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Internet Engineering Task Force T. Creighton
Internet-Draft C. Griffiths
Intended status: Informational J. Livingood
Expires: April 25, 2011 Comcast
R. Weber
Unaffiliated
October 22, 2010
DNS Redirect Use by Service Providers
draft-livingood-dns-redirect-03
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 servers, as well as to describe the recommended
practices regarding relating to DNS redirect. This document
specifically and narrowly addresses those cases where DNS Redirect is
being utilized to provide a web error redirect service to end users,
and describes the critical implications for DNS Redirect when DNSSEC
is deployed.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2011.
Copyright Notice
Copyright (c) 2010 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
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Provisions Relating to IETF Documents
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This document may contain material from IETF Documents or IETF
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Without obtaining an adequate license from the person(s) controlling
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Table of Contents
1. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Document Scope . . . . . . . . . . . . . . . . . . . . . . . . 4
4. DNSSEC Considerations and Implications . . . . . . . . . . . . 5
5. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Web Error Redirect . . . . . . . . . . . . . . . . . . . . . . 7
7. Opt-In or Opt-Out Mechanisms . . . . . . . . . . . . . . . . . 8
7.1. Opt-Out . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.2. Opt-In . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.3. Automated Mechanisms and Reasonable Processing Times . . . 9
7.4. Type of Opt-Out Method . . . . . . . . . . . . . . . . . . 9
8. Practices to Avoid . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Use of DNS Redirect with DNSSEC . . . . . . . . . . . . . 10
8.2. Improper Redirect of Valid Responses . . . . . . . . . . . 10
8.3. Redirect of SERVFAIL Responses . . . . . . . . . . . . . . 11
8.4. Routinely Broken, Purposefully Broken, and Otherwise
Unreliable Opt-Out Mechanisms . . . . . . . . . . . . . . 11
8.5. Markedly Slower DNS Query Performance . . . . . . . . . . 12
8.6. Override of a User's DNS Selection . . . . . . . . . . . . 12
9. Functional Design . . . . . . . . . . . . . . . . . . . . . . 12
10. Example DNS and HTTP Flows . . . . . . . . . . . . . . . . . . 15
11. Security Considerations . . . . . . . . . . . . . . . . . . . 18
12. Controvery Surrounding DNS Redirect . . . . . . . . . . . . . 19
13. Future Prospects for DNS Redirect . . . . . . . . . . . . . . 19
14. Why This Document Merits Publishing . . . . . . . . . . . . . 20
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
16. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 21
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
18.1. Normative References . . . . . . . . . . . . . . . . . . . 22
18.2. Informative References . . . . . . . . . . . . . . . . . . 22
Appendix A. Document Change Log . . . . . . . . . . . . . . . . . 23
Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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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 over time created " enhanced " DNS
services for their users, which often rely upon DNS Redirect
functionality. These enhanced services, which are offered on an
opt-in or opt-out basis, can perform a number of enhanced services
for users, such as attempting to interpret web address errors when an
invalid fully qualified domain name (FQDN, Section 5.1 of [RFC1035])
has been typed by a user.
This document describes the design and function of a DNS Redirect
service, as well as recommended practices and practices to avoid. It
also describes the critical implications for DNS Redirect when DNSSEC
is adopted, in Section 4.
3. Document Scope
This document focuses on the systems and 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.
There are several ways that such entities can provide users with
these enhanced DNS 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 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
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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.
Lastly, while Section 7 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.
4. DNSSEC Considerations and Implications
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 DNS Security-Aware
Authoritative Name Server that can be used by DNS Security-Aware
Resolvers to verify the answers.
DNSSEC is now in the process of being deployed on authoritative
servers, now that the DNS root has been signed and several key Top
Level Domains (TLDs) have been signed. DNSSEC is also starting to be
adopted by service providers, which are now in the process of adding
DNSSEC validation in DNS recursive resolvers.
It is critically important that service providers understand that
adoption of DNSSEC is technically incompatible with DNS redirect. As
such, in order to properly implement DNSSEC and maintain a valid
chain of trust, DNS redirect MUST NOT be used any longer. Thus, once
DNSSEC is in widespread use, this document should be considered
historical. That being said, sections of this document concerning
opt-in and opt-out practices may be useful for future reference in
other, unrelated documents.
5. Terminology
While these terms are generally well known, it is important to define
them in the context of this document.
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5.1. 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, or 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.
5.2. 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
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.
5.3. 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.
