draft-ietf-dnsop-bad-dns-res-01.txt   draft-ietf-dnsop-bad-dns-res-02.txt 
DNS Operations M. Larson DNS Operations M. Larson
Internet-Draft P. Barber Internet-Draft P. Barber
Expires: December 22, 2003 VeriSign Expires: January 17, 2005 VeriSign
June 23, 2003 July 19, 2004
Observed DNS Resolution Misbehavior Observed DNS Resolution Misbehavior
draft-ietf-dnsop-bad-dns-res-01 draft-ietf-dnsop-bad-dns-res-02
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract Abstract
This Internet-Draft describes DNS name server and stub resolver This memo describes DNS name server and resolver behavior that
behavior that results in a significant query volume sent to the root results in a significant query volume sent to the root and top-level
and top-level domain (TLD) name servers. In some cases we recommend domain (TLD) name servers. In some cases we recommend minor
minor additions to the DNS protocol specification and corresponding additions to the DNS protocol specification and corresponding changes
changes in name server implementations to alleviate these unnecessary in iterative resolver implementations to alleviate these unnecessary
queries. In one case, we have highlighted behavior of a popular name queries. The recommendations made in this document are a direct
server implementation that does not conform to the DNS specification. byproduct of observation and analysis of abnormal query traffic
The recommendations made in this document are a direct byproduct of patterns seen at two of the thirteen root name servers and all
observation and analysis of abnormal query traffic patterns seen at thirteen com/net TLD name servers.
two of the thirteen root name servers and all thirteen com/net TLD
name servers.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1]. document are to be interpreted as described in RFC 2119 [1].
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Observed name server misbehavior . . . . . . . . . . . . . . 4 1.1 A note about terminology in this memo . . . . . . . . . . 3
2.1 Aggressive requerying for delegation information . . . . . . 4 2. Observed name server misbehavior . . . . . . . . . . . . . . 5
2.1.1 Recommendation . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 Aggressive requerying for delegation information . . . . . 5
2.2 Repeated queries to lame servers . . . . . . . . . . . . . . 5 2.1.1 Recommendation . . . . . . . . . . . . . . . . . . . . 6
2.2.1 Recommendation . . . . . . . . . . . . . . . . . . . . . . . 6 2.2 Repeated queries to lame servers . . . . . . . . . . . . . 6
2.3 Incomplete negative caching implementation . . . . . . . . . 6 2.2.1 Recommendation . . . . . . . . . . . . . . . . . . . . 7
2.3.1 Recommendation . . . . . . . . . . . . . . . . . . . . . . . 6 2.3 Inability to follow multiple levels of out-of-zone glue . 7
2.4 Inability to follow multiple levels of out-of-zone glue . . 6 2.3.1 Recommendation . . . . . . . . . . . . . . . . . . . . 8
2.4.1 Recommendation . . . . . . . . . . . . . . . . . . . . . . . 7 2.4 Aggressive retransmission when fetching glue . . . . . . . 8
3. Observed client misbehavior . . . . . . . . . . . . . . . . 8 2.4.1 Recommendation . . . . . . . . . . . . . . . . . . . . 9
4. IANA considerations . . . . . . . . . . . . . . . . . . . . 9 2.5 Aggressive retransmission behind firewalls . . . . . . . . 9
5. Security considerations . . . . . . . . . . . . . . . . . . 10 2.5.1 Recommendation . . . . . . . . . . . . . . . . . . . . 9
6. Internationalization considerations . . . . . . . . . . . . 11 2.6 Misconfigured NS records . . . . . . . . . . . . . . . . . 10
Normative References . . . . . . . . . . . . . . . . . . . . 12 2.6.1 Recommendation . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 12 2.7 Name server records with zero TTL . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . 13 2.7.1 Recommendation . . . . . . . . . . . . . . . . . . . . 12
2.8 Unnecessary dynamic update messages . . . . . . . . . . . 12
2.8.1 Recommendation . . . . . . . . . . . . . . . . . . . . 13
2.9 Queries for domain names resembling IP addresses . . . . . 13
2.9.1 Recommendation . . . . . . . . . . . . . . . . . . . . 13
2.10 Misdirected recursive queries . . . . . . . . . . . . . 14
2.10.1 Recommendation . . . . . . . . . . . . . . . . . . . 14
2.11 Suboptimal name server selection algorithm . . . . . . . 14
2.11.1 Recommendation . . . . . . . . . . . . . . . . . . . 15
3. IANA considerations . . . . . . . . . . . . . . . . . . . . 16
4. Security considerations . . . . . . . . . . . . . . . . . . 17
5. Internationalization considerations . . . . . . . . . . . . 18
6. Normative References . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . 20
1. Introduction 1. Introduction
Observation of query traffic received by two root name servers and Observation of query traffic received by two root name servers and
the thirteen com/net TLD name servers has revealed that a large the thirteen com/net TLD name servers has revealed that a large
proportion of the total traffic often consists of "requeries". A proportion of the total traffic often consists of "requeries". A
requery is the same question (<qname, qtype, qclass>) asked requery is the same question (<qname, qtype, qclass>) asked
repeatedly at an unexpectedly high rate. We have observed requeries repeatedly at an unexpectedly high rate. We have observed requeries
from both a single IP address and multiple IP addresses. from both a single IP address and multiple IP addresses (i.e., the
same query received simultaneously from multiple IP addresses).
