Network Working Group                                          R. Bellis
Request for Comments: 5625                                    Nominet UK
BCP: 152                                                     August 2009
Category: Best Current Practice


                  DNS Proxy Implementation Guidelines

Abstract

   This document provides guidelines for the implementation of DNS
   proxies, as found in broadband gateways and other similar network
   devices.

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

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.

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. The Transparency Principle ......................................3
   4. Protocol Conformance ............................................4
      4.1. Unexpected Flags and Data ..................................4
      4.2. Label Compression ..........................................4
      4.3. Unknown Resource Record Types ..............................4
      4.4. Packet Size Limits .........................................4
           4.4.1. TCP Transport .......................................5
           4.4.2. Extension Mechanisms for DNS (EDNS0) ................6
           4.4.3. IP Fragmentation ....................................6
      4.5. Secret Key Transaction Authentication for DNS (TSIG) .......7
   5. DHCP's Interaction with DNS .....................................7
      5.1. Domain Name Server (DHCP Option 6) .........................7
      5.2. Domain Name (DHCP Option 15) ...............................8
      5.3. DHCP Leases ................................................8
   6. Security Considerations .........................................9
      6.1. Forgery Resilience .........................................9
      6.2. Interface Binding .........................................10
      6.3. Packet Filtering ..........................................10
   7. Acknowledgements ...............................................10
   8. References .....................................................11
      8.1. Normative References ......................................11
      8.2. Informative References ....................................12

1.  Introduction

   Research has found ([SAC035], [DOTSE]) that many commonly used
   broadband gateways (and similar devices) contain DNS proxies that are
   incompatible in various ways with current DNS standards.

   These proxies are usually simple DNS forwarders, but typically do not
   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.

   Note that to ensure full DNS protocol interoperability it is
   preferred that client stub resolvers should communicate directly with
   full-feature, upstream recursive resolvers wherever possible.

   That notwithstanding, this document describes the incompatibilities
   that have been discovered and offers guidelines to implementors on
   how to provide better interoperability in those cases where the
   client must use the broadband gateway's DNS proxy.

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 implement
   all current and future DNS features.

   There are several reasons why this is the case:

   o  Broadband gateways 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 or how they
      might be implemented.

   o  Doing so would substantially complicate the configuration user
      interface (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.

   [SAC035] shows that the more actively a proxy participates in the DNS
   protocol, the more likely it is that it will somehow interfere with
   the flow of messages between the DNS client and the upstream
   recursive resolvers.

   The role 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.

   Except when required to enforce an active security or network policy
   (such as maintaining a pre-authentication "walled garden"), end-users
   SHOULD be able to send their DNS queries to specified upstream

   resolvers, thereby bypassing the proxy altogether.  In this case, the
   gateway SHOULD NOT modify the DNS request or response packets in any
   way.

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, proxies MUST ignore any unknown DNS flags and proxy those
   packets as usual.

4.2.  Label Compression

   Compression of labels as per Section 4.1.4 of [RFC1035] is optional.

   Proxies MUST forward packets regardless of the presence or absence of
   compressed labels therein.

4.3.  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.

4.4.  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

   "TrunCation" (TC) bit in the DNS response header to indicate that
   truncation has occurred.  There are however two standard mechanisms
   (described in Sections 4.4.1 and 4.4.2) 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 remove the TC bit in server
   responses that 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.

   Since UDP packets larger than 512 octets are now expected in normal
   operation, proxies SHOULD NOT truncate UDP packets that exceed that
   size.  See Section 4.4.3 for recommendations for packet sizes
   exceeding the WAN MTU.

   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.

4.4.1.  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].

   Whilst TCP transport is not strictly mandatory, it is supported by
   the vast majority of stub resolvers and recursive servers.  Lack of
   support in the proxy prevents this fail-over mechanism from working.

   DNS proxies MUST 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 any 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.

4.4.2.  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
   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 MUST NOT refuse to proxy such packets.

4.4.3.  IP Fragmentation

   Support for UDP packet sizes exceeding the WAN MTU depends on the
   gateway's algorithm for handling fragmented IP packets.  Several
   methods are possible:

   1.  Fragments are dropped.

   2.  Fragments are forwarded individually as they're received.

   3.  Complete packets are reassembled on the gateway and then re-
       fragmented (if necessary) as they're forwarded to the client.

