DNS Response Policy Zones
draft-vixie-dns-rpz-02
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 state is "Replaced".
|
|
|---|---|---|---|
| Authors | Paul A. Vixie , Vernon Schryver | ||
| Last updated | 2016-12-16 | ||
| Replaced by | draft-ietf-dnsop-dns-rpz | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| 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) |
draft-vixie-dns-rpz-02
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
draft-vixie-dns-rpz-02
Abstract
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 http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 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
(http://trustee.ietf.org/license-info) 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 ("example.com") . . . . . . . . . . . 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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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. http://lists.redbarn.org/mailman/listinfo/dnsfirewalls offers
subscriptions and archives. See also https://dnsrpz.info/
[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
$ORIGIN RPZ.EXAMPLE.ORG.
ok.example.com CNAME rpz-passthru.
would exempt requests for ok.example.com from the NXDOMAIN policy or
action of the following record:
$ORIGIN RPZ.EXAMPLE.ORG.
example.com CNAME .
*.example.com 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
$ORIGIN RPZ.EXAMPLE.ORG.
example.com CNAME rpz-drop.
nothing is sent to a DNS client trying to resolve example.com, 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
"rpz-".
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
"NSIP".
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
addresses.
For example, the following would drop all requests from clients in
192.0.2.0/24 and give truthful answers to requests from a client at
2001:db8::3.
$ORIGIN RPZ.EXAMPLE.ORG.
24.0.2.0.192.rpz-client-ip 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 192.0.2.0/24 would be encoded as
"24.0.2.0.192".
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 ("example.com")
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 example.com.RPZ.EXAMPLE.ORG would affect responses to
requests about example.com.
Wildcards also work, and so the owner name
"*.example.com.RPZ.EXAMPLE.ORG" would trigger on queries to any
subdomain of example.com. 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 example.com and all subdomains of
example.com will result in synthetic NXDOMAIN responses.
$ORIGIN RPZ.EXAMPLE.ORG.
example.com CNAME .
*.example.com 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
192.0.2.0/24 unless address 192.0.2.2 is present, the RPZ would
contain these records:
$ORIGIN RPZ.EXAMPLE.ORG.
24.0.2.0.192.rpz-ip CNAME .
32.2.2.0.192.rpz-ip 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:
$ORIGIN RPZ.EXAMPLE.ORG.
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 4.4.1.1 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 ns.example.com, the RPZ
would contain the following:
$ORIGIN RPZ.EXAMPLE.ORG.
ns.example.com.rpz-nsdname CNAME .
4.4.1. Implementation considerations for NSDNAME triggers
Note that these considerations apply also to NSIP triggers described
in Section 4.5 below.
4.4.1.1. 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.
4.4.1.2. 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 "example.com 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
matched.
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 192.0.2.0/24 block, the RPZ would contain the
following:
$ORIGIN RPZ.EXAMPLE.ORG.
24.0.2.0.192.rpz-nsip 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
chain.
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
policies.
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 2.5.5.1 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
useful.
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
192.0.2.1. 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 redbarn.org and rhyolite.com 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
RPZ.
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,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
DOI 10.17487/RFC1995, August 1996,
<http://www.rfc-editor.org/info/rfc1995>.
[RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
August 1996, <http://www.rfc-editor.org/info/rfc1996>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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,
<http://www.rfc-editor.org/info/rfc2136>.
[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,
<http://www.rfc-editor.org/info/rfc2845>.
[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,
<http://www.rfc-editor.org/info/rfc4033>.
[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,
<http://www.rfc-editor.org/info/rfc4034>.
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, <http://www.rfc-editor.org/info/rfc4291>.
[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<http://www.rfc-editor.org/info/rfc5936>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<http://www.rfc-editor.org/info/rfc5952>.
12.2. Informative References
[ISC-ARM] Internet Systems Consortium, "BIND 9 Administrator
Reference Manual,
https://ftp.isc.org/isc/bind9/cur/9.10/doc/arm/
Bv9ARM.ch06.html#rpz", 2016.
[ISC-RPZ] Vixie, P. and V. Schryver, "DNS Response Policy Zones (DNS
RPZ, Format 3), https://ftp.isc.org/isc/dnsrpz/isc-tn-
2010-1.txt", 2010.
[ISC-RRL] Vixie, P. and V. Schryver, "DNS Response Rate Limiting
(DNS RRL), https://ftp.isc.org/isc/pubs/tn/isc-tn-
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:
bad.example.com CNAME walled-garden.example.net.
*.bad.example.com CNAME walled-garden.example.net.
If it is desirable to return NXDOMAIN for each domain (and its
subdomains in this example), try this:
bad.example.com CNAME .
*.bad.example.com 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
bad.example.com MX 0 wgmail.example.net.
bad.example.com A 192.0.2.66
*.bad.example.com MX 0 wgmail.example.net.
*.bad.example.com A 192.0.2.66
An extended example follows:
$ORIGIN rpz.example.net.
$TTL 1H
@ SOA LOCALHOST. named-mgr.example.net. (
1 1h 15m 30d 2h) NS LOCALHOST.
; QNAME policy records.
; There are no periods (.) after the relative owner names.
nxdomain.example.com CNAME . ; NXDOMAIN policy
nodata.example.com CNAME *. ; NODATA policy
; redirect to walled garden
bad.example.com A 10.0.0.1
AAAA 2001:db8::1
; do not rewrite OK.EXAMPLE.COM (PASSTHRU)
ok.example.com CNAME rpz-passthru.
bzone.example.com CNAME garden.example.net.
; redirect X.BZONE.EXAMPLE.COM to
; X.BZONE.EXAMPLE.COM.GARDEN.EXAMPLE.NET
*.bzone.example.com CNAME *.garden.example.net.
; rewrite all answers for 192.0.2.0/24 except 192.0.2.1
24.0.2.0.192.rpz-ip CNAME .
32.1.2.0.192.rpz-ip 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
ns.example.com.rpz-nsdname CNAME .
32.zz.db8.2001.rpz-nsip CNAME .
Authors' Addresses
Paul Vixie
Farsight Security, Inc.
Email: paul@redbarn.org
Vixie & Schryver Expires June 19, 2017 [Page 20]
Internet-Draft DNS RPZ December 2016
Vernon Schryver
Rhyolite Software
Email: vjs@rhyolite.com
Vixie & Schryver Expires June 19, 2017 [Page 21]