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RFC2308 - Negative Caching of DNS Queries (DNS NCACHE)

王朝other·作者佚名  2008-05-31
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Network Working Group M. Andrews

Request for Comments: 2308 CSIRO

Updates: 1034, 1035 March 1998

Category: Standards Track

Negative Caching of DNS Queries (DNS NCACHE)

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Abstract

[RFC1034] provided a description of how to cache negative responses.

It however had a fundamental flaw in that it did not allow a name

server to hand out those cached responses to other resolvers, thereby

greatly redUCing the effect of the caching. This document addresses

issues raise in the light of eXPerience and replaces [RFC1034 Section

4.3.4].

Negative caching was an optional part of the DNS specification and

deals with the caching of the non-existence of an RRset [RFC2181] or

domain name.

Negative caching is useful as it reduces the response time for

negative answers. It also reduces the number of messages that have

to be sent between resolvers and name servers hence overall network

traffic. A large proportion of DNS traffic on the Internet could be

eliminated if all resolvers implemented negative caching. With this

in mind negative caching should no longer be seen as an optional part

of a DNS resolver.

1 - Terminology

The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

document are to be interpreted as described in [RFC2119].

"Negative caching" - the storage of knowledge that something does not

exist. We can store the knowledge that a record has a particular

value. We can also do the reverse, that is, to store the knowledge

that a record does not exist. It is the storage of knowledge that

something does not exist, cannot or does not give an answer that we

call negative caching.

"QNAME" - the name in the query section of an answer, or where this

resolves to a CNAME, or CNAME chain, the data field of the last

CNAME. The last CNAME in this sense is that which contains a value

which does not resolve to another CNAME. Implementations should note

that including CNAME records in responses in order, so that the first

has the label from the query section, and then each in sequence has

the label from the data section of the previous (where more than one

CNAME is needed) allows the sequence to be processed in one pass, and

considerably eases the task of the receiver. Other relevant records

(such as SIG RRs [RFC2065]) can be interspersed amongst the CNAMEs.

"NXDOMAIN" - an alternate expression for the "Name Error" RCODE as

described in [RFC1035 Section 4.1.1] and the two terms are used

interchangeably in this document.

"NODATA" - a pseudo RCODE which indicates that the name is valid, for

the given class, but are no records of the given type. A NODATA

response has to be inferred from the answer.

"FORWARDER" - a nameserver used to resolve queries instead of

directly using the authoritative nameserver chain. The forwarder

typically either has better Access to the internet, or maintains a

bigger cache which may be shared amongst many resolvers. How a

server is identified as a FORWARDER, or knows it is a FORWARDER is

outside the scope of this document. However if you are being used as

a forwarder the query will have the recursion desired flag set.

An understanding of [RFC1034], [RFC1035] and [RFC2065] is expected

when reading this document.

2 - Negative Responses

The most common negative responses indicate that a particular RRset

does not exist in the DNS. The first sections of this document deal

with this case. Other negative responses can indicate failures of a

nameserver, those are dealt with in section 7 (Other Negative

Responses).

A negative response is indicated by one of the following conditions:

2.1 - Name Error

Name errors (NXDOMAIN) are indicated by the presence of "Name Error"

in the RCODE field. In this case the domain referred to by the QNAME

does not exist. Note: the answer section may have SIG and CNAME RRs

and the authority section may have SOA, NXT [RFC2065] and SIG RRsets.

It is possible to distinguish between a referral and a NXDOMAIN

response by the presense of NXDOMAIN in the RCODE regardless of the

presence of NS or SOA records in the authority section.

NXDOMAIN responses can be categorised into four types by the contents

of the authority section. These are shown below along with a

referral for comparison. Fields not mentioned are not important in

terms of the examples.

NXDOMAIN RESPONSE: TYPE 1.

Header:

RDCODE=NXDOMAIN

Query:

AN.EXAMPLE. A

Answer:

AN.EXAMPLE. CNAME TRIPPLE.XX.

