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DNS Extensions Working Group G. Sisson
Internet-Draft B. Laurie
Expires: January 11, 2006 Nominet
July 10, 2005
Derivation of DNS Name Predecessor and Successor
draft-ietf-dnsext-dns-name-p-s-00
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on January 11, 2006.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes two methods for deriving the canonically-
ordered predecessor and successor of a DNS name. These methods may
be used for dynamic NSEC resource record synthesis, enabling
security-aware name servers to provide authenticated denial of
existence without disclosing other owner names in a DNSSEC-secured
zone.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. Absolute Method . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Derivation of DNS Name Predecessor . . . . . . . . . . . . 4
3.2. Derivation of DNS Name Successor . . . . . . . . . . . . . 4
4. Modified Method . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Derivation of DNS Name Predecessor . . . . . . . . . . . . 6
4.2. Derivation of DNS Name Successor . . . . . . . . . . . . . 6
5. Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Case Considerations . . . . . . . . . . . . . . . . . . . 7
5.2. Choice of Range . . . . . . . . . . . . . . . . . . . . . 7
5.3. Wild Card Considerations . . . . . . . . . . . . . . . . . 8
5.4. Possible Modifications . . . . . . . . . . . . . . . . . . 8
5.4.1. Restriction of Effective Maximum DNS Name Length . . . 8
5.4.2. Use of Modified Method With Zones Containing
SRV RRs . . . . . . . . . . . . . . . . . . . . . . . 9
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Examples of Immediate Predecessors Using Absolute
Method . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.2. Examples of Immediate Successors Using Absolute Method . . 13
6.3. Examples of Predecessors Using Modified Method . . . . . . 19
6.4. Examples of Successors Using Modified Method . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
10.1. Normative References . . . . . . . . . . . . . . . . . . . 22
10.2. Informative References . . . . . . . . . . . . . . . . . . 22
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
Appendix A. Change History . . . . . . . . . . . . . . . . . . . 22
A.1. Changes from sisson-02 to ietf-00 . . . . . . . . . . . . 22
A.2. Changes from sisson-01 to sisson-02 . . . . . . . . . . . 23
A.3. Changes from sisson-00 to sisson-01 . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 25
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1. Introduction
One of the proposals for avoiding the exposure of zone information
during the deployment DNSSEC is dynamic NSEC resource record (RR)
synthesis. This technique is described in [I-D.ietf-dnsext-dnssec-
trans] and [I-D.ietf-dnsext-dnssec-online-signing], and involves the
generation of NSEC RRs that just span the query name for non-existent
owner names. In order to do this, the DNS names which would occur
just prior to and just following a given query name must be
calculated in real time, as maintaining a list of all possible owner
names that might occur in a zone would be impracticable.
Section 6.1 of [RFC4034] defines canonical DNS name order. This
document does not amend or modify this definition. However, the
derivation of immediate predecessor and successor, while trivial, is
non-obvious. Accordingly, several methods are described here as an
aid to implementors and a reference to other interested parties.
This document describes two methods:
1. An ``absolute method'', which returns the immediate predecessor
or successor of a domain name such that no valid DNS name could
exist between that DNS name and the predecessor or successor.
2. A ``modified method'', which returns a predecessor and successor
which are more economical in size and computation. This method
is restricted to use with zones consisting only of single-label
owner names where a maximum-length owner name would not result in
a DNS name exceeding the maximum DNS name length. This is,
however, the type of zone for which the technique of online-
signing is most likely to be used.
2. Notational Conventions
The following notational conventions are used in this document for
economy of expression:
N: An unspecified DNS name.
P(N): Immediate predecessor to N (absolute method).
S(N): Immediate successor to N (absolute method).
P'(N): Predecessor to N (modified method).
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S'(N): Successor to N (modified method).
3. Absolute Method
These derivations assume that all uppercase US-ASCII letters in N
have already been replaced by their corresponding lowercase
equivalents. Unless otherwise specified, processing stops after the
first step in which a condition is met.
3.1. Derivation of DNS Name Predecessor
To derive P(N):
1. If N is the same as the owner name of the zone apex, prepend N
repeatedly with labels of the maximum length possible consisting
of octets of the maximum sort value (e.g. 0xff) until N is the
maximum length possible; otherwise continue to the next step.
