Skip to main content

DNS Response Policy Zones

The information below is for an old version of the document.
Document Type This is an older version of an Internet-Draft whose latest revision is Replaced
Authors Paul A. Vixie , Vernon Schryver
Last updated 2016-12-16
Replaced by draft-ietf-dnsop-dns-rpz
Stream Internet Engineering Task Force (IETF)
Stream WG state Candidate for WG Adoption
Document shepherd (None)
IESG IESG state I-D Exists
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD (None)
Send notices to (None)
Domain Name System Operations                                   P. Vixie
Internet-Draft                                   Farsight Security, Inc.
Intended status: Informational                               V. Schryver
Expires: June 19, 2017                                 Rhyolite Software
                                                       December 16, 2016

                       DNS Response Policy Zones


   This document describes a method for expressing DNS response policy
   inside a specially constructed DNS zone, and for recursive name
   servers to use such poicy to return modified results to DNS clients.
   The modified DNS results can stop access to selected HTTP servers,
   redirect users to "walled gardens," block objectionable email, and
   otherwise defend against attack.  These "DNS Firewalls" are widely
   used in fighting Internet crime and abuse.

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

   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 June 19, 2017.

Copyright Notice

   Copyright (c) 2016 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
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

Vixie & Schryver          Expires June 19, 2017                 [Page 1]
Internet-Draft                   DNS RPZ                   December 2016

   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may not be modified, and derivative works of it may not
   be created, except to format it for publication as an RFC or to
   translate it into languages other than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Discussion Venue  . . . . . . . . . . . . . . . . . . . .   3
   2.  Zone Format . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Policy Actions  . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  The "NXDOMAIN" Action . . . . . . . . . . . . . . . . . .   4
     3.2.  The "NODATA" Action . . . . . . . . . . . . . . . . . . .   4
     3.3.  The "PASSTHRU" Action . . . . . . . . . . . . . . . . . .   5
     3.4.  The "DROP" Action . . . . . . . . . . . . . . . . . . . .   5
     3.5.  The "TCP-Only" Action . . . . . . . . . . . . . . . . . .   5
     3.6.  The "Local Data" Action . . . . . . . . . . . . . . . . .   6
   4.  Policy Triggers . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  The "Client IP Address" trigger (.rpz-client-ip)  . . . .   7
     4.2.  The "QNAME" trigger ("") . . . . . . . . . . .   8
     4.3.  The "Response IP address" trigger (.rpz-ip) . . . . . . .   8
     4.4.  The "NSDNAME" trigger (.rpz-nsdname)  . . . . . . . . . .   9
     4.5.  The "NSIP" trigger (.rpz-nsip)  . . . . . . . . . . . . .  10
   5.  Application of the Policy . . . . . . . . . . . . . . . . . .  11
     5.1.  Precedence Rules. . . . . . . . . . . . . . . . . . . . .  12
   6.  Subscriber Behavior . . . . . . . . . . . . . . . . . . . . .  14
   7.  Producer Behavior . . . . . . . . . . . . . . . . . . . . . .  15
   8.  History and Evolution . . . . . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     12.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   This document describes DNS Firewalls, a method of expressing DNS
   [RFC1034] policy information inside specially constructed DNS zones,

Vixie & Schryver          Expires June 19, 2017                 [Page 2]
Internet-Draft                   DNS RPZ                   December 2016

   known as Response Policy Zones (RPZs).  RPZs allow DNS reputation
   data producers and subscribers to cooperate in the application of
   policies to modify DNS responses in real time.  Using the policy
   information, DNS resolution for low-reputation DNS data can be made
   to deliberately fail or to return local data such as an alias to a
   "walled garden".

   A site's DNS response policy consists of the set of rules expressed
   in all of the RPZs that it uses.  Each rule, expressed as an RRset,
   consists of a trigger and an action.  A full description of the
   expressible policies is given in Section 3 (actions) and Section 4
   (triggers), while Section 6 explains how the rules are applied.

   Configuration examples are given using ISC BIND Version 9 (BIND9)
   [ISC-ARM] syntax, because work to add RPZ to that platform was
   started earliest (in 2009).  The RPZ specification itself is free to
   implement and free to use in operation.  It has been implemented in
   other name server software.  We expect that in time, additional
   recursive DNS implementations will also implement DNS Firewalls as
   described by this RPZ specification.

1.1.  Discussion Venue

   The discussion venue for this document is the DNS Firewalls mailing
   list. offers
   subscriptions and archives.  See also

   [NOTE TO EDITOR: This section must be removed before this Internet
   Draft is published as an RFC.]

2.  Zone Format

   A DNS Response Policy Zone (RPZ) is a DNS zone.  Like any DNS zone,
   an RPZ consists of RRsets or sets of resource records (RRs) with a
   common owner name and type.  RRsets other than SOA and NS specify
   actions and triggers.  The owner name (left hand side) of each RRset
   expresses a policy trigger, while the RDATA (right hand side) encodes
   the action to taken when the trigger matches.  Depending on the type
   of trigger (see Section 4), a particular characteristic of the DNS
   query or response is checked.