Authority:

XX. SOA NS1.XX. HOSTMASTER.NS1.XX. ....

XX. NS NS1.XX.

XX. NS NS2.XX.

Additional:

NS1.XX. A 127.0.0.2

NS2.XX. A 127.0.0.3

NXDOMAIN RESPONSE: TYPE 2.

Header:

RDCODE=NXDOMAIN

Query:

AN.EXAMPLE. A

Answer:

AN.EXAMPLE. CNAME TRIPPLE.XX.

Authority:

XX. SOA NS1.XX. HOSTMASTER.NS1.XX. ....

Additional:

<empty>

NXDOMAIN RESPONSE: TYPE 3.

Header:

RDCODE=NXDOMAIN

Query:

AN.EXAMPLE. A

Answer:

AN.EXAMPLE. CNAME TRIPPLE.XX.

Authority:

<empty>

Additional:

<empty>

NXDOMAIN RESPONSE: TYPE 4

Header:

RDCODE=NXDOMAIN

Query:

AN.EXAMPLE. A

Answer:

AN.EXAMPLE. CNAME TRIPPLE.XX.

Authority:

XX. NS NS1.XX.

XX. NS NS2.XX.

Additional:

NS1.XX. A 127.0.0.2

NS2.XX. A 127.0.0.3

REFERRAL RESPONSE.

Header:

RDCODE=NOERROR

Query:

AN.EXAMPLE. A

Answer:

AN.EXAMPLE. CNAME TRIPPLE.XX.

Authority:

XX. NS NS1.XX.

XX. NS NS2.XX.

Additional:

NS1.XX. A 127.0.0.2

NS2.XX. A 127.0.0.3

Note, in the four examples of NXDOMAIN responses, it is known that

the name "AN.EXAMPLE." exists, and has as its value a CNAME record.

The NXDOMAIN refers to "TRIPPLE.XX", which is then known not to

exist. On the other hand, in the referral example, it is shown that

"AN.EXAMPLE" exists, and has a CNAME RR as its value, but nothing is

known one way or the other about the existence of "TRIPPLE.XX", other

than that "NS1.XX" or "NS2.XX" can be consulted as the next step in

oBTaining information about it.

Where no CNAME records appear, the NXDOMAIN response refers to the

name in the label of the RR in the question section.

2.1.1 Special Handling of Name Error

This section deals with errors encountered when implementing negative

caching of NXDOMAIN responses.

There are a large number of resolvers currently in existence that

fail to correctly detect and process all forms of NXDOMAIN response.

Some resolvers treat a TYPE 1 NXDOMAIN response as a referral. To

alleviate this problem it is recommended that servers that are

authoritative for the NXDOMAIN response only send TYPE 2 NXDOMAIN

responses, that is the authority section contains a SOA record and no

NS records. If a non- authoritative server sends a type 1 NXDOMAIN

response to one of these old resolvers, the result will be an

unnecessary query to an authoritative server. This is undesirable,

but not fatal except when the server is being used a FORWARDER. If

however the resolver is using the server as a FORWARDER to such a

resolver it will be necessary to disable the sending of TYPE 1

NXDOMAIN response to it, use TYPE 2 NXDOMAIN instead.

Some resolvers incorrectly continue processing if the authoritative

answer flag is not set, looping until the query retry threshold is

exceeded and then returning SERVFAIL. This is a problem when your

nameserver is listed as a FORWARDER for such resolvers. If the

nameserver is used as a FORWARDER by such resolver, the authority

flag will have to be forced on for NXDOMAIN responses to these

resolvers. In practice this causes no problems even if turned on

always, and has been the default behaviour in BIND from 4.9.3

onwards.

2.2 - No Data

NODATA is indicated by an answer with the RCODE set to NOERROR and no

relevant answers in the answer section. The authority section will

contain an SOA record, or there will be no NS records there.

NODATA responses have to be algorithmically determined from the

response's contents as there is no RCODE value to indicate NODATA.