By analyzing requery events we have found that the cause of the By analyzing requery events we have found that the cause of the
duplicate traffic is almost always a deficient name server, stub duplicate traffic is almost always a deficient iterative resolver,
resolver and/or application implementation combined with an stub resolver and/or application implementation combined with an
operational anomaly. The implementation deficiencies we have operational anomaly. The implementation deficiencies we have
identified to date include well-intentioned recovery attempts gone identified to date include well-intentioned recovery attempts gone
awry, insufficient caching of failures, early abort when multiple awry, insufficient caching of failures, early abort when multiple
levels of glue records must be followed, and aggressive retry by stub levels of glue records must be followed, and aggressive retry by stub
resolvers and/or applications. Anomalies that we have seen trigger resolvers and/or applications. Anomalies that we have seen trigger
requery events include lame delegations, unusual glue records, and requery events include lame delegations, unusual glue records, and
anything that makes all authoritative name servers for a zone anything that makes all authoritative name servers for a zone
unreachable (DoS attacks, crashes, maintenance, routing failures, unreachable (DoS attacks, crashes, maintenance, routing failures,
congestion, etc.). congestion, etc.).
In the following sections, we provide a detailed explanation of the In the following sections, we provide a detailed explanation of the
observed behavior and recommend changes that will reduce the requery observed behavior and recommend changes that will reduce the requery
rate. Some of the changes recommended affect the core DNS protocol rate. Some of the changes recommended affect the core DNS protocol
specification, described principally in RFC 1034 [2], RFC 1035 [3] specification, described principally in RFC 1034 [2], RFC 1035 [3]
and RFC 2181 [4]. and RFC 2181 [4].
1.1 A note about terminology in this memo
To recast an old saying about standards, the nice thing about DNS
terms is that there are so many of them to choose from. Writing or
talking about DNS can be difficult and cause confusion resulting from
a lack of agreed-upon terms for its various components. Further
complicating matters are implementations that combine multiple roles
into one piece of software, which makes naming the result
problematic. An example is the entity that accepts recursive
queries, issues iterative queries as necessary to resolve them,
caches responses it receives, and which is also able answer questions
about certain zones authoritatively. Often called a "recursive name
server" or a "caching name server", it is in fact an iterative
resolver combined with an authoritative name server.
This memo is concerned principally with the behavior of iterative
resolvers, which are typically found as part of a recursive name
server. This memo uses the more precise term "iterative resolver",
because the focus is usually on that component, rather than the more
general term "recursive name server".
The advent of IPv6 requires mentioning AAAA records as well as A
records when discussing glue. To avoid continuous repetition and
qualification, this memo uses the general term "address records" to
encompass both A and AAAA records when a particular situation is
relevant to both types.
2. Observed name server misbehavior 2. Observed name server misbehavior
2.1 Aggressive requerying for delegation information 2.1 Aggressive requerying for delegation information
There can be times when every name server in a zone's NS RRset is There can be times when every name server in a zone's NS RRset is
unreachable (e.g., during a network outage), unavailable (e.g., the unreachable (e.g., during a network outage), unavailable (e.g., the
name server process is not running on the server host) or name server process is not running on the server host) or
misconfigured (e.g., the name server is not authoritative for the misconfigured (e.g., the name server is not authoritative for the
given zone, also known as "lame"). Consider a name server that given zone, also known as "lame"). Consider an iterative resolver
attempts to resolve a recursive query for a domain name in such a that attempts to resolve a query for a domain name in such a zone and
zone and discovers that none of the zone's name servers can provide discovers that none of the zone's name servers can provide an answer.
an answer. We have observed a recursive name server implementation We have observed an iterative resolver implementation that then
that then verifies the zone's NS RRset in its cache by querying for verifies the zone's NS RRset in its cache by querying for the zone's
the zone's delegation information: it sends a query for the zone's NS delegation information: it sends a query for the zone's NS RRset to
RRset to one of the parent zone's name servers. one of the parent zone's name servers.
