DNS Extensions Working Group G. Barwood
Internet-Draft
Intended status: Experimental 2 September 2009
Expires: March 2010
DNS Transport
draft-barwood-dnsext-dns-transport-03
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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 . . . . . . . . . . . . . . . . . . . . . . 6
3.5 Server response : single page . . . . . . . . . . . . . . . 6
3.6 Server response : multi page . . . . . . . . . . . . . . . . 7
3.7 Follow-up request . . . . . . . . . . . . . . . . . . . . . 8
3.8 Error response . . . . . . . . . . . . . . . . . . . . . . . 9
3.9 Congestion control . . . . . . . . . . . . . . . . . . . . . 9
3.10 Reserved areas . . . . . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
7. Normative References . . . . . . . . . . . . . . . . . . . . . 10
A. Implementation of Cookies . . . . . . . . . . . . . . . . . . 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 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 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 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 ( 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, 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 SERVERTOKEN,
SPORT 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 not occur - all known DNS
servers return a copy of the question, except for format error
responses, which should not occur.
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3.4 Initial request
To make a DNS request, a UDP packet is sent to SPORT, with format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | COUNT | MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QUERYID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
COUNT limits the number of pages the server will send.
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.
Servers should use a smaller MTU if the path MTU is less than the MTU
supplied by the client.
If there is only one page, the UDP response packet has format :
+-+-+-+-+-+-+-+-+
| 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QUERYID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
SERVERTOKEN is a copy of the SERVERTOKEN from the request.
QUERYID is a copy of QUERYID from the request.
DATA is the DNS payload.
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The client checks SERVERTOKEN, and then uses DATA as the normal DNS
response.
3.6 Server response : multi page
If there is more than one page, each UDP response packet has format :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | COUNT | PAGESIZE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PAGE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOTAL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COOKIE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QUERYID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
COUNT is the number of pages sent.
PAGESIZE is the size into which the response has been divided.
PAGE is the number of this page.
TOTAL is the size of the complete DNS payload.
SERVERTOKEN is a copy of SERVERTOKEN from the request.
COOKIE is used to request further pages.
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.
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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 format :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | COUNT | PAGESIZE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PAGE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COOKIE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QUERYID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ DATA \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
COUNT is the number of pages to be sent.
PAGESIZE is a copy of PAGESIZE from the server response.
PAGE is the 0-based number of the first page to be sent.
SERVERTOKEN is a copy of the SERVERTOKEN from the setup response.
COOKIE is a copy of COOKIE from the server response.
QUERYID identifies the request.
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.
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3.8 Error response.
If the server encounters an error condition, it sends an error
response, with format :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | ERRNUM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SERVERTOKEN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| QUERYID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where :
ERRNUM encodes the error condition, with values
0 Invalid SERVERTOKEN. The client should acquire
a new SERVERTOKEN and try again.
1 Invalid COOKIE/PAGESIZE/PAGE. The client
should send a new Initial Request.
SERVERTOKEN is a copy of the SERVERTOKEN from the setup response.
QUERYID is a copy of QUERYID from the request.
3.9 Congestion control
Clients should take into account estimated network performance when
calculating the COUNT field in requests. Factors are :
o The estimated round trip time.
o The estimated available bandwidth.
o The packet loss rate.
Servers may send a smaller number of pages than requested, for
policy reasons, or if there is local congestion, etc.
3.10 Reserved areas
Unused variable length areas after the defined fields are reserved
and must be set to zero length / zero by the sender and ignored by
the receiver.
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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 at present. If the protocol were to be become a standard,
there could be new registries for :
o Strings in setup response ( transport option codes ).
o Operation codes ( 1 - 5 have been used )
o Error codes
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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