In some cases to determine with certainty that NODATA is the correct

response it can be necessary to send another query.

The authority section may contain NXT and SIG RRsets in addition to

NS and SOA records. CNAME and SIG records may exist in the answer

section.

It is possible to distinguish between a NODATA and a referral

response by the presence of a SOA record in the authority section or

the absence of NS records in the authority section.

NODATA responses can be categorised into three types by the contents

of the authority section. These are shown below along with a

referral for comparison. Fields not mentioned are not important in

terms of the examples.

NODATA RESPONSE: TYPE 1.

Header:

RDCODE=NOERROR

Query:

ANOTHER.EXAMPLE. A

Answer:

<empty>

Authority:

EXAMPLE. SOA NS1.XX. HOSTMASTER.NS1.XX. ....

EXAMPLE. NS NS1.XX.

EXAMPLE. NS NS2.XX.

Additional:

NS1.XX. A 127.0.0.2

NS2.XX. A 127.0.0.3

NO DATA RESPONSE: TYPE 2.

Header:

RDCODE=NOERROR

Query:

ANOTHER.EXAMPLE. A

Answer:

<empty>

Authority:

EXAMPLE. SOA NS1.XX. HOSTMASTER.NS1.XX. ....

Additional:

<empty>

NO DATA RESPONSE: TYPE 3.

Header:

RDCODE=NOERROR

Query:

ANOTHER.EXAMPLE. A

Answer:

<empty>

Authority:

<empty>

Additional:

<empty>

REFERRAL RESPONSE.

Header:

RDCODE=NOERROR

Query:

ANOTHER.EXAMPLE. A

Answer:

<empty>

Authority:

EXAMPLE. NS NS1.XX.

EXAMPLE. NS NS2.XX.

Additional:

NS1.XX. A 127.0.0.2

NS2.XX. A 127.0.0.3

These examples, unlike the NXDOMAIN examples above, have no CNAME

records, however they could, in just the same way that the NXDOMAIN

examples did, in which case it would be the value of the last CNAME

(the QNAME) for which NODATA would be concluded.

2.2.1 - Special Handling of No Data

There are a large number of resolvers currently in existence that

fail to correctly detect and process all forms of NODATA response.

Some resolvers treat a TYPE 1 NODATA response as a referral. To

alleviate this problem it is recommended that servers that are

authoritative for the NODATA response only send TYPE 2 NODATA

responses, that is the authority section contains a SOA record and no

NS records. Sending a TYPE 1 NODATA response from a non-

authoritative server to one of these resolvers will only result in an

unnecessary query. If a server is listed as a FORWARDER for another

resolver it may also be necessary to disable the sending of TYPE 1

NODATA response for non-authoritative NODATA responses.

Some name servers fail to set the RCODE to NXDOMAIN in the presence

of CNAMEs in the answer section. If a definitive NXDOMAIN / NODATA

answer is required in this case the resolver must query again using

the QNAME as the query label.

3 - Negative Answers from Authoritative Servers

Name servers authoritative for a zone MUST include the SOA record of

the zone in the authority section of the response when reporting an

NXDOMAIN or indicating that no data of the requested type exists.

This is required so that the response may be cached. The TTL of this

record is set from the minimum of the MINIMUM field of the SOA record

and the TTL of the SOA itself, and indicates how long a resolver may

cache the negative answer. The TTL SIG record associated with the

SOA record should also be trimmed in line with the SOA's TTL.

If the containing zone is signed [RFC2065] the SOA and appropriate

NXT and SIG records MUST be added.

4 - SOA Minimum Field

The SOA minimum field has been overloaded in the past to have three

different meanings, the minimum TTL value of all RRs in a zone, the

default TTL of RRs which did not contain a TTL value and the TTL of

negative responses.

Despite being the original defined meaning, the first of these, the

minimum TTL value of all RRs in a zone, has never in practice been

used and is hereby deprecated.