For example, suppose that example.com has the following NS RRset: For example, suppose that "example.com" has the following NS RRset:
example.com. IN NS ns1.example.com. example.com. IN NS ns1.example.com.
example.com. IN NS ns2.example.com. example.com. IN NS ns2.example.com.
Upon receipt of a query for www.example.com and assuming that neither Upon receipt of a query for "www.example.com" and assuming that
ns1.example.com nor ns2.example.com can provide an answer, this neither "ns1.example.com" nor "ns2.example.com" can provide an
recursive name server implementation immediately queries a com zone answer, this iterative resolver implementation immediately queries a
name server for the example.com NS RRset to verify it has the proper "com" zone name server for the "example.com" NS RRset to verify it
delegation information. This name server implementation performs has the proper delegation information. This name server
this query to a zone's parent zone for each recursive query it implementation performs this query to a zone's parent zone for each
receives that fails because of a completely unresponsive set of name recursive query it receives that fails because of a completely
servers for the target zone. Consider the effect when a popular zone unresponsive set of name servers for the target zone. Consider the
experiences a catastrophic failure of all its name servers: now every effect when a popular zone experiences a catastrophic failure of all
recursive query for domain names in that zone sent to this name its name servers: now every recursive query for domain names in that
server implementation results in a query to the failed zone's parent zone sent to this name server implementation results in a query to
name servers. On one occasion when several dozen popular zones the failed zone's parent name servers. On one occasion when several
became unreachable, the query load to the com/net name servers dozen popular zones became unreachable, the query load on the com/net
increased by 50%. name servers increased by 50%.
We believe this verification query is not reasonable. Consider the We believe this verification query is not reasonable. Consider the
circumstances: When a recursing name server is resolving a query for circumstances: When an iterative resolver is resolving a query for a
a domain name in a zone it has not previously searched, it uses the domain name in a zone it has not previously searched, it uses the
list of name servers in the referral from the target zone's parent. list of name servers in the referral from the target zone's parent.
If on its first attempt to search the target zone, none of the name If on its first attempt to search the target zone, none of the name
servers in the referral are reachable, a verification query to the servers in the referral is reachable, a verification query to the
parent is pointless: this query to the parent would come so quickly parent is pointless: this query to the parent would come so quickly
on the heels of the referral that it would be almost certain to on the heels of the referral that it would be almost certain to
contain the same list of name servers. The chance of discovering any contain the same list of name servers. The chance of discovering any
new information is slim. new information is slim.
The other possibility is that the recursing name server successfully The other possibility is that the iterative resolver successfully
contacts one of the target zone's name servers and then caches the NS contacts one of the target zone's name servers and then caches the NS
RRset from the authority section of a response, the proper behavior RRset from the authority section of a response, the proper behavior
according to section 5.4.1 of RFC 2181 [4], because the NS RRset from according to section 5.4.1 of RFC 2181 [4], because the NS RRset from
the target zone is more trustworthy than delegation information from the target zone is more trustworthy than delegation information from
the parent zone. If, while processing a subsequent recursive query, the parent zone. If, while processing a subsequent recursive query,
the recursing name server discovers that none of the name servers the recursing name server discovers that none of the name servers
specified in the cached NS RRset is available or authoritative, specified in the cached NS RRset is available or authoritative,
querying the parent would be wrong. An NS RRset from the parent zone querying the parent would be wrong. An NS RRset from the parent zone
would now be less trustworthy than data already in the cache. would now be less trustworthy than data already in the cache.
For this query of the parent zone to be useful, the target zone's For this query of the parent zone to be useful, the target zone's
entire set of name servers would have to change AND the former set of entire set of name servers would have to change AND the former set of
name servers would have to be deconfigured and/or decomissioned AND name servers would have to be deconfigured and/or decommissioned AND
the delegation information in the parent zone would have to be the delegation information in the parent zone would have to be
updated with the new set of name servers, all within the TTL of the updated with the new set of name servers, all within the TTL of the
target zone's NS RRset. We believe this scenario is uncommon: target zone's NS RRset. We believe this scenario is uncommon:
administrative best practices dictate that changes to a zone's set of administrative best practices dictate that changes to a zone's set of
name servers happen gradually, with servers that are removed from the name servers happen gradually, with servers that are removed from the
NS RRset left authoritative for the zone as long as possible. The NS RRset left authoritative for the zone as long as possible. The
scenarios that we can envision that would benefit from the parent scenarios that we can envision that would benefit from the parent
requery behavior do not outweigh its damaging effects. requery behavior do not outweigh its damaging effects.
