instead, these should
simply be considered as alternate methods since, as with most any
technical problem, there are a variety of valid methods for solving a
problem.
While the Section 5.2 section indicates that users must be able to
opt into or out of DNS Redirect services, the reasons for why an ISP
or DNS ASP may choose one or the other as the default are out of
scope.
Lastly, in the Section 5.2 section of this document, the method by
which FQDNs, domains, and/or sites are added or removed from malware
lists is outside the scope of this document.
4. Terminology
While these terms are generally well known, it is important to define
them in the context of this document.
4.1. Internet Service Provider (ISP)
An Internet Service Provider, which provides Internet services,
including basic network connectivity. It is not germane to this
document what the method of connection is, such as wired or wireless,
what the speed of such a connection is, and what other services are
included or available to users. It is, however, assumed that the ISP
is providing recursive DNS services to their users and is in some
manner providing users with the IP addresses of these DNS servers,
whether via DHCP, static assignment by users, or some other method.
4.2. DNS Application Service Provider (ASP)
A DNS Application Service Provider, which provides managed and/or
hosted recursive DNS services (and possibly other DNS services) to
their users. In the case of managed services, the DNS ASP may
remotely manage the recursive DNS servers in a user's network. For a
hosted recursive DNS service, these servers are typically located
outside of the user's network and these hosted resources are shared
across multiple users. In most instances, these are hosted services
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and users are manually configuring either their DHCP server or their
individual computing devices with the IP addresses of the recursive
DNS servers operated by their ASP.
4.3. Internet User
An Internet user, which is generally a person using a computing
device to connect to and make use of the Internet. Such users are
typically connected at the edge of the network, though the method by
which they connect to the Internet is not particularly relevant to
this document.
4.4. DNS Recursive Resolver
A DNS recursive resolver processes fully qualified domain name
queries (FQDN, Section 5.1 of [RFC1035]) into IP addresses by finding
the resource records in the authoritative DNS servers for the domain
associated with the FQDN. The resource records are then cached on
the recursive server for future requests until an expiration timer
expires called time to live (TTL), as described in Section 5.2 of
[RFC2181]. These servers are in most cases provide by ISPs for name
resolution.
4.5. Web Browser
Client software operated by the user locally on their computing
device, such as Microsoft Internet Explorer, Mozilla Firefox, Apple
Safari, Google Chrome, etc.
4.6. Web Error Landing Server
The host that a user is directed to when the DNS Recursive Server
receives a NXDOMAIN response. The contents of the web page that the
web server sends the user varies widely across different ISPs and DNS
ASPs. In some cases it is simply a more descriptive error that the
user would otherwise receive, while in other cases it may include
links to sites similar to the URL attempted and/or a search page,
among many other possibilities.
4.7. Malicious Domain Web Error Landing Server
The web server that a user's web browser is directed to when the DNS
Recursive Server matches a DNS query to a malicious domain or FQDN.
The contents of the web page that the web server sends the user
varies widely across different ISPs and DNS ASPs. In most cases it
simply explains that the attempted URL contains malware and that
access has been prevented, though there are many other possibilities.
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4.8. User Options Web Server
The web server that a user is directed to via a link on a page served
by the Web Error Landing Server, the Malicious Domain Web Error
Landing Server, from another system such as an account management
system, or via direct access, which enables a user to control whether
or not they are opted into or opted out of DNS Redirect services.
This is described in additional detail in the Section 6 section.
4.9. NXDOMAIN Response
In this document, an NXDOMAIN (nonexistent domain) response can be
used interchangeably with an RCODE 3 response. The RCODE 3 response
was first documented in see Section 4.1.1 of [RFC1035]). Subsequent
RFCs introduced the term NXDOMAIN response, which is synonymous with
RCODE 3 and tends to be used more frequently, as noted in Section 2.2
of [RFC2136], Section 1 of [RFC2308], and Section 5.4 of [RFC2535].
5. Major Types of DNS Redirect Services
DNS Redirect services can be classified into several major
categories, as follows below.
5.1. Web Error Redirect
A web error redirect service enables an ISP or ASP to provide a user,
who is generally utilizing a web browser, with an improved user
experience when an attempt to reach a nonexistent domain is made.
5.1.1. Web Error Redirect Problem Statement
A user enters an incorrect URL into their web browser, such as
http://www.example.invalid, where .invalid is a nonexistent Top Level
Domain (TLD, see Section 2 of [RFC1591]). In such a case, a user
would typically receive an error.
5.1.2. Web Error Redirect Solution Description
When a recursive DNS server detects such a nonexistent domain error
(NXDOMAIN, see Section 4.1.1 of [RFC1035]), the ISP or ASP can
instead provide a IP address for a Web Error Landing Server that can
present the user with a list of suggested destinations rather than
simply an error page. This page must also provide the user with a
link to a method of opting out in the future. See Figure 1,
Figure 2, and Figure 8 for examples below.