The second, the default TTL of RRs which contain no explicit TTL in

the master zone file, is relevant only at the primary server. After

a zone transfer all RRs have explicit TTLs and it is impossible to

determine whether the TTL for a record was explicitly set or derived

from the default after a zone transfer. Where a server does not

require RRs to include the TTL value explicitly, it should provide a

mechanism, not being the value of the MINIMUM field of the SOA

record, from which the missing TTL values are obtained. How this is

done is implementation dependent.

The Master File format [RFC1035 Section 5] is extended to include

the following directive:

$TTL <TTL> [comment]

All resource records appearing after the directive, and which do not

explicitly include a TTL value, have their TTL set to the TTL given

in the $TTL directive. SIG records without a explicit TTL get their

TTL from the "original TTL" of the SIG record [RFC2065 Section 4.5].

The remaining of the current meanings, of being the TTL to be used

for negative responses, is the new defined meaning of the SOA minimum

field.

5 - Caching Negative Answers

Like normal answers negative answers have a time to live (TTL). As

there is no record in the answer section to which this TTL can be

applied, the TTL must be carried by another method. This is done by

including the SOA record from the zone in the authority section of

the reply. When the authoritative server creates this record its TTL

is taken from the minimum of the SOA.MINIMUM field and SOA's TTL.

This TTL decrements in a similar manner to a normal cached answer and

upon reaching zero (0) indicates the cached negative answer MUST NOT

be used again.

A negative answer that resulted from a name error (NXDOMAIN) should

be cached such that it can be retrieved and returned in response to

another query for the same <QNAME, QCLASS> that resulted in the

cached negative response.

A negative answer that resulted from a no data error (NODATA) should

be cached such that it can be retrieved and returned in response to

another query for the same <QNAME, QTYPE, QCLASS> that resulted in

the cached negative response.

The NXT record, if it exists in the authority section of a negative

answer received, MUST be stored such that it can be be located and

returned with SOA record in the authority section, as should any SIG

records in the authority section. For NXDOMAIN answers there is no

"necessary" obvious relationship between the NXT records and the

QNAME. The NXT record MUST have the same owner name as the query

name for NODATA responses.

Negative responses without SOA records SHOULD NOT be cached as there

is no way to prevent the negative responses looping forever between a

pair of servers even with a short TTL.

Despite the DNS forming a tree of servers, with various mis-

configurations it is possible to form a loop in the query graph, e.g.

two servers listing each other as forwarders, various lame server

configurations. Without a TTL count down a cache negative response

when received by the next server would have its TTL reset. This

negative indication could then live forever circulating between the

servers involved.

As with caching positive responses it is sensible for a resolver to

limit for how long it will cache a negative response as the protocol

supports caching for up to 68 years. Such a limit should not be

greater than that applied to positive answers and preferably be

tunable. Values of one to three hours have been found to work well

and would make sensible a default. Values exceeding one day have

been found to be problematic.

6 - Negative answers from the cache

When a server, in answering a query, encounters a cached negative

response it MUST add the cached SOA record to the authority section

of the response with the TTL decremented by the amount of time it was

stored in the cache. This allows the NXDOMAIN / NODATA response to

time out correctly.

If a NXT record was cached along with SOA record it MUST be added to

the authority section. If a SIG record was cached along with a NXT

record it SHOULD be added to the authority section.

As with all answers coming from the cache, negative answers SHOULD

have an implicit referral built into the answer. This enables the

resolver to locate an authoritative source. An implicit referral is

characterised by NS records in the authority section referring the

resolver towards a authoritative source. NXDOMAIN types 1 and 4

responses contain implicit referrals as does NODATA type 1 response.

7 - Other Negative Responses

Caching of other negative responses is not covered by any existing

RFC. There is no way to indicate a desired TTL in these responses.

Care needs to be taken to ensure that there are not forwarding loops.

