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Network Working Group R. Elz Request for Comments: 2182 University of Melbourne BCP: 16 R. Bush Category: Best Current Practice RGnet, Inc.

                                                            S. Bradner
                                                    Harvard University
                                                             M. Patton
                                                             July 1997
          Selection and Operation of Secondary DNS Servers

Status of this Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.


 The Domain Name System requires that multiple servers exist for every
 delegated domain (zone).  This document discusses the selection of
 secondary servers for DNS zones.  Both the physical and topological
 location of each server are material considerations when selecting
 secondary servers.  The number of servers appropriate for a zone is
 also discussed, and some general secondary server maintenance issues

Elz, et al. Best Current Practice [Page 1] RFC 2182 Selection and Operation of Secondary DNS Servers July 1997


     Abstract  ...................................................   1
  1  Introduction  ...............................................   2
  2  Definitions  ................................................   2
  3  Secondary Servers  ..........................................   3
  4  Unreachable servers  ........................................   5
  5  How many secondaries?  ......................................   7
  6  Finding Suitable Secondary Servers  .........................   8
  7  Serial Number Maintenance  ..................................   9
     Security Considerations  ....................................  11
     References  .................................................  11
     Acknowledgements  ...........................................  11
     Authors' Addresses  .........................................  11

1. Introduction

 A number of problems in DNS operations today are attributable to poor
 choices of secondary servers for DNS zones.  The geographic placement
 as well as the diversity of network connectivity exhibited by the set
 of DNS servers for a zone can increase the reliability of that zone
 as well as improve overall network performance and access
 characteristics.  Other considerations in server choice can
 unexpectedly lower reliability or impose extra demands on the
 This document discusses many of the issues that should be considered
 when selecting secondary servers for a zone.  It offers guidance in
 how to best choose servers to serve a given zone.

2. Definitions

 For the purposes of this document, and only this document, the
 following definitions apply:
 DNS                    The Domain Name System [RFC1034, RFC1035].
 Zone                   A part of the DNS tree, that is treated as a
 Forward Zone           A zone containing data mapping names to host
                        addresses, mail exchange targets, etc.

Elz, et al. Best Current Practice [Page 2] RFC 2182 Selection and Operation of Secondary DNS Servers July 1997

 Reverse Zone           A zone containing data used to map addresses
                        to names.
 Server                 An implementation of the DNS protocols able to
                        provide answers to queries.  Answers may be
                        from information known by the server, or
                        information obtained from another server.
 Authoritative Server   A server that knows the content of a DNS zone
                        from local knowledge, and thus can answer
                        queries about that zone without needing to
                        query other servers.
 Listed Server          An Authoritative Server for which there is an
                        "NS" resource record (RR) in the zone.
 Primary Server         An authoritative server for which the zone
                        information is locally configured.  Sometimes
                        known as a Master server.
 Secondary Server       An authoritative server that obtains
                        information about a zone from a Primary Server
                        via a zone transfer mechanism.  Sometimes
                        known as a Slave Server.
 Stealth Server         An authoritative server, usually secondary,
                        which is not a Listed Server.
 Resolver               A client of the DNS which seeks information
                        contained in a zone using the DNS protocols.

3. Secondary Servers

 A major reason for having multiple servers for each zone is to allow
 information from the zone to be available widely and reliably to
 clients throughout the Internet, that is, throughout the world, even
 when one server is unavailable or unreachable.
 Multiple servers also spread the name resolution load, and improve
 the overall efficiency of the system by placing servers nearer to the
 resolvers.  Those purposes are not treated further here.
 With multiple servers, usually one server will be the primary server,
 and others will be secondary servers.  Note that while some unusual
 configurations use multiple primary servers, that can result in data
 inconsistencies, and is not advisable.

Elz, et al. Best Current Practice [Page 3] RFC 2182 Selection and Operation of Secondary DNS Servers July 1997

 The distinction between primary and secondary servers is relevant
 only to the servers for the zone concerned, to the rest of the DNS
 there are simply multiple servers.  All are treated equally at first
 instance, even by the parent server that delegates the zone.
 Resolvers often measure the performance of the various servers,
 choose the "best", for some definition of best, and prefer that one
 for most queries.  That is automatic, and not considered here.
 The primary server holds the master copy of the zone file.  That is,
 the server where the data is entered into the DNS from some source
 outside the DNS.  Secondary servers obtain data for the zone using
 DNS protocol mechanisms to obtain the zone from the primary server.

