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DNS Operations M. Larson
Internet-Draft P. Barber
Expires: August 14, 2006 VeriSign
February 10, 2006
Observed DNS Resolution Misbehavior
draft-ietf-dnsop-bad-dns-res-05
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This memo describes DNS iterative resolver behavior that results in a
significant query volume sent to the root and top-level domain (TLD)
name servers. We offer implementation advice to iterative resolver
developers to alleviate these unnecessary queries. The
recommendations made in this document are a direct byproduct of
observation and analysis of abnormal query traffic patterns seen at
two of the thirteen root name servers and all thirteen com/net TLD
name servers.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. A note about terminology in this memo . . . . . . . . . . 3
2. Observed iterative resolver misbehavior . . . . . . . . . . . 5
2.1. Aggressive requerying for delegation information . . . . . 5
2.1.1. Recommendation . . . . . . . . . . . . . . . . . . . . 6
2.2. Repeated queries to lame servers . . . . . . . . . . . . . 7
2.2.1. Recommendation . . . . . . . . . . . . . . . . . . . . 7
2.3. Inability to follow multiple levels of indirection . . . . 8
2.3.1. Recommendation . . . . . . . . . . . . . . . . . . . . 9
2.4. Aggressive retransmission when fetching glue . . . . . . . 9
2.4.1. Recommendation . . . . . . . . . . . . . . . . . . . . 10
2.5. Aggressive retransmission behind firewalls . . . . . . . . 10
2.5.1. Recommendation . . . . . . . . . . . . . . . . . . . . 11
2.6. Misconfigured NS records . . . . . . . . . . . . . . . . . 11
2.6.1. Recommendation . . . . . . . . . . . . . . . . . . . . 12
2.7. Name server records with zero TTL . . . . . . . . . . . . 12
2.7.1. Recommendation . . . . . . . . . . . . . . . . . . . . 13
2.8. Unnecessary dynamic update messages . . . . . . . . . . . 13
2.8.1. Recommendation . . . . . . . . . . . . . . . . . . . . 14
2.9. Queries for domain names resembling IPv4 addresses . . . . 14
2.9.1. Recommendation . . . . . . . . . . . . . . . . . . . . 14
2.10. Misdirected recursive queries . . . . . . . . . . . . . . 15
2.10.1. Recommendation . . . . . . . . . . . . . . . . . . . . 15
2.11. Suboptimal name server selection algorithm . . . . . . . . 15
2.11.1. Recommendation . . . . . . . . . . . . . . . . . . . . 16
3. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
4. IANA considerations . . . . . . . . . . . . . . . . . . . . . 18
5. Security considerations . . . . . . . . . . . . . . . . . . . 19
6. Internationalization considerations . . . . . . . . . . . . . 20
7. Informative References . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . . . . 22
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1. Introduction
Observation of query traffic received by two root name servers and
the thirteen com/net TLD name servers has revealed that a large
proportion of the total traffic often consists of "requeries". A
requery is the same question (<QNAME, QTYPE, QCLASS>) asked
repeatedly at an unexpectedly high rate. We have observed requeries
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
duplicate traffic is almost always a deficient iterative resolver,
stub resolver or application implementation combined with an
operational anomaly. The implementation deficiencies we have
identified to date include well-intentioned recovery attempts gone
awry, insufficient caching of failures, early abort when multiple
levels of indirection must be followed, and aggressive retry by stub
resolvers or applications. Anomalies that we have seen trigger
requery events include lame delegations, unusual glue records, and
anything that makes all authoritative name servers for a zone
unreachable (DoS attacks, crashes, maintenance, routing failures,
congestion, etc.).
In the following sections, we provide a detailed explanation of the
observed behavior and recommend changes that will reduce the requery
rate. None of the changes recommended affects the core DNS protocol
specification; instead, this document consists of guidelines to
implementors of iterative resolvers.
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 the initial
recursive query, caches responses it receives, and which is also able
to answer questions about certain zones authoritatively. This entity
is an iterative resolver combined with an authoritative name server
and is often called a "recursive name server" or a "caching 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",
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because the focus is usually on that component. In instances where
the name server role of this entity requires mentioning, this memo
uses the term "recursive name server". As an example of the
difference, the name server component of a recursive name server
receives DNS queries and the iterative resolver component sends
queries.
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 record" to
encompass both A and AAAA records when a particular situation is
relevant to both types.
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2. Observed iterative resolver misbehavior
2.1. Aggressive requerying for delegation information
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
name server process is not running on the server host) or
misconfigured (e.g., the name server is not authoritative for the
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
discovers that none of the zone's name servers can provide an answer.
We have observed a recursive name server implementation whose
iterative resolver then verifies the zone's NS RRset in its cache by
querying for the zone's delegation information: it sends a query for
the zone's NS RRset to one of the parent zone's name servers. (Note
that queries with QTYPE=NS are not required by the standard
resolution algorithm described in section 4.3.2 of RFC 1034 [2].
These NS queries represent this implementation's addition to that
algorithm.)
