Internet Engineering Task Force M. Sivaraman
Internet-Draft Internet Systems Consortium
Intended status: Experimental S. Kerr
Expires: January 21, 2016 L. Song
Beijing Internet Institute
July 20, 2015
DNS message fragments
draft-muks-dns-message-fragments-00
Abstract
This document describes a method to transmit DNS messages over
multiple UDP datagrams by fragmenting them at the application layer.
The objective is to allow authoriative servers to successfully reply
to DNS queries via UDP using multiple smaller datagrams, where larger
datagrams may not pass through the network successfully.
Status of This Memo
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This Internet-Draft will expire on January 21, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
2. DNS Message Fragmentation Method . . . . . . . . . . . . . . 4
2.1. Client Behavior . . . . . . . . . . . . . . . . . . . . . 4
2.2. Server Behavior . . . . . . . . . . . . . . . . . . . . . 4
2.3. Other Notes . . . . . . . . . . . . . . . . . . . . . . . 6
3. The ALLOW-FRAGMENTS EDNS(0) Option . . . . . . . . . . . . . 7
3.1. Wire Format . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Option Fields . . . . . . . . . . . . . . . . . . . . . . 7
3.2.1. Maximum Fragment Size . . . . . . . . . . . . . . . . 7
3.3. Presentation Format . . . . . . . . . . . . . . . . . . . 7
4. The FRAGMENT EDNS(0) Option . . . . . . . . . . . . . . . . . 7
4.1. Wire Format . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Option Fields . . . . . . . . . . . . . . . . . . . . . . 7
4.2.1. Fragment Identifier . . . . . . . . . . . . . . . . . 7
4.2.2. Fragment Count . . . . . . . . . . . . . . . . . . . 8
4.3. Presentation Format . . . . . . . . . . . . . . . . . . . 8
5. Network Considerations . . . . . . . . . . . . . . . . . . . 8
5.1. Background . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Implementation Requirements . . . . . . . . . . . . . . . 9
6. Open Issues and Discussion . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Change History (to be removed before publication) . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
1.1. Background
[RFC1035] describes how DNS messages are to be transmitted over UDP.
A DNS query message is transmitted using one UDP datagram from client
to server, and a corresponding DNS reply message is transmitted using
one UDP datagram from server to client.
The upper limit on the size of a DNS message that can be transmitted
thus depends on the maximum size of the UDP datagram that can be
transmitted successfully from the sender to the receiver. Typically
any size limit only matters for DNS replies, as DNS queries are
usually small.
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As a UDP datagram is transmitted in a single IP PDU, in theory the
size of a UDP datagram (including various lower internet layer
headers) can be as large as 64 KiB. But practically, if the datagram
size exceeds the path MTU, then the datagram will either be
fragmented at the IP layer, or worse dropped, by a forwarder. In the
case of IPv6, DNS packets are fragmented by the sender only. If a
packet's size exceeds the path MTU, a Packet Too Big (PTB) ICMP
message will be received by sender without any clue to the sender to
reply again with a smaller sized message, due to the stateless
feature of DNS. In addition, IP-level fragmentation caused by large
DNS response packet will introduce risk of cache poisoning
[Fragment-Poisonous], in which the attacker can circumvent some
defense mechanisms (like port, IP, and query randomization
[RFC5452]).
As a result, a practical DNS payload size limitation is necessary.
[RFC1035] limited DNS message UDP datagram lengths to a maximum of
512 bytes. Although EDNS(0) [RFC6891] allows an initiator to
advertise the capability of receiving lager packets (up to 4096
bytes), it leads to fragmentation because practically most packets
are limited to 1500 byte size due to host Ethernet interfaces, or
1280 byte size due to minimum IPv6 MTU in the IPv6 stack [RFC3542].
