INTERNET-DRAFT DNS Protocol Security Extensions
20 November 1994
Expires 19 May 1995
Domain Name System Protocol Security Extensions
------ ---- ------ -------- -------- ----------
Donald E. Eastlake 3rd & Charles W. Kaufman
Status of This Document
This draft, file name draft-ietf-dnssec-secext-02.txt, is intended to
be become a standards track RFC. Distribution of this document is
unlimited. Comments should be sent to the DNS Security Working Group
mailing list <dns-security@tis.com> or to the authors.
This document is an Internet-Draft. 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. Internet-Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet-
Drafts as reference material or to cite them other than as a
``working draft'' or ``work in progress.''
To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or
munnari.oz.au.
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Abstract
The Domain Name System (DNS) has become a critical operational part
of the Internet infrastructure yet it has no strong security
mechanisms to assure data integrity or authentication. Extensions to
the DNS are described in detail that provide these services to
security aware resolvers or applications through the use of
cryptographic digital signatures. These digital signatures are
included in secured zones as resource records. Security can still be
provided even through non-security aware DNS servers.
A explanatory overview of the proposed extensions appears in draft-
ietf-dnssec-over-*.txt.
The extensions also provide for the storage of authenticated keys in
the DNS for DNS use and to support a general public key distribution
service. The stored keys enable security aware resolvers to learn
the authenticating key of zones in addition to keys for which they
are initially configured. Keys associated with DNS names can be
retrieved to support other protocols. Provision is made for a
variety of key types and algorithms.
In addition, the security extensions provide for the optional
authentication of DNS protocol transactions.
Acknowledgements
The significant contributions of the following persons (in alphabetic
order) to this draft are gratefully acknowledged: Madelyn Badger,
James M. Galvin, Olafur Gudmundsson, Sandy Murphy, Masataka Ohta,
Jeffrey I. Schiller.
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Table of Contents
Status of This Document....................................1
Abstract...................................................2
Acknowledgements...........................................2
Table of Contents..........................................3
1. Introduction............................................5
2. Brief Overview of the Extensions.......................6
2.1 Key Distribution.......................................6
2.2 Data Origin Authentication.............................6
2.2.1 Security Provided...................................6
2.2.2 The SIG Resource Record..............................7
2.2.3 The NXD Resource Record..............................7
2.2.4 Signers Other Than The Zone..........................8
2.2.5 Special Problems With Time-to-Live...................8
2.3 DNS Transaction Authentication.........................8
3. The KEY Resource Record................................10
3.1 KEY RDATA format......................................10
3.2 Object Types and DNS Names and Keys...................11
3.3 The KEY RR Flag Octet.................................11
3.4 The KEY Algorithm Version.............................12
3.5 KEY RRs in the Construction of Responses..............13
3.6 File Representation of KEY RRs........................13
4. The SIG Resource Record................................15
4.1 SIG RDATA Format......................................15
4.1.1 Signature Format....................................17
4.1.2 SIG RRs Covering Type ANY...........................18
4.1.3 Zone Transfer (AXFR) SIG............................18
4.1.4 Transaction SIGs....................................18
4.2 SIG RRs in the Construction of Responses..............19
4.3 Processing Responses with SIG RRs.....................19
4.4 File Representation of SIG RRs........................20
5. Non-existent Names.....................................22
5.1 The NXD Resource Record...............................22
5.2 NXD RDATA Format......................................23
5.3 Example...............................................23
5.4 Interaction of NXD RRs and Wildcard RRs...............23
5.5 Blocking NXD Pseudo-Zone Transfers....................24
6. How to Resolve Securely................................25
6.1 Boot File Format......................................25
6.2 Chaining Through Zones................................26
6.3 Secure Time...........................................27
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7. Operational Considerations.............................28
7.1 Key Size Considerations...............................28
7.2 Key Storage...........................................28
7.3 Key Generation........................................29
7.4 Key Lifetimes.........................................29
7.5 Revocation............................................29
7.6 Root..................................................30
8. Conformance............................................31
8.1 Server Conformance....................................31
8.2 Resolver Conformance..................................31
9. Security Considerations................................32
References................................................32
Authors Addresses.........................................33
Expiration and File Name..................................33
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1. Introduction
This draft provides detailed descriptions of the DNS protocol
extensions to support DNS security and public key distribution.
A broad overview of these extensions is provided in draft-ietf-
dnssec-over-*.txt. This draft assumes that the reader is familiar
with that overview and with the Domain Name System particularly as
described in RFCs 1034 and 1035. The requirements and goals of DNS
security are described in the overview document.
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2. Brief Overview of the Extensions
The DNS protocol extensions provide three distinct services: key
distribution as described in section 2.1 below, data origin
authentication as described in section 2.2 below, and transaction
authentication, described in section 2.3 below.
2.1 Key Distribution
The resource records are defined to associate keys with DNS names.
This permits the DNS to be used as a general public key distribution
mechanism in support of the data origin authentication and
transaction authentication DNS services as well as other security
services such as IP level security.
The syntax of a KEY resource record is described in Section 3. It
includes the name of the entity the key is associated with
(frequently but not always the KEY RR owner name), an algorithm
identifier, flags indicating the type of entity the key is associated
with or asserting that there is no key associated with that entity,
and the actual public key parameters.
2.2 Data Origin Authentication
Adding security requires no change to the "on-the-wire" DNS protocol
beyond the addition of the signature resource types (and, as a
practical matter, the key resource type needed for key distribution).
An additional domain name non-existence resource is added to extend
authentication to negative responses. This service can be supported
by existing resolver and server implementations so long as they could
support the additional resource types.
