One document matched: draft-barwood-dnsext-dns-transport-01.txt
Differences from draft-barwood-dnsext-dns-transport-00.txt
DNS Extensions Working Group G. Barwood
Internet-Draft
Intended status: Experimental 28 August 2009
Expires: March 2010
DNS Transport
draft-barwood-dnsext-dns-transport-01
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on March 1, 2010.
Copyright Notice
Copyright (c) 2009 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
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 DNS
query is a single round trip, after a setup exchange that establishes
a long term shared secret. The protocol may have other applications.
Barwood Expires March 2010 [Page 1]
Internet-Draft DNS Transport August 2009
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. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
A. Implementation of Cookies . . . . . . . . . . . . . . . . . . 9
Authors Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
Barwood Expires March 2010 [Page 2]
Internet-Draft DNS Transport August 2009
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.
Barwood Expires March 2010 [Page 3]
Internet-Draft DNS Transport August 2009
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.
Barwood Expires March 2010 [Page 4]
Internet-Draft DNS Transport August 2009
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.
Throughout, DNS Payload refers to a DNS Message [RFC1035], not
including the 16-bit ID field.
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 a TXT record as answer to the question, which
contains a string 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 a 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 obtained with a single query.
Barwood Expires March 2010 [Page 5]
Internet-Draft DNS Transport August 2009
3.4 Initial request
To make a DNS request, a UDP packet is sent to SPORT, with format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | | MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QUERYID | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
DATA is the DNS payload.
3.5 Server response : single page
The server checks SERVERTOKEN (it does not respond to invalid tokens),
and divides the DNS payload 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 request.
DATA is the DNS payload.
The client checks SERVERTOKEN, and then uses DATA as the normal DNS
response.
Barwood Expires March 2010 [Page 6]
Internet-Draft DNS Transport August 2009
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 payload.
The client checks SERVERTOKEN, allocates an assembly buffer of TOTAL
bytes (if not already allocated), and copies DATA into it at offset
PAGE x PAGESIZE. Once all the pages have been received, the assembly
buffer contains the DNS payload.
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 | PAGESIZE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COOKIE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QUERYID | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Barwood Expires March 2010 [Page 7]
Internet-Draft DNS Transport August 2009
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.
DATA is a copy of DATA from the initial 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 an initial 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 always 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. References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
Barwood Expires March 2010 [Page 8]
Internet-Draft DNS Transport August 2009
Appendix A : Implementation of Cookies
To show how server state is avoided or limited, two possible approaches
to the implementation of cookies are shown. These are illustrative, and
actual implementations are of course free to take a different approach.
(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
Barwood Expires March 2010 [Page 9]
| PAFTECH AB 2003-2026 | 2026-04-24 05:04:04 |