   Because an RPZ is a valid DNS zone, its contents can be transferred
   between DNS servers in whole (AXFR) [RFC5936] or incrementally as
   changes occur (IXFR) [RFC1995], authenticated and protected by TSIG
   transaction signatures [RFC2845] and expedited by real time change
   notifications (NOTIFY) [RFC1996], all subject to familiar DNS access
   controls.  An RPZ need not support query access since query access is
   never required.  It is the zone transfer of RPZ content from

Vixie & Schryver          Expires June 19, 2017                 [Page 3]
Internet-Draft                   DNS RPZ                   December 2016

   producers to subscribers which effectively publishes the policy data,
   and it is the subscriber's server configuration which promotes RPZ
   payload data into DNS control plane data.

   Any valid DNS zone (including an RPZ) is required to have an SOA
   record and at least one NS record at its apex, which is why the SOA
   and NS records of an RPZ cannot themselves be used to encode DNS
   response policy.

   The RPZ's SOA record is real, with a serial number used for NOTIFY
   and IXFR, and timers used for AXFR and IXFR.  The MNAME field or
   domain name of the primary source of the zone and the RNAME field or
   mailbox of the person responsible for the zone are often used by RPZ
   providers to label their policy zones.

   As for an RPZ's apex NS record(s), since query access is never
   required, they will never be used.  Similarly, no parent delegation
   is required for normal operation of the RPZ.  Thus, by convention, a
   single NS record having the deliberately bogus RDATA of "LOCALHOST."
   is used as a placeholder.

   The format of RPZs has undergone several revisions since work began
   (see Section 8).  All POLICY described here are from RPZ Format 1
   unless otherwise noted.  Policy triggers from a higher format number
   than a recursive name server's implementation level are expected to
   be invisible to that implementation.  Policy actions from a higher
   format number are likely to be misinterpreted as CNAME local data by
   older implementations.

3.  Policy Actions

   An RPZ resource record can specify any of six results or actions.

3.1.  The "NXDOMAIN" Action

   A single resource record (RR) consisting of a CNAME whose target is
   the root domain (.) will cause a response of NXDOMAIN to be returned.
   This is the most commonly used RPZ action.

3.2.  The "NODATA" Action

   A single RR consisting of a CNAME whose target is the wildcard top-
   level domain (*.) will cause a response of NODATA (ANCOUNT=0) to be
   returned regardless of query type.

Vixie & Schryver          Expires June 19, 2017                 [Page 4]
Internet-Draft                   DNS RPZ                   December 2016

3.3.  The "PASSTHRU" Action

   It is sometimes necessary to exempt some DNS responses from the
   response policy rule that covers an entire domain or a large IP
   address block.  Exempting some clients of a DNS resolver from all RPZ
   rewriting can also be useful for research into attackers and for
   debugging.  The PASSTHRU action is intended to override other,
   usually more general policies.  It should be written so that it
   appears at a higher precedence than the policies it must override.
   See Section 5.1 about the precedence rules.

   This policy zone record

       CNAME   rpz-passthru.

   would exempt requests for from the NXDOMAIN policy or
   action of the following record:

          CNAME   .
           *       CNAME   .

   The deprecated original encoding of the PASSTHRU action was a CNAME
   with a target equal to the QNAME field of the DNS request.  That
   encoding could not be used with some desirable triggers.

3.4.  The "DROP" Action

   The "DROP" policy that consists of discarding the request and
   response is specified by a CNAME whose target is "rpz-drop".  For
   example, with

          CNAME   rpz-drop.

   nothing is sent to a DNS client trying to resolve, not
   even a DNS error response.

3.5.  The "TCP-Only" Action

   The "TCP-Only" policy is specified by a CNAME whose target is
   "rpz-tcp-only".  It changes UDP responses to short, truncated DNS
   responses that require the DNS client to try again with TCP.  It is
   used to mitigate distributed DNS reflection attacks and is similar to
   the "slip" parameter of DNS Response Rate Limiting (RRL) [ISC-RRL].

Vixie & Schryver          Expires June 19, 2017                 [Page 5]
Internet-Draft                   DNS RPZ                   December 2016

3.6.  The "Local Data" Action

   An RRset that is neither a special RPZ encoding of an action nor one
   of several problematic record types specifies local data used to
   generate synthetic DNS responses.  The special RPZ encodings are
   CNAMEs with targets of NXDOMAIN (.), NODATA (*.), a top level domain
   starting with "rpz-", or a child of a top level domain starting with
   "rpz-".  Problematic record types include NS and DNSSEC (see
   [RFC4034]), because their appearance in responses would be invalid or
   confuse DNS clients.  Local data DNAME RRsets are also commonly
   rejected by RPZ subscribers for internal implementation and other
   reasons.  If any local data policy actions are present, then any
   request for an RR type that is not present in the local data is
   answered as NODATA (ANCOUNT=0) as if the recursive DNS server using
   RPZ were authoritative for the query name.

