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Versions: 00 01 02 03 04 RFC 2694

NAT Working Group                      P. Srisuresh, Lucent Technologies
INTERNET-DRAFT                             G. Tsirtsis,  BT Laboratories
Category: Informational                      P. Akkiraju,  Cisco Systems
Expire in six months                      A. Heffernan, Juniper Networks
                                                               June 1999

        DNS extensions to Network Address Translators (DNS_ALG)
                   <draft-ietf-nat-dns-alg-04.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC2026.  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.

Abstract

   Domain Name Service(DNS) provides name to address mapping within
   a routing class (ex: IP). Network Address Translators (NATs)
   attempt to provide transparent routing between hosts in disparate
   address realms of the same routing class. Typically, NATs exist at
   the border of a stub domain, hiding private addresses from external
   addresses. This document identifies the need for DNS extensions
   to NATs and outlines how a DNS Application Level Gateway (DNS_ALG)
   can meet the need. DNS_ALG modifies payload transparently to alter
   address mapping of hosts as DNS packets cross one address realm
   into another. The document also illustrates the operation of
   DNS_ALG with specific examples.

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1. Introduction

   Network Address Translators (NATs) are often used when network's
   internal IP addresses cannot be used outside the network either
   for privacy reasons or because they are invalid for use outside
   the network.

   Ideally speaking, a host name uniquely identifies a host and its
   address is used to locate routes to the host. However, host name
   and address are often not distinguished and used interchangeably
   by applications. Applications embed IP address instead of host
   name in payload. Examples would be e-mails that specify their MX
   server address (ex: user@666.42.7.11) instead of server name
   (ex: user@private.com) as sender ID; HTML files that include IP
   address instead of names in URLs, etc. Use of IP address in
   place of host name in payload represents a problem as the packet
   traverses a NAT device because NATs alter network and transport
   headers to suit an address realm, but not payload.

   DNS provides Name to address mapping. Whereas, NAT performs
   address translation (in network and transport headers) in
   datagrams traversing between private and external address realms.
   DNS Application Level Gateway (DNS_ALG) outlined in this document
   helps translate Name-to-Private-Address mapping in DNS payloads
   into Name-to-external-address mapping and vice versa using state
   information available on NAT.

   A Network Address Port Translator (NAPT) performs address and
   Transport level port translations (i.e, TCP, UDP ports and ICMP
   query IDs). DNS name mapping granularity, however, is limited to
   IP addresses and does not extend to transport level identifiers.
   As a result, the DNS_ALG processing for an NAPT configuration is
   simplified in that all host addresses in private network are
   bound to a single external address. The DNS name lookup for
   private hosts (from external hosts) do not mandate fresh
   private-external address binding, as all private hosts are bound
   to a single pre-defined external address. However, reverse name
   lookups for the NAPT external address will not map to any of
   the private hosts and will simply map to the NAPT router.
   Suffices to say, the processing requirements for a DNS_ALG
   supporting NAPT configuration are a mere subset of Basic NAT.
   Hence, the discussion in the remainder of the document will focus
   mainly on Basic NAT, Bi-directional NAT and Twice NAT
   configurations, with no specific reference to NAPT setup.

   Definitions for DNS and related terms may be found in [Ref 3] and
   [Ref 4]. Definitions for NAT related terms may be found in [Ref 1].

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2. Requirement for DNS extensions.

   There are many ways to ensure that a host name is mapped to an
   address relevant within an address realm. In the following
   sections, we will identify where DNS extensions would be needed.

   Typically, organizations have two types of authoritative name
   servers. Internal authoritative name servers identify all (or
   majority of) corporate resources within the organization. Only a
   portion of these hosts are allowed to be accessed by the external
   world. The remaining hosts and their names are unique to the
   private network. Hosts visible to the external world and the
   authoritative name server that maps their names to network
   addresses are often configured within a DMZ (De-Militarized Zone)
   in front of a firewall. We will refer the hosts and name servers
   within DMZ as DMZ hosts and DMZ name servers respectively. DMZ
   host names are end-to-end unique in that their FQDNs do not
   overlap with any end node that communicates with it .