According to DNS specifications [RFC1035], if the DNS response
message can not fit within the packet's size limit, the response is
truncated and the initiator will have to use TCP as a fallback to re-
query to receive large response. However, not to mention the high
setup cost introduced by TCP due to additional roundtrips, some
firewalls and middle boxes even block TCP/53 which cause no responses
to be received as well. It becomes a significant issue when the DNS
response size inevitably increases with DNSSEC deployment.
In this memo, DNS message fragmentation attempts to work around
middle box misbehavior by splitting a single DNS message across
multiple UDP datagrams. Note that to avoid DNS amplification and
reflection attacks, DNS cookies [I-D.ietf-dnsop-cookies] is a
mandatory requirement when using DNS message fragments.
1.2. Motivation
It is not a new topic regarding large DNS packets(>512B) issue
[I-D.ietf-dnsop-respsize], starting from introduction of IPv6,
EDNS(0) [SAC016], and DNSSEC deployment [SAC035]. In current
production networks, using DNSSEC with longer DNSKEYs (ZSK>1024B and
KSK>2048B) will result in response packets no smaller than 1500B
[T-DNS]. Especially during the KSK rollover process, responses to
the query of DNSKEY RRset will be enlarged as they contain both the
new and old KSK.
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When possible, we should avoid dropped packets as this means the
client must wait for a timeout, which incurs a high cost. For
example, a validator behind a firewall suffers waiting till the
timeout with no response, if the firewall drops large EDNS(0) packets
and IP fragments. It may even cause disaster when the validator can
not recieve response for new trust anchor KSK due to the extreme case
of bad middle boxes which also drop TCP/53.
Since UDP requires fewer packets on the wire and less state on
servers than TCP, in this memo we propose continuing to use UDP for
transmission but fragment the larger DNS packets into smaller DNS
packets at the application layer. We would like the fragments to
easily go through middle boxes and avoid falling back to TCP.
2. DNS Message Fragmentation Method
2.1. Client Behavior
Clients supporting DNS message fragmentation add an EDNS option to
their queries, which declares their support for this feature.
If a DNS reply is received that has been fragmented, it will consist
of multiple DNS message fragments (each transmitted in a respective
UDP packet), and every fragment contain an EDNS option which says how
many total fragments there are, and the identifier of the fragment
that the current packet represents. The client collects all of the
fragments and uses them to reconstruct the full DNS message. Clients
MUST maintain a timeout when waiting for the fragments to arrive.
Clients that support DNS message fragments MUST be able to reassemble
fragments into a DNS message of any size, up to the maximum of 64KiB.
The client MAY save information about what sizes of packets have been
received from a given server. If saved, this information MUST have a
limited duration.
Any DNSSEC validation is performed on the reassembled DNS message.
2.2. Server Behavior
Servers supporting DNS message fragmentation will look for the EDNS
option which declares client support for the feature. If not
present, the server MUST NOT use DNS message fragmentation. The
server MUST check that DNS cookies are supported. [**FIXME**]
Implementation of the first request case, where no existing
established cookie is available needs discussion; we want to avoid
additional round-trips here. Shane: don't cookies already handle
this case?
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The server prepares the response DNS message normally. If the
message exceeds the maximum UDP payload size specified by the client,
then it should fragment the message into multiple UDP datagrams.
Each fragment contains an identical DNS header with TC=1, possibly
varying only in the section counts. Setting the TC flag in this way
insures that clients which do not support DNS fragments can fallback
to TCP transparently.
As many RR are included in each fragment as are possible without
going over the desired size of the fragment. An EDNS option is added
to every fragment, that includes both the fragment identifier and the
total number of fragments.
The server needs to know how many total fragments there are to insert
into each fragment. A simple approach would be to generate all
fragments, and then count the total number at the end, and update the
previously-generated fragments with the total number of fragments.
Other techniques may be possible.
The server MUST limit the number of fragments that it uses in a
reply. (See "Open Issues and Discussion" for remaining work.)
The server MUST NOT exceed the maximum fragment size requested by a
client.