   The most common local data is a CNAME RR pointing to a walled garden.
   Such CNAME RRs are distinguishable from other rpz actions, because
   the CNAME target name will not be the root (.), nor the root wildcard
   (*.), nor be a subdomain of a top level domain that starts with

   A special form of local data involves a CNAME RR with a wildcarded
   target name.  Wildcards are not valid as CNAME targets in ordinary
   DNS zones.  This special form causes the QNAME to be prepended to the
   wildcarded target to communicate the triggering QNAME value to the
   walled garden DNS server.  For example a policy action of
   "CNAME *.EXAMPLE.COM" and a query name of "EVIL.ORG." will result in
   a synthetic response of "EVIL.ORG CNAME EVIL.ORG.EXAMPLE.COM."  The
   purpose for this special form is query logging in the walled garden's
   DNS server.

4.  Policy Triggers

   There are five types of RPZ triggers, and they are encoded by RRset
   owner names (left hand sides) in an RPZ.

   Two of these types of trigger match characteristics of the DNS query:
   "Client IP address" and "QNAME".  They are independent of cache
   contents or recursion results, but must be checked conceptually when
   the response is ready, including after any needed recursion.
   Recursion can sometimes be skipped, but only if the RPZ result would
   not be changed (see Section 5.1).

   The other three types of triggers are based on characteristics of the
   DNS response, that is, on the RDATA to be returned to the client, or
   in some cases, on NS-related RDATA used while recursively obtaining
   the final response, regardless of whether or not those NS records or

Vixie & Schryver          Expires June 19, 2017                 [Page 6]
Internet-Draft                   DNS RPZ                   December 2016

   additional data are themselves to be returned to the client.  These
   three trigger types are: "Response IP address", "NSDNAME", and

   All policies are conceptually applied after recursion, so that the
   recursive DNS resolver's cache contains either nothing or "truth,"
   even if this truth is hidden by current policy.  If the policy
   changes, the original, unmodified data is available for processing
   under the changed policy.

4.1.  The "Client IP Address" trigger (.rpz-client-ip)

   The IP addresses of DNS clients sending requests can be used as
   triggers, which can be useful for disabling RPZ rewriting for DNS
   clients used to test or investigate.  Client IP address policy RRsets
   have owner names that are subdomains of "rpz-client-ip" relativized
   to the RPZ apex name, preceded by an encoded IP address or block of

   For example, the following would drop all requests from clients in and give truthful answers to requests from a client at

       CNAME rpz-drop.
           128.3.zz.db8.2001.rpz-client-ip CNAME rpz-passthru.

4.1.1.  IP address encoding in triggers

   The IPv4 address (or address block) "B1.B2.B3.B4/prefix" is encoded
   in an RPZ trigger as "prefix.B4.B3.B2.B1", with the address octets
   reversed just as in the IN-ADDR.ARPA naming convention.  (See
   [RFC1034].)  The prefix length ("prefix") must be between 1 and 32.
   All four bytes, B4, B3, B2, and B1, must be present and must be
   written in decimal ASCII.

   For example, the address block would be encoded as

   The IPv6 address (or address block beginning at)
   "W1:W2:W3:W4:W5:W6:W7:W8" is encoded in a format similar to the
   standard IPv6 text representation (see [RFC5952]), again reversed:
   "prefix.W8.W7.W6.W5.W4.W3.W2.W1".  Each of W8,...,W1 is a one- to
   four-digit hexadecimal ASCII number representing 16 bits of the IPv6
   address with no leading zeroes.  All 8 words must be present unless a
   "zz" label is present.  The "zz" label is analogous to the double-
   colon (::) in the standard IPv6 address representation.  The "zz"
   label is expanded to zero-fill the middle portion of the IPv6

Vixie & Schryver          Expires June 19, 2017                 [Page 7]
Internet-Draft                   DNS RPZ                   December 2016

   address.  Exactly one "zz" label must be present if there are two or
   more consecutive zero words in the address.  The prefix length
   ("prefix") must be between 1 and 128

   For example, the address 2001:db8::3 (with implied prefix length 128)
   would be encoded as "128.3.zz.db8.2001".

4.2.  The "QNAME" trigger ("")

   The QNAME policy trigger matches on requested domains, the QNAME
   field in the question section of DNS requests and responses.  (See
   [RFC1035].)  The owner name of an RPZ QNAME policy RRset is the
   relativized name of the domain name about which policy is being
   expressed.  For example, if the RPZ apex name is RPZ.EXAMPLE.ORG, an
   RRset at would affect responses to
   requests about

   Wildcards also work, and so the owner name
   "*" would trigger on queries to any
   subdomain of  To control the policy for both a name and
   its subdomains, two policy RRsets must be used, one for the domain
   itself and another for a wildcard subdomain.  In the following
   example, queries for both and all subdomains of will result in synthetic NXDOMAIN responses.

