DNSEXT R. Bellis
Internet-Draft Nominet UK
Intended status: BCP November 26, 2008
Expires: May 30, 2009
DNS Proxy Implementation Guidelines
draft-ietf-dnsext-dnsproxy-00
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Abstract
This document provides guidelines for the implementation of DNS
proxies, as found in broadband routers and other similar network
devices.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The Transparency Principle . . . . . . . . . . . . . . . . . . 3
4. Protocol Conformance . . . . . . . . . . . . . . . . . . . . . 4
4.1. Unexpected Flags and Data . . . . . . . . . . . . . . . . 4
4.2. Unknown Resource Record Types . . . . . . . . . . . . . . 4
4.3. Packet Size Limits . . . . . . . . . . . . . . . . . . . . 4
4.3.1. TCP Transport . . . . . . . . . . . . . . . . . . . . 5
4.3.2. Extension Mechanisms for DNS (EDNS0) . . . . . . . . . 5
4.3.3. IP Fragmentation . . . . . . . . . . . . . . . . . . . 6
4.4. Secret Key Transaction Authentication for DNS (TSIG) . . . 6
5. DHCP's Interaction with DNS . . . . . . . . . . . . . . . . . 7
5.1. Domain Name Server (DHCP Option 6) . . . . . . . . . . . . 7
5.2. Domain Name (DHCP Option 15) . . . . . . . . . . . . . . . 7
5.3. DHCP Leases . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6.1. Forgery Resilience . . . . . . . . . . . . . . . . . . . . 8
6.2. Interface Binding . . . . . . . . . . . . . . . . . . . . 9
6.3. Packet Filtering . . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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1. Introduction
Recent research ([SAC035], [DOTSE]) has shown that many commonly-used
broadband routers (and similar devices) contain DNS proxies which are
incompatible in various ways with current DNS standards.
These proxies are usual simple DNS forwarders, but do not usually
have any caching capabilities. The proxy serves as a convenient
default DNS resolver for clients on the LAN, but relies on an
upstream resolver (e.g. at an ISP) to perform recursive DNS lookups.
This documents describes the incompatibilities that have been
discovered and offers guidelines to implementors on how to provide
maximum interoperability.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. The Transparency Principle
It is not considered practical for a simple DNS proxy to directly
implement all current and future DNS features.
There are several reasons why this is the case:
o broadband routers usually have limited hardware resources
o firmware upgrade cycles are long, and many users do not routinely
apply upgrades when they become available
o no-one knows what those future DNS features will be, nor how they
might be implemented
o it would substantially complicate the configuration UI of the
device
Furthermore some modern DNS protocol extensions (see e.g. EDNS0,
below) are intended to be used as "hop-by-hop" mechanisms. If the
DNS proxy is considered to be such a "hop" in the resolution chain
then for it to function correctly it would need to be fully compliant
with all such mechanisms.
Research has shown that the more actively a proxy participates in the
DNS protocol then the more likely it is that it will somehow
interfere with the flow of messages between the DNS client and the
upstream recursive resolvers.
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The task of the proxy SHOULD therefore be no more and no less than to
receive DNS requests from clients on the LAN side, forward those
verbatim to one of the known upstream recursive resolvers on the WAN
side, and ensure that the whole response is returned verbatim to the
original client.
It is RECOMMENDED that proxies should be as transparent as possible,
such that any "hop-by-hop" mechanisms or newly introduced protocol
extensions operate as if the proxy were not there.
4. Protocol Conformance
4.1. Unexpected Flags and Data
The Transparency Principle above, when combined with Postel's
Robustness Principle [RFC0793], suggests that DNS proxies should not
arbitrarily reject or otherwise drop requests or responses based on
perceived non-compliance with standards.
For example, some proxies have been observed to drop any packet
containing either the "Authentic Data" (AD) or "Checking Disabled"
(CD) bits from DNSSEC [RFC4035]. This may be because [RFC1035]
originally specified that these unused "Z" flag bits "MUST" be zero.
However these flag bits were always intended to be reserved for
future use, so refusing to proxy any packet containing these flags
(now that uses for those flags have indeed been defined) is not
appropriate.
Therefore it is RECOMMENDED that proxies SHOULD ignore any unknown
DNS flags and proxy those packets as usual.
4.2. Unknown Resource Record Types
[RFC3597] requires that resolvers MUST handle Resource Records (RRs)
of unknown type transparently.
All requests and responses MUST be proxied regardless of the values
of the QTYPE and QCLASS fields.
Similarly all responses MUST be proxied regardless of the values of
the TYPE and CLASS fields of any Resource Record therein.
4.3. Packet Size Limits
[RFC1035] specifies that the maximum size of the DNS payload in a UDP
packet is 512 octets. Where the required portions of a response
would not fit inside that limit the DNS server MUST set the
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"TrunCation" (TC) bit in the DNS response header to indicate that
truncation has occurred. There are however two standard mechanisms
(described below) for transporting responses larger than 512 octets.
Many proxies have been observed to truncate all responses at 512
octets, and others at a packet size related to the WAN MTU, in either
case doing so without correctly setting the TC bit.