The server should use the following sizes for each fragment in the
sequence in IPv4:
+-------------+---------------------------------+
| Fragment ID | Size |
+-------------+---------------------------------+
| 1 | min(512, client_specified_max) |
| 2 | min(1460, client_specified_max) |
| 3 | min(1480, client_specified_max) |
| N | min(1480, client_specified_max) |
+-------------+---------------------------------+
The rationale is that the first packet will always get through, since
if a 512 octet packet doesn't work, DNS cannot function. We then
increase to sizes that are likely to get through. 1460 is the 1500
octet Ethernet packet size, minus the IP header overhead and enough
space to support tunneled traffic. 1480 is the 1500 octet Ethernet
packet size, minus the IP header overhead. [**FIXME**] Why not add
1240 here? Shane answers: 1280 is not any kind of limit in IPv4, as
far as I know.
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The server should use the following sizes for each packet in the
sequence in IPv6:
+-------------+---------------------------------+
| Fragment ID | Size |
+-------------+---------------------------------+
| 1 | min(1240, client_specified_max) |
| 2 | min(1420, client_specified_max) |
| 3 | min(1460, client_specified_max) |
| N | min(1460, client_specified_max) |
+-------------+---------------------------------+
Like with IPv4, the idea is that the first packet will always get
through. In this case we use the IPv6-mandated 1280 octets, minus
the IP header overhead. We then increase to 1420, which is the 1500
octet Ethernet packet size, minus the IP header overhead and enough
space to support tunneled traffic. 1460 is the 1500 octet Ethernet
packet size, minus the IP header overhead.
2.3. Other Notes
o The FRAGMENT option MUST NOT be present in DNS query messages,
i.e., when QR=0. If a DNS implementation notices the FRAGMENT
option in a DNS query message, it MUST ignore it.
o In DNS reply messages, the FRAGMENT option MUST NOT be present in
datagrams when truncation is not done, i.e., when TC=0. If a DNS
implementation notices the FRAGMENT option in a DNS reply message
fragment datagram that is not truncated, i.e, when TC=0, it MUST
drop all DNS reply message fragment datagrams received so far
(awaiting assembly) for that message's corresponding question
tuple (server IP, port, message ID) without using any data from
them. [**FIXME**] Dropping fragments to be received yet will be
problematic for implementations, but dropping fragments received
so far ought to be sufficient.
o More than one FRAGMENT option MUST NOT be present in a DNS reply
message fragment datagram. If a DNS implementation notices
multiple FRAGMENT options in a DNS reply message fragment
datagram, it MUST drop all reply datagrams received for that
message's corresponding question tuple (server IP, port, message
ID) without using any data from them. [**FIXME**] Dropping
fragments to be received yet will be problematic for
implementations, but dropping fragments received so far ought to
be sufficient.
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3. The ALLOW-FRAGMENTS EDNS(0) Option
ALLOW-FRAGMENTS is an EDNS(0) [RFC6891] option that a client uses to
inform a server that it supports fragmented responses. [**FIXME**]
Why not simply use the FRAGMENT option here with count=0,
identifier=ignored and avoid using another option code? Shane: There
are no shortage of options. Plus, if we want to include a maximum
fragment size value in the ALLOW-FRAGMENTS then we really need a
separate option.
3.1. Wire Format
TBD.
3.2. Option Fields
3.2.1. Maximum Fragment Size
The Maximum Fragment Size field is represented as an unsigned 16-bit
integer. This is the maximum size used by any given fragment the
server returns. [**FIXME**] This field's purpose has to be explained.
Shane: discussed in the discussion section now.
3.3. Presentation Format
As with other EDNS(0) options, the ALLOW-FRAGMENTS option does not
have a presentation format.
4. The FRAGMENT EDNS(0) Option
FRAGMENT is an EDNS(0) [RFC6891] option that assists a client in
gathering the various fragments of a DNS message from multiple UDP
datagrams. It is described in a previous section. Here, its syntax
is provided.
