draft-ietf-dnsop-bad-dns-res-04.txt   draft-ietf-dnsop-bad-dns-res-05.txt 
DNS Operations M. Larson DNS Operations M. Larson
Internet-Draft P. Barber Internet-Draft P. Barber
Expires: January 18, 2006 VeriSign Expires: August 14, 2006 VeriSign
July 17, 2005 February 10, 2006
Observed DNS Resolution Misbehavior Observed DNS Resolution Misbehavior
draft-ietf-dnsop-bad-dns-res-04 draft-ietf-dnsop-bad-dns-res-05
Status of this Memo Status of this Memo
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This Internet-Draft will expire on January 18, 2006. This Internet-Draft will expire on August 14, 2006.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2006).
Abstract Abstract
This memo describes DNS iterative resolver behavior that results in a This memo describes DNS iterative resolver behavior that results in a
significant query volume sent to the root and top-level domain (TLD) significant query volume sent to the root and top-level domain (TLD)
name servers. We offer implementation advice to iterative resolver name servers. We offer implementation advice to iterative resolver
developers to alleviate these unnecessary queries. The developers to alleviate these unnecessary queries. The
recommendations made in this document are a direct byproduct of recommendations made in this document are a direct byproduct of
observation and analysis of abnormal query traffic patterns seen at observation and analysis of abnormal query traffic patterns seen at
two of the thirteen root name servers and all thirteen com/net TLD two of the thirteen root name servers and all thirteen com/net TLD
name servers. 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
1.1 A note about terminology in this memo . . . . . . . . . . 3 1.1. A note about terminology in this memo . . . . . . . . . . 3
2. Observed iterative resolver misbehavior . . . . . . . . . . 5 2. Observed iterative resolver misbehavior . . . . . . . . . . . 5
2.1 Aggressive requerying for delegation information . . . . . 5 2.1. Aggressive requerying for delegation information . . . . . 5
2.1.1 Recommendation . . . . . . . . . . . . . . . . . . . . 6 2.1.1. Recommendation . . . . . . . . . . . . . . . . . . . . 6
2.2 Repeated queries to lame servers . . . . . . . . . . . . . 7 2.2. Repeated queries to lame servers . . . . . . . . . . . . . 7
2.2.1 Recommendation . . . . . . . . . . . . . . . . . . . . 7 2.2.1. Recommendation . . . . . . . . . . . . . . . . . . . . 7
2.3 Inability to follow multiple levels of indirection . . . . 8 2.3. Inability to follow multiple levels of indirection . . . . 8
2.3.1 Recommendation . . . . . . . . . . . . . . . . . . . . 9 2.3.1. Recommendation . . . . . . . . . . . . . . . . . . . . 9
2.4 Aggressive retransmission when fetching glue . . . . . . . 9 2.4. Aggressive retransmission when fetching glue . . . . . . . 9
2.4.1 Recommendation . . . . . . . . . . . . . . . . . . . . 10 2.4.1. Recommendation . . . . . . . . . . . . . . . . . . . . 10
2.5 Aggressive retransmission behind firewalls . . . . . . . . 10 2.5. Aggressive retransmission behind firewalls . . . . . . . . 10
2.5.1 Recommendation . . . . . . . . . . . . . . . . . . . . 11 2.5.1. Recommendation . . . . . . . . . . . . . . . . . . . . 11
2.6 Misconfigured NS records . . . . . . . . . . . . . . . . . 11 2.6. Misconfigured NS records . . . . . . . . . . . . . . . . . 11
2.6.1 Recommendation . . . . . . . . . . . . . . . . . . . . 12 2.6.1. Recommendation . . . . . . . . . . . . . . . . . . . . 12
2.7 Name server records with zero TTL . . . . . . . . . . . . 12 2.7. Name server records with zero TTL . . . . . . . . . . . . 12
2.7.1 Recommendation . . . . . . . . . . . . . . . . . . . . 13 2.7.1. Recommendation . . . . . . . . . . . . . . . . . . . . 13
2.8 Unnecessary dynamic update messages . . . . . . . . . . . 13 2.8. Unnecessary dynamic update messages . . . . . . . . . . . 13
2.8.1 Recommendation . . . . . . . . . . . . . . . . . . . . 14 2.8.1. Recommendation . . . . . . . . . . . . . . . . . . . . 14
2.9 Queries for domain names resembling IPv4 addresses . . . . 14 2.9. Queries for domain names resembling IPv4 addresses . . . . 14
2.9.1 Recommendation . . . . . . . . . . . . . . . . . . . . 14 2.9.1. Recommendation . . . . . . . . . . . . . . . . . . . . 14
2.10 Misdirected recursive queries . . . . . . . . . . . . . 15 2.10. Misdirected recursive queries . . . . . . . . . . . . . . 15
2.10.1 Recommendation . . . . . . . . . . . . . . . . . . . 15 2.10.1. Recommendation . . . . . . . . . . . . . . . . . . . . 15
2.11 Suboptimal name server selection algorithm . . . . . . . 15 2.11. Suboptimal name server selection algorithm . . . . . . . . 15
2.11.1 Recommendation . . . . . . . . . . . . . . . . . . . 16 2.11.1. Recommendation . . . . . . . . . . . . . . . . . . . . 16
