Internet DRAFT - draft-muks-dns-message-checksums
draft-muks-dns-message-checksums
Internet Engineering Task Force M. Sivaraman
Internet-Draft Internet Systems Consortium
Intended status: Experimental September 27, 2015
Expires: March 30, 2016
DNS message checksums
draft-muks-dns-message-checksums-00
Abstract
This document describes a method for a client to be able to verify
that IP-layer PDU fragments of a UDP DNS message have not been
spoofed by an off-path attacker.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 30, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. DNS message checksum method . . . . . . . . . . . . . . . . . 3
3. The CHECKSUM EDNS(0) option . . . . . . . . . . . . . . . . . 4
3.1. Wire format . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Option fields . . . . . . . . . . . . . . . . . . . . . . 4
3.2.1. NONCE . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2.2. ALGORITHM . . . . . . . . . . . . . . . . . . . . . . 4
3.2.3. DIGEST . . . . . . . . . . . . . . . . . . . . . . . 4
3.2.4. NONCE-COPY . . . . . . . . . . . . . . . . . . . . . 5
3.3. Presentation format . . . . . . . . . . . . . . . . . . . 5
4. Checksum computation . . . . . . . . . . . . . . . . . . . . 5
5. Effects of using checksums . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Checksum algorithms . . . . . . . . . . . . . . . . 7
Appendix B. Change History (to be removed before publication) . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
[RFC1035] describes how DNS messages are to be transmitted over UDP.
A DNS query message is transmitted using one UDP datagram from client
to server, and a corresponding DNS reply message is transmitted using
one UDP datagram from server to client.
As a UDP datagram is transmitted in a single IP PDU, in theory the
size of a UDP datagram (including various lower internet layer
headers) can be as large as 64 KiB. But practically, if the datagram
size exceeds the path MTU, then the datagram will either be
fragmented at the IP layer, or dropped by a forwarder. In the case
of IPv4, DNS datagrams may be fragmented by a sender or a forwarder.
In the case of IPv6, DNS datagrams are fragmented by the sender only.
IP-layer fragmentation for large DNS response datagrams introduce
risk of cache poisoning by off-path attackers [Fragment-Poisonous] in
which an attacker can circumvent some defense mechanisms like port,
IP, and query randomization [RFC5452].
This memo introduces the concept of a DNS message checksum which may
be used to stop the effects of such off-path attacks.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. DNS message checksum method
Clients supporting DNS message checksums add an EDNS option to their
queries, which declares their support for this feature.
The CHECKSUM EDNS option contains 4 fields: NONCE, ALGORITHM, DIGEST,
and NONCE-COPY. These fields are described in Section 3.
It is OPTIONAL for a client to add a CHECKSUM EDNS option to DNS
query messages. If it adds such an option, it MUST set the NONCE
field to a random 64-bit unsigned integer. The ALGORITHM field MUST
be set to 0 and the DIGEST field MUST be left empty. The NONCE field
MUST be randomly generated (i.e., in no predictable sequence) for
each query for which the client uses a CHECKSUM EDNS option. The
NONCE-COPY field MUST be set identical to the value in the NONCE
field. The client is expected to remember the per-query NONCE
field's value to be used in verifying the reply to this query
message.
A client MUST NOT send multiple DNS query messages with the NONCE set
to a fixed unchanging value. Instead, it must not send the option at
all.
The server SHOULD add a CHECKSUM EDNS option in the reply message to
a corresponding query that arrived with this option present. The
NONCE field MUST be copied verbatim from the query message to the
corresponding reply message. A checksum is computed over the DNS
reply message as described in Section 4 and the ALGORITHM and DIGEST
fields MUST be set using the resulting checksum as described in
Section 3. The NONCE-COPY field MUST be set identical to the value
in the NONCE field. The server is at liberty to choose any checksum
algorithm it wants to. A list of algorithms is given in Appendix A.
When a client receives a reply message for which it sent a CHECKSUM
EDNS option in the corresponding query, it SHOULD look for the
presence of the CHECKSUM EDNS option in the reply. The client may
handle the lack of a CHECKSUM EDNS option in the reply as it chooses
to.
If a CHECKSUM EDNS option is present in the reply, the client SHOULD
first check and ensure that both the NONCE and NONCE-COPY fields
contain the same nonce value that was sent in the corresponding query
message. If the nonce is different in either of these two fields,
the reply message MUST be discarded. Afterwards, the client SHOULD
proceed to compute a checksum over the reply message as described in
Section 4 using the checksum algorithm in the ALGORITHM field. It
SHOULD then compare the checksum value with the value that was
received in the DIGEST field for equality. If they are not equal,
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the reply message MUST be discarded. If they are equal, the reply
message can be used normally as the client intends to use it.
3. The CHECKSUM EDNS(0) option
CHECKSUM is an EDNS(0) [RFC6891] option that is used to transmit a
digest of a DNS message in replies. Its use described in a previous
section. Here, its syntax is provided.
3.1. Wire format
The following describes the wire format of the OPTION-DATA field
[RFC6891] of the CHECKSUM EDNS option. All CHECKSUM option fields
must be represented in network byte order.