           CNAME   .
           *        CNAME   .

4.3.  The "Response IP address" trigger (.rpz-ip)

   The response IP policy trigger matches response contents (RDATA): it
   matches IP addresses that would otherwise appear in A and AAAA
   records in the answer section of a DNS response.  IP addresses in the
   authority and additional sections are not considered.  Response IP
   policy RRsets have owner names that are subdomains of "rpz-ip"
   relativized to the RPZ apex name, and an encoded IP address or block
   of addresses.  The IP address encodes are identical to those
   described in Section 4.1.1for Client IP Address triggers.

   For example, to force an NXDOMAIN response whenever a truthful
   response would contain an answer section A RRset having an address in unless address is present, the RPZ would
   contain these records:

    CNAME   .
    CNAME   rpz-passthru.

Vixie & Schryver          Expires June 19, 2017                 [Page 8]
Internet-Draft                   DNS RPZ                   December 2016

   In another example, to answer NODATA (ANCOUNT=0) whenever a truthful
   response would contain an answer AAAA RRset having an address
   2001:db8:101::/48 unless address 2001:db8:101::3 was present, the RPZ
   would contain these records:

           48.zz.101.db8.2001.rpz-ip       CNAME   *.
           128.3.zz.101:db8.2001.rpz-ip    CNAME   rpz-passthru.

   Please refer to Section 5.1 to understand how the above exception
   mechanims work.

4.4.  The "NSDNAME" trigger (.rpz-nsdname)

   The NSDNAME policy trigger matches name server names (NS RR) of any
   name server which is in the data path for an RRset present in the
   answer section of a DNS response.  The data path is all delegation
   points from (and including) the root zone to the closest enclosing NS
   RRset for the owner name of the answering RRset.

   In other words, an NSDNAME trigger is checked by first considering
   the named servers (domain names in the NS records) for the query
   domain (QNAME), then the name servers for the parent of the query
   domain name, and so on until the name servers for the root (.) have
   been checked or there fewer periods (.) in the domain name than the
   value of a local "min-ns-dots" parameter.  See Section below.

   NSDNAME policies are encoded as RRsets in subdomains of "rpz-nsdname"
   but otherwise much like QNAME policies (xref target="qname"/>).  For
   example, to force an NXDOMAIN answer whenever a name server for the
   requested domain or one of its parents is, the RPZ
   would contain the following:

    CNAME .

4.4.1.  Implementation considerations for NSDNAME triggers

   Note that these considerations apply also to NSIP triggers described
   in Section 4.5 below.  Performance issues

   The process of traversing the data path from the level nearest the
   queried record to the top (root domain) level can be expensive,
   especially when it comes to checking the many NS records for the top
   level domains and the root.  Because the name servers for the root
   and the TLDs are rarely used as RPZ triggers, some RPZ

Vixie & Schryver          Expires June 19, 2017                 [Page 9]
Internet-Draft                   DNS RPZ                   December 2016

   implementations have a "min-ns-dots" parameter that stops NSDNAME and
   NSIP checking early.

   Despite their costs, NSDNAME and NSIP triggers can be more effective
   than QNAME and IP triggers.  Miscreants can more easily change their
   direct domain names and IP addresses (which are detected by QNAME and
   IP triggers) than they can their change NS names and addresses
   (detected by NSDNAME and NSIP triggers) in parent domains such as
   TLDs.  Caching of NS records and related address data

   Some implementations of DNS RPZ will attempt to exhaustively discover
   all ancestral zone cuts above the query name and learn the NS RRset
   at the apex of each delegated zone.  Other implementations will know
   only the zone cut information which has naturally come into the
   cache, which will often include only name server names and addresses
   from the parent.  Apex ("below the cut") name server names and
   addresses often do not exactly match those from the parent.  Such
   inconsistencies can lead to instability in DNS RPZ behavior.  This
   potential inconsistency must be taken into account when designing a
   security policy or testing DNS RPZ.

   For NSDNAME and NSIP triggers, the BIND9 and Unbound RPZ
   implementations (as of 2016) match the NS, A, and AAAA RRsets already
   in their caches unless there are none, in which case they recurse.
   This strategy works well in practice, because their caches were
   likely recently populated while generating the unmodified response
   and checking QNAME and response IP address triggers.  In addition,
   the authoritative apex NS RRset (which, if obtained, would supersede
   the cached NS RRset from the delegating zone) of a domain operated by
   a malefactor is often peculiar.  Even when it is reasonable, the
   authoritative DNS servers for such a domain are often extremely slow
   or broken.  For example, RRs like " NS ." claiming root as
   the authoritative server for a second or lower level domain are
   popular choices in miscreant apex NS RRsets, as are imaginative name
   servers A and AAAA RRsets.