3. IANA considerations . . . . . . . . . . . . . . . . . . . . 17 3. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
4. Security considerations . . . . . . . . . . . . . . . . . . 18 4. IANA considerations . . . . . . . . . . . . . . . . . . . . . 18
5. Internationalization considerations . . . . . . . . . . . . 19 5. Security considerations . . . . . . . . . . . . . . . . . . . 19
6. Informative References . . . . . . . . . . . . . . . . . . . 19 6. Internationalization considerations . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 19 7. Informative References . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . . . . 22
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 (i.e., the from both a single IP address and multiple IP addresses (i.e., the
same query received simultaneously from multiple IP addresses). same query received simultaneously from multiple IP addresses).
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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. None of the changes recommended affects the core DNS protocol rate. None of the changes recommended affects the core DNS protocol
specification; instead, this document consists of guidelines to specification; instead, this document consists of guidelines to
implementors of iterative resolvers. implementors of iterative resolvers.
1.1 A note about terminology in this memo 1.1. A note about terminology in this memo
To recast an old saying about standards, the nice thing about DNS 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 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 talking about DNS can be difficult and cause confusion resulting from
a lack of agreed-upon terms for its various components. Further a lack of agreed-upon terms for its various components. Further
complicating matters are implementations that combine multiple roles complicating matters are implementations that combine multiple roles
into one piece of software, which makes naming the result into one piece of software, which makes naming the result
problematic. An example is the entity that accepts recursive problematic. An example is the entity that accepts recursive
queries, issues iterative queries as necessary to resolve the initial queries, issues iterative queries as necessary to resolve the initial
recursive query, caches responses it receives, and which is also able recursive query, caches responses it receives, and which is also able
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queries. queries.
The advent of IPv6 requires mentioning AAAA records as well as A The advent of IPv6 requires mentioning AAAA records as well as A
records when discussing glue. To avoid continuous repetition and records when discussing glue. To avoid continuous repetition and
qualification, this memo uses the general term "address record" to qualification, this memo uses the general term "address record" to
encompass both A and AAAA records when a particular situation is encompass both A and AAAA records when a particular situation is
relevant to both types. relevant to both types.
2. Observed iterative resolver misbehavior 2. Observed iterative resolver 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 an iterative resolver given zone, also known as "lame"). Consider an iterative resolver
that attempts to resolve a query for a domain name in such a zone and that attempts to resolve a query for a domain name in such a zone and
discovers that none of the zone's name servers can provide an answer. discovers that none of the zone's name servers can provide an answer.
We have observed a recursive name server implementation whose We have observed a recursive name server implementation whose
iterative resolver then verifies the zone's NS RRset in its cache by iterative resolver then verifies the zone's NS RRset in its cache by
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zone. For example, consider this NS record: zone. For example, consider this NS record:
example.com. IN NS ns.example.com. example.com. IN NS ns.example.com.
If a cache has this NS record but not the address record for If a cache has this NS record but not the address record for
"ns.example.com", it is unable to contact the "example.com" zone "ns.example.com", it is unable to contact the "example.com" zone
directly and must query the "com" zone to obtain the address record. directly and must query the "com" zone to obtain the address record.
Note, however, that such a query would not have QTYPE=NS according to Note, however, that such a query would not have QTYPE=NS according to
the standard resolution algorithm. the standard resolution algorithm.
