DNSOP Working Group W. Wijngaards
Internet-Draft NLnet Labs
Intended status: Standards Track G. Wiley
Expires: September 7, 2015 VeriSign, Inc.
March 6, 2015
Confidential DNS
draft-wijngaards-dnsop-confidentialdns-03
Abstract
This document offers opportunistic encryption to provide privacy for
DNS queries and responses.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on September 7, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
1. Introduction
The privacy of the Question, Answer, Authority and Additional
sections in DNS queries and responses is protected by the
confidential DNS protocol by encrypting the contents of each section.
The goal of this change to the DNS protocol is to make large scale
monitoring more expensive, see [draft-bortzmeyer-dnsop-dns-privacy]
and [draft-koch-perpass-dns-confidentiality]. Authenticity and
integrity may be provided by DNSSEC, this protocol does not change
DNSSEC and does not offer the means to authenticate responses.
Confidential communication between any pair of DNS servers is
supported, both between iterative resolvers and authoritative servers
and between stub resolvers and recursive resolvers.
The confidential DNS protocol has minimal impact on the number of
packets involved in a typical DNS query/response exchange by
leveraging a cacheable ENCRYPT Resource Record and an optionally
cacheable shared secret. The protocol supports selectable
cryptographic suites and parameters (such as key sizes).
The client fetches an ENCRYPT RR from the server that it wants to
contact. The public key retrieved in the ENCRYPT RR is used to
encrypt a shared secret or public key that the client uses to encrypt
the sections in the DNS query and which the name server uses to
encrypt the DNS response.
As this is opportunistic encryption, the key is (re-)fetched when the
exchange fails or after the TTL expires. If the key fetch fails or
the encrypted query fails, communication in the clear is performed.
The server advertises which crypto suites and key lengths may be used
in the ENCRYPT RR, the client then chooses a crypto suite from this
list and includes that selection in subsequent DNS queries.
The key from the server can be cached by the client, using the TTL
specified in the ENCRYPT RR, the IP address of the server
distinguishes keys in the cache. The server may also cache shared
secrets and keys from clients.
The optional authenticated mode of operation uses two mechanisms, one
for authoritative and one for recursive servers, that fetch the
public key for the server and sign it with DNSSEC. For authoritative
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servers, the key is included in an extra DS record in the parent's
delegation. For recursive servers the key is at the reverse IP
address location.
2. ENCRYPT RR Type
The RR type for confidential DNS is ENCRYPT, type TBD (decimal). The
presentation format is:
. ENCRYPT [flags] [algo] [id] [data]
The flags, algo and id are unsigned numbers in decimal and the data
is in base-64. The wireformat is: one octet flags, one octet algo,
one octet id and the remainder of the rdata is for the data. The
type is class independent. The domain name of the ENCRYPT record is
'.' (the root label) for hop-by-hop exchanges.
In the flags the least two bits are the usage value. The other flag
bits MUST be sent as zeroes, and the receiver MUST ignore RRs that
have other flag bits set.
o PAD (usage=0): the ENCRYPT contains padding material. Algo and id
are set to 0. Its data length varies (0-63 octets), and may
contain any value. It is used to pad packets to obscure the
packet length. Append such records to make the DNS message for
queries and answers a whole multiple of 64 bytes.
o KEY (usage=1): the ENCRYPT contains a public or symmetric key.
The algo field gives the algorithm. The id identifies the key,
this id is copied to ENCRYPT type RRS to identify which key to use
to decrypt the data. The data contains the key bits.
o RRS (usage=2): encrypted data. The data contains encrypted
resource records. The data is encrypted with the selected
algorithm and key id. The data contains resource records in DNS
wireformat [RFC1034], with a domain name, type, class, ttl,
rdatalength and rdata.
o SYM (usage=3): the ENCRYPT contains an encrypted symmetric key.
The contained, encrypted data is rdata of an ENCRYPT of type KEY
and has the symmetric key. The data is encrypted with the
algorithm and id indicated. The encrypted data encompasses the
flags, algo, id, data for the symmetric key.
The ENCRYPT RR type can contain keys. It uses the same format as the
DNSKEY record [RFC4034] for public keys. algo=0 is reserved for
future expansion of the algorithm number above 255. algo=1 is RSA,
the rdata determines the key size. algo=2 is AES, aes-cbc, size of
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the rdata determines the size of the key.
3. Server and Client Algorithm
If a clients wants to fetch the keys for the server from the server,
it performs a query with query type ENCRYPT and query name '.' (root
label). The reply contains the ENCRYPT (or multiple if a choice is
offered) in the answer section. These ENCRYPTs have the KEY usage.
If a client wants to perform an encrypted query, it sends an
unencrypted outer packet, with query type ENCRYPT and query name '.'
(root label). In the authority section it includes an ENCRYPT record
of type RRS. This encrypts a number of records, the first is a
query-section style query record, and then zero or more ENCRYPTs of
type KEY that the server uses to encrypt the reply. If the client
wants to use a symmetric key, it omits the KEYs, and instead includes
an ENCRYPT of type SYM in the authority section. The ENCRYPT of type
RRs then follows after the SYM and can be encrypted with the key from
that SYM.
If a server wants to encrypt a reply, it also uses the ENCRYPT type.
