One document matched: draft-barwood-dnsext-dns-transport-10.txt
Differences from draft-barwood-dnsext-dns-transport-09.txt
DNS Extensions Working Group G. Barwood
Internet-Draft
Intended status: Experimental 20 September 2009
Expires: March 2010
DNS Transport
draft-barwood-dnsext-dns-transport-10
Status of this Memo
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Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 exchange
that establishes a long term shared secret. 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. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Fragmentation. . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Spoofing . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3 Server state . . . . . . . . . . . . . . . . . . . . . . . . 4
2.4 Amplification attacks . . . . . . . . . . . . . . . . . . . 4
2.5 Packet retransmission . . . . . . . . . . . . . . . . . . . 4
2.6 Performance . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5
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 Error response . . . . . . . . . . . . . . . . . . . . . . . 8
3.9 Congestion control . . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
7. Normative References . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Implementation of Cookies . . . . . . . . . . . . . . 10
Appendix B. Anycast considerations . . . . . . . . . . . . . . . 10
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.
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 to solve these problems is proposed.
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2. Requirements
2.1 Fragmentation
As described in the introduction, fragmentation is undesirable.
However, fragmentation is unavoidable if the path MTU is too small.
Therefore, we require only that fragmentation does not occur provided
the actual path MTU is at least the MTU sent by the client.
2.2 Spoofing
Blind spoofing attacks must be prevented.
2.3 Server state
No per-client server state should be needed between transactions.
2.4 Amplification attacks
Amplification attacks against third parties must be prevented.
2.5 Packet re-transmission
Only lost IP packets must be re-transmitted.
This reduces problems due to network congestion.
2.6 Performance
Each transaction ( for moderate response sizes ) must be performed
in 1 RTT, after setup, provided that no IP packets are lost.
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3. Protocol
3.1 Overview
Communication is in two stages. First a long-lived SERVERTOKEN is
acquired by the client. Subsequent queries are protected by the
SERVERTOKEN.
Throughout, DNS Payload refers to a DNS Message [RFC1035], not including
the 16-bit ID field. DNS server support for the protocol is signaled by
a general purpose method [TPORT].
All numbers are unsigned integers, with the first bit being the most
significant. Reserved areas must be omitted by the sender and ignored
by the receiver.
Each packet starts with a 64-bit QUERYID that identifies the transaction
and ends with an 8-bit opcode, followed by a 16-bit port number,
which is 53 for DNS. These fields are not shown.
3.2 Setup request
The client acquires a SERVERTOKEN for a given Server by sending a
message with opcode 1. The variable length portion is reserved.
3.3 Setup response
The server replies with a message with opcode 2, format :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ RESERVED \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where SERVERTOKEN is a 32 bit value computed as a secure hash of the
client IP Address and a long-term server secret.
The client associates SERVERTOKEN, and the client IP address
( for multi-homed clients ) with the Server.
3.4 Initial request
To make a DNS request, a UDP packet is sent with opcode 3, format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU | COUNT |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
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SERVERTOKEN is a copy of SERVERTOKEN from the setup response.
DATA is the DNS payload.
MTU is a 16-bit number that limits the size of the IP packets
used to send the response. Must be at least 576.
COUNT limits the number of pages the server will send.
3.5 Server response : single page
The server checks SERVERTOKEN, and divides the response DNS payload
into equal size pages, so that the size of each IP packet is not greater
than MTU. Servers should use a smaller MTU if the path MTU is known to
be less than the MTU supplied by the client.
If there is only one page, the response has opcode 4, format :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where DATA is the DNS payload. The client uses DATA as the normal DNS
response.
3.6 Server response : multi page
If there is more than one page, the server sends multiple messages,
each with opcode 5 and format :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOTAL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COOKIE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COUNT | PAGE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PAGESIZE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
TOTAL is the size of the complete DNS payload.
COOKIE is used to request further pages ( see section 3.7 ).
COUNT is the number of pages sent.
PAGE is the 0-based number of this page.
DATA is part of the DNS payload.
PAGESIZE is the size into which the response has been divided.
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The client allocates an assembly buffer of TOTAL bytes (if not already
allocated), and copies DATA into it at offset PAGE x PAGESIZE.