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5.1.3. Web Error Redirect Solution Considerations
It is important to note that this technology can directly impact non-
web clients such as instant messaging, VPNs, FTP, email filters-
related DNS queries. Thus, special exclusions may need to be made in
order to prevent unintentional side effects. Design considerations
for the Web Error Search and Malicious Site Protection services
should include properly and promptly terminating non-HTTP connection
requests. Only A and AAAA resource records should be redirected, all
other resource record types must be answered as if there was no
redirection.
5.2. Malicious Site Protection
Malware websites have proliferated recently, making malware and bot
networks a major problem for users. In many cases, the initial
contact with a virus or malware occurs when an unsuspecting user
visits a particular website. This has even been observed to occur
when a user visits an otherwise legitimate website, which contains
external references that happen to contain malware, for example (such
as advertisements served by a third party). Many organizations
maintain lists of domains and FQDNs which host malware.
5.2.1. Malicious Site Protection Problem Statement
A user, malware agent, or bot requests a URL www.example.net or
domain example.net. This site is associated with distributing
malware or some other malicious activity that would not be desired by
the user. The correct IP address is returned by the DNS and the user
accesses the malware site or domain and their computer is infected
with a bot.
5.2.2. Malicious Site Protection Solution Description
By using Malicious Site Protection, a user may have their DNS
response redirected from the IP address for the malicious URL
www.example.net or domain example.net to a safe website that explains
why the user was redirected. Importantly, the application attempting
to access a malicious resource may or may not be a web browser and,
further, may be operating without the user's knowledge and/or
permission. This page on the aforementioned safe website that the
user is directed to may also provide the user with a link to a method
of opting out in the future. See Figure 3 and Figure 9 for examples
below. There may also be limited cases where it could be harmful to
the objective of Malicious Site Protection to redirect the user to a
safe website, in which case the user may not be directed to any
resource, and a NXDOMAIN response be provided.
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5.2.3. Malicious Site Protection Solution Considerations
It is important to note that this technology can directly impact non-
web clients such as instant messaging, VPNs, FTP, email filters-
related DNS queries. Thus, special exclusions may need to be made in
order to prevent unintentional side effects. Design considerations
for the Web Error Search and Malicious Site Protection services
should include properly and promptly terminating non-HTTP connection
requests. A range of resource records may be redirected, such as A,
AAAA, MX, SRV, and NAPTR records, in order to fulfill the objective
of preventing access to certain network elements containing malicious
content or which and in some way used to transmit, relay, or
otherwise transfer malicious content. All other resource record
types must be answered as if there was no redirection.
Malicious domain protection is also only effective if a user is
actually using the DNS IP addresses that have this functionality.
Thus, should a user's computer become compromised with some type of
bot or virus that changes their DNS IP addresses (typically without
their knowledge), the malicious domain protection would have no
effect since the user is now pointed to DNS servers which are
presumably in the control of a third party with malicious intentions.
5.3. Legally-Mandated DNS Redirect
A regulatory organization or other entities with law enforcement
authority over ISP businesses may in some cases mandate or otherwise
compel ISPs and/or DNS ASPs to perform DNS Redirection for specific
sites. For example, local laws may compel an ISP and/or DNS ASP to
attempt to protect/prevent users from viewing illegal content via a
mandate to redirect or block specific sites and/or domains. However,
it is out of the scope of this document to address the suitability of
DNS Redirect for this problem, how this may or may not affect user
rights/freedoms in various jurisdictions, problematic judgment areas
which may exist relating to management of applicable site lists,
unintended side effects of legally-mandated lists, etc.
5.3.1. Legally-Mandated DNS Redirect Problem Statement
Governments, whether via regulatory or law enforcement bodies, as
well as courts, sometimes require ISPs and/or DNS ASPs to block
access to certain sites or domains. In some cases, these entities
will provide a site (FQDN) and/or domain list to the ISP and/or DNS
ASP, and compel the ISP and/or DNS ASP to prevent access to these
sites and/or domains via the DNS.
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5.3.2. Legally-Mandated DNS Redirect Solution Description
By using legally-mandated domain redirection, a user may have their
DNS response redirected from the IP address for the URL
www.illegalcontent.example.net or domain illegalcontent.example to a
"safe" website that may explain why the user was redirected. See
Figure 4 and Figure 10 for examples below.
5.3.3. Legally-Mandated DNS Redirect Solution Considerations
It is important to note that these governmental and/or law
enforcement actions are frequently quite controversial in a given
jurisdiction, and also that there are many examples where lists have
inadvertently blocked legal content and therefore improperly blocked
the freedom of access to legitimate and legal content. A range of
resource records may be redirected, such as A, AAAA, SRV, and NAPTR
records, in order to fulfill particular legal mandates. All other
resource record types must be answered as if there was no
redirection.