5.4. 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 provided by ISPs for name
resolution.
5.5. 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.
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5.6. 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.
5.7. 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 7 section.
5.8. 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], and Section 1 of [RFC2308].
6. 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.
6.1. 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.
6.2. 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
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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 5 for examples below.
6.3. 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.
7. 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 8 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.
7.1. 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
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.
7.2. 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.
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7.3. 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.
7.4. 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
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.
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8. Practices to Avoid
This document primarily focuses on the recommended 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 catalogued below in
order to contrast them with recommended practices and provide
information which may be of interest and use to the community.
8.1. Use of DNS Redirect with DNSSEC
When DNSSEC has been implemented in a service provider's resolvers,
DNS redirect MUST NOT be used, as it is technically incompatible with
DNSSEC and breaks the chain of trust critical to proper DNSSEC
validation functionality.
8.2. 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 6 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
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.
Thus, to summarize, redirection of valid responses SHOULD NOT be
performed.
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8.3. 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.
8.4. 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 7. 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
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.
Thus, to summarize, the opt-out mechanism provided to users SHOULD be
reliable and SHOULD NOT be routinely broken, purposefully broken, or
otherwise unreliable.
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8.5. 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 a 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.
Thus, to summarize, when DNS redirect is performed, DNS query
performance SHOULD NOT suffer as a result, since this could provide
an incrementally inferior user experience as compared to when DNS
redirect is not performed.
8.6. 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.
An exception to this is when, unbeknownst to the user, malicious
software (malware) has changed the IP address of their DNS server to
a known malicious DNS server. In such cases, it may be in the best
interests of the user to take steps to ensure they do not use such a
malicious DNS server, particularly since they did not intend to do so
and may be infected with malware. While this is unrelated to DNS
Redirect per se, it merits mentioning based on feedback received from
the security community.
9. 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.
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9.1. 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).
Request Request
www.example.invalid www.example.invalid
+--------+ +--------+
++--++ ------------> | | ------------> | |
|| || | | | |
+-++--++-+ | | | |
+--------+ <------------ | | <------------ | |
Host NXDOMAIN +--------+ NXDOMAIN +--------+
Computer Response Recursive Response Authoritative
Server Server
Figure 1: DNS Redirect Response
9.2. 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.
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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
9.3. 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
9.4. Domain White List
There may be certain domains which should be not be redirected under
any circumstances for technical, 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.
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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.
10. Example DNS and HTTP Flows
This section shows several illustrated examples of DNS and HTTP
flows, in order to better explain certain DNS and HTTP use cases.
10.1. Successful DNS Lookup and HTTP Flow
This example represents a successful lookup of a valid DNS RR, and
the resulting HTTP transaction.
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 3: Successful DNS Lookup and HTTP Flow
10.2. Unsuccessful DNS Lookup and HTTP Flow
This example represents a lookup of a nonexistent DNS RR, and the
resulting HTTP transaction.
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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 4: Unsuccessful DNS Lookup and HTTP Flow
10.3. 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.
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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 5: DNS Redirect and HTTP Flow
10.4. 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 4 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
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even closely resemble the intended site, in the pattern used by
phishing sites).
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 6: Improper Redirect of Valid Response Redirect and HTTP Flow
11. Security Considerations
The critical considerations relating to DNS Security Extensions are
detailed in Section 4.
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.
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12. Controvery Surrounding DNS Redirect
It is clear based on the community feedback that this document has
elicited, and from debates which have occurred over the years prior
to this document, that DNS Redirect is a controversial topic. Views
on whether DNS Redirect should be performed or not vary widely. Some
feel strongly that it is a valid practice from which end users derive
benefits. Some others feel that DNS Redirect should not be performed
and that it puts at risk trust in and stability of the DNS. Others
in the community are neutral on the topic and have expressed the view
that as long as DNS Redirect does not slow the deployment of DNSSEC,
that it is transparently disclosed to end users, and that those end
users have easy opt-out methods, that it is an acceptable, or at
least tolerated, practice. This moderate view is probably the
majority view, though critics of DNS Redirect have expressed their
views firmly and many of those holding such strongly critical views
have played and continue to play a key role in DNS protocols and
other critical areas of the IETF and the Internet community. Some
strong critics also describe resolvers that perform DNS Redirect as
"lying resolvers", explaining that the accurate and therefore honest
response is an NXDOMAIN response and that anything else is not
intended and is considered a lie.