For example, suppose that "example.com" has the following NS RRset:
example.com. IN NS ns1.example.com.
example.com. IN NS ns2.example.com.
Upon receipt of a query for "www.example.com" and assuming that
neither "ns1.example.com" nor "ns2.example.com" can provide an
answer, this iterative resolver implementation immediately queries a
"com" zone name server for the "example.com" NS RRset to verify it
has the proper delegation information. This implementation performs
this query to a zone's parent zone for each recursive query it
receives that fails because of a completely unresponsive set of name
servers for the target zone. Consider the effect when a popular zone
experiences a catastrophic failure of all its name servers: now every
recursive query for domain names in that zone sent to this recursive
name server implementation results in a query to the failed zone's
parent name servers. On one occasion when several dozen popular
zones became unreachable, the query load on the com/net name servers
increased by 50%.
We believe this verification query is not reasonable. Consider the
circumstances: When an iterative resolver is resolving a query for a
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.
If on its first attempt to search the target zone, none of the name
servers in the referral is reachable, a verification query to the
parent would be pointless: this query to the parent would come so
quickly on the heels of the referral that it would be almost certain
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to contain the same list of name servers. The chance of discovering
any new information is slim.
The other possibility is that the iterative resolver successfully
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
according to section 5.4.1 of RFC 2181 [3], because the NS RRset from
the target zone is more trustworthy than delegation information from
the parent zone. If, while processing a subsequent recursive query,
the iterative resolver discovers that none of the name servers
specified in the cached NS RRset is available or authoritative,
querying the parent would be wrong. An NS RRset from the parent zone
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
entire set of name servers would have to change AND the former set of
name servers would have to be deconfigured or decommissioned AND 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 target
zone's NS RRset. We believe this scenario is uncommon:
administrative best practices dictate that changes to a zone's set of
name servers happen gradually when at all possible, with servers
removed from the NS RRset left authoritative for the zone as long as
possible. The scenarios that we can envision that would benefit from
the parent requery behavior do not outweigh its damaging effects.
This section should not be understood to claim that all queries to a
zone's parent are bad. In some cases, such queries are not only
reasonable but required. Consider the situation when required
information, such as the address of a name server (i.e., the address
record corresponding to the RDATA of an NS record), has timed out of
an iterative resolver's cache before the corresponding NS record. If
the name of the name server is below the apex of the zone, then the
name server's address record is only available as glue in the parent
zone. For example, consider this NS record:
example.com. IN NS ns.example.com.
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
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
the standard resolution algorithm.
2.1.1. Recommendation
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
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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
RRset:
1. is not authoritative for the zone (i.e., lame), or,
2. returns a server failure response (RCODE=2), or,
3. is dead or unreachable according to section 7.2 of RFC 2308 [4].
2.2. Repeated queries to lame servers
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.
A more common occurrence is when a subset of a zone's name servers
are unavailable or misconfigured. Different failure modes have
different expected durations. Some symptoms indicate problems that
are potentially transient; for example, various types of ICMP
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 query. Such responses could be the result of a host rebooting or
temporary outages; these events don't necessarily require any human
intervention and can be reasonably expected to be temporary.
Other symptoms clearly indicate a condition requiring human
intervention, such as lame server: if a name server is misconfigured
and not authoritative for a zone delegated to it, it is reasonable to
assume that this condition has potential to last longer than
unreachability or unresponsiveness. Consequently, repeated queries
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
to maintain a list of known lame servers and avoid querying them
repeatedly in a short interval.
It should also be noted, however, that some authoritative name server
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
the "lame" servers for other types of queries, particularly when all
known authoritative name servers appear to be "lame".
2.2.1. Recommendation
Iterative resolvers SHOULD cache name servers that they discover are
not authoritative for zones delegated to them (i.e. lame servers).
If this caching is performed, lame servers MUST be cached against the
specific query tuple <zone name, class, server IP address>. Zone
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.
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Implementations that perform lame server caching MUST refrain 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
thirty minutes is RECOMMENDED.
An exception to this recommendation occurs if all name servers for a
zone are marked lame. In that case, the iterative resolver SHOULD
temporarily ignore the servers' lameness status and query one or more
servers. This behavior is a workaround for the type-specific
lameness issue described in the previous section.
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
but not a child zone, e.g., lame for "example.com" but properly
authoritative for "sub.example.com". Therefore a name server should
not be automatically considered lame for subzones. In the case
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
NS record indicates it should be authoritative for that zone.
2.3. Inability to follow multiple levels of indirection
Some iterative resolver implementations are unable to follow
sufficient levels of indirection. For example, consider the
following delegations:
foo.example. IN NS ns1.example.com.
foo.example. IN NS ns2.example.com.
example.com. IN NS ns1.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 ns2.test.example.net.
An iterative resolver resolving the name "www.foo.example" must
follow two levels of indirection, first obtaining address records for
"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
to query those name servers for the address records of
"www.foo.example". While this situation may appear contrived, we
have seen multiple similar occurrences and expect more as new generic
top-level domains (gTLDs) become active. We anticipate many zones in
new gTLDs will use name servers in existing gTLDs, increasing the
number of delegations using out-of-zone name servers.