7.1 Server Failure (OPTIONAL)

Server failures fall into two major classes. The first is where a

server can determine that it has been misconfigured for a zone. This

may be where it has been listed as a server, but not configured to be

a server for the zone, or where it has been configured to be a server

for the zone, but cannot obtain the zone data for some reason. This

can occur either because the zone file does not exist or contains

errors, or because another server from which the zone should have

been available either did not respond or was unable or unwilling to

supply the zone.

The second class is where the server needs to obtain an answer from

elsewhere, but is unable to do so, due to network failures, other

servers that don't reply, or return server failure errors, or

similar.

In either case a resolver MAY cache a server failure response. If it

does so it MUST NOT cache it for longer than five (5) minutes, and it

MUST be cached against the specific query tuple <query name, type,

class, server IP address>.

7.2 Dead / Unreachable Server (OPTIONAL)

Dead / Unreachable servers are servers that fail to respond in any

way to a query or where the transport layer has provided an

indication that the server does not exist or is unreachable. A

server may be deemed to be dead or unreachable if it has not

responded to an outstanding query within 120 seconds.

Examples of transport layer indications are:

ICMP error messages indicating host, net or port unreachable.

TCP resets

IP stack error messages providing similar indications to those above.

A server MAY cache a dead server indication. If it does so it MUST

NOT be deemed dead for longer than five (5) minutes. The indication

MUST be stored against query tuple <query name, type, class, server

IP address> unless there was a transport layer indication that the

server does not exist, in which case it applies to all queries to

that specific IP address.

8 - Changes from RFC1034

Negative caching in resolvers is no-longer optional, if a resolver

caches anything it must also cache negative answers.

Non-authoritative negative answers MAY be cached.

The SOA record from the authority section MUST be cached. Name error

indications must be cached against the tuple <query name, QCLASS>.

No data indications must be cached against <query name, QTYPE,

QCLASS> tuple.

A cached SOA record must be added to the response. This was

explicitly not allowed because previously the distinction between a

normal cached SOA record, and the SOA cached as a result of a

negative response was not made, and simply extracting a normal cached

SOA and adding that to a cached negative response causes problems.

The $TTL TTL directive was added to the master file format.

9 - History of Negative Caching

This section presents a potted history of negative caching in the DNS

and forms no part of the technical specification of negative caching.

It is interesting to note that the same concepts were re-invented in

both the CHIVES and BIND servers.

The history of the early CHIVES work (Section 9.1) was supplied by

Rob Austein <sra@epilogue.com> and is reproduced here in the form in

which he supplied it [MPA].

Sometime around the spring of 1985, I mentioned to Paul Mockapetris

that our experience with his JEEVES DNS resolver had pointed out the

need for some kind of negative caching scheme. Paul suggested that

we simply cache authoritative errors, using the SOA MINIMUM value for

the zone that would have contained the target RRs. I'm pretty sure

that this conversation took place before RFC-973 was written, but it

was never clear to me whether this idea was something that Paul came

up with on the spot in response to my question or something he'd

already been planning to put into the document that became RFC-973.

In any case, neither of us was entirely sure that the SOA MINIMUM

value was really the right metric to use, but it was available and

was under the control of the administrator of the target zone, both

of which seemed to us at the time to be important feature.

Late in 1987, I released the initial beta-test version of CHIVES, the

DNS resolver I'd written to replace Paul's JEEVES resolver. CHIVES

included a search path mechanism that was used pretty heavily at

several sites (including my own), so CHIVES also included a negative

caching mechanism based on SOA MINIMUM values. The basic strategy

was to cache authoritative error codes keyed by the exact query

parameters (QNAME, QCLASS, and QTYPE), with a cache TTL equal to the

SOA MINIMUM value. CHIVES did not attempt to track down SOA RRs if

they weren't supplied in the authoritative response, so it never

managed to completely eliminate the gratuitous DNS error message

traffic, but it did help considerably. Keep in mind that this was

happening at about the same time as the near-collapse of the ARPANET

due to congestion caused by exponential growth and the the "old"

(pre-VJ) TCP retransmission algorithm, so negative caching resulted

in drasticly better DNS response time for our users, mailer daemons,

etcetera.