2. If the least significant (left-most) label of N consists of a
single octet of the minimum sort value (e.g. 0x00), remove that
label; otherwise continue to the next step.
3. If the least significant (right-most) octet in the least
significant (left-most) label of N is the minimum sort value,
remove the least significant octet and continue with step 5.
4. Decrement the value of the least significant (right-most) octet,
skipping any values that correspond to uppercase US-ASCII
letters, and then append the label with as many octets as
possible of the maximum sort value. Continue to the next step.
5. Prepend N repeatedly with labels of as long a length as possible
consisting of octets of the maximum sort value until N is the
maximum length possible.
3.2. Derivation of DNS Name Successor
To derive S(N):
1. If N is two or more octets shorter than the maximum DNS name
length, prepend N with a label containing a single octet of the
minimum sort value (e.g. 0x00); otherwise continue to the next
step.
2. If N is one or more octets shorter than the maximum DNS name
length and the least significant (left-most) label is one or more
octets shorter than the maximum label length, append an octet of
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the minimum sort value to the least significant label; otherwise
continue to the next step.
3. Increment the value of the least significant (right-most) octet
in the least significant (left-most) label that is less than the
maximum sort value (e.g. 0xff), skipping any values that
correspond to uppercase US-ASCII letters, and then remove any
octets to the right of that one. If all octets in the label are
the maximum sort value, then continue to the next step.
4. Remove the least significant (left-most) label. If N is now the
same as the owner name of the zone apex, do nothing. (This will
occur only if N is the maximum possible name in canonical DNS
name order, and thus has wrapped to the owner name of zone apex.)
Otherwise repeat starting at step 2.
4. Modified Method
This method is for use with zones consisting only of single-label
owner names where an owner name consisting of label of maximum length
would not result in a DNS name which exceeded the maximum DNS name
length. This method is computationally simpler and returns values
which are more economical in size than the absolute method. It
differs from the absolute method detailed above in the following
ways:
1. Step 1 of the derivation P(N) has been omitted as the existence
of the owner name of the zone apex never requires denial.
2. A new step 1 has been introduced which removes unnecessary
labels.
3. Step 4 of the derivation P(N) has been omitted as it is only
necessary for zones containing owner names consisting of more
than one label. This omission generally results in a significant
reduction of the length of derived predecessors.
4. Step 1 of the derivation S(N) had been omitted as it is only
necessary for zones containing owner names consisting of more
than one label. This omission results in a tiny reduction of the
length of derived successors, and maintains consistency with the
modification of step 4 of the derivation P(N) described above.
5. Steps 2 and 4 of the derivation S(N) have been modified to
eliminate checks for maximum DNS name length, as it is an
assumption of this method that no DNS name in the zone can exceed
the maximum DNS name length.
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These derivations assume that all uppercase US-ASCII letters in N
have already been replaced by their corresponding lowercase
equivalents. Unless otherwise specified, processing stops after the
first step in which a condition is met.
4.1. Derivation of DNS Name Predecessor
To derive P'(N):
1. If N has more labels than the number of labels in the owner name
of the apex + 1, repeatedly remove the least significant (left-
most) label until N has no more labels than the number of labels
in the owner name of the apex + 1; otherwise continue to next
step.
2. If the least significant (left-most) label of N consists of a
single octet of the minimum sort value (e.g. 0x00), remove that
label; otherwise continue to the next step.
3. If the least significant (right-most) octet in the least
significant (left-most) label of N is the minimum sort value,
remove the least significant octet.
4. Decrement the value of the least significant (right-most) octet,
skipping any values which correspond to uppercase US-ASCII
letters, and then append the label with as many octets as
possible of the maximum sort value.
4.2. Derivation of DNS Name Successor
To derive S'(N):
1. If N has more labels than the number of labels in the owner name
of the apex + 1, repeatedly remove the least significant (left-
most) label until N has no more labels than the number of labels
in the owner name of the apex + 1. Continue to next step.
2. If the least significant (left-most) label of N is one or more
octets shorter than the maximum label length, append an octet of
the minimum sort value to the least significant label; otherwise
continue to the next step.