Other proxies have been observed to incorrectly remove the TC bit in
server responses which correctly had the TC bit set by the server.
If a DNS response is truncated but the TC bit is not set then client
failures may result, in particular a naive DNS client library might
suffer crashes due to reading beyond the end of the data actually
received.
Therefore if a proxy must unilaterally truncate a response then the
proxy MUST set the TC bit. Similarly, proxies MUST NOT remove the TC
bit from responses.
4.3.1. TCP Transport
Should a UDP query fail because of truncation the standard fail-over
mechanism is to retry the query using TCP, as described in section
6.1.3.2 of [RFC1123] .
DNS proxies SHOULD therefore be prepared to receive and forward
queries over TCP.
Note that it is unlikely that a client would send a request over TCP
unless it had already received a truncated UDP response. Some
"smart" proxies have been observed to first forward a request
received over TCP to an upstream resolver over UDP, only for the
response to be truncated, causing the proxy to retry over TCP. Such
behaviour increases network traffic and causes delay in DNS
resolution since the initial UDP request is doomed to fail.
Therefore whenever a proxy receives a request over TCP, the proxy
SHOULD forward the query over TCP and SHOULD NOT attempt the same
query over UDP first.
4.3.2. Extension Mechanisms for DNS (EDNS0)
The Extension Mechanism for DNS [RFC2671] was introduced to allow the
transport of larger DNS packets over UDP and also to allow for
additional request and response flags.
A client may send an OPT Resource Record (OPT RR) in the Additional
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Section of a request to indicate that it supports a specific receive
buffer size. The OPT RR also includes the "DNSSEC OK" (DO) flag used
by DNSSEC to indicate that DNSSEC-related RRs should be returned to
the client.
However some proxies have been observed to either reject (with a
FORMERR response code) or black-hole any packet containing an OPT RR.
As per Section 4.1 proxies SHOULD NOT refuse to proxy such packets.
4.3.3. IP Fragmentation
Support for UDP packet sizes exceeding the WAN MTU depends on the
router's algorithm for handling fragmented IP packets. Several
options are possible:
1. fragments are dropped
2. fragments are forwarded individually as they're received
3. complete packets are reassembled on the router, and then re-
fragmented (if necessary) as they're forwarded to the client
Option 1 above will cause compatibility problems with EDNS0 unless
the DNS client is configured to advertise an EDNS0 buffer size
limited to 28 octets less than the MTU. Note that RFC 2671 does
recommend that the path MTU should be taken into account when using
EDNS0.
Also, whilst the EDNS0 specification allows for a buffer size of up
to 65535 octets, most common DNS server implementations do not
support a buffer size above 4096 octets.
Therefore it is RECOMMENDED (whichever of options 2 or 3 above is in
use) that routers SHOULD be capable of forwarding UDP packets up to a
payload size of at least 4096 octets.
4.4. Secret Key Transaction Authentication for DNS (TSIG)
[RFC2845] defines TSIG, which is a hop-by-hop mechanism for
authenticating DNS requests and responses at the packet level.
Whilst it's not impossible for a simple DNS proxy to implement TSIG
directly it is not advised since parsing and validating received
packets is a computationally intensive task, best left to full-
featured DNS clients.
DNS proxies SHOULD be transparent to TSIG signed packets.
Similarly, as per Section 4.2, DNS proxies SHOULD be capable to
proxying packets containing TKEY [RFC2930] Resource Records
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5. DHCP's Interaction with DNS
Whilst this document is primarily about DNS proxies, most consumers
rely on DHCP [RFC2131] to obtain network configuration settings.
Such settings include the client machine's IP address, subnet mask
and default gateway, but also include DNS related settings.
It is therefore appropriate to examine how DHCP affects client DNS
configuration.
5.1. Domain Name Server (DHCP Option 6)
Most routers default to supplying their own IP address in the DHCP
"Domain Name Server" option [RFC2132]. The net result is that
without explicit re-configuration many DNS clients will by default
send queries to the router's DNS proxy. This is understandable
behaviour given that the correct upstream settings are not usually
known at boot time.
Most routers learn their own DNS settings via values supplied by an
ISP via DHCP or PPP over the WAN interface. However whilst many
routers do allow the end-user to override those values, some routers
only use those end-user supplied values to affect the proxy's own
forwarding function, and do not offer these values via DHCP.
When using such a device the only way to avoid using the DNS proxy is
to hard-code the required values in the client operating system.
This may be acceptable for a desktop system but it is inappropriate
for mobile devices which are regularly used on many different
networks.
End users SHOULD be able to send their DNS queries directly to
specified upstream resolvers, ideally without hard-coding those
settings in their stub resolver.
It is therefore RECOMMENDED that routers SHOULD support end-user
configuration of values for the "Domain Name Server" DHCP option.
5.2. Domain Name (DHCP Option 15)
A significant amount of traffic to the DNS Root Name Servers is for
invalid top-level domain names, and some of that traffic can be
attributed to particular equipment vendors whose firmware defaults
this DHCP option to specific values.