4.5.  The "NSIP" trigger (.rpz-nsip)

   The NSIP policy trigger matches name server addresses, that is A or
   AAAA RRs referenced by an NS RRset.  NSIP is much like NSDNAME
   (described above) except that the matching is by name server address
   rather than name server name.  NSIP policies are expressed as
   subdomains of "rpz-nsip" and have the same subdomain naming
   convention as described for response IP policy triggers above
   (Section 4.1.1).

Vixie & Schryver          Expires June 19, 2017                [Page 10]
Internet-Draft                   DNS RPZ                   December 2016

   In a process very similar to that for an NSDNAME trigger
   (Section 4.4), an NSIP trigger is checked by first considering all of
   the IP addresses for all the named servers for the QNAME, then
   proceeding similarly parent of the QNAME, and so on until the name
   servers for the root (.) have been checked or a policy has been

   Policies are applied in order of precedence (see Section 5.1) at each
   level in the data path.  The data path traversal process stops
   immediately when there is at least one policy match at a given level.

   For example, to force an NXDNAME answer whenever one of the name
   servers for the requested domain (QNAME) or one of its parents has an
   address in the block, the RPZ would contain the

      CNAME   .

4.5.1.  Implementation considerations for NSIP triggers

   The performance and caching considerations for the implementation of
   NSIP triggers are essentially identical to those described for
   NSDNAME triggers (Section 4.4.1).

5.  Application of the Policy

   To enable the use of RPZs, the recursive name server's control plane
   is connected to the DNS RPZ data plane by supplying an ordered list
   of RPZs in the name server's configuration.  For each DNS RPZ in this
   list, it should be possible to specify an optional overriding policy
   action to be used for any policy triggers found in that RPZ.  These
   override policies should include NXDOMAIN, NODATA, PASSTHRU, DROP,
   TCP-ONLY, CNAME domain, GIVEN, and DISABLED.  The first five of these
   actions are defined in Section 3 above.  "CNAME domain" is a
   restricted case of the "Local Data" action (also defined in
   Section 3) in which the rewritten response is a CNAME RR targeting
   "domain."  GIVEN explicitly reaffirms the default, which is to
   respect all policy actions found in this DNS RPZ.  The overriding
   DISABLED action causes triggered actions in the zone to have no
   effect except to log what would have happened, provided sufficient
   logging is enabled; this is useful for debugging or previewing a
   policy zone.

   By default, policies are applied only on DNS requests that ask for
   recursion (RD=1).  Recursive DNS servers generally send their
   requests to authority servers without asking for recursion (RD=0),
   while stub resolvers ask for recursion (RD=1).  Thus, RPZ at a

Vixie & Schryver          Expires June 19, 2017                [Page 11]
Internet-Draft                   DNS RPZ                   December 2016

   recursive server by default only affects requests from stub
   resolvers.  This default can be overridden in some implementations
   with configuration statements such as "recursive-only no".

   Also by default, RPZ policies are only applied to responses to DNS
   requests that do not request DNSSEC metadata (DO=0) or for which no
   DNSSEC metadata exists.  This defaults can be overridden in some
   implementations with a configuration statement such "break-dnssec
   yes".  See Section 10 about the implications of responding with
   modified DNS responses when the DNS client seems to be expecting
   DNSSEC signatures.

   If a policy rule matches and results in a modified answer, then that
   modified answer will include in its authority section the SOA RR of
   the policy zone whose policy was used to generate the modified
   answer.  This SOA RR includes the name of the DNS RPZ and the serial
   number of the policy data which was connected to the DNS control
   plane when the answer was modified.

   Conceptually, policies MUST be applied after recursion is complete
   and all data needed to formulate a response is available.  However,
   implementations MAY short-circuit the process such as not waiting for
   recursion when it is clear which modification will be made to the
   response.  Nevertheless, it SHOULD be possible to configure the
   resolver to continue checking and filling its cache by recursion as
   if it had not already made its decision, in order to deny operators
   of authority servers for listed domains information about whether
   they are listed, that is, in order to minimize giving hints to
   miscreants about when to change their DNS data.  In BIND9, for
   example, this behavior is controlled with the "qname-wait-recurse"
   configuration option.