If signatures are always separately retrieved and verified when
retrieving the information they authenticate, there will be more
trips to the server and performance will suffer. To avoid this,
security aware servers mitigate that degradation by automatically
send exactly the signature(s) needed.
2.2.1 Security Provided
Security is provided by associating with resource records in the DNS
a cryptographically generated digital signature. Commonly, there
will be a single private key that signs for an entire zone. If a
security aware resolver reliably learns the public key of the zone,
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it can verify that all the data read was properly authorized and is
reasonably current. The expected implementation is for the zone
private key to be kept off-line and used to re-sign all of the
records in the zone periodically.
The data origin authentication key belongs to the zone and not to the
servers that store copies of the data. That means compromise of a
server or even all servers for a zone will not affect the degree of
assurance that a resolver has that the data is genuine.
A resolver can learn the public key of a zone either by reading it
from DNS or by having it staticly configured. To reliably learn the
public key by reading it from DNS, the key itself must be signed.
Thus, to provide a reasonable degree of security, the resolver must
be configured with at least the public key of one zone. From that,
it can securely read the public keys of other zones if the
intervening zones in the DNS tree are secure. It is in principle
more secure to have the resolver manually configured with the public
keys of multiple zones, since then the compromise of a single zone
would not permit the faking of information from other zones. It is
also more administratively cumbersome, however, particularly when
public keys change.
2.2.2 The SIG Resource Record
The syntax of a SIG resource record (signature) is described in
Section 4. It includes the type of the RR(s) being signed, the name
of the signer, the time at which the signature was created, the time
it expires (when it is no longer to be believed), its original time
to live (which may be longer than its current time to live but cannot
be shorter), the cryptographic algorithm in use, and the actual
signature.
Every name in a zone supporting signed data will have associated with
it a SIG resource records for each resource type under that name. A
security aware server supporting the performance enhanced version of
the DNS protocol security extensions will return with all records
retrieved the corresponding SIGs. If a server does not support the
protocol, the resolver must retrieve all the SIG records for a name
and select the one or ones that sign the resource record(s) that
resolver is interested in.
2.2.3 The NXD Resource Record
The above security mechanism provides only a way to sign existing RRs
in a zone. Data origin authentication is not obviously provided for
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the non-existence of a domain name in a zone. This gap is filled by
the NXD RR which authenticatably asserts a range of non-existent
names in a zone. The owner of the NXD RR is the start of such a
ranger and its RDATA is the end of the range; however, there are
additional complexities due to wildcards. Section 6 below covers the
NXD RR.
2.2.4 Signers Other Than The Zone
There are two general cases where a signature is generated by other
than the zone private key. One is for future support of dynamic
update where an entity is permitted to authenticate/update its own
record. The public key of the entity must be present in the DNS and
be appropriately signed but the other RR(s) may be signed with the
entity's key. The other is for support of transaction authentication
as described in 2.3 below.
2.2.5 Special Problems With Time-to-Live
A digital signature will fail to verify if any change has occurred to
the data between the time it was originally signed and the time the
signature is verified. This conflicts with our desire to have the
time-to-live field tick down when resource records are cached.
This could be avoided by leaving the time-to-live out of the digital
signature, but that would allow unscrupulous secondaries to set
arbitrarily long time to live values undetected. Instead, we include
the "original" time-to-live in the signature and communicate that
data in addition to the current time-to-live. Unscrupulous servers
under this scheme can manipulate the time to live but a security
aware resolver will bound the TTL value it uses at the original
signed value. Separately, signatures include a time signed and an
expiration time. A resolver that knows an absolute time can
determine securely whether a signature has expired.
2.3 DNS Transaction Authentication
The data origin authentication service described above protects
resource records but provides no protection for message headers. If
header bits are falsely set by a server, there is little that can be
done. However, it is possible to add transaction authentication.
Such authentication means that a resolver can be sure it is getting
messages from the server it thinks it queried and that the response
is from the query it sent and that these messages have not been
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diddled in transit.
This is accomplished by optionally adding a special SIG resource
record to end of the reply which digitally signs the concatenation of
the server's response and the resolver's query. The private key used
belongs to the host composing the reply, not to the zone being
queried. The corresponding public key is stored in and retrieved
from the DNS. Because replies are highly variable, message
authentication SIGs can not be precalculated. Thus it will be
necessary to keep the private key on-line, for example in software or
in a directly connected piece of hardware.
The best way to get the security provided by the transaction
authentication service would be to use a good IP level security
protocol. The authors of this draft decry the every growing number
of IP application level security protocols such as Telnet, NTP, FTP,
etc., etc. when a single IP-security protocol could secure most of
these applications.
Unfortunately, an IP-security standard has not yet been adopted. And
even if it had, there will be many systems for many years where it
will be hard to add IP security but relatively easy to replace the
DNS components. Furthermore, the data origin authentication service
requires the implementation of essentially all the mechanisms needed
for a rudimentary message authentication service. Thus a simple
transaction authentication service based on mechanisms already
required by DNS security is included as a strictly optional part of
these extensions.
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3. The KEY Resource Record
The KEY RR is used to document a key that is associated with a DNS
name. It will be a public key as only public keys are stored in the
DNS. This can be the public key of a zone owner, of a host or other
end entity, or a user. A KEY RR is, like any other RR, authenticated
by a SIG RR. Security aware DNS implementations should be designed
to handle at least two simultaneously valid keys of the same type
associated with a name.
The type number for the KEY RR is 25.
3.1 KEY RDATA format
The RDATA for a KEY RR consists of an object name, flags, the
algorithm version, and the public key itself. For the MD5/RSA
algorithm, that is the public exponent and the public modulus. The
format is as follows:
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /
+- object name +---------------+
/ | flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| algorithm | public key exponent |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| modulus length| |
+---------------+ public key modulus -+
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The object name, and the flags octets are described in Sections 3.2
and 3.3 below. The flags must be examined before any following data
as they control its the format and even whether there is any
following data.