2.1.1 Recommendation 2.1.1. Recommendation
An iterative resolver MUST NOT send a query for the NS RRset of a An iterative resolver MUST NOT send a query for the NS RRset of a
non-responsive zone to any of the name servers for that zone's parent non-responsive zone to any of the name servers for that zone's parent
zone. For the purposes of this injunction, a non-responsive zone is zone. For the purposes of this injunction, a non-responsive zone is
defined as a zone for which every name server listed in the zone's NS defined as a zone for which every name server listed in the zone's NS
RRset: RRset:
1. is not authoritative for the zone (i.e., lame), or, 1. is not authoritative for the zone (i.e., lame), or,
2. returns a server failure response (RCODE=2), or, 2. returns a server failure response (RCODE=2), or,
3. is dead or unreachable according to section 7.2 of RFC 2308 [4]. 3. is dead or unreachable according to section 7.2 of RFC 2308 [4].
2.2 Repeated queries to lame servers 2.2. Repeated queries to lame servers
Section 2.1 describes a catastrophic failure: when every name server Section 2.1 describes a catastrophic failure: when every name server
for a zone is unable to provide an answer for one reason or another. for a zone is unable to provide an answer for one reason or another.
A more common occurrence is when a subset of a zone's name servers A more common occurrence is when a subset of a zone's name servers
are unavailable or misconfigured. Different failure modes have are unavailable or misconfigured. Different failure modes have
different expected durations. Some symptoms indicate problems that different expected durations. Some symptoms indicate problems that
are potentially transient; for example, various types of ICMP are potentially transient; for example, various types of ICMP
unreachable messages because a name server process is not running or unreachable messages because a name server process is not running or
a host or network is unreachable, or a complete lack of a response to a host or network is unreachable, or a complete lack of a response to
a query. Such responses could be the result of a host rebooting or a query. Such responses could be the result of a host rebooting or
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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.
It should also be noted, however, that some authoritative name server It should also be noted, however, that some authoritative name server
implementations appear to be lame only for queries of certain types implementations appear to be lame only for queries of certain types
as described in RFC 4074 [5]. In this case, it makes sense to retry as described in RFC 4074 [5]. In this case, it makes sense to retry
the "lame" servers for other types of queries, particularly when all the "lame" servers for other types of queries, particularly when all
known authoritative name servers appear to be "lame". known authoritative name servers appear to be "lame".
2.2.1 Recommendation 2.2.1. Recommendation
Iterative resolvers SHOULD cache name servers that they discover are Iterative resolvers SHOULD cache name servers that they discover are
not authoritative for zones delegated to them (i.e. lame servers). not authoritative for zones delegated to them (i.e. lame servers).
If this caching is performed, lame servers MUST be cached against the If this caching is performed, lame servers MUST be cached against the
specific query tuple <zone name, class, server IP address>. Zone specific query tuple <zone name, class, server IP address>. Zone
name can be derived from the owner name of the NS record that was name can be derived from the owner name of the NS record that was
referenced to query the name server that was discovered to be lame. referenced to query the name server that was discovered to be lame.
Implementations that perform lame server caching MUST refrain from Implementations that perform lame server caching MUST refrain from
sending queries to known lame servers based on a time interval from sending queries to known lame servers based on a time interval from
when the server is discovered to be lame. A minimum interval of when the server is discovered to be lame. A minimum interval of
thirty minutes is RECOMMENDED. thirty minutes is RECOMMENDED.
An exception to this recommendation occurs if all name servers for a An exception to this recommendation occurs if all name servers for a
zone are marked lame. In that case, the iterative resolver SHOULD zone are marked lame. In that case, the iterative resolver SHOULD
temporarily ignore the servers' lameness status and query one or more temporarily ignore the servers' lameness status and query one or more
servers. This behavior is a workaround for the type-specific servers. This behavior is a workaround for the type-specific
lameness issue described in the previous section. lameness issue described in the previous section.
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Implementors should take care not to make lame server avoidance logic Implementors should take care not to make lame server avoidance logic
overly broad: note that a name server could be lame for a parent zone overly broad: note that a name server could be lame for a parent zone
but not a child zone, e.g., lame for "example.com" but properly but not a child zone, e.g., lame for "example.com" but properly
authoritative for "sub.example.com". Therefore a name server should authoritative for "sub.example.com". Therefore a name server should
not be automatically considered lame for subzones. In the case not be automatically considered lame for subzones. In the case
above, even if a name server is known to be lame for "example.com", above, even if a name server is known to be lame for "example.com",
it should be queried for QNAMEs at or below "sub.example.com" if an it should be queried for QNAMEs at or below "sub.example.com" if an
NS record indicates it should be authoritative for that zone. NS record indicates it should be authoritative for that zone.
2.3 Inability to follow multiple levels of indirection 2.3. Inability to follow multiple levels of indirection
Some iterative resolver implementations are unable to follow Some iterative resolver implementations are unable to follow
sufficient levels of indirection. For example, consider the sufficient levels of indirection. For example, consider the
following 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.