Depending upon government, law enforcement, and/or court-related
mandates/laws/rules, an ISP and/or DNS ASP performing Legally-
Mandated DNS Redirect may not provide an opt-out capability, and in
some jurisdictions they must not provide an opt-out capability. ISPs
should disclose openly that they have been compelled to perform
legally-mandated DNS Redirect, provided that such disclosure has not
been prohibited for some reason by any relevant regulator, court, or
law enforcement organization. For example, a page which may be
served in response to redirection should be a location at which such
a disclosure is made, in addition to the relevant sections of network
policy documents, etc.
In some cases where the such Legally-Mandated DNS Redirect is
required, there may be hosts with a mix of legal and illegal/
restricted content such that the redirect will not be to an error
page but will be instead to a proxy server, which will be capable of
performing a more fine-grained content analysis. The manner in which
such functionality might work is outside the scope of this document.
5.4. Content-Based Redirect
Content-Based Redirect is similar to content filtering, except that
it is more aptly "content avoidance." To explain this difference
more fully, no content is actually filtered by an ISP or DNS ASP
system before it is sent to the user. Instead, DNS responses can be
modified in order that a user is protected from content which they
deemed inappropriate.
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5.4.1. Content-Based Redirect Problem Statement
A user wishes to avoid visiting web sites with certain types of
content. For example, the user may have children in their household
and wished to prevent access to adult-themed content. Other examples
of the type of content that the user may wish to prevent access to
may include categories such as illicit drugs, alcohol, hate speech,
and weapons, among many other potential categories. The user in this
case may not exclusively be a residential user, but may also be the
network administrator for a small business, school, church, or other
organization. Thus, there may be a wide range of motivations for the
desire to prevent access to certain types of content.
5.4.2. Content-Based Redirect Solution Description
By using Content-Based Redirect, a user may have their DNS response
redirected from the IP address for the inappropriate URL
www.inappropriate.example.com to a safe website that explains why the
user was redirected. This page may also provide the user with a link
to a method of reconfiguring the service, in case it has unexpected
results and a site that the user wishes to access has been blocked.
In addition, the user should be able to fully configure Content-Based
Redirect via the User Options Web Server, such as electing which
categories of content they may wish to prevent access to. See
Figure 5 and Figure 11 for examples below.
5.5. Content-Based Redirect Solution Considerations
A range of resource records may be redirected, such as A, AAAA, SRV,
and NAPTR records, in order to fulfill the user-directed objective of
preventing access to certain types of content. All other resource
record types must be answered as if there was no redirection.
6. Opt-In or Opt-Out Mechanisms
ISPs and DNS ASPs must provide their users with a method to opt into
(opt-in) or out (opt-out) of some or all DNS Redirect services. Opt-
out and opt-in methods should be reliable and should take into
consideration the Section 7 section below. Whether such services are
offered on an opt-in or opt-out basis depends on a range of factors
which are outside of the scope of this document. The two different
methods, opt-out and opt-in, are described below.
6.1. Opt-Out
Opt-Out is used when the users are by default offered all or some DNS
Redirect services. As a result, the user must take an action to
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disable some or all such services. This is typically performed via a
User Options Web Server. Users that have chosen to opt-out should
receive DNS responses which are indistinguishable from those
responses provided by a DNS server with no DNS Redirect
functionality. In addition, opt-out should be persistent in nature,
which means that opt-out should be tied to a fixed credential or
attribute of some type, such as an account identifier, billing
identifier, or equipment identifier, which is not typically subject
to change on a regular basis.
6.2. Opt-In
Opt-In is used when the users are by default not offered any DNS
Redirect services. As a result, the user must take an action to
enable some or all such services. This is typically performed via a
User Options Web Server.
6.3. Automated Mechanisms and Reasonable Processing Times
Once a user has selected to opt-in or opt-out of DNS Redirect
services, such changes should occur automatically, when this is
technically possible, without requiring the user to manually change
any settings on their computing device. Such changes should also
occur within a reasonable period of time. In some cases, however, a
user may be offered the ability to speed the period of time for these
changes to take effect, such as by restarting the computing device or
a piece of network equipment which connects them to their ISP's
network, for example.
While an automated mechanism may be the easiest for users, since it
requires no manual reconfiguration of their network settings, the
authors also recognize that there may be extenuating circumstances
where this is not achievable. In such cases, which may for example
be due to the particular attributes of one or another ISP's network
design, a fully automated mechanism may not be possible. Another
example is where a user is switching from their ISP's DNS server IP
addresses to those of a DNS ASP. As a result, a user in all of these
cases, as well as other possible cases, must manually reconfigure
their network with different DNS IP addresses.