The public key exponent and modulus length fields show apply only if
the MD5/RSA algorithm is in use and a key is present. The public key
exponent is an unsigned 24 bit integer. The public key modulus field
is a multiprecision unsigned integer. The "modulus length" is an
unsigned octet which is the actual modulus length minus 64. This
limits keys to a maximum of 255+64 or 319 octets and a minimum of 64
octets. Although moduluses of less than 512 significant bits are not
recommended, due to the weak security they provide, they can be
represented by using leading zeros.
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3.2 Object Types and DNS Names and Keys
The public key in a KEY RR belongs to the object named in the object
name field. Frequently this will also be the owner name of the KEY
RR. But they will be different in the case of the key or keys stored
under a zone's name for the zone's superzone or keys that are stored
for cross certification.
The DNS object name may refer to up to three things. For example,
dee.lkg.dec.com could be (1) a zone, (2) a host, and (3) the mapping
into a DNS name of the user dee@lkg.dec.com . Thus, there are flags
in the KEY RR to indicate with which of these roles the object name
and public key are associated as described below. It is possible to
use the same key for these different things with the same name but
this is discouraged.
In addition, there are three control bits, the "no key" bit, the
"experimental" bit, and the "signatory" bit, as described in 3.3
below.
It would be desirable for the growth of DNS to be managed so that
additional possible simultaneous uses for names are NOT added. New
uses should be distinguished by exclusive domains. For example, all
IP autonomous system numbers have been mapped into the in-as.arpa
domain [draft-ietf-dnssec-as-map-*.txt] and all telephone numbers in
the world have been mapped into the tpc.int domain [...]. This is
preferable to having the same name possibly be an autonomous system
number, telephone number, and/or host as well as a zone and a user.
3.3 The KEY RR Flag Octet
In the "flags" field:
Bit 0 is the "no key" bit. If this bit is on, there is no key
information and the RR stopw with the flags octet.
Bits 1 is the "experimental" bit. Keys may be associated with
zones or entities for experimental, trial, or optional use, in which
case this bit will be one. If this bit is a zero, it means that the
use or availability of security based on the key is "mandatory".
Thus, if this bit is off for a zone, the zone should be assumed
secured by SIG RRs and any responses indicating the zone is not
secured should be considered bogus. Similarly, if this bit were off
for a host key and attempts to negotiate IP-security with the host
produced indications that IP-security was not supported, it should be
assumed that the host has been compromised or communications with it
are being spoofed. On the other hand, if this bit were a one, the
host might very well sometimes operate in a secure mode and at other
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times operate without the availability of IP-security. The
experimental bit, like all other aspects of the KEY RR, is only
effective if the KEY RR is appropriately signed by a SIG RR.
Bit 2 is the "signatory" bit. It indicates that the key can
validly sign RRs of the same name. If the owner name is a wildcard,
then RRs with any name which is in the wildcard's scope can be signed
including NS and corresponding KEY RRs to carve out a subzone. This
bit is meaningless for zone keys which always have authority to sign
any RRs in the zone. The signatory bit, like all other aspects of
the KEY RR, is only effective if the KEY RR is appropriately signed
by a SIG RR.
Bits 3-4 are reserved and must be zero. If they are found non-
zero, they should be ignored and the KEY RR used as indicated by the
other flags.
Bit 5 on indicates that this is a key associated with a "user"
or "account" at an end entity, usually a host. The coding of the
owner name is that used for the responsible individual mailbox in the
SOA record: The owner name is the user name as the name of a node
under the entity name. For example, "j.random_user" on
host.subdomain.domain could have a public key associated through an
KEY RR with name j\.random_user.host.subdomain.domain. It could be
used in an IP-security protocol where authentication of a user was
desired. This key would be useful in IP or other security for a user
level service such a telnet, ftp, rlogin, etc.
Bit 6 on indicates that this is a key associated with the non-
zone entity whose name is the RR owner name. This will commonly be a
host but could, in some parts of the DNS tree, be some other type of
entity such as an Autonomous System [draft-ietf-dnssec-as-map-*.txt].
This is the public key used in connection with the optional DNS
transaction authentication service that can be used if the owner name
is a DNS server host. It could also be used in an IP-security
protocol where authentication of a host was desired.
Bit 7 on indicates that this is a zone key for the zone whose
name is the RSA RR owner name. This is the fundamental type of DNS
data origin authentication public key.
3.4 The KEY Algorithm Version
This octet is the key algorithm version parallel to the same field
for the SIG resource. The MD5/RSA algorithm described in this draft
is number 1. Version numbers 2 through 253 are available for
assignment should sufficient reason arise. However, the designation
of a new version could have a major impact on interoperability
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requires an IETF standards action. Version 254 is reserved for
private use and will never be assigned a specific algorithm. For
version 254, the combined "public exponent", "modulus length" and
"modulus" area shown above will actually begin with an Object
Identifier (OID) indicating the private algorithm in use and the
remainder of the combined area is whatever is required by that
algorithm. Algorithm versions 0 and 255 are illegal.
3.5 KEY RRs in the Construction of Responses
A request for KEY RRs does not cause any special additional
information processing except, of course, for the corresponding SIG
RR from a security aware server.
Security aware DNS servers will include KEY RRs as additional
information in responses where appropriate as follows:
On the retrieval of NS RRs, the zone key KEY RR(s) for the zone
served by these name servers will be included. If not all additional
info will fit, the KEY RR(s) have lower priority than type A glue
RRs.
On retrieval of type A RRs, the end entity KEY RR(s) for the
host named will be included. On inclusion of A RRs as additional
information, their KEY RRs will also be included but with lower
priority than the relevant A RRs.