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2.3.1. Recommendation
Clearly constructing a delegation that relies on multiple levels of
indirection is not a good administrative practice. However, the
practice is widespread enough to require that iterative resolvers be
able to cope with it. Iterative resolvers SHOULD be able to handle
arbitrary levels of indirection resulting from out-of-zone name
servers. Iterative resolvers SHOULD implement a level-of-effort
counter to avoid loops or otherwise performing too much work in
resolving pathological cases.
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
example, if the zone is named "example.com", consider naming some of
the name servers "ns{1,2,...}.example.com" (or similar).
2.4. Aggressive retransmission when fetching glue
When an authoritative name server responds with a referral, it
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
server should also "put whatever addresses are available into the
additional section, using glue RRs if the addresses are not available
from authoritative data or the cache." Some name server
implementations take this address inclusion a step further with a
feature called "glue fetching". A name server that implements glue
fetching attempts to include address records for every NS record in
the authority section. If necessary, the name server issues multiple
queries of its own to obtain any missing address records.
Problems with glue fetching can arise in the context of
"authoritative-only" name servers, which only serve authoritative
data and ignore requests for recursion. Such an entity will not
normally generate any queries of its own. Instead it answers non-
recursive queries from iterative resolvers looking for information in
zones it serves. With glue fetching enabled, however, an
authoritative server invokes an iterative resolver to look up an
unknown address record to complete the additional section of a
response.
We have observed situations where the iterative resolver of a glue-
fetching name server can send queries that reach other name servers,
but is apparently prevented from receiving the responses. For
example, perhaps the name server is authoritative-only and therefore
its administrators expect it to receive only queries and not
responses. Perhaps unaware of glue fetching and presuming that the
name server's iterative resolver will generate no queries, its
administrators place the name server behind a network device that
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prevents it from receiving responses. If this is the case, all glue-
fetching queries will go answered.
We have observed name server implementations whose iterative
resolvers retry excessively when glue-fetching queries are
unanswered. A single com/net name server has received hundreds of
queries per second from a single such source. Judging from the
specific queries received and based on additional analysis, we
believe these queries result from overly aggressive glue fetching.
2.4.1. Recommendation
Implementers whose name servers support glue fetching 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.
2.5. 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, an
iterative resolver 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
recursive name servers, only a subset of whose iterative resolvers'
traffic is improperly filtered in this manner. Stub resolvers in the
organization could be configured to query multiple recursive name
servers. Consider the case where a stub resolver queries a filtered
recursive name server first. The iterative resolver of this
recursive name server sends one or more queries whose replies are
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
able to provide an answer. Since resolution ultimately succeeds the
underlying problem might not be recognized or corrected. A popular
stub resolver implementation has a very aggressive retransmission
schedule, including simultaneous queries to multiple recursive name
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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.
Iterative resolvers 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 delegation 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 [3], 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 overrides the
"example.com" NS RRset received moments ago from the "com"
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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 iterative resolver to attempt to use the incorrect NS
records and so it 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 iterative resolver 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 apex 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.
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
a corresponding address record does not exist in the zone AND there
are no delegated subzones where the address record could exist.
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 an iterative
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
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
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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 iterative resolvers 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 resulting from a zero TTL on a zone's NS RRset,
under such circumstances authoritative name servers SHOULD issue a
warning when loading a zone.
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-
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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 names 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 IPv4 addresses
The root name servers receive a significant number of A record
queries where the QNAME looks like an IPv4 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. An iterative
resolver 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
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bandwidth. A possible solution 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 AS 112 Project [8],
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. Of course, the proper and usual root zone
change procedures would have to be followed to make such a change to
the root zone.
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 offers 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 name server that supposedly offers recursion
is configured in a stub resolver using an interactive user interface,
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
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.
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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.
Apparently 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.
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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.
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4. IANA considerations
There are no new IANA considerations introduced by this memo.
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5. Security considerations
The iterative resolver misbehavior discussed in this document exposes
the root and TLD name servers to increased risk of both intentional
and unintentional denial of service attacks.
We believe that implementation of the recommendations offered in this
document will reduce the amount of unnecessary traffic seen at root
and TLD name servers, thus reducing the opportunity for an attacker
to use such queries to his or her advantage.
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6. Internationalization considerations
There are no new internationalization considerations introduced by
this memo.
7. Informative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[3] Elz, R. and R. Bush, "Clarifications to the DNS Specification",
RFC 2181, July 1997.
[4] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)",
RFC 2308, March 1998.
[5] Morishita, Y. and T. Jinmei, "Common Misbehavior Against DNS
Queries for IPv6 Addresses", RFC 4074, May 2005.
[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.
[8] <http://www.as112.net>
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Authors' Addresses
Matt Larson
VeriSign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166-6503
USA
Email: mlarson@verisign.com
Piet Barber
VeriSign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166-6503
USA
Email: pbarber@verisign.com
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