DNS proxy guidelines: DNS Proxy Implementation Guidelines
How home-router DNS proxies should pass queries along without breaking modern DNS.
Bellis, R., "DNS Proxy Implementation Guidelines", BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009, <https://www.rfc-editor.org/info/rfc5625>.
@misc{rfc5625,
series = {Request for Comments},
number = 5625,
howpublished = {RFC 5625},
publisher = {RFC Editor},
doi = {10.17487/RFC5625},
url = {https://www.rfc-editor.org/info/rfc5625},
author = {R. Bellis},
title = {{DNS Proxy Implementation Guidelines}},
pagetotal = 12,
year = 2009,
month = aug,
} RFC text
Contents
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/
AI ELI5
On this page
§1Introduction
Research cited by the RFC ([SAC035] and [DOTSE]) found that many commonly used broadband gateways contain DNS proxies that are incompatible in various ways with current DNS standards. These proxies are usually simple forwarders without caching. They act as a convenient default resolver for clients on the LAN but rely on an upstream resolver, for example at an ISP, to do recursive lookups.
The RFC says it is preferable for client stub resolvers to talk directly to full-feature upstream recursive resolvers wherever possible. Where the client must use the gateway's proxy, the document describes the incompatibilities found and gives implementors guidelines for better interoperability. It uses the RFC 2119 key words.
Many home routers pass your computer's DNS questions along, and many of them do it badly. This note explains how to do it right.
§3The Transparency Principle
A simple proxy cannot practically implement every current and future DNS feature. The RFC gives reasons: gateways have limited hardware, firmware upgrade cycles are long and many users do not apply upgrades, nobody knows what future DNS features will look like, and supporting them would complicate the configuration interface. Some extensions such as EDNS0 are also meant as hop-by-hop mechanisms, which would force a proxy that counts as a hop to be fully compliant.
Test results in [SAC035] show that the more actively a proxy takes part in the DNS protocol, the more likely it is to interfere with messages. So the proxy's role should be no more and no less than to receive requests from LAN clients, forward them verbatim to a known upstream recursive resolver on the WAN side, and return the whole response verbatim. Proxies are RECOMMENDED to be as transparent as possible.
Except when enforcing an active security or network policy, such as a pre-authentication "walled garden", end-users SHOULD be able to send queries to specified upstream resolvers and bypass the proxy altogether. In that case the gateway SHOULD NOT modify request or response packets in any way.
A good messenger carries your note exactly as written and brings back the full reply. The less the messenger tinkers, the fewer things go wrong.
§4.1Flags, Compression and Unknown Record Types
Combined with Postel's Robustness Principle (RFC 793), transparency means a proxy should not arbitrarily reject requests or responses it thinks are non-compliant. Some proxies were seen dropping any packet with the "Authentic Data" (AD) or "Checking Disabled" (CD) bits from DNSSEC. That may stem from RFC 1035 saying the unused "Z" bits MUST be zero, but those bits were always reserved for future use. Proxies MUST ignore unknown DNS flags and proxy the packets as usual.
Label compression (Section 4.1.4 of RFC 1035) is optional, and proxies MUST forward packets whether or not they contain compressed labels. RFC 3597 requires resolvers to handle unknown resource record types transparently, so all requests and responses MUST be proxied regardless of the QTYPE and QCLASS fields, and of the TYPE and CLASS fields of any record inside a response.
If a letter has a stamp or a sticker the post office has not seen before, it should still deliver the letter, not throw it away.
§4.4Packet Size, TCP and EDNS0
RFC 1035 limits a DNS payload in a UDP packet to 512 octets. When the required parts of a response do not fit, the server MUST set the "TrunCation" (TC) bit. Many proxies were seen truncating all responses at 512 octets, or at a size tied to the WAN MTU, without setting TC, and others stripping TC from correct server responses. A naive client library could then crash by reading past the data it actually received. Because larger UDP packets are now normal, proxies SHOULD NOT truncate UDP packets over 512 octets. If a proxy must truncate, it MUST set TC, and proxies MUST NOT remove TC from responses.
When a UDP query fails due to truncation, the standard fail-over is to retry over TCP (RFC 1123). Proxies MUST therefore be ready to receive and forward queries over TCP. Some "smart" proxies were seen sending a TCP request upstream over UDP first, which was doomed to be truncated. A proxy that receives a request over TCP SHOULD forward it over TCP and SHOULD NOT try UDP first.
