One document matched: draft-barwood-dnsext-dns-transport-12.txt
Differences from draft-barwood-dnsext-dns-transport-11.txt
DNS Extensions Working Group G. Barwood
Internet-Draft
Intended status: Experimental 3 October 2009
Expires: April 2010
DNS Transport
draft-barwood-dnsext-dns-transport-12
Status of this Memo
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Copyright (c) 2009 IETF Trust and the persons identified as the
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Abstract
Describes an experimental transport protocol for DNS.
IP fragmentation is avoided, blind spoofing, amplification attacks
and other denial of service attacks are prevented. Latency for a
typical DNS query is a single round trip, after a setup hadnshake.
No per-client server state is required between transactions.
The protocol may have other applications.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions and Objectives . . . . . . . . . . . . . . . . . . 3
2.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Security Objectives . . . . . . . . . . . . . . . . . . . . 4
2.3 Performance Objectives . . . . . . . . . . . . . . . . . . 4
3. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2 Setup request . . . . . . . . . . . . . . . . . . . . . . . 5
3.3 Setup response . . . . . . . . . . . . . . . . . . . . . . 5
3.4 Initial request . . . . . . . . . . . . . . . . . . . . . . 5
3.5 Server response : single page . . . . . . . . . . . . . . . 6
3.6 Server response : multi page . . . . . . . . . . . . . . . 6
3.7 Follow-up request . . . . . . . . . . . . . . . . . . . . . 7
3.8 Encryption and Authentication . . . . . . . . . . . . . . . 7
3.9 Congestion control . . . . . . . . . . . . . . . . . . . . 8
3.10 Status codes . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1 Normative References . . . . . . . . . . . . . . . . . . . . . 10
7.2 Informative References . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Implementation of Cookies . . . . . . . . . . . . . . 11
Appendix B. Anycast considerations . . . . . . . . . . . . . . . 11
Authors Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
DNSSEC implies that DNS responses may be large, possibly larger than the
de facto ~1500 byte internet MTU.
Large responses are a challenge for DNS transport. EDNS [RFC2671] was
introduced in 1999 to allow larger responses to be sent over UDP,
previously DNS/UDP was limited to a 512 bytes.
EDNS is problematic for several reasons:
(1) It allows amplification attacks against 3rd parties. DNS/UDP has
always been susceptible to these attacks, but EDNS has increased the
amplification factor by an order of magnitude.
(2) The IP protocol specifies a means by which large IP packets are
split into fragments and then re-assembled. However fragmented UDP
responses are undesirable for several reasons:
o Fragments may be spoofed. The DNS ID and port number are only
present in the first fragment, and the IP ID may be easy for an
attacker to predict.
o In practice fragmentation is not reliable, and large UDP packets may
fail to be delivered.
o If a single fragment is lost, the entire response must be re-sent.
o Re-assembling fragments requires buffer resources, which opens
up denial of service attacks [GONT].
Instead, it is possible to use TCP, but this is undesirable, as TCP
imposes increased latency and significant server state that may be
vulnerable to denial of service attack. In addition, support for TCP
is not universal.
Nearly all current DNS traffic is carried by UDP with a maximum size
of 512 bytes, and relying on TCP is a risk for the deployment of DNSSEC.
Therefore a new protocol is proposed, with mnenomic QRP, to stand for
"Quick Response Protocol".
2. Definitions and Objectives
2.1 Definitions
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].
DNS Payload A DNS Message [RFC1035], not including the 16-bit ID
field. For AXFR, the response messages are concatenated
without ID fields, to form a single DNS payload.
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Transaction A transaction is initiated by a client request packet
and the server responds with one or more response packets.
Transfer The requested transfer of a DNS Payload, using one or more
transactions as described in sections 3.6 and 3.7.
2.2 Security Objectives
Fragmentation must not occur provided the actual path MTU is at least
the MTU sent by the client or 600 bytes, whichever is larger.
Blind spoofing attacks must be prevented. Amplification attacks
against third parties must be prevented.
2.3 Performance Objectives
No per-client server state must be needed between transactions.
Each transaction ( for moderate response sizes ) must be performed
in a single round trip, after setup, provided that no IP packets are
lost.
Only lost IP packets are re-transmitted.
3. Protocol
3.1 Overview
Communication is over UDP [RFC768] in two stages. First a long-lived
SERVERTOKEN is acquired by the client. Subsequent queries are protected
against amplification attacks by the SERVERTOKEN.
DNS server support for the protocol is signaled by a general purpose
method [TPORT].
Each UDP packet starts with a 16-bit OPCODE, followed ( except as
described in section 3.8 ) by a 64-bit QUERYID that identifies the
transaction. These fields are not shown.
Fixed length field sizes are as shown in the diagrams. All numbers are
unsigned integers, with the first bit being the most significant.