2.1.1 Recommendation 2.1.1 Recommendation
skipping to change at page 6, line 19 skipping to change at page 7, line 15
and not authoritative for a zone delegated to it, it is reasonable to and not authoritative for a zone delegated to it, it is reasonable to
assume that this condition has potential to last longer than assume that this condition has potential to last longer than
unreachability or unresponsiveness. Consequently, repeated queries unreachability or unresponsiveness. Consequently, repeated queries
to known lame servers are not useful. In this case of a condition to known lame servers are not useful. In this case of a condition
with potential to persist for a long time, a better practice would be with potential to persist for a long time, a better practice would be
to maintain a list of known lame servers and avoid querying them to maintain a list of known lame servers and avoid querying them
repeatedly in a short interval. repeatedly in a short interval.
2.2.1 Recommendation 2.2.1 Recommendation
Name servers offering recursion SHOULD cache name servers that they Iterative resolvers SHOULD cache name servers that they discover are
discover are not authoritative for zones delegated to them (i.e. lame not authoritative for zones delegated to them (i.e. lame servers).
servers). Lame servers MUST be cached against the specific query Lame servers MUST be cached against the specific query tuple <zone
tuple <zone name, class, server IP address>. Zone name can be name, class, server IP address>. Zone name can be derived from the
derived from the owner name of the NS record that was referenced to owner name of the NS record that was referenced to query the name
query the name server that was discovered to be lame. server that was discovered to be lame. Implementations that perform
Implementations that perform lame server caching MUST refrain from lame server caching MUST refrain from sending queries to known lame
sending queries to known lame servers based on a time interval from servers based on a time interval from when the server is discovered
when the server is discovered to be lame. A minimum interval of to be lame. A minimum interval of thirty minutes is RECOMMENDED.
thirty minutes is RECOMMENDED.
2.3 Incomplete negative caching implementation
A widely distributed name server implementation does not properly
implement negative caching as described in RFC 2308 [5]. In
particular, this implementation does not cache NODATA responses.
Such a response indicates that the queried domain name exists but has
no records of the desired type. See Section 2.2 of RFC 2308 [5] for
information on how NODATA responses are indicated.
2.3.1 Recommendation
Vendors of any name server implementations that do not comply with
RFC 2308 [5] are encouraged to bring their software into conformance.
2.4 Inability to follow multiple levels of out-of-zone glue 2.3 Inability to follow multiple levels of out-of-zone glue
Some name server implementations are unable to follow more than one Some iterative resolver implementations are unable to follow more
level of out-of-zone glue. For example, consider the following than one level of out-of-zone glue. For example, consider the
delegations: following delegations:
foo.example. IN NS ns1.example.com. foo.example. IN NS ns1.example.com.
foo.example. IN NS ns2.example.com. foo.example. IN NS ns2.example.com.
example.com. IN NS ns1.test.example.net. example.com. IN NS ns1.test.example.net.
example.com. IN NS ns2.test.example.net. example.com. IN NS ns2.test.example.net.
test.example.net. IN NS ns1.test.example.net. test.example.net. IN NS ns1.test.example.net.
test.example.net. IN NS ns2.test.example.net. test.example.net. IN NS ns2.test.example.net.
A name server processing a recursive query for www.foo.example must A name server processing a recursive query for "www.foo.example" must
follow two levels of indirection, first obtaining address records for follow two levels of indirection, first obtaining address records for
ns1.test.example.net and/or ns2.test.example.net in order to obtain "ns1.test.example.net" and/or "ns2.test.example.net" in order to
address records for ns1.example.com and/or ns2.example.com in order obtain address records for "ns1.example.com" and/or "ns2.example.com"
to query those name servers for the address records of in order to query those name servers for the address records of
www.foo.example. While this situation may appear contrived, we have "www.foo.example". While this situation may appear contrived, we
seen multiple similar occurrences and expect more as the new generic have seen multiple similar occurrences and expect more as new generic
top-level domains (gTLDs) become active. We anticipate many zones in top-level domains (gTLDs) become active. We anticipate many zones in
the new gTLDs will use name servers in other gTLDs, increasing the the new gTLDs will use name servers in other gTLDs, increasing the
amount of inter-zone glue. amount of inter-zone glue.
2.4.1 Recommendation 2.3.1 Recommendation
Certainly constructing a delegation that relies on multiple levels of Clearly constructing a delegation that relies on multiple levels of
out-of-zone glue is not a good administrative practice. This issue out-of-zone glue is not a good administrative practice. This issue
could be mitigated with an operational injunction in an RFC to could be mitigated with an operational injunction in an RFC to
refrain from construction of such delegations. In our opinion the refrain from construction of such delegations. In our opinion the
practice is widespread enough to merit clarifications to the DNS practice is widespread enough to merit clarifications to the DNS
protocol specification to permit it on a limited basis. protocol specification to permit it on a limited basis.