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follow two levels of indirection, first obtaining address records for follow two levels of indirection, first obtaining address records for
"ns1.test.example.net" or "ns2.test.example.net" in order to obtain "ns1.test.example.net" or "ns2.test.example.net" in order to obtain
address records for "ns1.example.com" or "ns2.example.com" in order address records for "ns1.example.com" or "ns2.example.com" in order
to query those name servers for the address records of to query those name servers for the address records of
"www.foo.example". While this situation may appear contrived, we "www.foo.example". While this situation may appear contrived, we
have seen multiple similar occurrences and expect more as 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
new gTLDs will use name servers in existing gTLDs, increasing the new gTLDs will use name servers in existing gTLDs, increasing the
number of delegations using out-of-zone name servers. number of delegations using out-of-zone name servers.
2.3.1 Recommendation 2.3.1. Recommendation
Clearly constructing a delegation that relies on multiple levels of Clearly constructing a delegation that relies on multiple levels of
indirection is not a good administrative practice. However, the indirection is not a good administrative practice. However, the
practice is widespread enough to require that iterative resolvers be practice is widespread enough to require that iterative resolvers be
able to cope with it. Iterative resolvers SHOULD be able to handle able to cope with it. Iterative resolvers SHOULD be able to handle
arbitrary levels of indirection resulting from out-of-zone name arbitrary levels of indirection resulting from out-of-zone name
servers. Iterative resolvers SHOULD implement a level-of-effort servers. Iterative resolvers SHOULD implement a level-of-effort
counter to avoid loops or otherwise performing too much work in counter to avoid loops or otherwise performing too much work in
resolving pathological cases. resolving pathological cases.
A best practice that avoids this entire issue of indirection is to A best practice that avoids this entire issue of indirection is to
name one or more of a zone's name servers in the zone itself. For name one or more of a zone's name servers in the zone itself. For
example, if the zone is named "example.com", consider naming some of example, if the zone is named "example.com", consider naming some of
the name servers "ns{1,2,...}.example.com" (or similar). the name servers "ns{1,2,...}.example.com" (or similar).
2.4 Aggressive retransmission when fetching glue 2.4. Aggressive retransmission when fetching glue
When an authoritative name server responds with a referral, it When an authoritative name server responds with a referral, it
includes NS records in the authority section of the response. includes NS records in the authority section of the response.
According to the algorithm in section 4.3.2 of RFC 1034 [2], the name According to the algorithm in section 4.3.2 of RFC 1034 [2], the name
server should also "put whatever addresses are available into the server should also "put whatever addresses are available into the
additional section, using glue RRs if the addresses are not available additional section, using glue RRs if the addresses are not available
from authoritative data or the cache." Some name server from authoritative data or the cache." Some name server
implementations take this address inclusion a step further with a implementations take this address inclusion a step further with a
feature called "glue fetching". A name server that implements glue feature called "glue fetching". A name server that implements glue
fetching attempts to include address records for every NS record in fetching attempts to include address records for every NS record in
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prevents it from receiving responses. If this is the case, all glue- prevents it from receiving responses. If this is the case, all glue-
fetching queries will go answered. fetching queries will go answered.
We have observed name server implementations whose iterative We have observed name server implementations whose iterative
resolvers retry excessively when glue-fetching queries are resolvers retry excessively when glue-fetching queries are
unanswered. A single com/net name server has received hundreds of unanswered. A single com/net name server has received hundreds of
queries per second from a single such source. Judging from the queries per second from a single such source. Judging from the
specific queries received and based on additional analysis, we specific queries received and based on additional analysis, we
believe these queries result from overly aggressive glue fetching. believe these queries result from overly aggressive glue fetching.
2.4.1 Recommendation 2.4.1. Recommendation
Implementers whose name servers support glue fetching SHOULD take Implementers whose name servers support glue fetching SHOULD take
care to avoid sending queries at excessive rates. Implementations care to avoid sending queries at excessive rates. Implementations
SHOULD support throttling logic to detect when queries are sent but SHOULD support throttling logic to detect when queries are sent but
no responses are received. no responses are received.