3.6 File Representation of KEY RRs
KEY RRs may appear as lines in a zone data file.
The object name appears first.
The flag octet and algorithm version octets are then represented as
unsigned integers; however, if the "no key" flag is on in the flags,
nothing appears after the flag octet.
If the algorithm specified is the MD5/RSA algorithm, then the
exponent and modulus appear. The public key exponent is an unsigned
integer from 3 to 16777215. The public key modulus can be quite
large, up to 319 octets. It is the last data field and is
represented in hex and may be divided up into any number of white
space separated substrings, down to single hex digits, which are
concatenated to obtain the full signature. These hex substrings can
span lines using the standard parenthesis.
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If an algorithm from 2 through 253 is specified, the public key
parameters required by that algorithm are given. If the algorithm
specified is number 254, then an OID appears followed by whatever is
required for the private algorithm. Algorithm versions 0 and 255 are
illegal.
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4. The SIG Resource Record
The SIG or "signature" resource record (RR) is the fundamental way
that data is authenticated in the secure DNS. As such it is the
heart of the security provided.
The SIG RR unforgably authenticates other RRs of a particular type
and binds them to a time interval and the signer's fully qualified
domain name. This is done using cryptographic techniques and the
signer's private key. The signer is usually the owner of the zone
from which the RR originated.
4.1 SIG RDATA Format
The RDATA portion of a SIG RR is as shown below. The integrity of
the RDATA information and that of the SIG RRs owner, type, and class
are protected by the signature field.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| type covered | algorithm | labels |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| original TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| signature expiration |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| time signed |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| key footprint | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ signer's name /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| signature /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value of the SIG type is 24.
The "type covered" is the type of the other RRs covered by this SIG.
The algorithm version number is an octet specifying the digital
signature algorithm used. The MD5/RSA algorithm described in this
draft is version one. Version numbers 2 through 253 are available
for assignment should sufficient reason arise to allocate them.
However, the designation of a new version could have a major impact
on the interoperability of the global DNS systems and requires an
IETF standards action. Version 254 is reserved for private use and
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will never be assigned a specific algorithm. For version 254, the
"signature" area shown above will actually begin with an Object
Identified (OID) indicating the private algorithm in use and the
remainder of the signature area is whatever is required by that
algorithm. Version numbers 0 and 2555 are illegal.
The "labels" octet is an unsigned count of how many labels there are
in the original SIG RR owner name not counting the null label for
root and not counting any initial "*" for a wildcard. If, on
retrieval, the RR appears to have a longer name, the resolver can
tell it is the result of wildcard substitution. If the RR owner name
appears to be shorter than the labels count, the SIG RR should be
considered corrupt and ignored. The maximum number of labels
possible in the current DNS is 127 but the entire octet is reserved
and would be required should DNS names ever be expanded to 255
labels. If a secured retrieval is the result of wild card
substitution, it is necessary for the resolver to use the original
form of the name in verifying the digital signature. The field helps
optimize the determination of the original form.
The "original TTL" field is included in the RDATA portion to avoid
authentication problems that caching servers would otherwise cause by
decrementing the real TTL field and security problems that
unscrupulous servers could otherwise cause by manipulating the real
TTL field. This original TTL is protected by the signature while the
real TTL field is not.
The SIG is valid until the "signature expiration" time which is an
unsigned number of seconds since the start of 1 January 1970.
The "time signed" field is an unsigned number of seconds since the
start of 1 January 1970.
The "key footprint" is a 16 bit quantity that is used to help
efficiently select between multiple keys which may be applicable and
as a quick check that a public key about to be used for the
computationally expensive effort to decode the signature is possibly
valid. Its exact meaning is algorithm dependent. For the MD5/RSA
algorithm, it is the next to the bottom two octets of the public key
modulus needed to decode the signature field. That is to say, the
most significant 16 of the lest significant 24 bits of this quantity.
The "signer's name" field is the fully qualified domain name of the
signer generating the SIG RR. This is usually the zone which
contained the RR(s) being authenticated.
The structured of the "signature" field depends on the algorithm
chosen and is described below.
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4.1.1 Signature Format
The actual signature portion of the SIG RR binds the owner, signer,
class, original TTL, time signed, flags, and expiration time of the
SIG RR to all of the "type covered" RRs with that owner name. These
covered RRs are thereby authenticated. To accomplish this, a data
sequence is constructed as follows:
data = algorithm | original TTL | Expiration Time | Time signed |
Signer's Name | RR(s)...
where | is concatenation and RR(s) are all the expanded (no name
abbreviation) RR(s) of the type covered with the same owner name as
the SIG RR in canonical order. The canonical order for RRs is to
sort them in ascending order as left justified unsigned octet
sequences where a missing octet sorts before a zero octet.
How this data sequence is processed into the signature is algorithm
dependent.
For the MD5/RSA algorithm, the signature is as follows
hash = MD5 ( data )
signature = ( 01 | FF* | 00 | hash ) ** e (mod n)
where "|" is concatenation, "e" is the secret key exponent of the
signer, and "n" is the public modulus that is the signer's public
key. 01, FF, and 00 are fixed octets of the corresponding
hexadecimal value. The FF octet is repeated the maximum number of
times such that the value of the quantity being exponentiated is one
octet shorter than the value of n.
The size of n, including most and least significant bits (which will
be 1) SHALL be not less than 512 bits and not more than 2552 bits. n
and e MUST be chosen such that the public exponent is less than 2**24
- 1 and SHOULD be chosen such that the public exponent is small.
The above specifications are similar to Public Key Cryptographic
Standard #1 [PKCS1].