Thus, it is important for readers to understand that DNS Redirect
remains a practice which is subject to some controversy and that
there is not strong consensus in support of it. At best, there is
what could be described as grudging acceptance of the practice if it
has been implemented along the lines recommended in this document.
In addition, many critics take solace in the view that as DNSSEC is
increasingly deployed that DNS Redirect is likely to decline
correspondingly over time.
Finally, any provider implementing DNS Redirect is well advised to
follow the recommendations outlined herein. This is because many
critics of DNS Redirect have explained that their strong views
developed or deepened when they observed that some implementers have
deployed systems which fail to provide an easy and/or reliable opt-
out method, redirect valid responses, or follow practices noted as
ones to avoid in Section 8.
13. Future Prospects for DNS Redirect
As noted in Section 4, there exists today a technical incompatibility
between DNSSEC and DNS Redirect. While it is possible that some
provider implementing DNS Redirect today will uncover a method to
implement DNSSEC and DNS Redirect, currently these two functions do
not go well together. As such, providers will soon or are now facing
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a decision between embracing and deploying DNSSEC or continuing to
perform DNS Redirect. It is likely that many of these providers will
choose to deploy DNSSEC, as some already are doing [Comcast DNSSEC
Rollout Announced]. It is also possible that some others will deploy
DNSSEC-validating DNS recursive resolvers alongside those resolvers
performing DNS Redirect, giving their customers the choice between
the two. In this case, it is likely that over time customers will
express a preference for greater levels of Internet security,
including DNSSEC and other forms of security, and that their
respective service providers will evolve their offerings to satisfy
these customer needs.
14. Why This Document Merits Publishing
Documentation of DNS Redirect is beneficial for the Internet
community in and of itself. Prior to this document, the IETF lacked
a stable reference document that described how DNS Redirect was
designed and implemented, even though the practice has become
relatively widespread. As a result, one benefit of publishing this
is to document the design and implementation of DNS Redirect on the
Internet.
An additional benefit of the document is to guide those providers
which decide to implement DNS Redirect on what practices should be
avoided and what steps should be taken to ensure that end users are
able to easily and reliably opt out of such a system. While it does
not seem appropriate, based on the community response noted in
Section 12, to identify this as a Best Current Practices (BCP)
document, this can guide implementers on what to avoid and on "least
worst" practices as some in the community have described them.
Furthermore, this documents other key aspects of DNS Redirect that
are valuable. For example, this document encourages transparency and
disclosure of DNS Redirect practices on the Internet, and notes
community concerns and controversy regarding the practice, all of
which may be valuable to refer back to in future documents or to
refer to during future debates in the Internet community. There will
also be future systems which call for well functioning opt-out
systems. For such systems, the sections referring to automated opt-
out mechanisms and reasonable processing times of opt-out requests,
in Section 7.3, and the reliability of opt-out systems, in
Section 8.4, may be valuable. Finally, Section 8.6 describes that a
user's selection of an alternative DNS server IP address should not
be overridden, which seems an important principle to highlight.
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15. IANA Considerations
There are no IANA considerations in this document.
16. 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)
17. 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)
The authors and contributors also wish to thank ICANN's Security and
Stability Advisory Committee (SSAC) for their review and debate of
this document, as well as for raising important questions concerning
DNSSEC compatibility.
18. References
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18.1. 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.
[RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
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.
[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.
18.2. Informative References
[Comcast DNSSEC Rollout Announced]
Livingood, J., "DNS Security Rollout Begins", Comcast
Blog , October 2010, <http://blog.comcast.com/2010/10/
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dns-security-rollout-begins.html>.
Appendix A. Document Change Log
[RFC Editor: This section is to be removed before publication]
-03: Changed some text to make it more neutral, and introduced new
sections: Controvery Surrounding DNS Redirect, Future Prospects for
DNS Redirect, and Why This Document Merits Publishing. Also closed
out an open issue to remove references to RFC 2535, which is
obsolete. Lastly, updated the section 8.6 on override of user DNS
choices to note a malware case raised during a document review.
-02: Fixed some small grammatical nits.
-01: Removed sections regarding malicious domain protection, legally-
mandated redirect, and content-based redirect based on DNSOP WG
feedback to split those out into separate documents which will be
published in the future. Also significantly modified the DNSSEC
section and moved it to the top of the document. Also, capitalized
applicable 2119 language.
-00: First version published.
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. Ensure that references are in the appropriate section (normative
vs. informative).
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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
Jason Livingood
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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