2.5 Aggressive retransmission behind firewalls 2.5. Aggressive retransmission behind firewalls
A common occurrence and one of the largest sources of repeated A common occurrence and one of the largest sources of repeated
queries at the com/net and root name servers appears to result from queries at the com/net and root name servers appears to result from
resolvers behind misconfigured firewalls. In this situation, an resolvers behind misconfigured firewalls. In this situation, an
iterative resolver is apparently allowed to send queries through a iterative resolver is apparently allowed to send queries through a
firewall to other name servers, but not receive the responses. The firewall to other name servers, but not receive the responses. The
result is more queries than necessary because of retransmission, all result is more queries than necessary because of retransmission, all
of which are useless because the responses are never received. Just of which are useless because the responses are never received. Just
as with the glue-fetching scenario described in Section 2.4, the as with the glue-fetching scenario described in Section 2.4, the
queries are sometimes sent at excessive rates. To make matters queries are sometimes sent at excessive rates. To make matters
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recursive name server sends one or more queries whose replies are recursive name server sends one or more queries whose replies are
filtered, so it can't respond to the stub resolver, which times out. filtered, so it can't respond to the stub resolver, which times out.
Then the stub resolver retransmits to a recursive name server that is Then the stub resolver retransmits to a recursive name server that is
able to provide an answer. Since resolution ultimately succeeds the able to provide an answer. Since resolution ultimately succeeds the
underlying problem might not be recognized or corrected. A popular underlying problem might not be recognized or corrected. A popular
stub resolver implementation has a very aggressive retransmission stub resolver implementation has a very aggressive retransmission
schedule, including simultaneous queries to multiple recursive name schedule, including simultaneous queries to multiple recursive name
servers, which could explain how such a situation could persist servers, which could explain how such a situation could persist
without being detected. without being detected.
2.5.1 Recommendation 2.5.1. Recommendation
The most obvious recommendation is that administrators SHOULD take The most obvious recommendation is that administrators SHOULD take
care not to place iterative resolvers behind a firewall that allows care not to place iterative resolvers behind a firewall that allows
queries to pass through but not the resulting replies. queries to pass through but not the resulting replies.
Iterative resolvers SHOULD take care to avoid sending queries at Iterative resolvers SHOULD take care to avoid sending queries at
excessive rates. Implementations SHOULD support throttling logic to excessive rates. Implementations SHOULD support throttling logic to
detect when queries are sent but no responses are received. detect when queries are sent but no responses are received.
2.6 Misconfigured NS records 2.6. Misconfigured NS records
Sometimes a zone administrator forgets to add the trailing dot on the 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 domain names in the RDATA of a zone's NS records. Consider this
fragment of the zone file for "example.com": fragment of the zone file for "example.com":
$ORIGIN example.com. $ORIGIN example.com.
example.com. 3600 IN NS ns1.example.com ; Note missing example.com. 3600 IN NS ns1.example.com ; Note missing
example.com. 3600 IN NS ns2.example.com ; trailing dots example.com. 3600 IN NS ns2.example.com ; trailing dots
The zone's authoritative servers will parse the NS RDATA as The zone's authoritative servers will parse the NS RDATA as
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"ns1.example.com.example.com" and "ns2.example.com.example.com". In "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 this example, since all of the zone's name servers are named in the
zone itself (i.e., "ns1.example.com.example.com" and zone itself (i.e., "ns1.example.com.example.com" and
"ns2.example.com.example.com" both end in "example.com") and all are "ns2.example.com.example.com" both end in "example.com") and all are
bogus, the iterative resolver cannot reach any "example.com" name bogus, the iterative resolver cannot reach any "example.com" name
servers. Therefore attempts to resolve these names result in address servers. Therefore attempts to resolve these names result in address
record queries to the "com" authoritative servers. Queries for such record queries to the "com" authoritative servers. Queries for such
obviously bogus glue address records occur frequently at the com/net obviously bogus glue address records occur frequently at the com/net
name servers. name servers.
2.6.1 Recommendation 2.6.1. Recommendation
An authoritative server can detect this situation. A trailing dot An authoritative server can detect this situation. A trailing dot
missing from an NS record's RDATA always results by definition in a missing from an NS record's RDATA always results by definition in a
name server name that exists somewhere under the apex of the zone the name server name that exists somewhere under the apex of the zone the
NS record appears in. Note that further levels of delegation are NS record appears in. Note that further levels of delegation are
possible, so a missing trailing dot could inadvertently create a name possible, so a missing trailing dot could inadvertently create a name
server name that actually exists in a subzone. server name that actually exists in a subzone.
An authoritative name server SHOULD issue a warning when one of a An authoritative name server SHOULD issue a warning when one of a
zone's NS records references a name server below the zone's apex when zone's NS records references a name server below the zone's apex when
a corresponding address record does not exist in the zone AND there a corresponding address record does not exist in the zone AND there
are no delegated subzones where the address record could exist. are no delegated subzones where the address record could exist.