3.1. Selecting Secondary Servers

 When selecting secondary servers, attention should be given to the
 various likely failure modes.  Servers should be placed so that it is
 likely that at least one server will be available to all significant
 parts of the Internet, for any likely failure.
 Consequently, placing all servers at the local site, while easy to
 arrange, and easy to manage, is not a good policy.  Should a single
 link fail, or there be a site, or perhaps even building, or room,
 power failure, such a configuration can lead to all servers being
 disconnected from the Internet.
 Secondary servers must be placed at both topologically and
 geographically dispersed locations on the Internet, to minimise the
 likelihood of a single failure disabling all of them.
 That is, secondary servers should be at geographically distant
 locations, so it is unlikely that events like power loss, etc, will
 disrupt all of them simultaneously.  They should also be connected to
 the net via quite diverse paths.  This means that the failure of any
 one link, or of routing within some segment of the network (such as a
 service provider) will not make all of the servers unreachable.

3.2. Unsuitable Configurations

 While it is unfortunately quite common, servers for a zone should
 certainly not all be placed on the same LAN segment in the same room
 of the same building - or any of those.  Such a configuration almost
 defeats the requirement, and utility, of having multiple servers.
 The only redundancy usually provided in that configuration is for the
 case when one server is down, whereas there are many other possible
 failure modes, such as power failures, including lengthy ones, to

Elz, et al. Best Current Practice [Page 4] RFC 2182 Selection and Operation of Secondary DNS Servers July 1997

3.3. A Myth Exploded

 An argument is occasionally made that there is no need for the domain
 name servers for a domain to be accessible if the hosts in the domain
 are unreachable.  This argument is fallacious.
   + Clients react differently to inability to resolve than inability
     to connect, and reactions to the former are not always as
   + If the zone is resolvable yet the particular name is not, then a
     client can discard the transaction rather than retrying and
     creating undesirable load on the network.
   + While positive DNS results are usually cached, the lack of a
     result is not cached.  Thus, unnecessary inability to resolve
     creates an undesirable load on the net.
   + All names in the zone may not resolve to addresses within the
     detached network.  This becomes more likely over time.  Thus a
     basic assumption of the myth often becomes untrue.
 It is important that there be nameservers able to be queried,
 available always, for all forward zones.

4. Unreachable servers

 Another class of problems is caused by listing servers that cannot be
 reached from large parts of the network.  This could be listing the
 name of a machine that is completely isolated behind a firewall, or
 just a secondary address on a dual homed machine which is not
 accessible from outside.  The names of servers listed in NS records
 should resolve to addresses which are reachable from the region to
 which the NS records are being returned.  Including addresses which
 most of the network cannot reach does not add any reliability, and
 causes several problems, which may, in the end, lower the reliability
 of the zone.
 First, the only way the resolvers can determine that these addresses
 are, in fact, unreachable, is to try them.  They then need to wait on
 a lack of response timeout (or occasionally an ICMP error response)
 to know that the address cannot be used.  Further, even that is
 generally indistinguishable from a simple packet loss, so the
 sequence must be repeated, several times, to give any real evidence
 of an unreachable server.  All of this probing and timeout may take
 sufficiently long that the original client program or user will
 decide that no answer is available, leading to an apparent failure of
 the zone.  Additionally, the whole thing needs to be repeated from
 time to time to distinguish a permanently unreachable server from a
 temporarily unreachable one.

Elz, et al. Best Current Practice [Page 5] RFC 2182 Selection and Operation of Secondary DNS Servers July 1997

 And finally, all these steps will potentially need to be done by
 resolvers all over the network.  This will increase the traffic, and
 probably the load on the filters at whatever firewall is blocking
 this access.  All of this additional load does no more than
 effectively lower the reliability of the service.

