Network Working Group J. Levine
Internet-Draft Taughannock Networks
Intended status: Informational February 12, 2016
Expires: August 15, 2016
Publishing Organization Boundaries in the DNS
draft-levine-dbound-dns-00
Abstract
The organization that manages a subtree in the DNS is often different
from the one that manages the tree above it. We describe an
architecture to publish in the DNS the boundaries between
organizations that can be adapted to various policy models and can be
queried with a small number of DNS lookups.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Design Issues . . . . . . . . . . . . . . . . . . . . . . . . 2
3. RRTYPE format . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Lookup Process . . . . . . . . . . . . . . . . . . . . . . . 4
5. DNS Records . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Application scenarios . . . . . . . . . . . . . . . . . . . . 6
6.1. Cookies . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.2. SSL Certificates . . . . . . . . . . . . . . . . . . . . 6
6.3. DMARC . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Variations . . . . . . . . . . . . . . . . . . . . . . . . . 7
10. IANA considerations . . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . 9
11.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 9
A.1. Changes from -04 to new -00 . . . . . . . . . . . . . . . 9
A.2. Changes from -03 to -04 . . . . . . . . . . . . . . . . . 9
A.3. Changes from -02 to -03 . . . . . . . . . . . . . . . . . 10
A.4. Changes from -01 to -02 . . . . . . . . . . . . . . . . . 10
A.5. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Often, the organization that manages a subtree in the DNS is
different from the one that manages the tree above it. Many
applications use information about such boundaries to implement
security policies. For example, web browsers use them to limit the
names where web cookies can be set, and Secure Socket Layer (SSL)
certificate services use them to determine the party responsible for
the domain in a signing request. Some mail security applications
such as Domain-based Messaging Authentication, Reporting and
Conformance (DMARC) use them to locate an organization's policy
records in the DNS.
[[Please direct discussion of this draft to the dbound working group
at dbound@ietf.org.]]
2. Design Issues
Organization boundaries can be assigned on what one could call an
opt-in or opt-out basis. "Opt-in" means that two names are only
managed by the same organization if both actively assert that they
are related. "Opt-out" means that if there is any boundary
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information at all for a DNS subtree, each name is assumed to be
under the same management as its parent unless there is a boundary
assertion to the contrary. This design describes an opt-out model.
Within the opt-out model, this design can adapt to a variety of
scenarios:
o Policies can be published by the domains themselves, or by a third
party. In the former case, each domain might assert its own
boundary policies. In the latter case, the third party makes the
assertions, which may or may not agree with what the domains
themselves would want.
o Multiple levels of delegation may be implemented, which is
different from irregular boundaries. For example, "ca", "on.ca",
and "toronto.on.ca" are irregular boundaries, because they're all
handled by the Canadian Internet Registration Authority (CIRA).
CentralNIC's "uk.com" would be a second level of delegation below
Verisign's com.
o Different sets of boundary rules can be published for different
applications. For example, the boundaries for SSL certificates
might be different from the boundaries for e-mail policies, or for
web cookie setting policies.
In the lookup process below, the boundary point data is stored in the
DNS tree in a new BOUND RRTYPE. The boundary is considered to be
directly below the name that the process returns, similarly to the
names in the PSL [PSL]. If the process returned "abc.example", then
"foo.abc.example" and "bar.abc.example" are separated by the
boundary, but "foo.abc.example" and "bar.foo.abc.example" are not.
Each domain publishes its own policies.
3. RRTYPE format
The BOUND record contains two 16-bit numeric values and an
uncompressed domain name. In a master file, they are written as two
decimal values and a domain name.
The first numeric value is a bit mask expressing policy options. The
only bits currently assigned is 0x0001 (NOLOWER) which means that no
lower level boundaries can exist below this one, and 0x0002 (NOBOUND)
which means that this name is not a boundary for this application.
The second numeric value is a number identifying the application to
which this boundary applies. The number zero is a default for any
applications not otherwise specified.
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4. Lookup Process
In general, the lookup process takes as input a domain name an
application number. It returns the name of the boundary node in the
DNS. This may be the domain itself or a parent. If there is no
policy for the domain the lookup fails; there are no defaults, and
the DNS root is not within any organization boundary. (Applications
may apply defaults of their own, but that is beyond the scope of this
specification.)
Names of boundary information records use the tag "_bound" which is
intended to be unique.
For the first lookup, the client extracts the top level component
(i.e., the rightmost label, as "label" is defined in Section 3 of
[RFC1034]) of the domain name from the subcomponents, if any, and
inserts the prefix in front of that component, after other components
if any. For example, if the domain to be checked is "example.com" or
"www.example.com", the client issues a DNS query for
"example._bound.com" or "www.example._bound.com". If the domain is a
dotless one such as "example", the client looks up "_bound.example".