(A public exponent of 3 minimizes the effort needed to decode a
signature. Use of 3 as the public exponent may be weak for
confidentiality uses since, if the same data can be collected
encrypted under three different keys with an exponent of 3 then,
using the Chinese Remainder Theorem [ref], the original plain text
can be easily recovered. This weakness is not significant for DNS
because we seek only authentication, not confidentiality.)
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4.1.2 SIG RRs Covering Type ANY
The SIG RR described above protects all the RRs with a particular
owner name and type. Thus a server must supply them all to convince
a security aware resolver. However, an unscrupulous server could
claim there were no RRs of a particular type under an owner name
while presenting signed RRs of other types. To provide a means of
protection against this, one SIG RR is added for each owner name that
covers the type ANY. It is calculated as indicated above except that
all RRs for that owner name, except the SIG RR covering type ANY
itself, are included in the data string which is processed into the
signature.
4.1.3 Zone Transfer (AXFR) SIG
The above SIG mechanisms assure the authentication of all the RRs of
a particular name and type and all the RRs of a particular name and
any type. However, to secure complete zone transfers, a SIG RR owned
by the zone name must be created with a type covered of AXFR that
covers all other zone signed RRs. It will be calculated by hashing
together all other static zone RRs, including SIGs. The RRs are
ordered and concatenated for hashing as described in Section 4.1.1.
This SIG, other than having to be calculated last of all zone key
signed SIGs in the zone, is the same as any other SIG.
Dynamic zone RRs which might be added by some future dynamic zone
update protocol and signed by an end entity or user key rather than a
zone key (see Section 3.2) are not included. They originate in the
network and will not, in general, be migrated to the recommended off
line zone signing procedure (see Section 8.2). Thus such dynamic RRs
are not directly signed by the zone and are not generally protected
against omission during zone transfers.
4.1.4 Transaction SIGs
A response message from a security aware server may optionally
contain a special SIG as the last item in the additional information
section to authenticate the transaction.
This SIG has a "type covered" field of zero, which is not a valid RR
type. It is calculated by using a "data" (see section 4.1.1) of the
entire preceding DNS reply message, including header, concatenated
with the entire DNS query message that produced this response,
including the query's header. That is
data = full response (less trailing message SIG) | full query
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Verification of the message SIG (which is signed by the server host
key, not the zone key) by the requesting resolver shows that the
query and response were not tampered with in transit and that the
response corresponds to the intended query.
4.2 SIG RRs in the Construction of Responses
Security aware servers MUST, for every RR the query will return,
attempt to send the available SIG RRs which authenticate the
requested RR. If multiple such SIGs are available, there may be
insufficient space in the response to include them all. In this
case, SIGs whose signer is the zone containing the RR MUST be given
highest priority and retained even if SIGs with other signers must be
dropped.
To minimize possible truncation problems, if a SIG covers any RR that
would be in the answer section of the response, its automatic
inclusion MUST be the answer section. If it covers an RR that would
appear in the authority section, its automatic inclusion MUST be in
the authority section. If it covers an RR that would appear in the
additional information section it MUST appear in the additional
information section.
Optionally, DNS transactions may be authenticated by a SIG RR at the
end of the response in the additional information section (section
4.1.4). Such SIG RRs are signed by the DNS server originating the
response. Although the signer field must be the name of the
originating server host, the name, class, TTL, and original TTL, are
meaningless. The class and TTL fields can be zero. To save space,
the name should be root (a single zero octet).
[There may be a problem with SIG and NXD RR's associated with domain
names that are CNAMEs. The DNS RFCs prohibit other types of RRs
appearing with a CNAME RR. This problem is being ignored until it is
clear if DNS servers will really have a problem with this.]
4.3 Processing Responses with SIG RRs
If SIG RRs are received in response to a query specifying the SIG
type, no special processing is required but a security aware client
may wish to authenticate them by decoding the signature and applying
consistency checks.
If SIG RRs are received in any other response, a security aware
client should decoded them using the public key of the signer. The
result of such decoding should thenbe verified against the
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appropriate other RRs retrieved.
If the message does not pass reasonable checks or the SIG does not
check against the signed RRs, the SIG RR is invalid and should be
ignored. The time of receipt of the SIG RR must be in the inclusive
range of the time signed and the signature expiration but the SIG can
be retained and remains locally valid until the expiration time plus
the authenticated TTL.
If the SIG RR is the last RR in a response in the additional
information section and has a type covered of zero, it is a
transaction signature of the the response and the query that produced
the response. It may be optionally checked and the message rejected
if the checks fail. But even it the checks succeed, such a
transaction authentication SIG does NOT authenticate any RRs in the
message. Only proper direct or hashed RR signets signed by the zone
can authenticate RRs. If a resolver does not implement transaction
SIGs, it MUST at least ignore them without error.
If all reasonable checks indicate that the SIG RR is valid then RRs
verified by it should be considered authenticated and all other RRs
in the response should be considered with suspicion.
4.4 File Representation of SIG RRs
A SIG RR covering RRs can be represented as a single logical line in
a zone data file [RFC1033] but there are some special problems as
described below. (It does not make sense to include a transaction
authenticating SIG RR in a file as it is a transient authentication
that must be calculated in real time by the DNS server.)
There is no particular problem with the signer, covered type, and
times. The time fields appears in the form YYYYMMDDHHMMSS where YYYY
is the year, the first MM is the month number (01-12), DD is the day
of the month (01-31), HH is the hour in 24 hours notation (00-23),
the second MM is the minute (00-59), and SS is the second (00-59).
The original TTL and algorithm fields appears as unsigned integers.
The "labels" field does not appear in the file representation as it
can be calculated from the owner name.
The key footprint appears as an eight digit unsigned hexadecimal
number.