   Method 1 above will cause compatibility problems with EDNS0 unless
   the DNS client is configured to advertise an EDNS0 buffer size
   limited to the WAN MTU less the size of the IP header.  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 (irrespective of which of the above methods is in use),
   proxies SHOULD be capable of forwarding UDP packets up to a payload
   size of at least 4096 octets.

   NB: in theory, IP fragmentation may also occur if the LAN MTU is
   smaller than the WAN MTU, although the author has not observed such a
   configuration in use on any residential broadband service.

4.5.  Secret Key Transaction Authentication for DNS (TSIG)

   [RFC2845] defines TSIG, which is a mechanism for authenticating DNS
   requests and responses at the packet level.

   Any modifications made to the DNS portions of a TSIG-signed query or
   response packet (with the exception of the Query ID) will cause a
   TSIG authentication failure.

   DNS proxies MUST implement Section 4.7 of [RFC2845] and either
   forward packets unchanged (as recommended above) or fully implement
   TSIG.

   As per Section 4.3, DNS proxies MUST be capable of proxying packets
   containing TKEY [RFC2930] Resource Records.

   NB: any DNS proxy (such as those commonly found in WiFi hotspot
   "walled gardens") that transparently intercepts all DNS queries and
   that returns unsigned responses to signed queries, will also cause
   TSIG authentication failures.

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 gateways 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 gateway's DNS proxy.  This is understandable
   behaviour given that the correct upstream settings are not usually
   known at boot time.

   Most gateways learn their own DNS settings via values supplied by an
   ISP via DHCP or PPP over the WAN interface.  However, whilst many
   gateways do allow the device administrator to override those values,
   some gateways only use those 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 that are regularly used on many different
   networks.

   As per Section 3, 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 gateways SHOULD support device-
   administrator 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 MUST NOT be sent to clients when no value
   is configured.

5.3.  DHCP Leases

   It is noted that some DHCP servers in broadband gateways offer, by
   default, their own IP address for the "Domain Name Server" option (as
   described above) but then automatically start offering the upstream
   servers' addresses once they've been learnt over the WAN interface.

   In general, this behaviour is highly 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 to whether
   some mechanism for forcing a DHCP lease renewal 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.

   Alternatively, the DHCP server might only issue very short (i.e., 60
   second) leases while the WAN link is down, only reverting to more
   typical lease lengths once the WAN link is up and the upstream DNS
   servers are known.  Indeed, with such a configuration it may be
   possible to avoid the need to implement a DNS proxy function in the
   broadband gateway at all.

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.

   NB: changing the Query ID is acceptable and compatible with proxying
   TSIG-signed packets since the TSIG signature calculation is based on
   the original message ID, which is carried in the TSIG RR.

   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.

   It is strongly RECOMMENDED that DNS proxies follow the relevant
   recommendations in [RFC5452], particularly those in Section 9.2
   relating to randomisation of Query IDs and source ports.  This also
   applies to source port selection within any NAT function.

   If a DNS proxy is running on a broadband gateway with NAT that is
   compliant with [RFC4787], then it SHOULD also follow the
   recommendations in Section 10 of [RFC5452] concerning how long DNS
   state is kept.

6.2.  Interface Binding

   Some gateways 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 gateway 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 that MAY be dropped include:

   o  invalid compression pointers (i.e., those that point outside of
      the current packet or that 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)

   Dropped packets will cause the client to repeatedly retransmit the
   original request, with the client only detecting the error after
   several retransmit intervals.

   In these circumstances, proxies SHOULD synthesise a suitable DNS
   error response to the client (i.e., SERVFAIL) instead of dropping the
   packet completely.  This will allow the client to detect the error
   immediately.

7.  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.

8.  References

8.1.  Normative References

   [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.

   [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.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [RFC5358]  Damas, J. and F. Neves, "Preventing Use of Recursive
              Nameservers in Reflector Attacks", BCP 140, RFC 5358,
              October 2008.

   [RFC5452]  Hubert, A. and R. van Mook, "Measures for Making DNS More
              Resilient against Forged Answers", RFC 5452, January 2009.

8.2.  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/