EDNS0 (RFC 2671) lets a client put an OPT record in the Additional Section to advertise a receive buffer size and the "DNSSEC OK" (DO) flag. Some proxies rejected such packets with a FORMERR response code or silently discarded them. Proxies MUST NOT refuse to proxy them.
Large UDP packets can exceed the WAN MTU, and gateways handle fragments in one of three ways: drop them, forward them individually, or reassemble and re-fragment. Dropping fragments breaks EDNS0 unless the client advertises a buffer size limited to the WAN MTU less the IP header size. EDNS0 allows buffers up to 65535 octets, but most common servers do not support above 4096, so proxies SHOULD forward UDP packets with payloads of at least 4096 octets whichever method they use.
Short messages fit on a postcard. Long ones need a bigger envelope. If the postcard is cut short, the messenger must say so, and must be able to carry the big envelope too.
| Observed behaviour | Requirement |
|---|---|
| Truncating UDP responses at 512 octets without setting TC | SHOULD NOT truncate; if it must, MUST set TC |
| Removing the TC bit from server responses | MUST NOT remove TC |
| No TCP support, or retrying a TCP request over UDP first | MUST forward over TCP; SHOULD NOT try UDP first |
| Rejecting or black-holing packets with an OPT record | MUST NOT refuse to proxy them |
| Dropping fragmented large responses | SHOULD forward UDP payloads of at least 4096 octets |
§4.5TSIG Signed Messages
TSIG (RFC 2845) authenticates DNS requests and responses at the packet level. Any change to the DNS portions of a signed packet, other than the Query ID, causes an authentication failure.
Proxies MUST implement Section 4.7 of RFC 2845 and either forward packets unchanged or fully implement TSIG. They MUST also be able to proxy packets containing TKEY (RFC 2930) records. The RFC adds that a proxy which transparently intercepts all DNS queries and returns unsigned answers to signed queries, as commonly found in WiFi hotspot walled gardens, will also cause TSIG failures.
A signed letter has a wax seal. If the messenger changes the words, the seal breaks, so the messenger must leave signed letters alone.
§5How DHCP Points Clients at the Proxy
Most consumers get their network settings from DHCP (RFC 2131), including DNS settings. Most gateways default to supplying their own address in the "Domain Name Server" option (Option 6, RFC 2132), so clients use the proxy unless reconfigured. Some gateways use the ISP-supplied DNS values only for their own forwarding and do not offer them over DHCP, leaving hard-coding in the client as the only escape, which is poor for mobile devices. Gateways are therefore RECOMMENDED to let the device administrator configure the Option 6 values.
For the "Domain Name" option (Option 15), the RFC notes that a significant amount of traffic to the DNS Root Name Servers is for invalid top-level domain names, some traceable to vendor firmware defaults. Since there is no standard for a local suffix, the default SHOULD be empty and the option MUST NOT be sent when no value is configured.
Lease timing matters too. A lease obtained while the WAN link is down will carry the gateway's own address until renewed. The RFC offers no firm rule, but suggests considering lease length, forcing a renewal, persisting learned upstream values (at the risk of stale values if the device moves to another ISP), or issuing only 60 second leases while the WAN is down. With that last approach the RFC says a DNS proxy might not be needed at all.
When your device joins the network, the router tells it who to ask for directions. The router should let its owner change that answer and should not hand out made-up names.
§6Security Considerations
The document introduces no new protocols but lists vendor recommendations. On forgery resilience, many proxies pick a new Query ID for outbound requests so answers go to the right client; that is acceptable even with TSIG, because the signature uses the original message ID carried in the TSIG record. However, many proxies were seen using sequential Query IDs. Proxies are strongly RECOMMENDED to follow RFC 5452, particularly Section 9.2 on randomising Query IDs and source ports, including source ports in any NAT function. A proxy on a gateway with NAT compliant with RFC 4787 SHOULD also follow Section 10 of RFC 5452 on how long DNS state is kept.
Some gateways listen on both LAN and WAN interfaces, which allows reflector attacks as described in RFC 5358. The proxy SHOULD NOT, by default, be reachable from the WAN.
Deep packet inspection sits uneasily with transparency, but the RFC separates malformed traffic from merely unexpected data. Packets that MAY be dropped include those with invalid compression pointers (outside the packet or causing a parsing loop) and incorrect Question, Answer, Authority or Additional counts, with care where truncation is possible. Silent drops make clients retransmit and notice the error late, so proxies SHOULD synthesise an error response such as SERVFAIL instead.
The messenger should not use easy-to-guess ticket numbers, should only serve people inside the house, and should say "something is wrong" rather than staying silent.