3. Increment the value of the least significant (right-most) octet
in the least significant (left-most) label that is less than the
maximum sort value (e.g. 0xff), skipping any values which
correspond to uppercase US-ASCII letters, and then remove any
octets to the right of that one. If all octets in the label are
the maximum sort value, then continue to the next step.
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4. Remove the least significant (left-most) label. (This will occur
only if the least significant label is the maximum label length
and consists entirely of octets of the maximum sort value, and
thus has wrapped to the owner name of the zone apex.)
5. Notes
5.1. Case Considerations
Section 3.5 of [RFC1034] specifies that "while upper and lower case
letters are allowed in [DNS] names, no significance is attached to
the case". Additionally, Section 6.1 of [RFC4034] states that when
determining canonical DNS name order, "uppercase US-ASCII letters are
treated as if they were lowercase US-ASCII letters". Consequently,
values corresponding to US-ASCII uppercase letters must be skipped
when decrementing and incrementing octets in the derivations
described in Section 3.1 and Section 3.2.
The following pseudo-code is illustrative:
Decrement the value of an octet:
if (octet == '[') // '[' is just after uppercase 'Z'
octet = '@'; // '@' is just prior to uppercase 'A'
else
octet--;
Increment the value of an octet:
if (octet == '@') // '@' is just prior to uppercase 'A'
octet = '['; // '[' is just after uppercase 'Z'
else
octet++;
5.2. Choice of Range
[RFC2181] makes the clarification that "any binary string whatever
can be used as the label of any resource record". Consequently the
minimum sort value may be set as 0x00 and the maximum sort value as
0xff, and the range of possible values will be any DNS name which
contains octets of any value other than those corresponding to
uppercase US-ASCII letters.
However, if all owner names in a zone are in the letter-digit-hyphen,
or LDH, format specified in [RFC1034], it may be desirable to
restrict the range of possible values to DNS names containing only
LDH values. This has the effect of:
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1. making the output of tools such as `dig' and `nslookup' less
subject to confusion;
2. minimising the impact that NSEC RRs containing DNS names with
non-LDH values (or non-printable values) might have on faulty DNS
resolver implementations; and
3. preventing the possibility of results which are wildcard DNS
names (see Section 5.3).
This may be accomplished by using a minimum sort value of 0x1f (US-
ASCII character `-') and a maximum sort value of 0x7a (US-ASCII
character lowercase `z'), and then skipping non-LDH, non-lowercase
values when incrementing or decrementing octets.
5.3. Wild Card Considerations
Neither derivation avoids the possibility that the result may be a
DNS name containing a wildcard label, i.e. a label containing a
single octet with the value 0x2a (US-ASCII character `*'). With
additional tests, wildcard DNS names may be explicitly avoided;
alternatively, if the range of octet values can be restricted to
those corresponding to letter-digit-hyphen, or LDH, characters (see
Section 5.2), such DNS names will not occur.
Note that it is improbable that a result which is a wildcard DNS name
will occur unintentionally; even if one does occur either as the
owner name of, or in the RDATA of an NSEC RR, it is treated as a
literal DNS name with no special meaning.
5.4. Possible Modifications
5.4.1. Restriction of Effective Maximum DNS Name Length
[RFC1034] specifies that "the total number of octets that represent a
[DNS] name (i.e., the sum of all label octets and label lengths) is
limited to 255", including the null (zero-length) label which
represents the root. For the purpose of deriving predecessors and
successors during NSEC RR synthesis, the maximum DNS name length may
be effectively restricted to the length of the longest DNS name in
the zone. This will minimise the size of responses containing
synthesised NSEC RRs but, especially in the case of the modified
method, may result in some additional computational complexity.
Note that this modification will have the effect of revealing
information about the longest name in the zone. Moreover, when the
contents of the zone changes, e.g. during dynamic updates and zone
transfers, care must be taken to ensure that the effective maximum
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DNS name length agrees with the new contents.