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 request with opcode 6, format :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COOKIE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ RANGES \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PAGESIZE | NRANGE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
SERVERTOKEN is a copy of the SERVERTOKEN from the setup response.
COOKIE is a copy of COOKIE from the server response.
RANGES is an array of ranges. Each range is an 8 bit count
field (the number of pages in the range) followed by a
24-bit page number which specifies the first page in the
range. The ranges must not overlap, and must be sorted
in ascending page number order.
DATA is a copy of DATA from the initial request.
PAGESIZE is a copy of PAGESIZE from the server response.
NRANGE is the number of ranges.
The server response is the same as in section 3.6.
Again, servers may send fewer pages than requested. The pages sent have
smaller numbers than any pages not sent. For example if in a follow-up
request the client requests ranges 2-3, 4-6 (5 pages), and the server
only sends 3 pages (COUNT=3), the pages sent are 2,3 and 4.
Once a client has received all pages, it processes the complete
assembled response as normal.
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3.8 Error response.
If the server encounters an error condition, it sends an error
response, with opcode 7, format :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ RESERVED \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ERRNUM |
+-+-+-+-+-+-+-+-+
where :
ERRNUM encodes the error condition, with values
0 Invalid SERVERTOKEN. The client should acquire
a new SERVERTOKEN and try again.
1 Invalid COOKIE The client should send a new
Initial Request.
2 Protocol error. Should not occur if the
protocol is correctly implemented.
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.
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
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, and
this should be sufficient to send the complete response in a single
round trip, assuming the MTU is 1500 bytes.
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4. Security Considerations
Fragmented responses are vulnerable to blind spoofing, therefore
fragmented responses should be avoided if possible. Clients should
check for fragmented responses if possible, and should not use
records from fragmented responses unless they can be verified with
DNSSEC.
To prevent an attacker generating a large number of IP packets from a
single request, a check should be made that the MTU is at least 584.
Clients should impose limits on the maximum size response (TOTAL) they
will accept, to prevent attacks by malicious servers.
Secret values, that is the long term server secret and QUERYID should
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.
The hash function used to compute SERVERTOKEN must be
cryptographically secure.
Amplification attacks from previous users of the client IP address on
the current user are not prevented by the protocol. Therefore servers
should change their long term secret occasionally to invalidate old
SERVERTOKENs.
5. IANA Considerations
A UDP port to which requests are sent.
A [TPORT] DNS transport protocol identifier.
6. Acknowledgments
Mark Andrews, Alex Bligh, Robert Elz, Alfred Hines, Douglas Otis,
Nicholas Weaver and Wouter Wijngaards were each instrumental in
creating and refining this specification.
7. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
[TPORT] Barwood, G., "DNS Transport Signal", IETF dnsext draft,
September 2009.
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Appendix A. Implementation of Cookies
To show how server state is avoided or limited, two possible approaches
to the implementation of cookies are given. These are illustrative, and
actual implementations are of course free to take a different approach.
(1) The COOKIE is a local DNS database version number. The database
is structured so that old queries may be replayed, with the database
version number being supplied as a parameter, or a COOKIE error is
returned if the database is updated while a transfer is in progress.
(2) The server maintains a list of recent multi-page responses:
COOKIE DATA ACCESSTIME
1 .... 10:25:11
2 .... 10:25:16
.....
If a response is multi-page, the list is checked to see if there is an
existing entry that can be used ( hashing techniques are used to make
the search efficient ).
Entries that have not been accessed for more than 5 seconds may be
deleted.
Some care should be taken to ensure that on server restart, old cookie
values are not re-used. Periodically, a new range of cookies should be
issued, and the new allocation value recorded in permanent storage.
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. They also
issue identical responses for a given query, assuming the zone version
is the same. The cookie is the zone version number. If the Anycast
routing changes and the new server does not have the required zone
version, a COOKIE error will result, and the client has to restart
the query. Such errors can be avoided by not serving a new zone
until all the Anycast servers have received a copy.
By incorporating the software version into the SERVERTOKEN, it should be
possible to smoothly update the system, effectively switching to
solution (a) while the software update is in progress.
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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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