One document matched: draft-barwood-dnsext-dns-transport-02.txt
Differences from draft-barwood-dnsext-dns-transport-01.txt
DNS Extensions Working Group G. Barwood
Internet-Draft
Intended status: Experimental 31 August 2009
Expires: March 2010
DNS Transport
draft-barwood-dnsext-dns-transport-02
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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 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 . . . . . . . . . . . . . . . . . . . . . . 6
3.5 Server response : single page . . . . . . . . . . . . . . . 6
3.6 Server response : multi page . . . . . . . . . . . . . . . . 7
3.7 Retry request . . . . . . . . . . . . . . . . . . . . . . . 7
3.8 Error response . . . . . . . . . . . . . . . . . . . . . . . 8
3.9 Reserved areas . . . . . . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
7. Normative References . . . . . . . . . . . . . . . . . . . . . 9
A. Implementation of Cookies . . . . . . . . . . . . . . . . . . 10
Authors Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
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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 reponses 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 two orders 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 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.
o In practise 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 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.
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 (there may be other strings) 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 long-term 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.
If the label of the TXT record is not as expected, the response is
discarded as bogus ( a spoofing attempt ).
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3.4 Initial request
To make a DNS request, a UDP packet is sent to SPORT, with format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QUERYID | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
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.
QUERYID identifies the request.
DATA is the DNS payload.
3.5 Server response : single page
The server checks SERVERTOKEN, 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.
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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 DNS payload.
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 with format :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | PAGE | PAGESIZE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COOKIE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QUERYID | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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where :
PAGE is the 0-based number of the page.
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.
QUERYID is a copy of QUERYID from the initial request.
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 If the server encounters an error condition, it responds with
an error response, with format :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | ERRNUM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QUERYID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
ERRNUM encodes the error condition.
SERVERTOKEN is a copy of the SERVERTOKEN from the setup response.
QUERYID is a copy of QUERYID from the initial request.
The only currently defined value for ERRNUM is
0 Invalid SERVERTOKEN. This allows the client to rapidly
recover from a situation where the SERVERTOKEN has changed
- either because the long-term secret has been compromised,
or the server has restarted, and the server long-term
secret has changed, or the IP address has been re-assigned.
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3.9 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.
Secret values ( the long term server secret, the client secret, QUERYID
) must be generated so that an attacker cannot easily guess them, by
using cryptographic hash functions and 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, although a relatively weak function may be sufficient, since
acquiring large numbers of input/output pairs in order to deduce the
long term server secret is not easy for an attacker.
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. 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.
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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 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, or use a new long-term secret to generate
SERVERTOKENs.
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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