   When the QNAME is resolved with CNAME or DNAME, there are no response
   IP address that might match a response IP address trigger, but NSIP
   and NSDNAME triggers might be matched.  Unless the original query
   type is ANY, CNAME, or DNAME, the resolver will start over and try to
   resolve the target of the CNAME.  RPZ is also restarted and the CNAME
   target is matched against CNAME policy rules resolved IP addresses
   (if any) are matched with response IP address policy triggers, and so
   forth.  This process is repeated as the resolver follows the CNAME

5.1.  Precedence Rules.

   More than one policy trigger among the various DNS RPZs connected to
   the name server's control plane can match a given DNS response, but
   only a single policy rule can affect the response.  In theory and for
   standardization, all possible rules are considered simultaneously and

Vixie & Schryver          Expires June 19, 2017                [Page 12]
Internet-Draft                   DNS RPZ                   December 2016

   the following precedence rules are used to choose the single best RPZ
   rule.  In implementations, policy triggers are usually considered in
   a sequence that mirrors the process of generating the DNS response,
   because checking RPZ triggers is conveniently made a part of that
   process.  For example, client IP RPZ address triggers are often
   checked early as the DNS request is being received and the client IP
   address is checked in the access control list (ACL) that determines
   which DNS client IP addresses can ask for recursion.  The QNAME is
   available for RPZ trigger matching before any response IP addresses
   are known and so QNAME poliocy triggers are usually checked
   immediately after client IP address triggers and before response IP
   address triggers.  NSIP and NSDNAME triggers are often checked last.
   As far as the DNS client can determine, it MUST seem that all
   matching triggers are found and weighed using the precedence rules,
   but in practice, shortcuts are taken.  For example, according to the
   precedence rules, a matched QNAME trigger in the first policy zone
   makes all response IP address, NSIP, and NSDNAME triggers moot.
   There is no need to look for those matches, because they will not
   affect the response.

   The following list is ordered so that rules listed early override
   rules listed later.

   RPZ Ordering
      Changes triggered by records in RPZs specified earlier in the
      ordered set of DNS RPZs are applied instead of triggers in policy
      zone specified later.

   Within An RPZ
      Among policies from a single RPZ, client IP address policies are
      chosen instead of QNAME policies, QNAME policies are preferred to
      IP, IP policies are preferred to NSDNAME, and NSDNAME policies are
      preferred to NSIP.

   Exact name match
      As in exact versus wildcard domain name matching at authority
      servers, exact domain name QNAME or NSDNAME triggers are preferred
      to wildcards.

   Name Length
      The preceding rule implies QNAME policies are preferred to NSDNAME

      Among triggered QNAME or NSDNAME policies within an RPZ, choose
      the policy that matches the triggering domain name that appears
      earlier in the sorting using the DNSSEC canonical DNS name order
      described in section 6.1 of [RFC4034].  The last labels of domain
      names are most significant in that ordering.  A domain name that

Vixie & Schryver          Expires June 19, 2017                [Page 13]
Internet-Draft                   DNS RPZ                   December 2016

      is a parent of another appears before the child.  Labels are
      compared as if they were words in a dictionary so that a label
      that is a prefix of a second label appears before the second.
      Characters in labels are sorted by their values as US-ASCII
      characters except that uppercase letters are treated as if they
      were lowercase.

   Prefix Length
      A preceding rule implies that IP policies within an RPZ are
      preferred to NSIP policies.

      Among triggered IP or NSIP policies, use the policy (not the
      matched IP address) with the longest internal policy zone prefix
      length.  The internal prefix length of an IPv6 address trigger is
      the numeric value of the first label that defines it as described
      in Section 4.  The internal prefix length of an IPv4 trigger is
      the numeric value of its first label plus 112.  This adjustment
      makes IPv4 triggers work the same as their equivalent
      IPv4-compatible IPv6 address triggers.  It also tends to favor
      IPv4 triggers over IPv6 triggers.  (See section of
      [RFC4291] about IPv4-compatible IPv6 addresses.)

   Tie Breaker
      Given equal internal prefix lengths, use the IP or NSIP policy
      that matches the first IP address.  Addresses with equal trigger
      internal prefix lengths are ordered by their representations as
      domain names described in Section 4, including the leading,
      unadjusted prefix length.  Because this tie breaking considers the
      matched IP addresses instead of the triggered policy rules, the
      first or least significant label of an IPv6 address is always
      "128", and the first label of an IPv4 address is always "32".

6.  Subscriber Behavior

   RPZs must be primary or secondary zones at subscriber recursive
   resolvers.  They can be searched only in a recursive server's own
   storage, because additional network transactions for DNS resolvers
   are extremely undesirable.

   Response policy zones are loaded in the usual way.  For primary zones
   this may mean loading the contents of a local file into memory, or
   connecting to a database.  For secondary zones this means
   transferring the zone from the primary server using zone transfer
   such as IXFR [RFC1995] or AXFR [RFC5936].  It is strongly recommended
   that all secondary zone transfer relationships be protected with
   transaction signatures (DNS TSIG) and that real time change
   notification be enabled using the DNS NOTIFY protocol [RFC1996].