Variable length reserved areas MUST be omitted by the sender.
Fixed length reserved ares MUST be set to zero by the sender.
All reserved areas MUST be ignored by the receiver.
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3.2 Setup request
The client acquires a SERVERTOKEN for a given Server IP address by
sending a packet with OPCODE 1.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ RESERVED \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.3 Setup response
The server response has OPCODE 1, format :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| STATUS | RESERVED \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
SERVERTOKEN is a 32 bit value computed as a secure hash of the
client IP Address and a long term server secret.
Servers MUST change their long term secret
occasionally ( at least once every 4 weeks ).
STATUS is an 8 bit status code, see section 3.10.
The client associates SERVERTOKEN, and the client IP address
( for multi-homed clients ) with the Server IP address.
3.4 Initial request
To make a DNS request, a packet is sent with OPCODE 2, format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU | COUNT | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
SERVERTOKEN is a copy of SERVERTOKEN from the setup response.
MTU limits the size in bytes of the IP packets used to
send the response. MUST be at least 600.
COUNT limits the number of pages the server will send.
DATA is the DNS payload.
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3.5 Server response : single page
The server checks SERVERTOKEN, and obtains the DNS response payload.
If the requested MTU is less than 600 bytes, the server SHOULD set
MTU to 600 bytes. If the path MTU is known to be less than the value
supplied by the client, MTU is reduced to that value ( but not to
less than 600 bytes ).
If the DNS payload size plus IP/UDP/QRP overhead is not greater than
MTU, the server sends a single page response, OPCODE 2, format :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where DATA is the DNS payload. The client uses DATA as the normal DNS
response.
3.6 Server response : multi page
Otherwise, the server divides the DNS payload into equal size pages,
so that each IP response packet does not exceed MTU, and sends
multiple packets, each with OPCODE 3 and format :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOTAL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COOKIE |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COUNT | PAGE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PAGESIZE | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
TOTAL is the size of the complete DNS payload.
COOKIE is a 64-bit value used to request further pages.
COUNT is the number of pages sent.
PAGE is the 24-bit zero-based number of this page.
PAGESIZE is the size into which the response has been divided.
DATA is part of the DNS payload.
The client allocates an assembly buffer of TOTAL bytes (if not already
allocated), and copies DATA into it at offset PAGE x PAGESIZE.
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Clients SHOULD impose limits on the maximum size response (TOTAL) they
will accept, to prevent attacks by malicious servers.
Servers MAY send a smaller number of pages than requested, for
policy reasons, or if there is local congestion. The pages sent have
numbers 0 .. COUNT-1.
3.7 Follow-up request
If the client does not receive a page, due to packet loss or not
all pages being sent, it sends a packet with OPCODE 3, format :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COOKIE |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COUNT | PAGE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PAGESIZE | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
SERVERTOKEN is a copy of the SERVERTOKEN from the setup response.
COOKIE is a copy of COOKIE from the server response.
COUNT is the number of pages requested.
PAGE is the number of the first page to be sent.
PAGESIZE is a copy of PAGESIZE from the server response.
DATA is a copy of DATA from the initial request.
The server response is the same as in section 3.6.
Once a client has received all pages, it processes the complete
assembled response as normal.
If the server encounters an error condition, such as an invalid
SERVERTOKEN or COOKIE, it sends a setup response (section 3.3),
and the client retries with a new initial request (section 3.4).
If a server has more than one IP address, a client MAY attempt
to use a SERVERTOKEN it has previously acquired from another
IP address. A client MAY also attempt to continue a failed transfer on
an alternate server IP address using the same SERVERTOKEN and COOKIE.
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3.8 Encryption and Authentication.
OPCODE 4 is used to optionally encrypt and authenticate packets.
The encryption and authentication algorithms are described in [NACL].
Public keys are 255 bits. The wire format is 32 bytes, with the unused
first bit set to zero. Public key tags are the first 4 bytes of the wire
format public key.
For client requests, the OPCODE is followed by
o A 12 byte client nonce.
o A 32 byte client public key.
o A 4 byte server public key tag.
o A cryptographic box containing a 16 byte MAC and the encrypted packet.
For server responses, the OPCODE is followed by
o A 12 byte client nonce ( copied from the request ).
o A 12 byte server nonce.
o A cryptographic box containing a 16 byte MAC and the encrypted packet.
In both cases the 64-bit QUERYID is omitted from the unencrypted packet,
which starts with the underlying OPCODE. The 12 byte client nonce is
used instead to identify the transaction.
Public keys are stored in QRPK resource records, which are included in
the Authority section of the response whenever an A or AAAA record for
the same domain is sent. The wire format is as above, the presentation
format is the 50 character base32 encoded string of the 255-bit public
key, using "0123456789BCDFGHJKLMNPQRSTUVWXYZ" as the alphabet for the
encoding, which is case insensitive.