+--------------+------------------+--------------------+
| Option field | Type | Field size |
+--------------+------------------+--------------------+
| NONCE | unsigned integer | 64 bits (8 octets) |
| ALGORITHM | unsigned integer | 16 bits (2 octets) |
| DIGEST | byte array | Variable length |
| NONCE-COPY | unsigned integer | 64 bits (8 octets) |
+--------------+------------------+--------------------+
3.2. Option fields
3.2.1. NONCE
The NONCE field is represented as an unsigned 64-bit integer in
network byte order. It MUST be randomly computed for each query
message which a client sends out, and is copied verbatim from the
query to the corresponding reply DNS message by the server.
3.2.2. ALGORITHM
The ALGORITHM field is represented as an unsigned 16-bit integer in
network byte order. In query messages, it MUST be set to 0. In
reply messages, it MUST contain the numeric value of the algorithm
used to compute the DIGEST field. A list of algorithms and their
values is given in Appendix A.
3.2.3. DIGEST
The DIGEST field is represented as a sequence of octets present after
the NONCE and ALGORITHM fields. Its size is implicitly computed from
the value in the OPTION-LENGTH field [RFC6891] for the CHECKSUM EDNS
option minus the size of the NONCE, ALGORITHM and NONCE-COPY fields.
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In query messages, it MUST be empty. In reply messages, it MUST
contain the digest of the reply message which is computed as
described in Section 3.
3.2.4. NONCE-COPY
The NONCE-COPY field is represented as an unsigned 64-bit integer in
network byte order. Its value MUST be set to be identical to the
NONCE field.
3.3. Presentation format
As with other EDNS(0) options, the CHECKSUM EDNS option does not have
a presentation format.
4. Checksum computation
The NONCE and NONCE-COPY fields are present on either side of the
DIGEST field on purpose, so that an IP-layer PDU fragment will
contain both the DIGEST (in full or part) and at least one of NONCE
or NONCE-COPY fields. Suitable checksum algorithms MUST be chosen so
that the DIGEST field is not so large that this property is violated.
To generate the checksum digest to be placed in the DIGEST field,
first the entire DNS message must be prepared (rendered) along with
the CHECKSUM option embedded in it to the point that it is ready to
be sent out on the wire. In this CHECKSUM option, initially the
DIGEST field must be filled with zero values and its size must be
reserved equal to the size expected for the digest from the checksum
algorithm intended to be used. The NONCE and NONCE-COPY fields MUST
be set to the value of the nonce from the query DNS message. The
ALGORITHM field MUST be set to the checksum algorithm intended to be
used. After this, the whole message contents (from the start of the
DNS message header onwards) must be input to the checksum algorithm
and the calculated checksum must be patched into the DIGEST field,
space for which was reserved before.
To verify the checksum digest from a DNS message that was received,
first the DIGEST field is copied to a temporary location and the
DIGEST field in the message is patched with zero values. After this,
the whole message contents (from the start of the DNS message header
onwards) must be input to the checksum algorithm specified in the
ALGORITHM field. The calculated checksum must be compared for
equality with the checksum originally received in the DIGEST field,
the content of which was earlier saved to a temporary location. If
both are equal, the checksum matches.
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5. Effects of using checksums
The methods in this memo are designed to thwart off-path spoofing
attacks which may lead to cache-poisoning, including the specific
case when IP-layer PDU fragmentation occurs.
The CHECKSUM EDNS option is not designed to offer any protection
against on-path attackers. Very little can be done without using
strong cryptographic methods for this case.
Checksum computation may increase resource usage on servers and
clients. It is thus desirable to use fast checksum algorithms which
provide ample security to verify a short-lived DNS message.
The entropy source used for generating random values for use in the
NONCE field may be chosen similarly to provide ample security to
verify a short-lived DNS message.
As a side-effect of using checksums, resolver cache poisoning attacks
are made more difficult due to the presence of the NONCE field.
6. IANA Considerations
The CHECKSUM EDNS(0) option requires an option code to be assigned
for it. Checksum algorithms in Appendix A need to be registered as
well.
7. Acknowledgements
TBD.
8. References
[Fragment-Poisonous]
Herzberg, A. and H. Shulman, "Fragmentation Considered
Poisonous", 2012.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5452] Hubert, A. and R. van Mook, "Measures for Making DNS More
Resilient against Forged Answers", RFC 5452, DOI 10.17487/
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RFC5452, January 2009,
<http://www.rfc-editor.org/info/rfc5452>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/
RFC6891, April 2013,
<http://www.rfc-editor.org/info/rfc6891>.
Appendix A. Checksum algorithms
TBD. This section will list checksum algorithms in a later version
of the draft, after discussion.
Appendix B. Change History (to be removed before publication)
o draft-muks-dns-message-checksums-00
Initial draft.
Author's Address
Mukund Sivaraman
Internet Systems Consortium
950 Charter Street
Redwood City, CA 94063
US
Email: muks@isc.org
URI: http://www.isc.org/
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