6.4. Type of Opt-Out Method
There are several workable methods that can be employed to effect the
actual opt-out for a given user. These include setting a local user
application attribute, such as via a cookie in a web browser, as well
as setting a network attribute, via a DHCP change or manually
configuring the DNS IP addresses (in the operating system, modem,
home gateway device, or router) in order to change the DNS IP
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addresses for a particular user.
While all of these methods are workable and can be made reliable, the
best current method is via a network-based change of some sort. In
this way, all Internet-connected computing devices within a given
household are included in the opt-out (these devices are generally
connected in some manner to the LAN side of some type of customer
premise device, such as a cable modem or DSL modem). This is in
contrast to a method which uses a local user application attribute,
such as a cookie in a web browser, where deletion of cookies, upgrade
to a new operating system, upgrade to a new web browser, use of a
different web browser, or countless other factors on that device
could cause the user to be opted back into a DNS Redirect service.
Thus, a network-based approach which sets opt-out-related attributes
at the device, or household level, is the most inclusive and
persistent method for providing a reliable opt-out method, and is the
recommended practice.
7. Practices to Avoid
This document primarily focuses on the best current practices for an
ISP or ASP to provide users with DNS Redirect services. However, it
is important to note that some entities may not operate in accordance
with such practices. As such, some of these are cataloged below in
order to contrast them with best practices and provide information
which may be of interest and use to the community.
7.1. Improper Redirect of Valid Responses
It has been observed that some service providers improperly utilize
DNS Redirect services when there is a valid DNS resource record
returned in response to a DNS recursive query. The effect is to
redirect users to a server not maintained by the intended
destination, such as a web site that looks like the intended web site
but is not actually the intended site and is instead controlled by
the service provider. For example a DNS query for www.example.com
results in a valid A record response, but this valid response is
instead replaced with an A record controlled by the service provider.
In this case the intended server identified with the valid A record
contained valid, lawful, non-malicious content, and there would
otherwise appear to be no valid justification for a redirect to
occur. See Figure 12 for an example below.
If there is a valid and reasonable justification for such a redirect
to occur, examples of which are not currently known by the authors of
this document, then the resulting connection to the server that the
user has been redirected to should clearly and prominently disclose
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that this is not the intended site. For example, in the case of an
attempt by a user to connect to a web site, the site may contain a
banner or frame which indicates that this is not the intended site or
that the site is in some manner controlled by the service provider.
In addition, such a notice should also offer a clear method to opt-
out of this redirect function.
7.2. Redirect of SERVFAIL Responses
Redirection of SERVFAIL responses should not occur. SERVFAIL
responses may occur intermittently in an operational network for a
variety of highly transient reasons. As a result, a DNS Redirect
should not be performed when a SERVFAIL response is received, as
normal retry a short time later is likely to result in a valid
response.
7.3. Routinely Broken, Purposefully Broken, and Otherwise Unreliable
Opt-Out Mechanisms
There are several well known and dependable methods of opt-out
mechanisms that ISPs and DNS ASPs can deploy for users to opt-out of
their DNS Redirect services. These methods can rather easily be
employed and are highly recommended, as noted in Section 6. However,
some ISPs and DNS ASPs may instead choose to employ a less dependable
mechanism, which routinely fails to work as expected by users or is
known not to function properly.
For example, one routinely unreliable method for opt-out is the
cookie-based method. When a user opts out of a DNS Redirect service,
a cookie is installed in their web browser. The problem with this
method occurs when a user clears their cookies or the cookies are
deleted for some reason. In some cases, users may configure their
web browsers to clear all cookies every time the close their web
browser. Thus, one possible effect upon the user in this case is
that they are once again opted into the redirect service.
Furthermore, a cookie-based method has the effect of only opting out
browser-based protocols (generally HTTP and HTTPS), which means that
the user may have non-web applications affected by DNS Redirect, even
though they believe they have opted-out. As a result, there is no
assured permanency with this opt-out method, nor does it work
consistently across all applications and protocols, which can be
aggravating to users who do not wish to utilize DNS Redirect
services.
Another example of an unreliable method for opt-out is one where opt-
out is tied to the IP address of the user, where that address may be
subject to change on a regular basis, such as via an ISP-based DHCP
lease. In such a case, if opt-out was tied to what can be considered
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a largely dynamic IP address, then the user would be opted-in every
time they received a new IP address, forcing them to repeatedly opt-
out.
7.4. Markedly Slower DNS Query Performance
An ISP or DNS ASP should also understand that DNS query latency, the
time between when a user's stub resolver issues and DNS query and
receives a DNS response, should be kept as low as is reasonably
possible. High DNS query latency is often perceived by users, and
can have an adverse effect on a variety of applications where low DNS
query latency may be especially important. Any additional processing
which must be performed in order to provide DNS Redirect services
should be monitored closely, in order that DNS Redirect functionality
does not markedly slow DNS query performance.