The reply looks like a normal DNS packet, i.e. it has a normal
unencrypted outer DNS packet. Because the query name and query type
have been encrypted, the outer packet has a query name of '.' and
query type of ENCRYPT and the reply has an ENCRYPT type RRS in the
answer section. The reply RRs have been encrypted into the data of
the ENCRYPT record. The RRS data starts with 10 bytes of header; the
flags and section counts.
The client may lookup keys whenever it wants to. It may cache the
keys for the server, using the TTL of those ENCRYPT records. It
should also cache failures to lookup the ENCRYPT record for some
time. If the client fails to look up the ENCRYPT records it MUST
fall back to unencrypted communication (this is the opportunistic
encryption case). The result of an encrypted query may also be
timeouts, errors or replies with mangled contents, in that case the
client MUST fall back to unencrypted communication (this is the
opportunistic encryption case).
If some middlebox removes the ENCRYPT from the authority section of
an encrypted query, the query looks like a . ENCRYPT lookup and
likely a reply with ENCRYPTs of type KEY is returned instead of the
encrypted reply with an ENCRYPT of type RRS, and again the client
does the unencrypted fallback (this is the opportunistic encryption
case). If the server has changed its keys and does not recognize the
keys in an encrypted query, it should return an ENCRYPT record of
type PAD with no data. A server may decide it does not (any longer)
have the resources for encryption and reply with SERVFAIL to
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encrypted queries, forcing unencrypted fallback (this is the
opportunistic encryption case). Keys for unknown algorithms should
be ignored by the client, if no usable keys remain, fallback to
insecure (this is for both opportunistic and authenticated).
The client may cache the ENCRYPT of type SYM for a server together
with the symmetric secret, this is better for performance, as public-
key operations can be avoided for repeated queries. The server may
also cache the ENCRYPTs of type SYM with the decoded secret,
associating a lookup for the rdata of the SYM record with the decoded
secret, avoiding public-key operations for repeated queries. This is
why the SYM record is sent separately in the authority section in
queries (it is identical and can be used for cache lookups).
Key rollover is possible, support the old key for its TTL, while
advertising the new key, for the servers. For clients, generate a
new public or symmetric key and use it.
4. Authenticated Operation
The previous documented the opportunistic operation, where deployment
is easier, but security is weaker. This documents options for
authenticated operation. The client selects if encryption is
authenticated, opportunistic, or disabled in its local policy
(configuration).
The authentication happens with a DNSSEC signed DS record that
carries the key for confidential DNS. This removes a full roundtrip
from the connection setup cost. The DS has hash type TBDhashtype,
that is specific for confidential DNS. The DS record carries a flag
byte and the public key (in DNSKEY's wireformat) in its rdata. This
means that the confidential DNS keys are acquired with a referral to
the zone and are secured with DNSSEC.
Because the key itself is carried, the probe sequence can be omitted
and an encrypted query can be sent to the delegated server straight
away. The nameservers for that zone then MUST support using that key
for encrypting packets. The servers have the same key with
authenticated mode, where with the opportunistic mode, every server
could have its own key.
Validators do not know or support the DS with ENCRYPT hash type,
those validators ignore them and continue to DNSSEC validate the
zone. Validators that support the new hash type should use them to
encrypt messages and use the remaining DS records to DNSSEC validate
the zone.
This changes the opportunistic encryption to authenticated
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encryption. The fallback to insecure is still possible and this may
make deployment easier. The one byte at the start of the base64
data, in its least significant bit, signals if fallback to insecure
is allowed (value 0x01). That gives the zone owner the option to
enable fallback to insecure or if it should be disabled. The
remainder of the DS base64 data contains a public key in the same
format as when sent in the rdata of ENCRYPT KEY. The type of the key
is in the key type field of this DS record. With fallback to
insecure disabled and the keys authenticated the confidential DNS
query and response should be fully secure (i.e. not
'Opportunistically' secure).
With fallback to insecure disabled, queries fail instead of falling
back to insecure. This means no answer is acquired, and DNS lookups
for that zone fail because the security failed.
The DS method works for authority servers. Recursors need another
method. The client looks up reverse-of-recursors-IP.arpa ENCRYPT and
gets the keys signed with DNSSEC from there (type ENCRYPT KEY
lookup). If there is no dnssec secure answer with a key, the
opportunistic key exchange is attempted. Do this for DNSSEC-insecure
answers, if there is no trust anchor, or when no such name and
ENCRYPT are present. If it is dnssec bogus, then authentication
failed and it is not possible to communicate with the server (with
the authenticated communication mode selected by the client).
5. IANA Considerations
An RR type registration for type ENCRYPT with number TBD and it
references this document [[to be done when this becomes RFC]].
A DS record hash type is registered TBDhashtype that references this
document. It is for the confidential DNS public key, acronym
ENCRYPT.
6. Security Considerations
Opportunistic encryption can be configured. Opportunistic encryption
has many drawbacks against active intrusion, but it works against
pervasive passive surveillance, and thus it improves privacy.
With authentication (if selected by the client) the key is secured
with DNSSEC.
This technique encrypts DNS queries and answers, but other data
sources, such as timing, IP addresses, and the packet size can be
observed. These could provide almost all the information that was
encrypted.
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7. Acknowledgments
Roy Arends
8. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
Authors' Addresses
Wouter Wijngaards
NLnet Labs
Science Park 140
Amsterdam 1098 XH
The Netherlands
EMail: wouter@nlnetlabs.nl
Glen Wiley
VeriSign, Inc.
Reston, VA
USA
EMail: gwiley@verisign.com
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