2.7 Name server records with zero TTL 2.7. Name server records with zero TTL
Sometimes a popular com/net subdomain's zone is configured with a 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 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 being cached and will result in a higher query volume to the zone's
authoritative servers. The zone's administrator should understand authoritative servers. The zone's administrator should understand
the consequences of such a configuration and provision resources the consequences of such a configuration and provision resources
accordingly. A zero TTL on the zone's NS RRset, however, carries accordingly. A zero TTL on the zone's NS RRset, however, carries
additional consequences beyond the zone itself: if an iterative additional consequences beyond the zone itself: if an iterative
resolver cannot cache a zone's NS records because of a zero TTL, it resolver 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 will be forced to query that zone's parent's name servers each time
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some delay involved in effecting changes. Further, changes to the some delay involved in effecting changes. Further, changes to the
set of a zone's authoritative name servers (and therefore to the set of a zone's authoritative name servers (and therefore to the
zone's NS RRset) are typically relatively rare: providing reliable zone's NS RRset) are typically relatively rare: providing reliable
authoritative service requires a reasonably stable set of servers. authoritative service requires a reasonably stable set of servers.
Therefore an extremely low or zero TTL on a zone's NS RRset rarely 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 makes sense, except in anticipation of an upcoming change. In this
case, when the zone's administrator has planned a change and does not case, when the zone's administrator has planned a change and does not
want iterative resolvers throughout the Internet to cache the NS want iterative resolvers throughout the Internet to cache the NS
RRset for a long period of time, a low TTL is reasonable. RRset for a long period of time, a low TTL is reasonable.
2.7.1 Recommendation 2.7.1. Recommendation
Because of the additional load placed on a zone's parent's Because of the additional load placed on a zone's parent's
authoritative servers resulting from a zero TTL on a zone's NS RRset, authoritative servers resulting from a zero TTL on a zone's NS RRset,
under such circumstances authoritative name servers SHOULD issue a under such circumstances authoritative name servers SHOULD issue a
warning when loading a zone. warning when loading a zone.
2.8 Unnecessary dynamic update messages 2.8. Unnecessary dynamic update messages
The UPDATE message specified in RFC 2136 [6] allows an authorized 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 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 DNS message sent over the network. Consider the case of an agent
desiring to add a particular resource record. Because of zone cuts, desiring to add a particular resource record. Because of zone cuts,
the agent does not necessarily know the proper zone to which the the agent does not necessarily know the proper zone to which the
record should be added. The dynamic update process requires that the record should be added. The dynamic update process requires that the
agent determine the appropriate zone so the UPDATE message can be agent determine the appropriate zone so the UPDATE message can be
sent to one of the zone's authoritative servers (typically the sent to one of the zone's authoritative servers (typically the
primary master as specified in the zone's SOA MNAME field). primary master as specified in the zone's SOA MNAME field).
skipping to change at page 14, line 21 skipping to change at page 14, line 21
with queries for SOA or NS rather than "probing" with UPDATE with queries for SOA or NS rather than "probing" with UPDATE
messages. Once the appropriate zone is found, an UPDATE message can messages. Once the appropriate zone is found, an UPDATE message can
be sent. In addition, the results of these queries can be cached to be sent. In addition, the results of these queries can be cached to
aid in determining closest enclosing zones for future updates. Once aid in determining closest enclosing zones for future updates. Once
the closest enclosing zone is determined with this method, the update the closest enclosing zone is determined with this method, the update
will either succeed or fail and there is no need to send further will either succeed or fail and there is no need to send further
updates to higher-level zones. The important point is that walking updates to higher-level zones. The important point is that walking
up the tree with queries yields cacheable information, whereas up the tree with queries yields cacheable information, whereas
walking up the tree by sending UPDATE messages does not. walking up the tree by sending UPDATE messages does not.
2.8.1 Recommendation 2.8.1. Recommendation
Dynamic update agents SHOULD send SOA or NS queries to progressively Dynamic update agents SHOULD send SOA or NS queries to progressively
higher-level names to find the closest enclosing zone for a given higher-level names to find the closest enclosing zone for a given
name to update. Only after the appropriate zone is found should the name to update. Only after the appropriate zone is found should the
client send an UPDATE message to one of the zone's authoritative client send an UPDATE message to one of the zone's authoritative
servers. Update clients SHOULD NOT "probe" using UPDATE messages by servers. Update clients SHOULD NOT "probe" using UPDATE messages by
walking up the tree to progressively higher-level zones. walking up the tree to progressively higher-level zones.