Name servers offering recursion SHOULD be able to handle at least Name servers offering recursion SHOULD be able to handle at least
three levels of indirection resulting from out-of-zone glue. three levels of indirection resulting from out-of-zone glue.
3. Observed client misbehavior 2.4 Aggressive retransmission when fetching glue
We have observed situations where a zone's name servers are When an authoritative name server responds with a referral, it
misconfigured or unavailable, resulting in a SERVFAIL response from a includes NS records in the authority section of the response.
recursive name server in response to queries for domain names in that According to the algorithm in section 4.3.2 of RFC 1034 [2], the name
zone. In some instances, we then observe many repeated queries (on server should also "put whatever addresses are available into the
the order of hundreds per second) to the com/net name servers for additional section, using glue RRs if the addresses are not available
domain names in the affected zones. Sometimes the queries originate from authoritative data or the cache." Some name server
from multiple source IP addresses, while at other times a single implementations take this address inclusion a step further with a
source address sends many repeated queries. This behavior appears to feature called "glue fetching". A name server that implements glue
be triggered by a SERVFAIL response (i.e., upon investigation, the fetching attempts to include A records for every NS record in the
<qname, qtype, qclass> of a repeated query at the com/net name authority section. If necessary, the name server issues multiple
servers produces a SERVFAIL response when sent to a local recursive queries of its own to obtain any missing address records.
name server.)
We suspect that some DNS clients (i.e., stub resolvers) and/or Problems with glue fetching can arise in the context of
application programs have overzealous retransmission algorithms that "authoritative-only" name servers, which only serve authoritative
are trigged by a SERVFAIL response. Unfortunately, we have not been data and ignore requests for recursion. Such a server will not
able to isolate particular implementations. The authors encourage generate any queries of its own. Instead it answers non-recursive
and welcome reports of DNS clients and applications with overzealous queries from resolvers looking for information in zones it serves.
retransmission algorithms. With glue fetching enabled, however, an authoritative server will
generate queries whenever it needs to look up an unknown address
record to complete the additional section of a response.
4. IANA considerations We have observed situations where a glue-fetching name server can
send queries that reach other name servers, but apparently is
prevented from receiving the responses. For example, perhaps the
name server is authoritative-only and therefore its administrators
expect it to receive only queries. Perhaps unaware of glue fetching
and presuming that the name server will generate no queries, its
administrators place the name server behind a network device that
prevents it from receiving responses. If this is the case, all
glue-fetching queries will go answered.
There are no new IANA considerations introduced by this We have observed name server implementations that retry excessively
Internet-Draft. when glue-fetching queries are unanswered. A single com/net name
server has received hundreds of queries per second from a single name
server. Judging from the specific queries received and based on
additional analysis, we believe these queries result from overly
aggressive glue fetching.
5. Security considerations 2.4.1 Recommendation
Nameserver, stub resolver and application misbehaviors identical or Implementers whose name servers support glue fetching SHOULD take
similar to those observed and discussed in this document expose root care to avoid sending queries at excessive rates. Implementations
and TLD name server constellations to increased risk of both SHOULD support throttling logic to detect when queries are sent but
intentional and unintentional denial of service. no responses are received.
2.5 Aggressive retransmission behind firewalls
A common occurrence and one of the largest sources of repeated
queries at the com/net and root name servers appears to result from
resolvers behind misconfigured firewalls. In this situation, a
recursive name server is apparently allowed to send queries through a
firewall to other name servers, but not receive the responses. The
result is more queries than necessary because of retransmission, all
of which are useless because the responses are never received. Just
as with the glue-fetching scenario described in Section 2.4, the
queries are sometimes sent at excessive rates. To make matters
worse, sometimes the responses, sent in reply to legitimate queries,
trigger an alarm on the originator's intrusion detection system. We
are frequently contacted by administrators responding to such alarms
who believe our name servers are attacking their systems.
Not only do some resolvers in this situation retransmit queries at an
excessive rate, but they continue to do so for days or even weeks.
This scenario could result from an organization with multiple
iterative resolvers, only a subset of whose traffic is improperly
filtered in this manner. Stub resolvers in the organization could be
configured to query multiple name servers. Consider the case where a
stub resolver queries a filtered name server first. This name server
sends one or more queries whose replies are filtered, so it can't
respond to the stub resolver, which times out. The resolver
retransmits to a name server that is able to provide an answer.
Since resolution ultimately succeeds the underlying problem might not
be recognized or corrected. A popular stub resolver has a very
aggressive retransmission schedule, including simultaneous queries to
multiple name servers, which could explain how such a situation could
persist without being detected.
