One document matched: draft-barwood-dnsext-dns-transport-00.txt
DNS Extensions Working Group G. Barwood
Internet-Draft
Intended status: Experimental 27 August 2009
Expires: February 2010
DNS Transport
draft-barwood-dnsext-dns-transport-00
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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, but high
performance is neverthelesss achieved. Latency for a DNS query is a
single round trip, after a setup exchange that establishes a
long term shared secret.
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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 . . . . . . . . . . . . . . . . . . . . . . 6
3.5 Server response : single page . . . . . . . . . . . . . . . 6
3.6 Server response : multi page . . . . . . . . . . . . . . . . 7
3.7 Retry request . . . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
7. Informative References . . . . . . . . . . . . . . . . . . . . 8
A. Implementation of Cookies . . . . . . . . . . . . . . . . . . 9
Authors Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
DNSSEC implies that DNS responses may be large, possibly larger than the
de facto ~1500 byte internet MTU. The IP protocol specifies a means by
which large IP packets are split into fragments and then re-assembled.
Fragmented UDP responses are undesirable for several reasons:
(1) Fragments can easily be spoofed. The DNS ID and port number are only
present in the first fragment, and the IP ID is usually easy for an
attacker to predict.
(2) In practise fragmentation is not reliable, and large UDP packets may
fail to be delivered.
(3) If a single fragment is lost, the entire response must be re-sent.
(4) Re-assembling fragments requires buffer resources, which opens
up denial of service attacks.
Instead, it is possible to use TCP for large responses, but this is
undesirable, as TCP imposes significant overhead and state that may
be vulnerable to denial of service attack.
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 protocol to allow large DNS responses to be sent using small
UDP packets 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 dos 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 is required 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 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, using a standard DNS lookup. Subsequent
queries are sent using a different port, and are protected by
the SERVERTOKEN.
3.2 Setup request
The client acquires a SERVERTOKEN for a given Server IP address by
sending a special question to the server, with
QTYPE = TXT
QCLASS = IN
QNAME = TRANSPORT.<Client Secret>.LOCAL
where <Client Secret> is a secret label chosen to prevent spoofing
of the response.
3.3 Setup response
The server returns one or more TXT records as answer to the question,
exactly one of which has data with format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | SPORT |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SPORT is a 16 bit UDP port number, to which requests are sent.
SERVERTOKEN is a 32 bit value computed as a hash of the client IP
Address and permanent server secret.
The TTL should be at least 1 day. The client associates the SERVERTOKEN
and the client IP address ( for multi-homed clients ) with the Server
IP address.
If the TXT record is not returned, the server does not have support,
and this fact should be cached, with a TTL of at least 1 day.
If a copy of the Question is not returned, extra queries need to
be sent to prevent spoofing. This should be very unusual - all known
DNS servers return a copy of the question, except for format error
responses, which should not occur.
This format is intended to be extensible, so that multiple transport
options can be requested with a single query.
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3.4 Initial request
To make a DNS request, a UDP packet is sent to SPORT, with format:
+-+-+-+-+-+-+-+-+
| 0 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QUERYID | MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ QUESTION \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
QUERYID identifies the request.
MTU limits the size of the IP packets used to send the
response. Must be at least 576 bytes.
SERVERTOKEN is a copy of the SERVERTOKEN from the setup response.
QUESTION is QNAME, QTYPE, QCLASS, with usual DNS format.
3.5 Server response : single page
The server checks SERVERTOKEN (it does not respond to invalid tokens),
and divides the DNS response (excluding the ID field) into equal size
pages, so that the size of each IP packet is not greater than MTU.
If there is only one page, the UDP response packet has format :
+-+-+-+-+-+-+-+-+
| 0 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QUERYID | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where
SERVERTOKEN is a copy of the SERVERTOKEN from the initial request.
QUERYID is a copy of QUERYID from the initial reuqest
DATA is the DNS response.
The client checks SERVERTOKEN, and then uses DATA as the normal DNS
response.
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3.6 Server response : multi page
If there is more than one page, each UDP response packet has format
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | | PAGE | PAGESIZE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COOKIE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOTAL | QUERYID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
PAGE is the 0-based number of the page.
PAGESIZE is the size into which the reponse has been divided.
SERVERTOKEN is a copy of SERVERTOKEN from the request.
COOKIE is used to re-try missing pages.
TOTAL is the size of the complete response ( excluding tyhe ID ).
QUERYID is a copy of QUERYID from the request.
DATA is part of the DNS response.
The client checks SERVERTOKEN, allocates a buffer of TOTAL bytes
(if not already allocated), and copies DATA into it at offset
PAGE x PAGESIZE.
3.7 Retry request
If the client fails to receive a page, due to packet loss, it sends
a retry request as follows :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | | PAGE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QUERYID | PAGESIZE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COOKIE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ QUESTION \
+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++
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where :
PAGE is the 0-based number of the page.
QUERYID is a copy of QUERYID from the initial request.
PAGESIZE is a copy of PAGESIZE from a server response.
SERVERTOKEN is a copy of the SERVERTOKEN from the setup response.
COOKIE is a copy of COOKIE from the server response.
QUESTION is a copy of QUESTION from the inital request.
Responses are the same as in section 3.6. If the response COOKIE
changes, the existing pages are discarded, and retry requests are
issued for the pages that have not been fetched, using a new QUERYID.
If a client request times out on 2 consecutive occasions (using a
given SERVERTOKEN), the SERVERTOKEN is deleted, and a new SERVERTOKEN
must be acquired.
3.8 Reserved areas
Reserved areas and undefined bits must be set to zero length / zero by
the sender and must be ignored by the receiver. Variable length areas
after the end of the defined fields are reserved.
4. Security Considerations
Fragmented responses are vulnerable to blind spoofing, therefore
fragmented responses should be avoided if possible.
A check must be made that the MTU is at least 584, to prevent an
attacker generating a large number of IP packets from a single request.
5. IANA Considerations
None
6. Acknowledgments
Mark Andrews, Alex Bligh, Robert Elz, Douglas Otis,
Wouter Wijngaards and Nicholas Weaver were each instrumental in
creating and refining this specification.
7. Informative References
None
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Appendix A : Implementation of Cookies
To show how server state is avoided, two possible approaches to the
implementation of cookies are given.
(1) The server maintains a DNS database version number, which is
incremented when the database is updated. The DNS database is stuctured
so that old queries may be replayed, with the database version number
being supplied as a parameter. COOKIE is simply the DNS database
version number.
(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.
Alternatively, the server should wait 10 seconds after restarting before
issuing any cookies.
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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