5.4.2. Use of Modified Method With Zones Containing SRV RRs
Normally the modified method cannot be used in zones that contain
SRV RRs [RFC2782], as SRV RRs have owner names which contain multiple
labels. However the use of SRV RRs can be accommodated by various
techniques. There are at least four possible ways to do this:
1. Use conventional NSEC RRs for the region of the zone that
contains first-level labels beginning with the underscore (`_')
character. For the purposes of generating these NSEC RRs, the
existence of (possibly fictional) ownernames `9{63}' and `a'
could be assumed, providing a lower and upper bound for this
region. Then all queries where the QNAME doesn't exist but
contains a first-level label beginning with an underscore could
be handled using the normal DNSSEC protocol.
This approach would make it possible to enumerate all DNS names
in the zone containing a first-level label beginning with
underscore, including all SRV RRs, but this may be of less a
concern to the zone administrator than incurring the overhead of
the absolute method or of the following variants of the modified
method.
2. The absolute method could be used for synthesising NSEC RRs for
all queries where the QNAME contains a leading underscore.
However this re-introduces the susceptibility of the absolute
method to denial of service activity, as an attacker could send
queries for an effectively inexhaustible supply of domain names
beginning with a leading underscore.
3. A variant of the modified method could be used for synthesising
NSEC RRs for all queries where the QNAME contains a leading
underscore. This variant would assume that all predecessors and
successors to queries where the QNAME contains a leading
underscore may consist of two lablels rather than only one. This
introduces a little additional complexity without incurring the
full increase in response size and computational complexity as
the absolute method.
4. Finally, a variant the modified method which assumes that all
owner names in the zone consist of one or two labels could be
used. However this negates much of the reduction in response
size of the modified method and may be nearly as computationally
complex as the absolute method.
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6. Examples
In the following examples:
the owner name of the zone apex is "example.com.";
the range of octet values is 0x00 - 0xff excluding values
corresponding to uppercase US-ASCII letters; and
non-printable octet values are expressed as three-digit decimal
numbers preceded by a backslash (as specified in Section 5.1 of
[RFC1035]).
6.1. Examples of Immediate Predecessors Using Absolute Method
Example of typical case:
P(foo.example.com.) =
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255.\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255.\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255.fon\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255.example.com.
or, in alternate notation:
\255{49}.\255{63}.\255{63}.fon\255{60}.example.com.
where {n} represents the number of repetitions of an octet.
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Example where least significant (left-most) label of DNS name
consists of a single octet of the minimum sort value:
P(\000.foo.example.com.) = foo.example.com.
Example where least significant (right-most) octet of least
significant (left-most) label has the minimum sort value:
P(foo\000.example.com.) =
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255.\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255.\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255.\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255.foo.example.com.
or, in alternate notation:
\255{45}.\255{63}.\255{63}.\255{63}.foo.example.com.
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Example where DNS name contains an octet which must be decremented by
skipping values corresponding to US-ASCII uppercase letters:
P(fo\[.example.com.) =
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255.\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255.\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255.fo\@\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255.example.com.
or, in alternate notation:
\255{49}.\255{63}.\255{63}.fo\@\255{60}.example.com.
where {n} represents the number of repetitions of an octet.
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Example where DNS name is the owner name of the zone apex, and
consequently wraps to the DNS name with the maximum possible sort
order in the zone:
P(example.com.) =
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255.\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255.\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255.\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255.example.com.
or, in alternate notation:
\255{49}.\255{63}.\255{63}.\255{63}.example.com.
6.2. Examples of Immediate Successors Using Absolute Method
Example of typical case:
S(foo.example.com.) = \000.foo.example.com.
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Example where DNS name is one octet short of the maximum DNS name
length:
N = fooooooooooooooooooooooooooooooooooooooooooooooo
.ooooooooooooooooooooooooooooooooooooooooooooooo
oooooooooooooooo.ooooooooooooooooooooooooooooooo
oooooooooooooooooooooooooooooooo.ooooooooooooooo
oooooooooooooooooooooooooooooooooooooooooooooooo.example.com.
or, in alternate notation:
fo{47}.o{63}.o{63}.o{63}.example.com.
S(N) =
fooooooooooooooooooooooooooooooooooooooooooooooo
\000.ooooooooooooooooooooooooooooooooooooooooooo
oooooooooooooooooooo.ooooooooooooooooooooooooooo
oooooooooooooooooooooooooooooooooooo.ooooooooooo
oooooooooooooooooooooooooooooooooooooooooooooooo
oooo.example.com.
or, in alternate notation:
fo{47}\000.o{63}.o{63}.o{63}.example.com.