Vixie & Schryver          Expires June 19, 2017                [Page 14]
Internet-Draft                   DNS RPZ                   December 2016

   DNS resolvers often have limited or no notion of a DNS zone or zone
   file.  They sometimes have special local zones, but generally have no
   implementations of IXFR, AXFR, or NOTIFY.  Therefore, an external
   module or daemon that maintains local copies of policy zones can be

7.  Producer Behavior

   A DNS RPZ producer should make every effort to ensure that
   incremental zone transfer (IXFR [RFC1995]) rather than full zone
   transfer (AXFR [RFC5936]) is used to move new policy data toward
   subscribers.  Also, real time zone change notifications (DNS NOTIFY
   [RFC1996]) should be enabled and tested.  DNS RPZ subscribers are
   "stealth slaves" as described in RFC 1996, and each such server must
   be explicitly listed in the master server's configuration as
   necessary to allow zone transfers from the stealth slave, as well to
   ensure that zone change notifications are sent to it.  Because DNS
   NOTIFY is a lazy protocol, it may be necessary to explicitly trigger
   the master server's "notify" logic after each change of the DNS RPZ.
   These operational guidelines are to limit policy data latency, since
   minimal latency is critical to both prevention of crime and abuse,
   and to withdrawal of erroneous or outdated policy.

   In the data feed for disreputable domains, each addition or deletion
   or expiration can be handled using DNS UPDATE [RFC2136] to trigger
   normal DNS NOTIFY and subsequent DNS IXFR activity which can keep the
   subscribing servers well synchronized to the master RPZ.
   Alternatively, on some primary name servers (such as ISC BIND) it is
   possible to generate an entirely new primary RPZ file and have the
   server compute the differences between each new version and its
   predecessor.  In ISC BIND this option is called "ixfr-from-
   differences" and is known to be performant even for million-rule DNS
   RPZ's with significant churn on a minute by minute basis.

   It is good operational practice to include test records in each DNS
   RPZ to help that DNS RPZ's subscribers verify that response policy
   rewriting is working.  For example, a DNS RPZ might include a QNAME
   policy record for BAD.EXAMPLE.COM and an IP policy record for  A subscriber can verify the correctness of their
   installation by querying for BAD.EXAMPLE.COM which does not exist in
   real DNS.  If an answer is received it will be from the DNS RPZ.
   That answer will contain an SOA RR denoting the fully qualified name
   of the DNS RPZ itself.

Vixie & Schryver          Expires June 19, 2017                [Page 15]
Internet-Draft                   DNS RPZ                   December 2016

8.  History and Evolution

   RPZ was previously described in a technical note from Internet
   Systems Consortium [ISC-RPZ].  A more up to date description appeared
   in chapter 6 of the "BIND 9 Administrator Reference Manual" [ISC-ARM]
   as of 2010.

   RPZ was designed by Paul Vixie and Vernon Schryver in 2009.  The
   initial implementation and first patch adding it to BIND were written
   by Vernon Schryver in late 2009.  Patches for various versions of
   BIND9 including 9.4, 9.6, and 9.7 were distributed from FTP servers
   at and starting in 2010.

   If all RPZ triggers and actions had been foreseen at the start in
   2009, they would probably have been encoded differently.  Instead RPZ
   grew incrementally, and upward compatibility required support of the
   original encodings.  The initial specification or Format 1 contained
   only QNAME triggers.  Changes for Format 2 included adding triggers
   based on response contents (RDATA), the targets of NS records
   (NSDNAME), and contents of A and AAAA records that resolve NS records
   (NSIP).  Format 3 included "rpz-passthru" for the PASSTHRU action and
   wildcards in CNAME targets to synthesize local data.

   Today, with a number of commercial RPZ providers with many users and
   no functional problems with the encodings, any lack of aesthetic
   appeal is balanced by the ever increasing weight of the installed
   base.  For example, it is impossible to replace the original QNAME
   trigger encoding NXDOMAIN and NODATA policy action encodings with
   encodings that involve rpz-* pseudo-TLDs at RPZ providers without
   breaking the many existing RPZ subscriber installations.  The
   original, deprecated PASSTHRU encoding of a CNAME pointing to the
   trigger QNAME might still be in use in local, private policy zones,
   and so it is still recognized by RPZ subscriber implementations.

   The initial RPZ idea was only to deny the existence of objectionable
   domain names, and so there were only QNAME triggers and NXDOMAIN
   actions.  Given that single kind of trigger, encoding it as the owner
   name of a policy record was clearly best.  A CNAME pointing to the
   root domain (.) is a legal and valid but not generally useful record,
   and so that became the encoding for the NXDOMAIN action.  The
   encoding of the NODATA action as "CNAME *." followed similar
   reasoning.  Requests for more kinds of triggers and actions required
   a more general scheme, and so they are encoded as CNAMES with targets
   in bogus TLDs owner names with DNS labels that start with "rpz_".

Vixie & Schryver          Expires June 19, 2017                [Page 16]
Internet-Draft                   DNS RPZ                   December 2016

9.  IANA Considerations

   No actions are required from IANA as result of the publication of
   this document.