2.9 Queries for domain names resembling IPv4 addresses 2.9. Queries for domain names resembling IPv4 addresses
The root name servers receive a significant number of A record The root name servers receive a significant number of A record
queries where the QNAME looks like an IPv4 address. The source of queries where the QNAME looks like an IPv4 address. The source of
these queries is unknown. It could be attributed to situations where these queries is unknown. It could be attributed to situations where
a user believes an application will accept either a domain name or an 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 IP address in a given configuration option. The user enters an IP
address, but the application assumes any input is a domain name and address, but the application assumes any input is a domain name and
attempts to resolve it, resulting in an A record lookup. There could attempts to resolve it, resulting in an A record lookup. There could
also be applications that produce such queries in a misguided attempt also be applications that produce such queries in a misguided attempt
to reverse map IP addresses. to reverse map IP addresses.
These queries result in Name Error (RCODE=3) responses. An iterative These queries result in Name Error (RCODE=3) responses. An iterative
resolver can negatively cache such responses, but each response resolver can negatively cache such responses, but each response
requires a separate cache entry, i.e., a negative cache entry for the 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 "192.0.2.1" does not prevent a subsequent query for the
domain name "192.0.2.2". domain name "192.0.2.2".
2.9.1 Recommendation 2.9.1. Recommendation
It would be desirable for the root name servers not to have to answer It would be desirable for the root name servers not to have to answer
these queries: they unnecessarily consume CPU resources and network these queries: they unnecessarily consume CPU resources and network
bandwidth. A possible solution is to delegate these numeric TLDs bandwidth. A possible solution is to delegate these numeric TLDs
from the root zone to a separate set of servers to absorb the from the root zone to a separate set of servers to absorb the
traffic. The "black hole servers" used by the AS 112 Project [8], traffic. The "black hole servers" used by the AS 112 Project [8],
which are currently delegated the in-addr.arpa zones corresponding to which are currently delegated the in-addr.arpa zones corresponding to
RFC 1918 [7] private use address space, would be a possible choice to RFC 1918 [7] private use address space, would be a possible choice to
receive these delegations. Of course, the proper and usual root zone receive these delegations. Of course, the proper and usual root zone
change procedures would have to be followed to make such a change to change procedures would have to be followed to make such a change to
the root zone. the root zone.
2.10 Misdirected recursive queries 2.10. Misdirected recursive queries
The root name servers receive a significant number of recursive The root name servers receive a significant number of recursive
queries (i.e., queries with the RD bit set in the header). Since queries (i.e., queries with the RD bit set in the header). Since
none of the root servers offers recursion, the servers' response in none of the root servers offers recursion, the servers' response in
such a situation ignores the request for recursion and the response such a situation ignores the request for recursion and the response
probably does not contain the data the querier anticipated. Some of probably does not contain the data the querier anticipated. Some of
these queries result from users configuring stub resolvers to query a these queries result from users configuring stub resolvers to query a
root server. (This situation is not hypothetical: we have received root server. (This situation is not hypothetical: we have received
complaints from users when this configuration does not work as complaints from users when this configuration does not work as
hoped.) Of course, users should not direct stub resolvers to use 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 name servers that do not offer recursion, but we are not aware of any
stub resolver implementation that offers any feedback to the user stub resolver implementation that offers any feedback to the user
when so configured, aside from simply "not working". when so configured, aside from simply "not working".
2.10.1 Recommendation 2.10.1. Recommendation
When the IP address of a name server that supposedly offers recursion When the IP address of a name server that supposedly offers recursion
is configured in a stub resolver using an interactive user interface, is configured in a stub resolver using an interactive user interface,
the resolver could send a test query to verify that the server indeed the resolver could send a test query to verify that the server indeed
supports recursion (i.e., verify that the response has the RA bit set supports recursion (i.e., verify that the response has the RA bit set
in the header). The user could be immediately notified if the server in the header). The user could be immediately notified if the server
is non-recursive. is non-recursive.
The stub resolver could also report an error, either through a user 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 interface or in a log file, if the queried server does not support
recursion. Error reporting SHOULD be throttled to avoid a recursion. Error reporting SHOULD be throttled to avoid a
notification or log message for every response from a non-recursive notification or log message for every response from a non-recursive
server. server.