7.5. Override of a User's DNS Selection
Some users may decide to use the DNS server IP addresses of a DNS ASP
or other non-ISP-provided DNS servers. Such selections should be
preserved as the free choice of a user, particularly when DNS
Redirect services are offered. Thus, an ISP should not redirect port
53 DNS traffic from servers intended by the user via their selection
of non-ISP DNS servers to the DNS servers of the ISP, except in
reasonable and justifiable cases where a user has been placed into a
so-called "walled garden" for reasons of abuse, security compromise,
account non-payment, new service activation, etc.
8. Functional Design
The functional design described in this section is intended to be
generally representative of the many different ways that DNS Redirect
services are deployed today. As such, they are necessarily high
level and different implementations may vary somewhat, due to any
number of factors.
8.1. DNS Recursive Resolver
The DNS Recursive Resolver is used by the host computer to translate
fully qualified domain names into IP addresses, according to Section
3.6.1 of [RFC1034]. When a FQDN does not exist in authoritative DNS
a NXDOMAIN response, as described in Section 4.1.1 of [RFC1035] is
normally returned (see Figure 1). In the case of DNS Redirect, the
NXDOMAIN response is changed to reply with a resource record (RR)
response which instructs the host computer to send the original
request to a new IP address (see Figure 1).
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Request Request
www.example.invalid www.example.invalid
+--------+ +--------+
++--++ ------------> | | ------------> | |
|| || | | | |
+-++--++-+ | | | |
+--------+ <------------ | | <------------ | |
Host NXDOMAIN +--------+ NXDOMAIN +--------+
Computer Response Recursive Response Authoritative
Server Server
Figure 1: DNS Redirect Response
8.2. Web Error Landing Server
When a user requests an invalid URL or Domain, their web client is
redirected to a Web Error Landing Server which presents several
possible helpful website views (see Figure 2). The first is "Did you
mean..." response which presents the user with possible correct
results based on their original invalid request. The search server
can also present search engine results to the user.
Request Request
www.example.invalid www.example.invalid
+--------+ +--------+
++--++ ---------------> | | --------------->| |
|| || | | | |
+-++--++-+ | | | |
+--------+ <-------------- | | <------------- | |
Host Redirect +--------+ NXDOMAIN +--------+
Computer IP Address Recursive Response Authoritative
Server Server
|
| ___________________________________
| +--------+ | Web Response: |
| | | | "Did you mean...www.example.com"|
+------> | | ------> |__________________________________|
| | | Search result: #1 |
| | | Search result: #2 |
+--------+ |__________________________________|
Web Server
Landing Page
Figure 2: Web Error Landing Server
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8.3. Web Browser Client
The Web Browser Client is redirected to a Web Server Landing Page
instead of presenting an error page when there is no valid DNS record
present.
Examples of common Web Browser Clients include:
o Microsoft Internet Explorer
o Mozilla Firefox
o Apple Safari
o Google Chrome
o Opera
8.4. Domain White List
There may be certain domains which should be not be redirected under
any circumstances for legal, business, or other reasons. The Domain
White List can contain both domains, such as *.example.com, as well
as specific FQDNs, such as www.example.com. For instance, the owner
of example.com may request that the ISP or DNS ASP not perform DNS
Redirect for the example.com domain, so that there is no DNS Redirect
resulting from queries to nonexistent names, such as
invalid.example.com.
8.5. Malicious Domain List
Using a Malicious Domain List, a DNS server can redirect DNS requests
that were intended for malicious websites or domains to a web server
landing page (see Figure 3). The Malicious Domain List can contain
both domains, such as *.example.net, as well as specific FQDNs, such
as www.example.net.
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Request Request
www.example.net www.example.net
+--------+ +--------+
++--++ ---------------> | | --------------->| |
|| || | | | |
+-++--++-+ | | | |
+--------+ <-------------- | | <------------- | |
Host Redirect +--------+ Response +--------+
Computer IP Address Recursive IP Address Authoritative
Server Server
|
| ___________________________________
| +--------+ | Web Response: |
| | | | "Malware software alert!" |
+-------> | | ------> |__________________________________|
| | | The site you attempted to access |
| | | is known to host malware that |
+--------+ | could damage your computer. |
Web Server |__________________________________|
Landing Page
Figure 3: Malicious Domain Response
8.6. Legally-Mandated DNS Redirect Domain List
Using a Malicious Domain List, a DNS server can redirect DNS requests
that were intended for malicious websites or domains to a web server
landing page (see Figure 3). The Legally-Mandated DNS Redirect
Domain List can contain both domains, such as
*.illegalcontent.example, as well as specific FQDNs, such as
www.illegalcontent.example.net.