2.5.1 Recommendation
The most obvious recommendation is that administrators SHOULD take
care not to place iterative resolvers behind a firewall that allows
queries to pass through but not the resulting replies.
Name servers SHOULD take care to avoid sending queries at excessive
rates. Implementations SHOULD support throttling logic to detect
when queries are sent but no responses are received.
2.6 Misconfigured NS records
Sometimes a zone administrator forgets to add the trailing dot on the
domain names in the RDATA of a zone's NS records. Consider this
fragment of the zone file for "example.com":
$ORIGIN example.com.
example.com. 3600 IN NS ns1.example.com ; Note missing
example.com. 3600 IN NS ns2.example.com ; trailing dots
The zone's authoritative servers will parse the NS RDATA as
"ns1.example.com.example.com" and "ns2.example.com.example.com" and
return NS records with this incorrect RDATA in responses, including
typically the authority section of every response containing records
from the "example.com" zone.
Now consider a typical sequence of queries. An iterative resolver
attempting to resolve address records for "www.example.com" with no
cached information for this zone will query a "com" authoritative
server. The "com" server responds with a referral to the
"example.com" zone, consisting of NS records with valid RDATA and
associated glue records. (This example assumes that the
"example.com" zone information is correct in the "com" zone.) The
iterative resolver caches the NS RRset from the "com" server and
follows the referral by querying one of the "example.com"
authoritative servers. This server responds with the
"www.example.com" address record in the answer section and,
typically, the "example.com" NS records in the authority section and,
if space in the message remains, glue address records in the
additional section. According to Section 5.4 of RFC 2181 [4], NS
records in the authority section of an authoritative answer are more
trustworthy than NS records from the authority section of a
non-authoritative answer. Thus the "example.com" NS RRset just
received from the "example.com" authoritative server displaces the
"example.com" NS RRset received moments ago from the "com"
authoritative server.
But the "example.com" zone contains the erroneous NS RRset as shown
in the example above. Subsequent queries for names in "example.com"
will cause the server to attempt to use the incorrect NS records and
so the server will try to resolve the nonexistent names
"ns1.example.com.example.com" and "ns2.example.com.example.com". In
this example, since all of the zone's name servers are named in the
zone itself (i.e., "ns1.example.com.example.com" and
"ns2.example.com.example.com" both end in "example.com") and all are
bogus, the recursive server cannot reach any "example.com" name
servers. Therefore attempts to resolve these names result in address
record queries to the "com" authoritative servers. Queries for such
obviously bogus glue address records occur frequently at the com/net
name servers.
2.6.1 Recommendation
An authoritative server can detect this situation. A trailing dot
missing from an NS record's RDATA always results by definition in a
name server name that exists somewhere under the SOA of the zone the
NS record appears in. (Note that further levels of delegation are
possible, so a missing trailing dot could inadvertently create a name
server name that actually exists in a subzone.) But in any case, the
address record must be present in this zone, either as authoritative
data or glue.
An authoritative name server SHOULD report an error when one of a
zone's NS records references a name server below the zone's SOA when
a corresponding address record does not exist in the zone.
2.7 Name server records with zero TTL
Sometimes a popular com/net subdomain's zone is configured with a TTL
of zero on the zone's NS records, which prohibits these records from
being cached and will result in a higher query volume to the zone's
authoritative servers. The zone's administrator should understand
the consequences of such a configuration and provision resources
accordingly. A zero TTL on the zone's NS RRset, however, carries
additional consequences beyond the zone itself: if a recursive name
server cannot cache a zone's NS records because of a zero TTL, it
will be forced to query that zone's parent's name servers each time
it resolves a name in the zone. The com/net authoritative servers do
see an increased query load when a popular com/net subdomain's zone
is configured with a TTL of zero on the zone's NS records.
A zero TTL on an RRset expected to change frequently is extreme but
permissible. A zone's NS RRset is a special case, however, because
changes to it must be coordinated with the zone's parent. In most
zone parent/child relationships we are aware of, there is typically
some delay involved in effecting changes. Further, changes to the
set of a zone's authoritative name servers (and therefore to the
zone's NS RRset) are typically relatively rare: providing reliable
authoritative service requires a reasonably stable set of servers.
Therefore an extremely low or zero TTL on a zone's NS RRset rarely
makes sense, except in anticipation of an upcoming change. In this
case, when the zone's administrator has planned a change and does not
want recursive name servers throughout the Internet to cache the NS
RRset for a long period of time, a low TTL is reasonable.
2.7.1 Recommendation
Because of the additional load placed on a zone's parent's
authoritative servers imposed by a zero TTL on a zone's NS RRset,
under such circumstances authoritative name servers SHOULD issue a
warning when loading a zone or refuse to load the zone altogether.