2.11 Suboptimal name server selection algorithm 2.11. Suboptimal name server selection algorithm
An entire document could be devoted to the topic of problems with An entire document could be devoted to the topic of problems with
different implementations of the recursive resolution algorithm. The different implementations of the recursive resolution algorithm. The
entire process of recursion is woefully under specified, requiring entire process of recursion is woefully under specified, requiring
each implementor to design an algorithm. Sometimes implementors make each implementor to design an algorithm. Sometimes implementors make
poor design choices that could be avoided if a suggested algorithm poor design choices that could be avoided if a suggested algorithm
and best practices were documented, but that is a topic for another and best practices were documented, but that is a topic for another
document. document.
Some deficiencies cause significant operational impact and are Some deficiencies cause significant operational impact and are
therefore worth mentioning here. One of these is name server therefore worth mentioning here. One of these is name server
selection by an iterative resolver. When an iterative resolver wants selection by an iterative resolver. When an iterative resolver wants
to contact one of a zone's authoritative name servers, how does it 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 choose from the NS records listed in the zone's NS RRset? If the
selection mechanism is suboptimal, queries are not spread evenly selection mechanism is suboptimal, queries are not spread evenly
among a zone's authoritative servers. The details of the selection among a zone's authoritative servers. The details of the selection
mechanism are up to the implementor, but we offer some suggestions. mechanism are up to the implementor, but we offer some suggestions.
2.11.1 Recommendation 2.11.1. Recommendation
This list is not conclusive, but reflects the changes that would This list is not conclusive, but reflects the changes that would
produce the most impact in terms of reducing disproportionate query produce the most impact in terms of reducing disproportionate query
load among a zone's authoritative servers. I.e., these changes would load among a zone's authoritative servers. I.e., these changes would
help spread the query load evenly. help spread the query load evenly.
o Do not make assumptions based on NS RRset order: all NS RRs SHOULD 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, be treated equally. (In the case of the "com" zone, for example,
most of the root servers return the NS record for "a.gtld- most of the root servers return the NS record for "a.gtld-
servers.net" first in the authority section of referrals. servers.net" first in the authority section of referrals.
skipping to change at page 17, line 5 skipping to change at page 17, line 5
o Maintain state and favor the best-performing of a zone's o Maintain state and favor the best-performing of a zone's
authoritative servers. A good definition of performance is authoritative servers. A good definition of performance is
response time. Non-responsive servers can be penalized with an response time. Non-responsive servers can be penalized with an
extremely high response time. extremely high response time.
o Do not lock onto the best-performing of a zone's name servers. An o Do not lock onto the best-performing of a zone's name servers. An
iterative resolver SHOULD periodically check the performance of iterative resolver SHOULD periodically check the performance of
all of a zone's name servers to adjust its determination of the all of a zone's name servers to adjust its determination of the
best-performing one. best-performing one.
3. IANA considerations 3. Acknowledgments
The authors would like to thank the following people for their
comments that improved this document: Andras Salamon, Dave Meyer,
Doug Barton, Jaap Akkerhuis, Jinmei Tatuya, John Brady, Kevin Darcy,
Olafur Gudmundsson, Pekka Savola, Peter Koch and Rob Austein. We
apologize if we have omitted anyone; any oversight was unintentional.
4. IANA considerations
There are no new IANA considerations introduced by this memo. There are no new IANA considerations introduced by this memo.
4. Security considerations 5. Security considerations
The iterative resolver misbehavior discussed in this document exposes The iterative resolver misbehavior discussed in this document exposes
the root and TLD name servers to increased risk of both intentional the root and TLD name servers to increased risk of both intentional
and unintentional denial of service attacks. and unintentional denial of service attacks.
We believe that implementation of the recommendations offered in this We believe that implementation of the recommendations offered in this
document will reduce the amount of unnecessary traffic seen at root document will reduce the amount of unnecessary traffic seen at root
and TLD name servers, thus reducing the opportunity for an attacker and TLD name servers, thus reducing the opportunity for an attacker
to use such queries to his or her advantage. to use such queries to his or her advantage.
5. Internationalization considerations 6. Internationalization considerations
There are no new internationalization considerations introduced by There are no new internationalization considerations introduced by
this memo. this memo.
6. Informative References 7. Informative 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", [2] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[3] Elz, R. and R. Bush, "Clarifications to the DNS Specification", [3] Elz, R. and R. Bush, "Clarifications to the DNS Specification",
RFC 2181, July 1997. RFC 2181, July 1997.
skipping to change at page 21, line 41 skipping to change at page 22, line 41
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights. except as set forth therein, the authors retain all their rights.
Acknowledgment Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
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