Since no standard exists for a "local" scoped domain name suffix it
is RECOMMENDED that the default value for this option SHOULD be
empty, and that this option SHOULD NOT be sent to clients when no
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value is configured.
5.3. DHCP Leases
It is noted that some DHCP servers in broadband gateways by default
offer their own IP address for the "Domain Name Server" option (as
describe above) but then automatically start offering the upstream
settings once they've been learnt over the WAN interface.
In general this behaviour is desirable, but the effect for the end-
user is that the settings used depend on whether the DHCP lease was
obtained before or after the WAN link was established.
If the DHCP lease is obtained whilst the WAN link is down then the
DHCP client (and hence the DNS client) will not receive the correct
values until the DHCP lease is renewed.
Whilst no specific recommendations are given here, vendors may wish
to give consideration to the length of DHCP leases, and whether some
mechanism for forcing a DHCP lease renewal (i.e. by toggling the LAN
port link state whenever the WAN link state changes from DOWN to UP)
might be appropriate.
Another possibility is that the learnt upstream values might be
persisted in non-volatile memory such that on reboot the same values
can be automatically offered via DHCP. However this does run the
risk that incorrect values are initially offered if the device is
moved or connected to another ISP.
6. Security Considerations
This document introduces no new protocols. However there are some
security related recommendations for vendors that are listed here.
6.1. Forgery Resilience
Whilst DNS proxies are not usually full-feature resolvers they
nevertheless share some characteristics with them.
Notwithstanding the recommendations above about transparency many DNS
proxies are observed to pick a new Query ID for outbound requests to
ensure that responses are directed to the correct client.
It has been standard guidance for many years that each DNS query
should use a randomly generated Query ID. However many proxies have
been observed picking sequential Query IDs for successive requests.
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DNS proxies SHOULD follow the relevant recommendations in
[I-D.ietf-dnsext-forgery-resilience], particularly those in Section
9.2 relating to randomisation of Query IDs and source ports.
NB: Changing the Query ID is incompatible with transparent proxying
of any TSIG records since the TSIG signature includes the Query ID.
It SHOULD be avoided wherever possible.
6.2. Interface Binding
Some routers have been observed to have their DNS proxy listening on
both internal (LAN) and external (WAN) interfaces. In this
configuration it is possible for the proxy to be used to mount
reflector attacks as described in [RFC5358]
The DNS proxy in a router SHOULD NOT by default be accessible from
the WAN interfaces of the device.
6.3. Packet Filtering
The Transparency and Robustness Principles are not entirely
compatible with the Deep Packet Inspection features of security
appliances such as firewalls which are intended to protect systems on
the inside of a network from rogue traffic.
However a clear distinction may be made between traffic that is
intrinsically malformed and that which merely contains unexpected
data.
Examples of malformed packets which MAY be dropped include:
o invalid compression pointers (i.e. those that run forward of the
current packet offset, or which might cause a parsing loop).
o incorrect counts for the Question, Answer, Authority and
Additional Sections (although care should be taken where
truncation is a possibility).
Since dropped packets will cause the client to repeatedly retransmit
the original request, it is RECOMMENDED that proxies SHOULD instead
return a suitable DNS error response to the client (i.e. SERVFAIL)
instead of dropping the packet completely.
7. IANA Considerations
This document requests no IANA actions.
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8. Change Log
draft-ietf-dnsproxy-00
Changed recommended DPI error to SERVFAIL (from Jelte)
Changed example for invalid compression pointers (from Wouter).
Note about TSIG implications of changing Query ID (from Wouter).
Clarified TC-bit text (from Wouter)
Extra text about proxy bypass (Nicholas W.)
draft-bellis-dnsproxy-00
Initial draft
9. Acknowledgements
The author would particularly like to acknowledge the assistance of
Lisa Phifer of Core Competence. In addition the author is grateful
for the feedback from the members of the DNSEXT Working Group.
10. References
10.1. Normative References
[I-D.ietf-dnsext-forgery-resilience]
Hubert, B. and R. Mook, "Measures for making DNS more
resilient against forged answers",
draft-ietf-dnsext-forgery-resilience-09 (work in
progress), November 2008.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
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[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, May 2000.
[RFC2930] Eastlake, D., "Secret Key Establishment for DNS (TKEY
RR)", RFC 2930, September 2000.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, September 2003.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive
Nameservers in Reflector Attacks", BCP 140, RFC 5358,
October 2008.
10.2. Informative References
[DOTSE] Ahlund and Wallstrom, "DNSSEC Tests of Consumer Broadband
Routers", February 2008,
<http://www.iis.se/docs/Routertester_en.pdf>.
[SAC035] Bellis, R. and L. Phifer, "Test Report: DNSSEC Impact on
Broadband Routers and Firewalls", September 2008,
<http://www.icann.org/committees/security/sac035.pdf>.
Author's Address
Ray Bellis
Nominet UK
Edmund Halley Road
Oxford OX4 4DQ
United Kingdom
Phone: +44 1865 332211
Email: ray.bellis@nominet.org.uk
URI: http://www.nominet.org.uk/
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