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Request Request
www.illegalcontent.example.net www.illegalcontent.example.net
+--------+ +--------+
++--++ ---------------> | | --------------->| |
|| || | | | |
+-++--++-+ | | | |
+--------+ <-------------- | | <------------- | |
Host Redirect +--------+ Response +--------+
Computer IP Address Recursive IP Address Authoritative
Server Server
|
| ___________________________________
| +--------+ | Web Response: |
| | | | "Illegal content alert!" |
+-------> | | ------> |__________________________________|
| | | The site you attempted to access |
| | | has been designated as hosting |
+--------+ | illegal content under XYZ law of |
Web Server | 2009, which your ISP is legally |
Landing Page | compelled to block or be fined. |
|__________________________________|
Figure 4: Legally-Mandated DNS Redirect Domain Response
8.7. Content-Based DNS Redirect Domain List
Using a Content Protection List, a DNS server can redirect DNS
requests that were intended for websites or domains containing
content deemed inappropriate by a user, to a web server landing page
(see Figure 5). The Legally-Mandated DNS Redirect Domain List can
contain both domains, such as *.inappropriate.example, as well as
specific FQDNs, such as www.inappropriate.example.com.
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Request Request
www.inappropriate.example.com www.inappropriate.example.com
+--------+ +--------+
++--++ ---------------> | | --------------->| |
|| || | | | |
+-++--++-+ | | | |
+--------+ <-------------- | | <------------- | |
Host Redirect +--------+ Response +--------+
Computer IP Address Recursive IP Address Authoritative
Server Server
|
| ___________________________________
| +--------+ | Web Response: |
| | | | "Inappropriate content alert!" |
+------> | | ------> |__________________________________|
| | | The site you attempted to access |
| | | has been blocked by you or |
+--------+ | your network administrator. |
Web Server |__________________________________|
Landing Page
Figure 5: Content-Based Redirect Domain Response
9. Example DNS and HTTP Flows
9.1. Successful DNS Lookup and HTTP Flow
This example represents a successful lookup of a valid DNS RR, and
the resulting HTTP transaction.
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Web DNS R DNS A DNS Web Server
Browser Client Server Server 10.1.10.10
| Request | A | | |
|www.example. |Record Query | A | |
| com |www.example. |Record Query | |
|------------>| com |www.example. | |
| |------------>| com | |
| | |------------>| |
| | | A Record | |
| | A Record | 10.1.10.10 | |
| DNS Response| 10.1.10.10 |<------------| |
| 10.1.10.10 |<------------| | |
|<------------| | | |
| HTTP GET | | | |
| 10.1.10.10 | | | |
|------------------------------------------------------>|
| | | | |
| | | | |
| | | | |
Figure 6: Successful DNS Lookup and HTTP Flow
9.2. Unsuccessful DNS Lookup and HTTP Flow
This example represents a lookup of a nonexistent DNS RR, and the
resulting HTTP transaction.
Web DNS R DNS A DNS
Browser Client Server Server
| Request | A | |
|www.example. |Record Query | A |
| invalid |www.example. |Record Query |
|------------>| invalid |www.example. |
| |------------>| invalid |
| | |------------>|
| | | NXDOMAIN |
| | NXDOMAIN |<------------|
| NXDOMAIN |<------------| |
|<------------| | |
| | | |
Figure 7: Unsuccessful DNS Lookup and HTTP Flow
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9.3. DNS Redirect and HTTP Flow
This example represents a lookup of a non-existing DNS RR, and the
HTTP transition that results from a typical DNS Redirect service.
Redirect
Host R DNS A DNS Web Server Web Server
Computer Server Server 10.2.20.20 10.1.10.10
| A | | | |
|Record Query | A | | |
|www.example. |Record Query | | |
| invalid |www.example. | | |
|------------>| invalid | | |
| |------------>| | |
| A Record | NXDOMAIN | | |
| 10.2.20.20 |<------------| | |
|<------------| | | |
| HTTP GET | | | |
| 10.2.20.20 | | | |
|---------------------------------------->| |
| | | HTTP 200 OK | |
|<----------------------------------------| |
| A | | | |
|Record Query | A | | |
|www.example. |Record Query | | |
| com |www.example. | | |
|------------>| com | | |
| |------------>| | |
| | A Record | | |
| A Record | 10.1.10.10 | | |
| 10.1.10.10 |<------------| | |
|<------------| | | |
| HTTP GET | | | |
| 10.1.10.10 | | | |
|------------------------------------------------------>|
| | | | HTTP 200 OK |
|<------------------------------------------------------|
| | | | |
Figure 8: DNS Redirect and HTTP Flow
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9.4. Malicious Site Redirect and HTTP Flow
This example represents a lookup of a valid RR which hosts malware,
and the HTTP transaction that results from a typical Malicious Site
Protection service.