10.  Security Considerations

   RPZ is a mechanism for providing "untruthful" DNS results from
   recursive servers.  Nevertheless, RPZ does not exacerbate the
   existing vulnerability of recursive servers to falsified DNS data.
   RPZ merely formalizes and facilitates modifying DNS data on its way
   from DNS authority servers to clients.  However, the use of DNSSEC
   (see [RFC4033] and [RFC4034]) prevents such changes to DNS data by

   Therefore, by default, DNS resolvers using RPZ avoid modifying DNS
   results when DNSSEC signatures are available and are requested by the
   DNS client.  However, when the common "break-dnssec" configuration
   setting is used, RPZ-using resolvers rewrite responses.  They omit
   DNSSEC RRsets, because the modified responses cannot be verified by
   DNSSEC signatures.  This renders the results invalid according to
   DNSSEC.  In such a case, a querying client which checks DNSSEC will
   ignore the invalid result, and will effectively be blocked from
   miscreant domains; this behaviour is functionally similar to that
   caused by an RPZ NXDOMAIN policy action.

   The policy zones might be considered sensitive, because they contain
   information about miscreants.  Like other DNS zones in most
   situations, RPZs are transferred from sources to subscribers as
   cleartext vulnerable to observation.  However, TSIG transaction
   signatures [RFC2845] SHOULD be used to authenticate and protect RPZ
   contents from modification.

   Recursive servers using RPZ are often configured to complete
   recursion even if a policy trigger provides a rewritten answer
   without needing recursion.  This impedes miscreants observing
   requests from their own authority servers from inferring whether RPZ
   is in use and whether their RRs are listed.  "qname-wait-recurse" is
   a common configuration switch that controls this behavior.  See
   Section 5.

11.  Acknowledgements

   The authors gratefully acknowledge the substantial contributed
   material and editorial scrutiny of Anne Bennett.  She directed the
   reorganization and clarification of the entire document.

Vixie & Schryver          Expires June 19, 2017                [Page 17]
Internet-Draft                   DNS RPZ                   December 2016

   Eric Ziegast, Jeroen Massar, and Ben April provided improvements to
   the document and caught errors.

12.  References

12.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
              DOI 10.17487/RFC1995, August 1996,

   [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone
              Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
              August 1996, <>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,

   [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
              Wellington, "Secret Key Transaction Authentication for DNS
              (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000,

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,

Vixie & Schryver          Expires June 19, 2017                [Page 18]
Internet-Draft                   DNS RPZ                   December 2016

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <>.

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952,
              DOI 10.17487/RFC5952, August 2010,

12.2.  Informative References

   [ISC-ARM]  Internet Systems Consortium, "BIND 9 Administrator
              Reference Manual,
              Bv9ARM.ch06.html#rpz", 2016.

   [ISC-RPZ]  Vixie, P. and V. Schryver, "DNS Response Policy Zones (DNS
              RPZ, Format 3),
              2010-1.txt", 2010.

   [ISC-RRL]  Vixie, P. and V. Schryver, "DNS Response Rate Limiting
              (DNS RRL),
              2012-1.txt", 2012.

Appendix A.  Examples

   An existing data feed capable of producing an RHSBL can be trivially
   used to generate a DNS RPZ.  If the desired policy is to alias
   targeted domains to a local walled garden, then for each domain name,
   generate the following records, one for the name itself and perhaps
   also one for its subdomains:
         *     CNAME

   If it is desirable to return NXDOMAIN for each domain (and its
   subdomains in this example), try this:
       CNAME   .
         *     CNAME   .

   Try this if there are walled gardens for mail versus everything else:

Vixie & Schryver          Expires June 19, 2017                [Page 19]
Internet-Draft                   DNS RPZ                   December 2016
       MX      0       A
         *     MX      0
         *     A

   An extended example follows:

         $TTL 1H
         @                     SOA LOCALHOST. (
                                   1 1h 15m 30d 2h) NS LOCALHOST.

         ; QNAME policy records.
         ; There are no periods (.) after the relative owner names.  CNAME   .           ; NXDOMAIN policy    CNAME   *.          ; NODATA policy

         ; redirect to walled garden       A
                               AAAA    2001:db8::1

         ; do not rewrite OK.EXAMPLE.COM (PASSTHRU)        CNAME   rpz-passthru.     CNAME

         ; redirect X.BZONE.EXAMPLE.COM to
         *   CNAME   *

         ; rewrite all answers for except   CNAME   .   CNAME   rpz-passthru.

         ; rewrite to NXDOMAIN all responses; for domains for which
         ; NS.EXAMPLE.COM is an authoritative DNS server or a server
         ; for a parent) or that have an authoritative server
         ; in 2001:db8::/32    CNAME   .
         32.zz.db8.2001.rpz-nsip       CNAME   .

Authors' Addresses

   Paul Vixie
   Farsight Security, Inc.


Vixie & Schryver          Expires June 19, 2017                [Page 20]
Internet-Draft                   DNS RPZ                   December 2016

   Vernon Schryver
   Rhyolite Software


Vixie & Schryver          Expires June 19, 2017                [Page 21]