2.8 Unnecessary dynamic update messages
The UPDATE message specified in RFC 2136 [6] allows an authorized
agent to update a zone's data on an authoritative name server using a
DNS message sent over the network. Consider the case of an agent
desiring to add a particular resource record. Because of zone cuts,
the agent does not necessarily know the proper zone to which the
record should be added. The dynamic update process requires that the
agent determine the appropriate zone so the UPDATE message can be
sent to one of the zone's authoritative servers (typically the
primary master as specified in the zone's SOA MNAME field).
The appropriate zone to update is the closest enclosing zone, which
cannot be determined only by inspecting the domain name of the record
to be updated, since zone cuts can occur anywhere. One way to
determine the closest enclosing zone entails walking up the name
space tree by sending repeated UPDATE messages until success. For
example, consider an agent attempting to add an address record with
the name "foo.bar.example.com". The agent could first attempt to
update the "foo.bar.example.com" zone. If the attempt failed, the
update could be directed to the "bar.example.com" zone, then the
"example.com" zone, then the "com" zone, and finally the root zone.
A popular dynamic agent follows this algorithm. The result is many
UPDATE messages received by the root name servers, the com/net
authoritative servers, and presumably other TLD authoritative
servers. A valid question is why the algorithm proceeds to send
updates all the way to TLD and root name servers. This behavior is
not entirely unreasonable: in enterprise DNS architectures with an
"internal root" design, there could conceivably be private,
non-public TLD or root zones that would be the appropriate targets
for a dynamic update.
A significant deficiency with this algorithm is that knowledge of a
given UPDATE message's failure is not helpful in directing future
UPDATE messages to the appropriate servers. A better algorithm would
be to find the closest enclosing zone by walking up the name space
with queries for SOA or NS rather than "probing" with UPDATE
messages. Once the appropriate zone is found, an UPDATE message can
be sent. In addition, the results of these queries can be cached to
aid in determining closest enclosing zones for future updates. Once
the closest enclosing zone is determined with this method, the update
will either succeed or fail and there is no need to send further
updates to higher-level zones. The important point is that walking
up the tree with queries yields cacheable information, whereas
walking up the tree by sending UPDATE messages does not.
2.8.1 Recommendation
Dynamic update agents SHOULD send SOA or NS queries to progressively
higher-level zones to find the closest enclosing zone for a given
name to update. Only after the appropriate zone is found should the
client send an UPDATE message to one of the zone's authoritative
servers. Update clients SHOULD NOT "probe" using UPDATE messages by
walking up the tree to progressively higher-level zones.
2.9 Queries for domain names resembling IP addresses
The root name servers receive a significant number of A record
queries where the qname is an IP address. The source of these
queries is unknown. It could be attributed to situations where a
user believes an application will accept either a domain name or an
IP address in a given configuration option. The user enters an IP
address, but the application assumes any input is a domain name and
attempts to resolve it, resulting in an A record lookup. There could
also be applications that produce such queries in a misguided attempt
to reverse map IP addresses.
These queries result in Name Error (RCODE=3) responses. A recursive
name server can negatively cache such responses, but each response
requires a separate cache entry, i.e., a negative cache entry for the
domain name "192.0.2.1" does not prevent a subsequent query for the
domain name "192.0.2.2".
2.9.1 Recommendation
It would be desirable for the root name servers not to have to answer
these queries: they unnecessarily consume CPU resources and network
bandwidth. One possibility is for iterative resolver implementations
to produce the Name Error response directly. We suggest that
implementors consider the option of synthesizing Name Error responses
at the iterative resolver. The server could claim authority for
synthesized TLD zones corresponding to the first octet of every
possible IP address, e.g. 1., 2., through 255. This behavior could
be configurable in the (probably unlikely) event that numeric TLDs
are ever put into use.
Another option is to delegate these numeric TLDs from the root zone
to a separate set of servers to absorb the traffic. The "black hole
servers" used by the <http://www.as112.net>, which are currently
delegated the in-addr.arpa zones corresponding to RFC 1918 [7]
private use address space, would be a possible choice to receive
these delegations.
2.10 Misdirected recursive queries
The root name servers receive a significant number of recursive
queries (i.e., queries with the RD bit set in the header). Since
none of the root servers offer recursion, the servers' response in
such a situation ignores the request for recursion and the response
probably does not contain the data the querier anticipated. Some of
these queries result from users configuring stub resolvers to query a
root server. (This situation is not hypothetical: we have received
complaints from users when this configuration does not work as
hoped.) Of course, users should not direct stub resolvers to use name
servers that do not offer recursion, but we are not aware of any stub
resolver implementation that offers any feedback to the user when so
configured, aside from simply "not working".