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Example where DNS name is the maximum DNS name length:
N = fooooooooooooooooooooooooooooooooooooooooooooooo
o.oooooooooooooooooooooooooooooooooooooooooooooo
ooooooooooooooooo.oooooooooooooooooooooooooooooo
ooooooooooooooooooooooooooooooooo.oooooooooooooo
oooooooooooooooooooooooooooooooooooooooooooooooo
o.example.com.
or, in alternate notation:
fo{48}.o{63}.o{63}.o{63}.example.com.
S(N) =
fooooooooooooooooooooooooooooooooooooooooooooooo
p.oooooooooooooooooooooooooooooooooooooooooooooo
ooooooooooooooooo.oooooooooooooooooooooooooooooo
ooooooooooooooooooooooooooooooooo.oooooooooooooo
oooooooooooooooooooooooooooooooooooooooooooooooo
o.example.com.
or, in alternate notation:
fo{47}p.o{63}.o{63}.o{63}.example.com.
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Example where DNS name is the maximum DNS name length and the least
significant (left-most) label has the maximum sort value:
N = \255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255.ooooooooooooooooooooooooooooooooooooooooooo
oooooooooooooooooooo.ooooooooooooooooooooooooooo
oooooooooooooooooooooooooooooooooooo.ooooooooooo
oooooooooooooooooooooooooooooooooooooooooooooooo
oooo.example.com.
or, in alternate notation:
\255{49}.o{63}.o{63}.o{63}.example.com.
S(N) =
oooooooooooooooooooooooooooooooooooooooooooooooo
oooooooooooooop.oooooooooooooooooooooooooooooooo
ooooooooooooooooooooooooooooooo.oooooooooooooooo
ooooooooooooooooooooooooooooooooooooooooooooooo.
example.com.
or, in alternate notation:
o{62}p.o{63}.o{63}.example.com.
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Example where DNS name is the maximum DNS name length and the eight
least significant (right-most) octets of the least significant (left-
most) label have the maximum sort value:
N = foooooooooooooooooooooooooooooooooooooooo\255
\255\255\255\255\255\255\255.ooooooooooooooooooo
oooooooooooooooooooooooooooooooooooooooooooo.ooo
oooooooooooooooooooooooooooooooooooooooooooooooo
oooooooooooo.ooooooooooooooooooooooooooooooooooo
oooooooooooooooooooooooooooo.example.com.
or, in alternate notation:
fo{40}\255{8}.o{63}.o{63}.o{63}.example.com.
S(N) =
fooooooooooooooooooooooooooooooooooooooop.oooooo
oooooooooooooooooooooooooooooooooooooooooooooooo
ooooooooo.oooooooooooooooooooooooooooooooooooooo
ooooooooooooooooooooooooo.oooooooooooooooooooooo
ooooooooooooooooooooooooooooooooooooooooo.example.com.
or, in alternate notation:
fo{39}p.o{63}.o{63}.o{63}.example.com.
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Example where DNS name is the maximum DNS name length and contains an
octet which must be incremented by skipping values corresponding to
US-ASCII uppercase letters:
N = fooooooooooooooooooooooooooooooooooooooooooooooo
\@.ooooooooooooooooooooooooooooooooooooooooooooo
oooooooooooooooooo.ooooooooooooooooooooooooooooo
oooooooooooooooooooooooooooooooooo.ooooooooooooo
oooooooooooooooooooooooooooooooooooooooooooooooo
oo.example.com.
or, in alternate notation:
fo{47}\@.o{63}.o{63}.o{63}.example.com.
S(N) =
fooooooooooooooooooooooooooooooooooooooooooooooo
\[.ooooooooooooooooooooooooooooooooooooooooooooo
oooooooooooooooooo.ooooooooooooooooooooooooooooo
oooooooooooooooooooooooooooooooooo.ooooooooooooo
oooooooooooooooooooooooooooooooooooooooooooooooo
oo.example.com.
or, in alternate notation:
fo{47}\[.o{63}.o{63}.o{63}.example.com.
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Example where DNS name has the maximum possible sort order in the
zone, and consequently wraps to the owner name of the zone apex:
N = \255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255.\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255.\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255.\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255.example.com.
or, in alternate notation:
\255{49}.\255{63}.\255{63}.\255{63}.example.com.