As far as I know, CHIVES was the first resolver to implement negative

caching. CHIVES was developed during the twilight years of TOPS-20,

so it never ran on very many machines, but the few machines that it

did run on were the ones that were too critical to shut down quickly

no matter how much it cost to keep them running. So what few users

we did have tended to drive CHIVES pretty hard. Several interesting

bits of DNS technology resulted from that, but the one that's

relevant here is the MAXTTL configuration parameter.

Experience with JEEVES had already shown that RRs often showed up

with ridiculously long TTLs (99999999 was particularly popular for

many years, due to bugs in the code and documentation of several

early versions of BIND), and that robust software that blindly

believed such TTLs could create so many strange failures that it was

often necessary to reboot the resolver frequently just to clear this

garbage out of the cache. So CHIVES had a configuration parameter

"MAXTTL", which specified the maximum "reasonable" TTL in a received

RR. RRs with TTLs greater than MAXTTL would either have their TTLs

reduced to MAXTTL or would be discarded entirely, depending on the

setting of another configuration parameter.

When we started getting field experience with CHIVES's negative

caching code, it became clear that the SOA MINIMUM value was often

large enough to cause the same kinds of problems for negative caching

as the huge TTLs in RRs had for normal caching (again, this was in

part due to a bug in several early versions of BIND, where a

secondary server would authoritatively deny all knowledge of its

zones if it couldn't contact the primaries on reboot). So we started

running the negative cache TTLs through the MAXTTL check too, and

continued to experiment.

The configuration that seemed to work best on WSMR-SIMTEL20.ARMY.MIL

(last of the major Internet TOPS-20 machines to be shut down, thus

the last major user of CHIVES, thus the place where we had the

longest experimental baseline) was to set MAXTTL to about three days.

Most of the traffic initiated by SIMTEL20 in its last years was

mail-related, and the mail queue timeout was set to one week, so this

gave a "stuck" message several tries at complete DNS resolution,

without bogging down the system with a lot of useless queries. Since

(for reasons that now escape me) we only had the single MAXTTL

parameter rather than separate ones for positive and negative

caching, it's not clear how much effect this setting of MAXTTL had on

the negative caching code.

CHIVES also included a second, somewhat controversial mechanism which

took the place of negative caching in some cases. The CHIVES

resolver daemon could be configured to load DNS master files, giving

it the ability to act as what today would be called a "stealth

secondary". That is, when configured in this way, the resolver had

direct access to authoritative information for heavily-used zones.

The search path mechanisms in CHIVES reflected this: there were

actually two separate search paths, one of which only searched local

authoritative zone data, and one which could generate normal

iterative queries. This cut down on the need for negative caching in

cases where usage was predictably heavy (e.g., the resolver on

XX.LCS.MIT.EDU always loaded the zone files for both LCS.MIT.EDU and

AI.MIT.EDU and put both of these suffixes into the "local" search

path, since between them the hosts in these two zones accounted for

the bulk of the DNS traffic). Not all sites running CHIVES chose to

use this feature; C.CS.CMU.EDU, for example, chose to use the

"remote" search path for everything because there were too many

different sub-zones at CMU for zone shadowing to be practical for

them, so they relied pretty heavily on negative caching even for

local traffic.

Overall, I still think the basic design we used for negative caching

was pretty reasonable: the zone administrator specified how long to

cache negative answers, and the resolver configuration chose the

actual cache time from the range between zero and the period

specified by the zone administrator. There are a lot of details I'd

do differently now (like using a new SOA field instead of overloading

the MINIMUM field), but after more than a decade, I'd be more worried

if we couldn't think of at least a few improvements.

9.2 BIND

While not the first attempt to get negative caching into BIND, in

July 1993, BIND 4.9.2 ALPHA, Anant Kumar of ISI supplied code that

implemented, validation and negative caching (NCACHE). This code had

a 10 minute TTL for negative caching and only cached the indication

that there was a negative response, NXDOMAIN or NOERROR_NODATA. This

is the origin of the NODATA pseudo response code mentioned above.