4.1. Wire Format
TBD.
4.2. Option Fields
4.2.1. Fragment Identifier
The Fragment Identifier field is represented as an unsigned 8-bit
integer. The first fragment is identified as 1. Values in the range
[1,255] can be used to identify the various fragments. Value 0 is
used for signalling purposes.
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4.2.2. Fragment Count
The Fragment Count field is represented as an unsigned 8-bit integer.
It contains the number of fragments in the range [1,255] that make up
the DNS message. Value 0 is used for signalling purposes.
4.3. Presentation Format
As with other EDNS(0) options, the FRAGMENT option does not have a
presentation format.
5. Network Considerations
5.1. Background
TCP-based application protocols co-exist well with competing traffic
flows in the internet due to congestion control methods such as in
[RFC5681] that are present in TCP implementations.
UDP-based application protocols have no restrictions in lower layers
to stop them from flooding datagrams into a network and causing
congestion. So applications that use UDP have to check themselves
from causing congestion so that their traffic is not disruptive.
In the case of [RFC1035], only one reply UDP datagram was sent per
request UDP datagram, and so the lock-step flow control automatically
ensured that UDP DNS traffic didn't lead to congestion. When DNS
clients didn't hear back from the server, and had to retransmit the
question, they typically paced themselves by using methods such as a
retransmission timer based on a smoothed round-trip time between
client and server.
Due to the message fragmentation described in this document, when a
DNS query causes multiple DNS reply datagrams to be sent back to the
client, there is a risk that without effective control of flow, DNS
traffic could cause problems to competing flows along the network
path.
Because UDP does not guarantee delivery of datagrams, there is a
possibility that one or more fragments of a DNS message will be lost
during transfer. This is especially a problem on some wireless
networks where a rate of datagrams can continually be lost due to
interference and other environmental factors. With larger numbers of
message fragments, the probability of fragment loss increases.
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5.2. Implementation Requirements
TBD.
6. Open Issues and Discussion
1. Resolver behavior
We need some more discussion of resolver behavior in general, at
least to the point of making things clear to an implementor.
2. The use of DNS fragments mechanism
Is this mechanism designed for all DNS transactions, or only
used in some event or special cases like a key rollover process?
If the mechanism is designed for general DNS transactions, when
is it triggered and how is it integrated with existing patterns?
One option is that DNS fragments mechanism works as a backup
with EDNS, and triggered only when a larger packet fails in the
middle. It will be orthogonal with TCP which provide additional
context that TC bit will be used in server side.
3. What is the size of fragments?
Generally speaking the number of fragment increases if fragment
size is small (512 bytes, or other empirical value), which makes
the mechanism less efficient. If the size can changed
dynamically according to negotiation or some detection, it will
introduce more cost and round trip time.
4. What happens if a client that does not support DNS fragments
receives an out-of-order or partial fragment?
We need to consider what happens when a client that does not
support DNS fragments gets a partial response, possibly even out
of order.
5. We should explain risk of congestion, packet loss, etc. when
introducing the limit on the number of fragments. We might also
set specific upper limits for number of fragments.
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6. EDNS buffer sizes vs. maximum fragmentation sizes
Mukund: We need further discussion about the sizes; also an
upper limit for each *fragment* has to be the client's UDP
payload size as it is the driver and it alone knows the ultimate
success/failure of message delivery. So if it sets a maximum
payload size of 1200, there's no point in trying 1460. Clients
that support DNS message fragments (and signal support using the
EDNS option) should adapt their UDP payload size discovery
algorithm to work with this feature, as the following splits on
sizes will assist PMTU discovery.
Shane: I think we need to separate the EDNS maximum UDP payload
size from the maximum fragment size. I think that it is quite
likely that (for example) we will want to restrict each fragment
to 1480 bytes, but that the EDNS buffer size might remain at 4
kibibytes.