2.10.1 Recommendation
When the IP address of a (supposedly) iterative resolver is
configured in a stub resolver using an interactive user interface,
the resolver could send a test query to verify that the server
supports recursion (i.e., the response has the RA bit set in the
header). The user could be immediately notified if the server is
non-recursive.
The stub resolver could also report an error, either through a user
interface or in a log file, if the queried server does not support
recursion. Error reporting SHOULD be throttled to avoid a
notification or log message for every response from a non-recursive
server.
2.11 Suboptimal name server selection algorithm
An entire document could be devoted to the topic of problems with
different implementations of the recursive resolution algorithm. The
entire process of recursion is woefully under specified, requiring
each implementor to design an algorithm. Sometimes implementors make
poor design choices that could be avoided if a suggested algorithm
and best practices were documented, but that is a topic for another
document.
Some deficiencies cause significant operational impact and are
therefore worth mentioning here. One of these is name server
selection by an iterative resolver. When an iterative resolver wants
to contact one of a zone's authoritative name servers, how does it
choose from the NS records listed in the zone's NS RRset? If the
selection mechanism is suboptimal, queries are not spread evenly
among a zone's authoritative servers. The details of the selection
mechanism are up to the implementor, but we offer some suggestions.
2.11.1 Recommendation
This list is not conclusive, but reflects the changes that would
produce the most impact in terms of reducing disproportionate query
load among a zone's authoritative servers. I.e., these changes would
help spread the query load evenly.
o Do not make assumptions based on NS RRset order: all NS RRs SHOULD
be treated equally. (In the case of the "com" zone, for example,
most of the root servers return the NS record for
"a.gtld-servers.net" first in the authority section of referrals.
As a result, this server receives disproportionately more traffic
than the other 12 authoritative servers for "com".)
o Use all NS records in an RRset. (For example, we are aware of
implementations that hard-coded information for a subset of the
root servers.)
o Maintain state and favor the best-performing of a zone's
authoritative servers. A good definition of performance is
response time. Non-responsive servers can be penalized with an
extremely high response time.
o Do not lock onto the best-performing of a zone's name servers. An
iterative resolver SHOULD periodically check the performance of
all of a zone's name servers to adjust its determination of the
best-performing one.
3. IANA considerations
There are no new IANA considerations introduced by this memo.
4. Security considerations
Name server and resolver misbehaviors identical or similar to those
discussed in this document expose the root and TLD name servers to
increased risk of both intentional and unintentional denial of
service.
We believe that implementation of the recommendations offered in this We believe that implementation of the recommendations offered in this
document will reduce the requery traffic seen at root and TLD name document will reduce the amount of unnecessary traffic seen at root
servers, thus reducing the opportunity for an attacker to use such and TLD name servers, thus reducing the opportunity for an attacker
requerying to his or her advantage. to use such queries to his or her advantage.
6. Internationalization considerations 5. Internationalization considerations
We do not believe this document introduces any new We do not believe this document introduces any new
internationalization considerations to the DNS protocol internationalization considerations to the DNS protocol
specification. specification.
Normative References 6 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[2] Mockapetris, P., "Domain names - concepts and facilities", STD [2] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987. 13, RFC 1034, November 1987.
[3] Mockapetris, P., "Domain names - implementation and [3] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[4] Elz, R. and R. Bush, "Clarifications to the DNS Specification", [4] Elz, R. and R. Bush, "Clarifications to the DNS Specification",
RFC 2181, July 1997. RFC 2181, July 1997.
[5] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC [5] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC
2308, March 1998. 2308, March 1998.
[6] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April
1997.
[7] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and E.
Lear, "Address Allocation for Private Internets", BCP 5, RFC
1918, February 1996.
Authors' Addresses Authors' Addresses
Matt Larson Matt Larson
VeriSign, Inc. VeriSign, Inc.
21345 Ridgetop Circle 21345 Ridgetop Circle
Dulles, VA 20166-6503 Dulles, VA 20166-6503
USA USA
EMail: mlarson@verisign.com EMail: mlarson@verisign.com
Piet Barber Piet Barber
VeriSign, Inc. VeriSign, Inc.
21345 Ridgetop Circle 21345 Ridgetop Circle
Dulles, VA 20166-6503 Dulles, VA 20166-6503
USA USA
EMail: pbarber@verisign.com EMail: pbarber@verisign.com
Intellectual Property Statement Intellectual Property Statement
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be obtained from the IETF Secretariat. be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive this standard. Please address the information to the IETF Executive
Director. Director.
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
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and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
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