Mark Andrews of CSIRO added code (RETURNSOA) that stored the SOA

record such that it could be retrieved by a similar query. UUnet

complained that they were getting old answers after loading a new

zone, and the option was turned off, BIND 4.9.3-alpha5, April 1994.

In reality this indicated that the named needed to purge the space

the zone would occupy. Functionality to do this was added in BIND

4.9.3 BETA11 patch2, December 1994.

RETURNSOA was re-enabled by default, BIND 4.9.5-T1A, August 1996.

10 Example

The following example is based on a signed zone that is empty apart

from the nameservers. We will query for WWW.XX.EXAMPLE showing

initial response and again 10 minutes later. Note 1: during the

intervening 10 minutes the NS records for XX.EXAMPLE have expired.

Note 2: the TTL of the SIG records are not explicitly set in the zone

file and are hence the TTL of the RRset they are the signature for.

Zone File:

$TTL 86400

$ORIGIN XX.EXAMPLE.

@ IN SOA NS1.XX.EXAMPLE. HOSTMATER.XX.EXAMPLE. (

1997102000 ; serial

1800 ; refresh (30 mins)

900 ; retry (15 mins)

604800 ; expire (7 days)

1200 ) ; minimum (20 mins)

IN SIG SOA ...

1200 IN NXT NS1.XX.EXAMPLE. A NXT SIG SOA NS KEY

IN SIG NXT ... XX.EXAMPLE. ...

300 IN NS NS1.XX.EXAMPLE.

300 IN NS NS2.XX.EXAMPLE.

IN SIG NS ... XX.EXAMPLE. ...

IN KEY 0x4100 1 1 ...

IN SIG KEY ... XX.EXAMPLE. ...

IN SIG KEY ... EXAMPLE. ...

NS1 IN A 10.0.0.1

IN SIG A ... XX.EXAMPLE. ...

1200 IN NXT NS2.XX.EXAMPLE. A NXT SIG

IN SIG NXT ...

NS2 IN A 10.0.0.2

IN SIG A ... XX.EXAMPLE. ...

1200 IN NXT XX.EXAMPLE. A NXT SIG

IN SIG NXT ... XX.EXAMPLE. ...

Initial Response:

Header:

RDCODE=NXDOMAIN, AA=1, QR=1, TC=0

Query:

WWW.XX.EXAMPLE. IN A

Answer:

<empty>

Authority:

XX.EXAMPLE. 1200 IN SOA NS1.XX.EXAMPLE. ...

XX.EXAMPLE. 1200 IN SIG SOA ... XX.EXAMPLE. ...

NS2.XX.EXAMPLE. 1200 IN NXT XX.EXAMPLE. NXT A NXT SIG

NS2.XX.EXAMPLE. 1200 IN SIG NXT ... XX.EXAMPLE. ...

XX.EXAMPLE. 86400 IN NS NS1.XX.EXAMPLE.

XX.EXAMPLE. 86400 IN NS NS2.XX.EXAMPLE.

XX.EXAMPLE. 86400 IN SIG NS ... XX.EXAMPLE. ...

Additional

XX.EXAMPLE. 86400 IN KEY 0x4100 1 1 ...

XX.EXAMPLE. 86400 IN SIG KEY ... EXAMPLE. ...

NS1.XX.EXAMPLE. 86400 IN A 10.0.0.1

NS1.XX.EXAMPLE. 86400 IN SIG A ... XX.EXAMPLE. ...

NS2.XX.EXAMPLE. 86400 IN A 10.0.0.2

NS3.XX.EXAMPLE. 86400 IN SIG A ... XX.EXAMPLE. ...

After 10 Minutes:

Header:

RDCODE=NXDOMAIN, AA=0, QR=1, TC=0

Query:

WWW.XX.EXAMPLE. IN A

Answer:

<empty>

Authority:

XX.EXAMPLE. 600 IN SOA NS1.XX.EXAMPLE. ...