7. TSIG should be addressed
We need to document how to handle TSIG, even though this is not
likely to be a real-world issue. Probably each fragment should
be TSIG signed, as this makes it harder for an attacker to
inject bogus packets that a client will have to process.
8. RR splitting should be addressed
We need to document whether or not RR can be split. Probably it
makes sense not to allow this, although this will reduce the
effectiveness of the fragmentation, as the units that can be
packed into each fragment will be bigger.
9. We need to document that some messages may not be possible to
split.
Some messages may be too large to split. A trivial example is a
TXT record that is larger than the buffer size. Probably the
best behavior here is to truncate.
10. DNSSEC checks
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DNSSEC checks should be done on the final reassembled packet.
This needs to be documented.
11. Name compression
Name compression should be done on the each fragment separately.
This needs to be documented.
12. OPT-RR
Some OPT-RR seem to be oriented at the entire message, others
make more sense per packet. This needs to be sorted out. Also
we need to investigate the edge case where fragments have
conflicting options (Mukund thinks that we can copy the approach
in the EDNS specification and use the same rules about
conflicting OPT-RR that it uses.)
7. Security Considerations
To avoid DNS amplification or reflection attacks, DNS cookies
[I-D.ietf-dnsop-cookies] must be used. The DNS cookie EDNS option is
identical in all fragments that make up a DNS message. The
duplication of the same cookie values in all fragments that make up
the message is not expected to introduce a security weakness in the
case of off-path attacks.
8. IANA Considerations
The ALLOW-FRAGMENTS and FRAGMENT EDNS(0) options require option codes
to be assigned for them.
9. Acknowledgements
Thanks to Stephen Morris, JINMEI Tatuya, Paul Vixie, Mark Andrews,
and David Dragon for reviewing a pre-draft proposal and providing
support, comments and suggestions.
10. References
[Fragment-Poisonous]
Herzberg, A. and H. Shulman, "Fragmentation Considered
Poisonous", 2012.
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[I-D.ietf-dnsop-cookies]
Eastlake, D. and M. Andrews, "Domain Name System (DNS)
Cookies", draft-ietf-dnsop-cookies-04 (work in progress),
July 2015.
[I-D.ietf-dnsop-respsize]
Vixie, P., Kato, A., and J. Abley, "DNS Referral Response
Size Issues", draft-ietf-dnsop-respsize-15 (work in
progress), February 2014.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, May 2003.
[RFC5452] Hubert, A. and R. van Mook, "Measures for Making DNS More
Resilient against Forged Answers", RFC 5452, January 2009.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
[SAC016] ICANN Security and Stability Advisory Committee, "Testing
Firewalls for IPv6 and EDNS0 Support", 2007.
[SAC035] ICANN Security and Stability Advisory Committee, "DNSSEC
Impact on Broadband Routers and Firewalls", 2008.
[T-DNS] Zhu, L., Hu, Z., and J. Heidemann, "T-DNS: Connection-
Oriented DNS to Improve Privacy and Security (extended)",
2007, <http://www.isi.edu/~johnh/PAPERS/Zhu14b.pdf>.
Appendix A. Change History (to be removed before publication)
o draft-muks-dns-message-fragments-00
Initial draft.
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Authors' Addresses
Mukund Sivaraman
Internet Systems Consortium
950 Charter Street
Redwood City, CA 94063
US
Email: muks@isc.org
URI: http://www.isc.org/
Shane Kerr
Beijing Internet Institute
2/F, Building 5, No.58 Jinghai Road, BDA
Beijing 100176
CN
Email: shane@biigroup.cn
URI: http://www.biigroup.com/
Linjian Song
Beijing Internet Institute
2/F, Building 5, No.58 Jinghai Road, BDA
Beijing 100176
CN
Email: songlinjian@gmail.com
URI: http://www.biigroup.com/
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