R DNS Mal Redirect
Host R DNS Server Web Server
Computer Server Mal List 10.2.20.20
| A | | |
|Record Query | Mal List | |
|www.example. | Query | |
| net |www.example. | |
|------------>| net | |
| |------------>| |
| | Postivie | |
| A Record | Match | |
| 10.2.20.20 |<------------| |
|<------------| | |
| HTTP GET | | |
| 10.2.20.20 | | |
|---------------------------------------->|
| | | HTTP 200 OK |
|<----------------------------------------|
| | | |
Figure 9: Malicious Site Redirect and HTTP Flow
9.5. Legally-Mandated Redirect and HTTP Flow
This example represents a lookup of a valid RR which hosts illegal
content, and the HTTP transaction that results from a typical
Legally-Mandated DNS Redirect function.
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R DNS Illegal Content
Server Redirect
Host R DNS Illegal Web Server
Computer Server Content List 10.2.20.20
| A | | |
|Record Query | Illegal | |
| www. | Content List | |
|illegalcontent.| | |
|example.net | Query | |
| | www. | |
| |illegalcontent.| |
| |example.net | |
|-------------->| | |
| |-------------->| |
| | Postivie | |
| A Record | Match | |
| 10.2.20.20 |<--------------| |
|<--------------| | |
| HTTP GET | | |
| 10.2.20.20 | | |
|---------------------------------------------->|
| | | HTTP 200 OK |
|<----------------------------------------------|
| | | |
Figure 10: Legally-Mandated Redirect and HTTP Flow
9.6. Content-Based Redirect and HTTP Flow
This example represents a lookup of a valid RR which hosts content
which has been deemed inappropriate by a user, and the HTTP
transaction that results from a typical Content-Based Redirect
function.
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R DNS Inappropriate
Server Content Redirect
Host R DNS Inappropriate Web Server
Computer Server Content List 10.2.20.20
| A | | |
|Record Query | Illegal | |
| www. | Content List | |
|inappropriate. | | |
|example.com | Query | |
| | www. | |
| |inappropriate. | |
| |example.com | |
|-------------->| | |
| |-------------->| |
| | Postivie | |
| A Record | Match | |
| 10.2.20.20 |<--------------| |
|<--------------| | |
| HTTP GET | | |
| 10.2.20.20 | | |
|---------------------------------------------->|
| | | HTTP 200 OK |
|<----------------------------------------------|
| | | |
Figure 11: Content-Based Redirect and HTTP Flow
9.7. Improper Redirect of Valid Response Redirect and HTTP Flow
This example represents an improper redirect occurring when a valid
DNS RR should have been returned in response to a DNS recursive query
for an example website, the resulting HTTP transaction, and that no
DNS query or HTTP traffic was sent to the valid authoritative DNS
server and valid web server. Section 10 below shows one of the
reasons why this practice is problematic. Another reason is that a
user intends to visit a valid resource with lawful and legitimate
content, such as a web site, and is instead sent to a different
destination (which may even closely resemble the intended site, in
the pattern used by phishing sites).
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R DNS Improper Valid
Server Redirect Valid Web
Host R DNS Improper Web Server A DNS Server
Computer Server Reirect List 10.2.20.20 Server 10.1.10.10
| A | Improper | | | |
|Record Query |Redirect List| | | |
|www.example. | Query | | | |
| com |www.example. | | | |
|------------>| com | | | |
| |------------>| | | |
| | Postivie | | | |
| A Record | Match | | | |
| 10.2.20.20 |<------------| | | |
|<------------| | | | |
| HTTP GET | | | | |
| 10.2.20.20 | | | | |
|-------------------------------------->| | |
| | |HTTP 200 OK| | |
|<--------------------------------------| | |
| | | | | |
Figure 12: Improper Redirect of Valid Response Redirect and HTTP Flow
10. DNSSEC Considerations
DNS security extensions defined in [RFC4033], [RFC4034], and
[RFC4035] use cryptographic digital signatures to provide origin
authentication and integrity assurance for DNS data. This is done by
creating signatures for DNS data on a Security-Aware Name Server that
can be used by Security-Aware Resolvers to verify the answers. As
the DNS redirections described herein take place on the recursive
server there is no need to look into the communication between the
recursive resolvers and name servers. Depending on the security
awareness of recursive server used to perform DNS Redirect services,
as well as the security awareness of the stub resolvers the following
impact is observed on DNS Redirect when giving out answers to fully
secured zones or parent zones:
o Security-Oblivious Resolver with Validating and Non-Validating
Stub Resolvers: Since the resolver is not security-aware, it will
not pass any DNSSEC-related packets and, thus, even the validating
stub resolver cannot validate the records and will present the DNS
Redirect answers to the application.