4.1. Servers behind intermittent connections

 A similar problem occurs with DNS servers located in parts of the net
 that are often disconnected from the Internet as a whole.  For
 example, those which connect via an intermittent connection that is
 often down.  Such servers should usually be treated as if they were
 behind a firewall, and unreachable to the network at any time.

4.2. Other problem cases

 Similar problems occur when a Network Address Translator (NAT)
 [RFC1631] exists between a resolver and server.  Despite what
 [RFC1631] suggests, NATs in practice do not translate addresses
 embedded in packets, only those in the headers.  As [RFC1631]
 suggests, this is somewhat of a problem for the DNS.  This can
 sometimes be overcome if the NAT is accompanied by, or replaced with,
 an Application Layer Gateway (ALG).  Such a device would understand
 the DNS protocol and translate all the addresses as appropriate as
 packets pass through.  Even with such a device, it is likely to be
 better in any of these cases to adopt the solution described in the
 following section.

4.3. A Solution

 To avoid these problems, NS records for a zone returned in any
 response should list only servers that the resolver requesting the
 information, is likely to be able to reach.  Some resolvers are
 simultaneously servers performing lookups on behalf of other
 resolvers.  The NS records returned should be reachable not only by
 the resolver that requested the information, but any other resolver
 that may be forwarded the information.  All the addresses of all the
 servers returned must be reachable.  As the addresses of each server
 form a Resource Record Set [RFC2181], all must be returned (or none),
 thus it is not acceptable to elide addresses of servers that are
 unreachable, or to return them with a low TTL (while returning others
 with a higher TTL).
 In particular, when some servers are behind a firewall, intermittent
 connection, or NAT, which disallows, or has problems with, DNS
 queries or responses, their names, or addresses, should not be
 returned to clients outside the firewall.  Similarly, servers outside
 the firewall should not be made known to clients inside it, if the

Elz, et al. Best Current Practice [Page 6] RFC 2182 Selection and Operation of Secondary DNS Servers July 1997

 clients would be unable to query those servers.  Implementing this
 usually requires dual DNS setups, one for internal use, the other for
 external use.  Such a setup often solves other problems with
 environments like this.
 When a server is at a firewall boundary, reachable from both sides,
 but using different addresses, that server should be given two names,
 each name associated with appropriate A records, such that each
 appears to be reachable only on the appropriate side of the firewall.
 This should then be treated just like two servers, one on each side
 of the firewall.  A server implemented in an ALG will usually be such
 a case.  Special care will need to be taken to allow such a server to
 return the correct responses to clients on each side.  That is,
 return only information about hosts reachable from that side and the
 correct IP address(es) for the host when viewed from that side.
 Servers in this environment often need special provision to give them
 access to the root servers.  Often this is accomplished via "fake
 root" configurations.  In such a case the servers should be kept well
 isolated from the rest of the DNS, lest their unusual configuration
 pollute others.

5. How many secondaries?

 The DNS specification and domain name registration rules require at
 least two servers for every zone.  That is, usually, the primary and
 one secondary.  While two, carefully placed, are often sufficient,
 occasions where two are insufficient are frequent enough that we
 advise the use of more than two listed servers.  Various problems can
 cause a server to be unavailable for extended periods - during such a
 period, a zone with only two listed servers is actually running with
 just one.  Since any server may occasionally be unavailable, for all
 kinds of reasons, this zone is likely, at times, to have no
 functional servers at all.
 On the other hand, having large numbers of servers adds little
 benefit, while adding costs.  At the simplest, more servers cause
 packets to be larger, so requiring more bandwidth.  This may seem,
 and realistically is, trivial.  However there is a limit to the size
 of a DNS packet, and causing that limit to be reached has more
 serious performance implications.  It is wise to stay well clear of
 it.  More servers also increase the likelihood that one server will
 be misconfigured, or malfunction, without being detected.
 It is recommended that three servers be provided for most
 organisation level zones, with at least one which must be well
 removed from the others.  For zones where even higher reliability is
 required, four, or even five, servers may be desirable.  Two, or