S(N) = example.com.
6.3. Examples of Predecessors Using Modified Method
Example of typical case:
P'(foo.example.com.) =
fon\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255.example.com.
or, in alternate notation:
fon\255{60}.example.com.
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Example where DNS name contains more labels than DNS names in the
zone:
P'(bar.foo.example.com.) = foo.example.com.
Example where least significant (right-most) octet of least
significant (left-most) label has the minimum sort value:
P'(foo\000.example.com.) = foo.example.com.
Example where least significant (left-most) label has the minimum
sort value:
P'(\000.example.com.) = example.com.
Example where DNS name is the owner name of the zone apex, and
consequently wraps to the DNS name with the maximum possible sort
order in the zone:
P'(example.com.) =
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255.example.com.
or, in alternate notation:
\255{63}.example.com.
6.4. Examples of Successors Using Modified Method
Example of typical case:
S'(foo.example.com.) = foo\000.example.com.
Example where DNS name contains more labels than DNS names in the
zone:
S'(bar.foo.example.com.) = foo\000.example.com.
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Example where least significant (left-most) label has the maximum
sort value, and consequently wraps to the owner name of the zone
apex:
N = \255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255\255\255\255\255\255\255\255\255\255
\255\255\255.example.com.
or, in alternate notation:
\255{63}.example.com.
S'(N) = example.com.
7. Security Considerations
The derivation of some predecessors/successors requires the testing
of more conditions than others. Consequently the effectiveness of a
denial-of-service attack may be enhanced by sending queries that
require more conditions to be tested. The modified method involves
the testing of fewer conditions than the absolute method and
consequently is somewhat less susceptible to this exposure.
8. IANA Considerations
This document has no IANA actions.
Note to RFC Editor: This section is included to make it clear during
pre-publication review that this document has no IANA actions. It
may therefore be removed should it be published as an RFC.
9. Acknowledgments
The authors would like to thank Olaf Kolkman, Olafur Gudmundsson and
Niall O'Reilly for their review and input.
10. References
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10.1 Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
10.2 Informative References
[I-D.ietf-dnsext-dnssec-online-signing]
Ihren, J. and S. Weiler, "Minimally Covering NSEC Records
and DNSSEC On-line Signing",
draft-ietf-dnsext-dnssec-online-signing-00 (work in
progress), May 2005.
[I-D.ietf-dnsext-dnssec-trans]
Arends, R., Koch, P., and J. Schlyter, "Evaluating DNSSEC
Transition Mechanisms",
draft-ietf-dnsext-dnssec-trans-02 (work in progress),
February 2005.
Appendix A. Change History
A.1. Changes from sisson-02 to ietf-00
o Added notes on use of SRV RRs with modified method.
o Changed reference from weiler-dnssec-online-signing to ietf-
dnsext-dnssec-online-signing.
o Changed reference from ietf-dnsext-dnssec-records to RFC 4034.
o Miscellaneous minor changes to text.
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A.2. Changes from sisson-01 to sisson-02
o Added modified version of derivation (with supporting examples).
o Introduced notational conventions N, P(N), S(N), P'(N) and S'(N).
o Added clarification to derivations about when processing stops.
o Miscellaneous minor changes to text.
A.3. Changes from sisson-00 to sisson-01
o Split step 3 of derivation of DNS name predecessor into two
distinct steps for clarity.
o Added clarifying text and examples related to the requirement to
avoid uppercase characters when decrementing or incrementing
octets.
o Added optimisation using restriction of effective maximum DNS name
length.
o Changed examples to use decimal rather than octal notation as per
[RFC1035].
o Corrected DNS name length of some examples.
o Added reference to weiler-dnssec-online-signing.
o Miscellaneous minor changes to text.
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Authors' Addresses
Geoffrey Sisson
Nominet
Sandford Gate
Sandy Lane West
Oxford
OX4 6LB
GB
Phone: +44 1865 332339
Email: geoff@nominet.org.uk
Ben Laurie
Nominet
17 Perryn Road
London
W3 7LR
GB
Phone: +44 20 8735 0686
Email: ben@algroup.co.uk
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