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diff --git a/contrib/bind9/doc/draft/draft-ietf-dnsop-bad-dns-res-05.txt b/contrib/bind9/doc/draft/draft-ietf-dnsop-bad-dns-res-05.txt deleted file mode 100644 index 0855ba3..0000000 --- a/contrib/bind9/doc/draft/draft-ietf-dnsop-bad-dns-res-05.txt +++ /dev/null @@ -1,1232 +0,0 @@ - - - -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 - -Status of this Memo - - By submitting this Internet-Draft, each author represents that any - applicable patent or other IPR claims of which he or she is aware - have been or will be disclosed, and any of which he or she becomes - aware will be disclosed, in accordance with Section 6 of BCP 79. - - Internet-Drafts are working documents of the Internet Engineering - Task Force (IETF), its areas, and its working groups. Note that - other groups may also distribute working documents as Internet- - Drafts. - - Internet-Drafts are draft documents valid for a maximum of six months - and may be updated, replaced, or obsoleted by other documents at any - time. It is inappropriate to use Internet-Drafts as reference - material or to cite them other than as "work in progress." - - The list of current Internet-Drafts can be accessed at - http://www.ietf.org/ietf/1id-abstracts.txt. - - The list of Internet-Draft Shadow Directories can be accessed at - http://www.ietf.org/shadow.html. - - This Internet-Draft will expire on August 14, 2006. - -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. - - - -Larson & Barber Expires August 14, 2006 [Page 1] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - - 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 - - - - - - - - - - - - -Larson & Barber Expires August 14, 2006 [Page 2] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - -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", - - - -Larson & Barber Expires August 14, 2006 [Page 3] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - - 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. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Larson & Barber Expires August 14, 2006 [Page 4] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - -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 - - - -Larson & Barber Expires August 14, 2006 [Page 5] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - - 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 - - - -Larson & Barber Expires August 14, 2006 [Page 6] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - - 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. - - - -Larson & Barber Expires August 14, 2006 [Page 7] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - - 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. - - - - - - -Larson & Barber Expires August 14, 2006 [Page 8] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - -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 - - - -Larson & Barber Expires August 14, 2006 [Page 9] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - - 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 - - - -Larson & Barber Expires August 14, 2006 [Page 10] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - - 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" - - - -Larson & Barber Expires August 14, 2006 [Page 11] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - - 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 - - - -Larson & Barber Expires August 14, 2006 [Page 12] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - - 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- - - - -Larson & Barber Expires August 14, 2006 [Page 13] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - - 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 - - - -Larson & Barber Expires August 14, 2006 [Page 14] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - - 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. - - - -Larson & Barber Expires August 14, 2006 [Page 15] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - - 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. - - - - - - - - - - - - - - - -Larson & Barber Expires August 14, 2006 [Page 16] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - -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. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Larson & Barber Expires August 14, 2006 [Page 17] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - -4. IANA considerations - - There are no new IANA considerations introduced by this memo. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Larson & Barber Expires August 14, 2006 [Page 18] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - -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. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Larson & Barber Expires August 14, 2006 [Page 19] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - -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> - - - - - - - - - - - - - - - - - - - - -Larson & Barber Expires August 14, 2006 [Page 20] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - -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 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Larson & Barber Expires August 14, 2006 [Page 21] - -Internet-Draft Observed DNS Resolution Misbehavior February 2006 - - -Intellectual Property Statement - - The IETF takes no position regarding the validity or scope of any - Intellectual Property Rights or other rights that might be claimed to - pertain to the implementation or use of the technology described in - this document or the extent to which any license under such rights - might or might not be available; nor does it represent that it has - made any independent effort to identify any such rights. Information - on the procedures with respect to rights in RFC documents can be - found in BCP 78 and BCP 79. - - Copies of IPR disclosures made to the IETF Secretariat and any - assurances of licenses to be made available, or the result of an - attempt made to obtain a general license or permission for the use of - such proprietary rights by implementers or users of this - specification can be obtained from the IETF on-line IPR repository at - http://www.ietf.org/ipr. - - The IETF invites any interested party to bring to its attention any - copyrights, patents or patent applications, or other proprietary - rights that may cover technology that may be required to implement - this standard. Please address the information to the IETF at - ietf-ipr@ietf.org. - - -Disclaimer of Validity - - This document and the information contained herein are provided on an - "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS - OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET - ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, - INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE - INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED - WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. - - -Copyright Statement - - Copyright (C) The Internet Society (2006). This document is subject - to the rights, licenses and restrictions contained in BCP 78, and - except as set forth therein, the authors retain all their rights. - - -Acknowledgment - - Funding for the RFC Editor function is currently provided by the - Internet Society. - - - - -Larson & Barber Expires August 14, 2006 [Page 22] - |