Elz, et al. Best Current Practice [Page 7] RFC 2182 Selection and Operation of Secondary DNS Servers July 1997

 occasionally three of five, would be at the local site, with the
 others not geographically or topologically close to the site, or each
 Reverse zones, that is, sub-domains of .IN-ADDR.ARPA, tend to be less
 crucial, and less servers, less distributed, will often suffice.
 This is because address to name translations are typically needed
 only when packets are being received from the address in question,
 and only by resolvers at or near the destination of the packets.
 This gives some assurances that servers located at or near the packet
 source, for example, on the the same network, will be reachable from
 the resolvers that need to perform the lookups.  Thus some of the
 failure modes that need to be considered when planning servers for
 forward zones may be less relevant when reverse zones are being

5.1. Stealth Servers

 Servers which are authoritative for the zone, but not listed in NS
 records (also known as "stealth" servers) are not included in the
 count of servers.
 It can often be useful for all servers at a site to be authoritative
 (secondary), but only one or two be listed servers, the rest being
 unlisted servers for all local zones, that is, to be stealth servers.
 This allows those servers to provide answers to local queries
 directly, without needing to consult another server.  If it were
 necessary to consult another server, it would usually be necessary
 for the root servers to be consulted, in order to follow the
 delegation tree - that the zone is local would not be known.  This
 would mean that some local queries may not be able to be answered if
 external communications were disrupted.
 Listing all such servers in NS records, if more than one or two,
 would cause the rest of the Internet to spend unnecessary effort
 attempting to contact all servers at the site when the whole site is
 inaccessible due to link or routing failures.

6. Finding Suitable Secondary Servers

 Operating a secondary server is usually an almost automatic task.
 Once established, the server generally runs itself, based upon the
 actions of the primary server.  Because of this, large numbers of
 organisations are willing to provide a secondary server, if
 requested.  The best approach is usually to find an organisation of
 similar size, and agree to swap secondary zones - each organisation
 agrees to provide a server to act as a secondary server for the other

Elz, et al. Best Current Practice [Page 8] RFC 2182 Selection and Operation of Secondary DNS Servers July 1997

 organisation's zones.  Note that there is no loss of confidential
 data here, the data set exchanged would be available publically
 whatever the servers are.

7. Serial Number Maintenance

 Secondary servers use the serial number in the SOA record of the zone
 to determine when it is necessary to update their local copy of the
 zone.  Serial numbers are basically just 32 bit unsigned integers
 that wrap around from the biggest possible value to zero again.  See
 [RFC1982] for a more rigorous definition of the serial number.
 The serial number must be incremented every time a change, or group
 of changes, is made to the zone on the primary server.  This informs
 secondary servers they need update their copies of the zone.  Note
 that it is not possible to decrement a serial number, increments are
 the only defined modification.
 Occasionally due to editing errors, or other factors, it may be
 necessary to cause a serial number to become smaller.  Never simply
 decrease the serial number.  Secondary servers will ignore that
 change, and further, will ignore any later increments until the
 earlier large value is exceeded.
 Instead, given that serial numbers wrap from large to small, in
 absolute terms, increment the serial number, several times, until it
 has reached the value desired.  At each step, wait until all
 secondary servers have updated to the new value before proceeding.
 For example, assume that the serial number of a zone was 10, but has
 accidentally been set to 1000, and it is desired to set it back to
 11.  Do not simply change the value from 1000 to 11.  A secondary
 server that has seen the 1000 value (and in practice, there is always
 at least one) will ignore this change, and continue to use the
 version of the zone with serial number 1000, until the primary
 server's serial number exceeds that value.  This may be a long time -
 in fact, the secondary often expires its copy of the zone before the
 zone is ever updated again.
 Instead, for this example, set the primary's serial number to
 2000000000, and wait for the secondary servers to update to that
 zone.  The value 2000000000 is chosen as a value a lot bigger than
 the current value, but less that 2^31 bigger (2^31 is 2147483648).
 This is then an increment of the serial number [RFC1982].
 Next, after all servers needing updating have the zone with that
 serial number, the serial number can be set to 4000000000.
 4000000000 is 2000000000 more than 2000000000 (fairly clearly), and