Where the parent zone does not have QRPK support, or the domain owner
is unable to upload a QRPK record to the parent zone, public keys MAY
be encoded in a label of a name server, using the prefix "QK-" followed
by the presentation format encoding of the public key.
3.9 Congestion control
Clients SHOULD take into account estimated network performance when
requesting pages. Factors are :
o The round trip time (RTT).
o The time to transmit 1 packet due to limited bandwidth (PT).
The total number of pages requested but not received or lost (INFLIGHT)
SHOULD NOT be more than max( RTT / PT, 4 ) at any time.
Packets are deemed lost if they do not arrive within 4 x RTT, or 500
milli-seconds, which ever is greater.
For example, if RTT = 150 milli-seconds, the bandwidth is 300 kilobytes
per second and the packet size is 1500 bytes, then we have PT = 5
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milli-seconds and INFLIGHT = 30.
The setup response SHOULD be used to initialize RTT to a value not
exceeding 30 milli-seconds. Clients MUST use a conservative value for
PT, not less than 5 milli-seconds, until a proper estimate is available.
In practice for normal DNS usage, INFLIGHT can simply be set to 4.
3.10 Status Codes
The following values are defined:
0 No error
1 Invalid SERVERTOKEN
2 Invalid COOKIE
11 Invalid OPCODE
12 End of packet error
13 Other format error
31 Invalid PAGESIZE
32 Invalid PAGE
41 Invalid Public Key Tag
42 Authentication error
Only codes 0-2 will occur if the protocol is correctly implemented,
in the absence of network errors or attacks.
Responses with codes greater than 10 may be logged or used for debugging
purposes, but MUST otherwise be ignored.
4. Security Considerations
Fragmented responses are vulnerable to blind spoofing. If the path
MTU is less than the value supplied by the client, denial of service
attacks are possible, and data can be altered unless authenticated
by other means.
Amplification attacks from previous users of the client IP address on
the current user are not prevented by the protocol until the long term
server secret is changed, as described in section 3.3. In-path
(man-in-the-middle) amplification attacks are not prevented, however
such attacks are relatively difficult to carry out, requiring
the attacker to have network access close to the victim.
Transactions not protected as described in section 3.8 are vulnerable
to data alteration. Such attacks may be prevented by the use of DNSSEC.
Secret values need to be generated so that an attacker cannot easily
guess them, by using cryptographic random number generators seeded
from data that cannot be guessed by an attacker, such as thermal
noise or other random physical fluctuations.
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5. IANA Considerations
The following values may be used for private testing only :
53000 for the UDP Port to which requests are sent.
252 for the DNS transport protocol identifier ( mnemomic QRP ).
65356 for the TPORT resource record type identifier.
65357 for the QRPK recorce record type identifier.
IANA is requested to make official reservations, to allow public
operation.
6. Acknowledgments
Mark Andrews, Alex Bligh, Robert Elz, Alfred Hoenes, Douglas Otis,
Nicholas Weaver and Wouter Wijngaards were each instrumental in
creating and refining this specification.
7. References
7.1 Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC768] J. Postel, "User Datagram Protocol", RFC 768,
USC/Information Sciences Institute, August 1980.
[TPORT] Barwood, G., "DNS Transport Signal", IETF dnsext draft,
October 2009.
[NACL] Bernstein, D., "Cryptography in NaCl", March 2009.
7.2 Informative References
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
[GONT] Gont, F., "Security Assessment of the Internet Protocol
version 4", August 2009.
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Appendix A. Implementation of Cookies
The suggested implementation of cookies is by version numbers. Each
RRset has a version number assigned from a 64-bit clock that is
increased whenever the DNS database is updated. The version of a
response is the largest version number of the associated RRsets. The
cookie is the version number.
If the database is updated while a transfer is progress, a COOKIE error
occurs, and the client restarts the transfer.
Alternatively, if old queries may be replayed, COOKIE errors may be
avoided( however such errors should be rare ).
Appendix B. Anycast considerations
Anycast DNS servers need to operate consistently.
There are (at least ) two possibilities:
(a) Each server within the Anycast system issues distinct SERVERTOKENS.
If the Anycast routing changes, a SERVERTOKEN error occurs, and the
client restarts the query.
(b) Each server within the Anycast system has the same long term secret,
and thus issues the same SERVERTOKEN to a given client. A global clock
is used for issuing updates. If the Anycast routing changes and an
update is in progress, a COOKIE error may occur, and the client has to
restart the query. Such errors can be avoided by not serving updates
until all the Anycast servers have received a copy.
Author's Address
George Barwood
33 Sandpiper Close
Gloucester
GL2 4LZ
United Kingdom
Phone: +44 452 722670
EMail: george.barwood@blueyonder.co.uk
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