However, the signature itself can be very long. It is the last data
field and is represented in hex and may be divided up into any number
of white space separated substrings, down to single hex digits, which
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are concatenated to obtain the full signature. These hex substrings
can be split between lines using the standard parenthesis.
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5. Non-existent Names
The SIG RR mechanism described in section 4 above provides strong
authentication of RRs that exist in a zone. But is it not
immediately clear how to authenticatably deny the existence of a name
in a zone.
The nonexistence of a name in a zone is indicates by the NXD RR for a
name interval containing the nonexistent name. An NXD RR and its SIG
are returned in the additional information section, along with the
error, if the resolver is security aware. NXD RRs can also be
returned if an explicit query is made for the NXD type.
The existence of a complete set of NXD records in a zone means that
any query for any name to a security aware server serving the zone
should result in an reply containing at least one signed RR.
5.1 The NXD Resource Record
The NXD resource record is used to securely indicate that no RRs with
an owner name in a certain name interval exist in a zone.
The owner name of the NXD RR is an existing name in the zone. It's
RDATA is another existing name in the zone. The presence of the NXD
RR means that no name between its owner name and the name in its
RDATA area exists. This implies a canonical ordering of all domain
names in a zone.
The ordering is to sort labels as unsigned left justified octet
strings where the absence of a byte sorts before a zero byte. Names
are then sorted by sorting on the highest level label and then,
within those names with the same highest level label by the next
lower label, etc. Since we are talking about a zone, the zone name
itself always exists and all other names are the zone name with some
prefix of lower level labels. Thus the zone name itself always sorts
first.
There is a slight problem with the last NXD in a zone as it wants to
have an owner name which is the last existing name in sort order,
which is easy, but it is not obvious what name to put in its RDATA to
indicate the entire remainder of the name space. This is handled by
treating the name space as circular and putting the zone name in the
RDATA of the last NXD.
There are additional complexities due to interaction with wildcards
as explained below.
The NXD RRs for a zone can be automatically calculated and added to
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the zone by the same recommended off-line process that signs the
zone. The NXD RRs TTL should not exceed the zone minimum TTL.
The type number for the NXD RR is xxx.
5.2 NXD RDATA Format
The RDATA for an NXD RR consists simply of a domain name.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| next domain name /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.3 Example
Assume a zone has entries for
big.foo.bar
medium.foo.bar
small.foo.bar
tiny.foo.bar
Then a query to a security aware server for huge.foo.bar would
produce an error reply with the additional information section
containing
big.foo.bar NXD medium.foo.bar
5.4 Interaction of NXD RRs and Wildcard RRs
As a wildcard RR causes all possible names in an interval to exist,
there should not be an NXD record that would cover any part of this
interval. Thus if *.X.ZONE exists you would expect an NXD RR that
ends at X.ZONE and one that starts with the last name covered by
*.X.ZONE. However, this "last name covered" is something very ugly
and long like \255\255\255....X.zone. So the NXD for the interval
following is simply given the owner name *.X.zone. This "*" type
name is not expanded when the NXD is returned as additional
information in connection with a query for a non-existent name.
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If there could be any wildcard RRs in a zone and thus wildcard NXDs,
care must be taken in interpreting the results of explicit NXD
retrievals as the owner name may be a wildcard expansion.
The existence of one or more wildcard RRs covering a name interval
makes it possible for a malicious server to hide any more specificly
named RRs in the internal. The server can just falsely return the
wildcard match NXD instead of the more specificly named RRs. If
there is a zone wide wildcard, there will be only one NXD RR whose
owner name and RDATA are both the zone name. In this case a server
could conceal the existence of any more specific RRs in the zone. It
would be possible to make a more complex NXD feature, taking into
account the types of RRs that did not exist in a name interval, and
the like, which would eliminate this possibility. But it would be
more complex and would be so constraining as to make any future
dynamic update feature that could create new names very difficult
(see Section 3.2).
5.5 Blocking NXD Pseudo-Zone Transfers
In a secure zone, a resolver can query for the initial NXD associated
with the zone name. Using the RDATA field from that RR, it can query
for the next NXD RR. By repeating this, it can walk through all the
NXDs in the zone. If there are no wildcards, it can use this
technique to find all names in a zone. If it does type ANY queries,
it can incrementally get all information in the zone and perhaps
defeat attempts to administratively block zone transfers.
If there are any wildcards, this NXD walking technique will not find
any more specific RR names in the part of the name space the wildcard
covers. By doing explicit retrievals for wildcard names, a resolver
could determine what intervals are covered by wildcards but still
could not, with these techniques, find any names inside such
intervals except by trying every name.
If it is desired to block NXD walking, the recommended method is to
add a zone wide wildcard of the KEY type which asserts the
nonexistence of any keys. This will cause there to be only one NXD
RR in the zone for the zone name and leak no information about what
real names exist in the zone. This protection from pseudo-zone
transfers is bought at the expense of eliminating the data origin
authentication of the non-existence of names that NXD RRs can
provide. If an entire zone is covered by a wildcard, a malicious
server can return an RR produced by matching that wildcard and can
thus hide all the real data and delegations with more specific names
in the zone.
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6. How to Resolve Securely
Retrieving or resolving authentic data from the DNS involves starting
with one or more trusted public keys and, in general, progressing
securely from them through the DNS zone structure to the zone of
interest. Such trusted public keys would normally be configured in a
manner similar to that described in section 6.1. However, as a
practical matter, a security aware resolver would still gain some
confidence in the results it returns even if it was not configured
with any keys but trusted what it got from a local well known server
as a starting point.