XX.EXAMPLE. 600 IN SIG SOA ... XX.EXAMPLE. ...

NS2.XX.EXAMPLE. 600 IN NXT XX.EXAMPLE. NXT A NXT SIG

NS2.XX.EXAMPLE. 600 IN SIG NXT ... XX.EXAMPLE. ...

EXAMPLE. 65799 IN NS NS1.YY.EXAMPLE.

EXAMPLE. 65799 IN NS NS2.YY.EXAMPLE.

EXAMPLE. 65799 IN SIG NS ... XX.EXAMPLE. ...

Additional

XX.EXAMPLE. 65800 IN KEY 0x4100 1 1 ...

XX.EXAMPLE. 65800 IN SIG KEY ... EXAMPLE. ...

NS1.YY.EXAMPLE. 65799 IN A 10.100.0.1

NS1.YY.EXAMPLE. 65799 IN SIG A ... EXAMPLE. ...

NS2.YY.EXAMPLE. 65799 IN A 10.100.0.2

NS3.YY.EXAMPLE. 65799 IN SIG A ... EXAMPLE. ...

EXAMPLE. 65799 IN KEY 0x4100 1 1 ...

EXAMPLE. 65799 IN SIG KEY ... . ...

11 Security Considerations

It is believed that this document does not introduce any significant

additional security threats other that those that already exist when

using data from the DNS.

With negative caching it might be possible to propagate a denial of

service attack by spreading a NXDOMAIN message with a very high TTL.

Without negative caching that would be much harder. A similar effect

could be achieved previously by spreading a bad A record, so that the

server could not be reached - which is almost the same. It has the

same effect as far as what the end user is able to do, but with a

different psychological effect. With the bad A, I feel "damn the

network is broken again" and try again tomorrow. With the "NXDOMAIN"

I feel "Oh, they've turned off the server and it doesn't exist any

more" and probably never bother trying this server again.

A practical example of this is a SMTP server where this behaviour is

encoded. With a NXDOMAIN attack the mail message would bounce

immediately, where as with a bad A attack the mail would be queued

and could potentially get through after the attack was suspended.

For such an attack to be successful, the NXDOMAIN indiction must be

injected into a parent server (or a busy caching resolver). One way

this might be done by the use of a CNAME which results in the parent

server querying an attackers server. Resolvers that wish to prevent

such attacks can query again the final QNAME ignoring any NS data in

the query responses it has received for this query.

Implementing TTL sanity checking will reduce the effectiveness of

such an attack, because a successful attack would require re-

injection of the bogus data at more frequent intervals.

DNS Security [RFC2065] provides a mechanism to verify whether a

negative response is valid or not, through the use of NXT and SIG

records. This document supports the use of that mechanism by

promoting the transmission of the relevant security records even in a

non security aware server.

Acknowledgments

I would like to thank Rob Austein for his history of the CHIVES

nameserver. The DNSIND working group, in particular Robert Elz for

his valuable technical and editorial contributions to this document.

References

[RFC1034]

Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES,"

STD 13, RFC1034, November 1987.

[RFC1035]

Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND

SPECIFICATION," STD 13, RFC1035, November 1987.

[RFC2065]

Eastlake, D., and C. Kaufman, "Domain Name System Security

Extensions," RFC2065, January 1997.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels," BCP 14, RFC2119, March 1997.

[RFC2181]

Elz, R., and R. Bush, "Clarifications to the DNS

Specification," RFC2181, July 1997.

Author's Address

Mark Andrews

CSIRO - Mathematical and Information Sciences

Locked Bag 17

North Ryde NSW 2113

AUSTRALIA

Phone: +61 2 9325 3148

EMail: Mark.Andrews@cmis.csiro.au

Full Copyright Statement

Copyright (C) The Internet Society (1998). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS 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.

 
 
 
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