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o Security-Aware Resolver with Non-Validating Stub Resolver: As the
security-aware resolver does not have the key generated the
resource record signatures (RRSIG) for it's response it should
give the redirected answer out as indeterminate or insecure. As
the stub resolver is not doing any validation it will use the DNS
Redirect response and pass them on to the application.
o Security-Aware Resolver with Validating Stub Resolver: As the
security-aware resolver does not have the key generated the
resource record signatures (RRSIG) for it's response it should
give the redirected answer out as indeterminate or insecure.
However, as the validating stub has a DNSKEY record for the zone
or a DS record for the parent zone it cannot validate the answer
and will report it as bogus to the application leading to non-
resolution for that domain.
So the only case where DNS security extensions cause problems for DNS
Redirect is with a validating stub resolver. This case doesn't have
widespread deployment now and could be mitigated by using trust
anchor, configured by the applicable ISP or DNS ASP, that could be
used to sign the redirected answers. As noted above in Section 9.7,
such improper redirection of valid responses may also cause DNSSEC
trust verification problems.
11. Security Considerations
Security best practices should be followed regarding access to the
opt-in and opt-out functions, in order that someone other than the
user is able to change the user's DNS Redirect settings. For
example, the User Options Web Server must not permit someone to
modify a page URI to access and change another user's options. Thus,
if the URI is
"http://www.example.net/redirect-options.php?account=1234", someone
must not be able to modify the account to be "=1235" and then be able
to change the options for a different user with some other additional
validation being performed. While web site security practices are
outside the scope of this document, the authors believe it is
important to identify such problematic use cases to any ISPs and DNS
ASPs offering and/or implementing DNS Redirect functionality.
12. IANA Considerations
There are no IANA considerations in this document.
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13. Contributors
The following people made significant textual contributions to this
document and played an important role in the development and
evolution of this document:
Don Bowman, Sandvine (don@sandvine.com)
Rick Hiester, Verizon (richard.hiester@verizon.com)
Chris Roosenraad, Time Warner Cable (chris.roosenraad@twcable.com)
David Ulevitch, OpenDNS (david@opendns.com)
14. Acknowledgements
The authors and contributors also wish to acknowledge the assistance
of the following individuals in helping us to develop and/or review
this document:
John Barnitz, Comcast Cable Communications
(john_barnitz@cable.comcast.com)
Mike Burns, Cablevision (mburns@cablevision.com)
Phil Marcella, Comcast Interactive Media
(phillip_marcella@cable.comcast.com)
Luis Uribarri, Comcast Cable Communications
(luis_uribarri@cable.comcast.com)
Sandy Wilbourn, Nominum (sandy.wilbourn@nominum.com)
Matt Williams, Cox Cable (matt.williams@cox.com)
And another contributor...
15. 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.
[RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
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Miller, "Common DNS Implementation Errors and Suggested
Fixes", RFC 1536, October 1993.
[RFC1591] Postel, J., "Domain Name System Structure and Delegation",
RFC 1591, March 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
Appendix A. Document Change Log
[RFC Editor: This section is to be removed before publication]
-00 version:
o first version published
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Appendix B. Open Issues
[RFC Editor: This section is to be removed before publication]
1. RW: Consider whether it is a good idea to add to section 4.9
(NXDOMAIN RESPONSE) a reference to Authenticated Denial of
Existence described in RFC4035 section 5.4 as these should be
also redirected.
2. MB: Consider addressing how opt-out works when a user roams
across a shared WiFi AP.
3. RH: Update reference to RFC2535, which is obsoleted by RFCs 4033,
4034, 4035.
4. JL: Consider capitalizing RFC 2119 language used.
5. JL: Need additional review and development of the DNSSEC section.
Could probably benefit by having a DNSSEC expert perform a review
of that section and offer suggestions.
Authors' Addresses
Tom Creighton
Comcast Cable Communications
One Comcast Center
1701 John F. Kennedy Boulevard
Philadelphia, PA 19103
US
Email: tom_creighton@cable.comcast.com
URI: http://www.comcast.com
Chris Griffiths
Comcast Cable Communications
One Comcast Center
1701 John F. Kennedy Boulevard
Philadelphia, PA 19103
US
Email: chris_griffiths@cable.comcast.com
URI: http://www.comcast.com
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Jason Livingood (editor)
Comcast Cable Communications
One Comcast Center
1701 John F. Kennedy Boulevard
Philadelphia, PA 19103
US
Email: jason_livingood@cable.comcast.com
URI: http://www.comcast.com
Ralf Weber
Unaffiliated
Bleichgarten 1
Hohenahr-Hohensolms 35644
Germany
Email: rw@hohensolms.de
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