6.1 Boot File Format
The recommended format for a boot file line to configure starting
keys is as follows:
zonekey f.q.d.n algorithm [exponent modulus]
for a zone public key or
hostkey f.q.d.n algorithm [exponent modulus]
for a host public key. f.q.d.n is the domain name of the zone or
host. Algorithm is the algorithm in use where one is the MD5/RSA
algorithm and 254 indicates a private algorithm. The material after
the algorithm is algorithm dependent and, for private algorithms,
starts with the algorithm's identifying OID. For the RSA algorithm,
it is the public key exponent as a decimal number between 3 and
16777215, and the modulus in hex.
While it might seem logical for everyone to start with the key for
the root zone, this has problems. The logistics of updating every
DNS resolver in the world when the root key changes would be
excessive. It may be some time before there even is a root key.
Furthermore, many organizations will explicitly wish their "interior"
DNS implementations to completely trust only their own zone. These
interior resolvers can then go through the organization's zone server
to access data outsize the organization's domain.
If desired, the IP address for the f.q.d.n's with configured keys can
generally also be configured via an /etc/hosts or similar local file.
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6.2 Chaining Through Zones
Starting with one trusted zone key, it is possible to retrieve signed
keys for subzones which have a key. Every secure zone (except root)
should also include the RSA record for its super-zone signed by the
secure zone. This makes it possible to climb the tree of zones if
one starts below root. A secure sub-zone is generally indicated by a
KEY RR appearing with the NS RRs for the sub-zone. These make it
possible to descend within the tree of zones.
A resolver should keep track of the number of successive secure zones
traversed from a starting point to any secure zone it can reach. In
general, the lower such a distance number is, the greater the
confidence in the data. Data configured via a boot file should be
given a distance number of zero. Should a query encounter different
data with different distance values, that with a larger value should
be ignored.
A security conscious resolver should completely refuse to step from a
secure zone into a non-secure zone unless the non-secure zone is
certified to be non-secure or only experimentally secure by the
presence of an authenticated KEY RR for the non-secure zone with a no
key flag or the presence of a KEY RR with the experimental bit set.
Otherwise the resolver is probably getting completely bogus or
spoofed data.
If legitimate non-secure zones are encountered in traversing the DNS
tree, then no zone can be trusted as secure that can be reached only
via information from such non-secure zones. Since the non-secure zone
data could have been spoofed, the "secure" zone reach via it could be
counterfeit. The "distance" to data in such zones or zones reached
via such zones could be set to 512 or more as this exceeds the
largest possible distance through secure zones in the DNS.
Nevertheless, continuing to apply secure checks within "secure" zones
reached via non-secure zones will, as a practical matter, provide
some increase in security.
The existence of cross certifications adds further policy questions.
Assume we have a zone B.A and a zone Y.X. Many possibilities exist
for a secure resolver configured with the B.A key to get to Y.X. If
all the zones along the way are secure, it could climb to root and
then descend the other side, a total of four hops (B.A -> A -> . -> X
-> Y.X). If the B.A administrator had installed a cross certified
key for Y.X in the B.A zone, using that would be a shorter and
presumably more secure way to find Y.X's key which would be immune to
the non-security or even compromise of the servers for A or root or
X. But what if some less trusted subzone of B.A, say C.B.A installed
a cross certified key for Y.X? If there is a conflict, should this
be preferred to a hierarhically derived key obtained by climbing to
root and descending? Such questions are a matter of local resolver
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policy.
6.3 Secure Time
Coordinated interpretation of the time fields in SIG RRs requires
that reasonably consistent time be available to the hosts
implementing the DNS security extensions.
A variety of time synchronization protocols exist including the
Network Time Protocol (NTP, RFC1305). If such protocols are used,
they should be used securely so that time can not be spoofed.
Otherwise, for example, a host could get its clock turned back and
might then believe old SIG and KEY RRs which were valid but no longer
are.
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7. Operational Considerations
This section discusses a variety of considerations in secure
operation of DNS using these proposed protocol extensions.
7.1 Key Size Considerations
There are a number of factors that effect public key size choice for
use in the DNS security extension. Unfortunately, these factors
usually do not all point in the same direction. Choice of a key size
should be made by the zone administrator depending on their local
conditions.
For most schemes, larger keys are more secure but slower.
Verification (the most common operation) for the MD5/RSA algorithm
will vary roughly with the square of the modulus length, signing will
vary with the cube of the modulus length, and key generation (the
least common operation) will vary with the fourth power of the
modulus length. The current best algorithms for factoring a modulus
and breaking RSA security vary roughly with the square of the modulus
itself. Thus going from a 640 bit modulus to a 1280 bit modulus only
increases the verification time by a factor of 4 but vastly increases
the work factor of breaking the key. [RSA FAQ]
However, larger keys increase size of the KEY RR and usually the SIG
RR. This increases the change of UDP packet flow and the possible
necessity for using higher overhead TCP.
The recommended minimum RSA algorithm modulus size, 640 bits, is
believed by the authors to be secure at this time and for some years
but high level nodes in the DNS tree may wish to set a higher
minimum, perhaps 1000 bits, for security reasons. (Since the United
States National Security Agency generally permits export of
encryption systems using an RSA modulus of up to 512 bits, use of
that small a modulus, i.e. n, must be considered weak.)
7.2 Key Storage
It is strongly recommended that zone private keys and the zone file
master copy be kept and used in off-line non-network connected
physically secure machines only. Periodically an application can be
run to re-sign the RRs in a zone by adding NXD and SIG RRs. Then the
augmented file can be transferred, perhaps by sneaker-net, to the
networked zone primary server machine.
The idea is to have a one way information flow to the network to
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avoid the possibility of tampering from the network. Keeping the
zone master file on-line on the network and simply cycling it through
an off-line signer does not do this. The on-line version could still
be tampered with if the host it resides on is compromised. The
master copy of the zone file should be off net and should not be
updated based on an unsecured network mediated communication.