The client does a DNS lookup of BOUND records at that name, which
will return zero or more BOUND records. A failure such as NXDOMAIN
is considered to return zero records. A lookup can return multiple
records if different applications have different boundaries or policy
options.
If a relevant policy record is returned, and the record does not
contain the NOBOUND bit, the domain name in the record is the policy
boundary. A policy record is relevant if it its application number
is the application's number, or its application number is zero and
there is no record with the application's number. For example, a
check for a boundary above "example.com" would be issued at
"example._bound.com", and the expected response could be "BOUND 0 0
com".
If there are no boundaries below the queried point, the policy record
contains "BOUND 1 0 ." indicating the root. For example, if all
subdomains of the "example" top-level domain (TLD) are under the same
management as the TLD itself, checks for "_bound.example" or
"www._bound.example" would return "BOUND 1 0 .".
If the relevant record has the NOLOWER bit set, the process stops.
Otherwise, the client inserts the prefix tag into the name just below
(i.e., to the left of) the name at the largest matching boundary
indicated in the lookup result, and repeats the lookup. For example:
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o When evaluating "www.foo.example.com", the first query would be to
"www.foo.example._bound.com". If the reply to this is "BOUND 0 0
com", then the second query would go to
"www.foo._bound.example.com".
o When evaluating "www.example.on.ca", the first query would be to
"www.example.on._bound.ca". If the reply to this is "BOUND 0 0
on.ca", the next lookup would be to "www._bound.example.on.ca".
This process repeats until a DNS lookup returns a relevant record
with the NOLOWER bit set, or a lookup returns no relevant records, at
which point the boundary is the domain name in the last retrieved
relevant record.
If an otherwise relevant record has the NOBOUND bit set, the process
continues if the NOLOWER bit is not set but there is no boundary at
the name with the NOBOUND bit. They exist so that a name in the
hierarchy can be a boundary for some applications but not for others.
5. DNS Records
The publishing entity uses wildcards and prefixed names that parallel
the regular names under a TLD to cover the domain's name space.
If there is a boundary at a given name, an entry in the TLD record
covers the names below it. For example, if there is a boundary at
".TEST", a suitable record would be:
*._bound.test IN BOUND 0 0 test"
If the boundary is above the TEST domain, i.e., TEST is under the
same management as FOO.TEST, the record would indicate no boundaries,
and an additional non-wildcard record is needed to cover TEST itself:
*._bound.test IN BOUND 0 0 .
_bound.test IN BOUND 0 0 .
In domains with irregular policy boundaries, multiple records in the
record describe the boundary points. For example, in the CA (Canada)
TLD, for national organizations there might be a boundary directly
below the national TLD; for provincial organizations there might be a
boundary below a provincial subdomain such as "on.ca"; and for local
(e.g., municipal) organizations, a boundary below a municipal
subdomain such as "toronto.on.ca" might exist. A suitable set of of
records covers this structure. The closest encloser rule in RFC 4592
[RFC4592] makes the wildcards match the appropriate names.
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*._bound.ca IN BOUND 0 0 ca
*.on._bound.ca IN BOUND 0 0 on.ca
*.toronto.on._bound.ca IN BOUND 0 0 toronto.on.ca"
For any set of policy boundaries in a tree of DNS names, a suitable
set of policy records can describe the boundaries, so a client can
find the boundary for any name in the tree with a single policy
lookup per level of delegation.
Since the delegation structure is unlikely to change frequently, long
time-to-live (TTL) values in the DBOUND records are appropriate.
If different applications have different boundaries or policy
options, the policy records for each application are put at the
appropriate names for the boundaries. Due to the way DNS wildcards
work, each name with any policy records MUST have records for all
policies, with the NOBOUND bit for policies for which the name is not
in fact a boundary.
6. Application scenarios
Here are some ways that applications might use BOUND data.
6.1. Cookies
If an http request attempts to set a cookie for a domain other than
the request's own domain, the client would do boundary check for the
"cookie" application for both the request's domain and the cookie
domain. If they are not separated by a boundary, the request is
allowed.
6.2. SSL Certificates
The client would do a boundary check for the domain name in an normal
certificate, or the name after the "*." in a wildcard certificate for
the "cert" application. If the boundary is above the name, the name
is allowed.
6.3. DMARC
If a DMARC lookup for the domain in a message's From: header fails,
the client would do a boundary check for the domain name using the
"dmarc" application. The organizational domain is the immediate
subdomain of the boundary domain. (Note that the boundary will
always be the one looked up or an ancestor.)