Elz, et al. Best Current Practice [Page 9] RFC 2182 Selection and Operation of Secondary DNS Servers July 1997

 is thus another increment (the value added is less than 2^31).
 Once this copy of the zone file exists at all servers, the serial
 number can simply be set to 11.  In serial number arithmetic, a
 change from 4000000000 to 11 is an increment.  Serial numbers wrap at
 2^32 (4294967296), so 11 is identical to 4294967307 (4294967296 +
  11).  4294967307 is just 294967307 greater than 4000000000, and
 294967307 is well under 2^31, this is therefore an increment.
 When following this procedure, it is essential to verify that all
 relevant servers have been updated at each step, never assume
 anything.  Failing to do this can result in a worse mess than existed
 before the attempted correction.  Also beware that it is the
 relationship between the values of the various serial numbers that is
 important, not the absolute values.  The values used above are
 correct for that one example only.
 It is possible in essentially all cases to correct the serial number
 in two steps by being more aggressive in the choices of the serial
 numbers.  This however causes the numbers used to be less "nice", and
 requires considerably more care.
 Also, note that not all nameserver implementations correctly
 implement serial number operations.  With such servers as secondaries
 there is typically no way to cause the serial number to become
 smaller, other than contacting the administrator of the server and
 requesting that all existing data for the zone be purged.  Then that
 the secondary be loaded again from the primary, as if for the first
 It remains safe to carry out the above procedure, as the
 malfunctioning servers will need manual attention in any case.  After
 the sequence of serial number changes described above, conforming
 secondary servers will have been reset.  Then when the primary server
 has the correct (desired) serial number, contact the remaining
 secondary servers and request their understanding of the correct
 serial number be manually corrected.  Perhaps also suggest that they
 upgrade their software to a standards conforming implementation.
 A server which does not implement this algorithm is defective, and
 may be detected as follows.  At some stage, usually when the absolute
 integral value of the serial number becomes smaller, a server with
 this particular defect will ignore the change.  Servers with this
 type of defect can be detected by waiting for at least the time
 specified in the SOA refresh field and then sending a query for the
 SOA.  Servers with this defect will still have the old serial number.
 We are not aware of other means to detect this defect.

Elz, et al. Best Current Practice [Page 10] RFC 2182 Selection and Operation of Secondary DNS Servers July 1997

Security Considerations

 It is not believed that anything in this document adds to any
 security issues that may exist with the DNS, nor does it do anything
 to lessen them.
 Administrators should be aware, however, that compromise of a server
 for a domain can, in some situations, compromise the security of
 hosts in the domain.  Care should be taken in choosing secondary
 servers so that this threat is minimised.


 [RFC1034]   Mockapetris, P., "Domain Names - Concepts and Facilities",
             STD 13, RFC 1034, November 1987.
 [RFC1035]   Mockapetris, P., "Domain Names - Implementation and
             Specification", STD 13, RFC 1035, November 1987
 [RFC1631]   Egevang, K., Francis, P., "The IP Network Address Translator
             (NAT)", RFC 1631, May 1994
 [RFC1982]   Elz, R., Bush, R., "Serial Number Arithmetic",
             RFC 1982, August 1996.
 [RFC2181]   Elz, R., Bush, R., "Clarifications to the DNS specification",
             RFC 2181, July 1997.


 Brian Carpenter and Yakov Rekhter suggested mentioning NATs and ALGs
 as a companion to the firewall text.  Dave Crocker suggested
 explicitly exploding the myth.

Authors' Addresses

 Robert Elz
 Computer Science
 University of Melbourne
 Parkville, Vic,  3052
 EMail: kre@munnari.OZ.AU

Elz, et al. Best Current Practice [Page 11] RFC 2182 Selection and Operation of Secondary DNS Servers July 1997

 Randy Bush
 RGnet, Inc.
 5147 Crystal Springs Drive NE
 Bainbridge Island, Washington,  98110
 United States.
 Scott Bradner
 Harvard University
 1350 Mass Ave
 Cambridge, MA,  02138
 United States.
 Michael A. Patton
 33 Blanchard Road
 Cambridge, MA,  02138
 United States.

Elz, et al. Best Current Practice [Page 12]

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