Non-zone private keys, such as host keys, may have to be kept on line
to be used for real-time purposes such a IP secure session set-up.
7.3 Key Generation
Careful key generation is a sometimes over looked but absolutely
essential element in any cryptographically secure system. The
strongest algorithms used with the longest keys are still of no use
if an adversary can guess enough to lower the size of the likely key
space so that it can be exhaustively searched. Suggestions will be
found in draft-ietf-security-randomness-*.txt.
It is strongly recommended that key generation also occur off-line,
perhaps on the machine used to sign zones (see Section 7.2).
7.4 Key Lifetimes
No key should be used forever. The longer a key is in use, the
greater the probability that it will have been compromised through
carelessness, accident, espionage, or cryptanalysis.
No zone key should have a lifetime significantly over five years.
The recommended maximum lifetime for zone keys that are kept off-line
and carefully guarded is 13 months with the intent that they be
replaced every year. The recommended maximum lifetime for end entity
keys that are used for IP-security or the like and are kept on line
is 36 days with the intent that they be replaced monthly or more
often. In some cases, an entity key lifetime of a little over a day
may be reasonable.
7.5 Revocation
[This is new and should probably be moved to someplace above...]
Data in the DNS is authenticated via keyed digital signatures. Thus
to revoke data, we need to alter the time period of the key's
effectiveness to not include the date signed of the data to be
remoked. Note that the case of a compromised key, where we wish to
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make its period of effectiveness null, we generally need only think
of the KEY RR as data and make the period of effectiveness of the
superzone's key not include the time of the previous superzone
signing of the key.
Normally, a KEY retrieved from the DNS appears to be effective from
the beginning of time until the expiration of the SIG that
authenticated it. (If this were not so, a secure zone would have to
be resigned every time its superzone is resigned.) When data is
being revoked, a zone can be resigned with a later date signed but a
mechanism is need to make the earlier SIG of the now revoked data not
longer valid.
A dawn for the effectiveness of a node's KEY RR is provided, when
desired, by having the key appear self-signed. This declares the
time signed of the self signature as the time of effectiveness of the
key(s). Note this is entirely under the control of a zone and
asynchronous with its superzone's signings.
This technique covers non-zone entities as well. For an owner to
have a dawn to the effectiveness of its delegated key (which is
signed by the zone key), it need only self-sign its key and the date
signed is the dawn.
7.6 Root
The root zone includes its own key self-signed.
It should also be noted that in DNS the root is a zone unto itself.
Thus the root zone key should only be see signing itself or signing
RRs with names one level below root, such as .aq, .edu, or .arpa.
Implementations MAY reject as bogus any purported root signature of
records with a name more than one level below root.
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8. Conformance
Several levels of server and resolver conformance are defined.
8.1 Server Conformance
Three levels of server conformance are defined as follows:
Zilch server compliance is the ability to store and retrieve
(including zone transfer) SIG, KEY, and NXD RRs. It is believed that
most current servers meet this level of compliance.
Minimal server compliance adds the following to zilch
compliance: (1) ability to read SIG, KEY, and NXD RRs in zone files
and (2) ability, given a zone file and private key, to add
appropriate SIG and NXD RRs, possibly via a separate application.
Primary servers for secure zones must be at least minimally
compliant.
Full server compliance is ability to automatically include SIG,
KEY, and NXD RRs in responses as appropriate, as well as meeting
minimal compliance.
8.2 Resolver Conformance
Two levels of resolver compliance are defined:
A Zilch compliance resolver can handle SIG, KEY, and NXD RRs
when they are explicitly requested. It is believed that current
resolvers are Zilch compliant.
A fully compliant resolver understands KEY, SIG, and NXD RRs,
maintains appropriate information in its local caches and database to
indicate which RRs have been authenticated and to what extent they
have been authenticated, and performs additional queries as necessary
to obtain KEY, SIG, or NXD RRs from non-security aware servers.
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9. Security Considerations
This document concerns technical details of extensions to the Domain
Name System (DNS) protocol to provide data integrity and data origin
authentication, public key distribution, and optional transaction
security.
If should be noted that, at most, these extensions guarantee the
validity of resource records, including KEY resource records,
retrieved from the DNS. They do not magically solve other security
problems. For example, using secure DNS you can have high confidence
in the IP address you retrieve for a host; however, this does not
stop someone for substituting an unauthorized host at that address or
capturing packets sent to that address and responding with packets
apparently from that address. Any reasonably complete security
system will require the protection of many other facets of the
Internet.
References
[PKCS1] - PKCS #1: RSA Encryption Standard, RSA Data Security, Inc.,
3 June 1991, Version 1.4.
[RFC1032] - Domain Administrators Guide, M. Stahl, November 1987
[RFC1033] - Domain Administrators Operations Guide, M. Lottor,
November 1987
[RFC1034] - Domain Names - Concepts and Facilities, P. Mockapetris,
November 1987
[RFC1035] - Domain Names - Implementation and Specifications
[RFC1321] - The MD5 Message-Digest Algorithm, R. Rivest, April 16
1992.
[RSA FAQ] - RSADSI Frequently Asked Questions periodic posting.
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Authors Addresses
Donald E. Eastlake 3rd
Digital Equipment Corporation
550 King Street, LKG2-1/BB3
Littleton, MA 01460
Telephone: +1 508 486 6577(w) +1 508 287 4877(h)
EMail: dee@lkg.dec.com
Charles W. Kaufman
Iris Associates
Telephone: +1 508-392-5276
EMail: charlie_kaufman@iris.com
Expiration and File Name
This draft expires 19 May 1995.
Its file name is draft-ietf-dnssec-secext-02.txt.
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