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7. Discussion
The total number of DNS lookups is the number of levels of boundary
delegation, plus one if the last boundary doesn't have the NOLOWER
flag. That is unlikely to be more than 2 or 3 in realistic
scenarios, and depends on the number of boundaries, not the number of
components in the names that are looked up.
Some domains have very irregular boundaries. This may require a
relatively large number of records to describe all the boundaries,
perhaps several hundred, but it doesn't seem like a number that would
challenge modern DNS servers.
The wildcard lookup means that each time an application looks up the
boundaries for a hostname, the lookup results use DNS cache entries
that will not be reused other than for subsequent lookups for the
identical hostname. This might cause cache churn, but it seems at
worst no more than we already tolerate from DNSBL lookups.
8. Security Considerations
The purpose of publishing organization boundaries is to provide
advice to third parties that wish to know whether two names are
managed by the same organization, allowing those names to be treated
"as the same" in some sense. Clients that rely on published
boundaries are outsourcing some part of their own security policy to
the publisher, so their own security depends on the publisher's
boundaries being accurate.
Although in some sense domains are always in control of their
subdomains, there are many situations in which parent domains are not
expected to influence subdomains. For example, second level domains
in global TLDs (gTLDs) operated by registries with contracts with the
Internet Corporation for Assigned Names and Numers (ICANN) Since
there is no technical bar to a parent publishing records that shadow
part or all of the boundary record namespace for delegated
subdomains, correct operation depends on the parent and subdomains
agreeing about who publishes what.
The DNS is subject to a variety of attacks. DBOUND records are
subject to the same ones as any other bit of the DNS, and the same
responses, such as using DNSSEC, apply.
9. Variations
Since nothing but BOUND records should be published at names with
_bound components, one could get the same effect with TXT records,
e.g.:
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*.toronto.on._bound.ca IN TXT "bound=1 0 0 toronto.on.ca"
The "bound=1" tag is to prevent confusion when a domain publishes a
wildcard such as *.example.com that could match a _bound name.
If third parties wanted to publish boundary information, they could
do it in their own subtree of the DNS. For example, if
policy.example were publishing boundary information about boundaries,
the records for the test domain described above would be:
*._bound.test.policy.exaple IN BOUND 0 0 .
_bound.test.policy.example IN BOUND 0 0 .
10. IANA considerations
This document defines a new DBOUND RRTYPE. IANA has assigned value
TBD.
This document requests that IANA create a registry of BOUND Flag
Bits. Its registration policy is IETF Review. Its initial contents
are as follows. [[NOTE: new flags are likely to change the lookup
algorithm]]
+------------------+-----------------+--------------------------+
| Bit | Reference | Description |
+------------------+-----------------+--------------------------+
| 0x0001 (NOLOWER) | (this document) | No lower level policies |
| 0x0002 (NOBOUND) | (this document) | No boundary at this name |
+------------------+-----------------+--------------------------+
Table 1: BOUND Flag Bits Initial Values
This document requests that IANA create a registry of BOUND
Applications. Its registration policy is First Come First Served.
Its initial contents are as follows. [[Note: New applications don't
affect the lookup process, and shouldn't affect existing
applications.]]
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+----------+--------------+-----------------------------------------+
| Value | Reference | Description |
+----------+--------------+-----------------------------------------+
| 0 (Any) | (this | Any application without a specific |
| | document) | boundary record |
| 1 | (this | HTTP web cookies |
| (Cookie) | document) | |
| 2 (Cert) | (this | SSL certificate authorities |
| | document) | |
| 3 | (this | DMARC organizational domains |
| (DMARC) | document) | |
+----------+--------------+-----------------------------------------+
Table 2: BOUND Applications Initial Values
11. References
11.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name
System", RFC 4592, July 2006.
11.2. Informative References
[PSL] Mozilla Foundation, "Public Suffix List", Nov 2015.
Appendix A. Change Log
*NOTE TO RFC EDITOR: This section may be removed upon publication of
this document as an RFC.*
A.1. Changes from -04 to new -00
Add NOBOUND record to make wildcard matches do the right thing
Rename to match WG name
A.2. Changes from -03 to -04
Editorial changes, fix speling errors.
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A.3. Changes from -02 to -03
New BOUND record type and modified lookup procedure.
A.4. Changes from -01 to -02
Add ABNF.
MSK overhaul of the middle part.
Put the wildcards back.
A.5. Changes from -00 to -01
Take out wildcards and put everything in one record.
Add DNS nits.
Author's Address
John Levine
Taughannock Networks
PO Box 727
Trumansburg, NY 14886
Phone: +1 831 480 2300
Email: standards@taugh.com
URI: http://jl.ly
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