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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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   This Internet-Draft will expire on January 7, 2010.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the



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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

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 &quot enhanced &quot 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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