                                   \ | /
                           +-----------------------+
                           |Service Provider Router|
                           +-----------------------+
                            WAN  |
               Stub A .........|\|....
                               |
                     +-----------------+
                     |Stub Router w/NAT|
                     +-----------------+
                         |
                         |   DMZ - Network
   ------------------------------------------------------------
      |         |              |            |             |
     +--+      +--+           +--+         +--+      +----------+
     |__|      |__|           |__|         |__|      | Firewall |
    /____\    /____\         /____\       /____\     +----------+
   DMZ-Host1  DMZ-Host2 ...  DMZ-Name     DMZ-Web       |
                             Server       Server etc.   |
                                                        |
     Internal hosts (Private IP network)                |
   ------------------------------------------------------------
       |             |                 |           |
      +--+         +--+               +--+       +--+
      |__|         |__|               |__|       |__|
     /____\       /____\             /____\     /____\
    Int-Host1    Int-Host2  .....   Int-Hostn   Int-Name Server

    Figure 1: DMZ network configuration of a private Network.

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   Figure 1 above illustrates configuration of a private network which
   includes a DMZ. Actual configurations may vary. Internal name servers
   are accessed by users within the private network only. Internal DNS
   queries and responses do not cross the private network boundary. DMZ
   name servers and DMZ hosts on the other hand are end-to-end unique
   and could be accessed by external as well as internal hosts.
   Throughout this document, our focus will be limited to DMZ hosts and
   DMZ name servers and will not include internal hosts and internal
   name servers, unless they happen to be same.

2.1. DMZ hosts assigned static external addresses on NAT

   Take the case where DMZ hosts are assigned static external
   addresses on the NAT device. Note, all hosts within private domain,
   including the DMZ hosts are identified by their private addresses.
   Static mapping on the NAT device allows the DMZ hosts to be
   identified by their public addresses in the external domain.

2.1.1. Private networks with no DMZ name servers

   Take the case where a private network has no DMZ name server
   for itself. If the private network is connected to a single service
   provider for external connectivity, the DMZ hosts may be listed
   by their external addresses in the authoritative name servers of
   the service provider within their forward and in-add.arpa reverse
   zones.

   If the network is connected to multiple service providers, the
   DMZ host names may be listed by their external address(es) within
   the authoritative name servers of each of the service providers.
   This is particularly significant in the case of in-addr.arpa reverse
   zones, as  the private network may be assigned different address
   prefixes by the service providers.

   In both cases, externally generated DNS lookups will not reach the
   private network.  A large number of NAT based private domains
   pursue this option to have their DMZ hosts listed by their
   external addresses on service provider's name servers.

2.1.2. Private networks with DMZ name servers

   Take the case where a private network opts to keep an authoritative
   DMZ name server for the zone within the network itself. If the
   network is connected to a single service provider, the DMZ name
   server may be configured to obviate DNS payload interceptions as
   follows. The hosts in DMZ name server must be mapped to their

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   statically assigned external addresses and the internal name server
   must be configured to bypass the DMZ name server for queries
   concerning external hosts. This scheme ensures that DMZ name
   servers are set for exclusive access to external hosts alone (not
   even to the DMZ hosts) and hence can be configured with external
   addresses only.

   The above scheme requires careful administrative planning to ensure
   that DMZ name servers are not contacted by the private hosts
   directly or indirectly (through the internal name servers). Using
   DNS-ALG would obviate the administrative ordeals with this approach.

2.2. DMZ hosts assigned external addresses dynamically on NAT

   Take the case where DMZ hosts in a private network are assigned
   external addresses dynamically by NAT. While the addresses issued
   to these hosts are fixed within the private network, their
   externally known addresses are ephemeral, as determined by NAT.
   In such a scenario, it is mandatory for the private organization
   to have a DMZ name server in order to allow access to DMZ hosts
   by their name.

   The DMZ name server would be configured with private addresses
   for DMZ hosts. DNS Application Level Gateway (DNS_ALG) residing
   on NAT device will intercept the DNS packets directed to or from
   the DMZ name server(s) and perform transparent payload translations
   so that a DMZ host name has the right address mapping within
   each address realm (i.e., private or external).

3. Interactions between NAT and DNS_ALG

   This document operates on the paradigm that interconnecting address
   realms may have overlapping address space. But, names of hosts
   within interconnected realms must be end-to-end unique in order for
   them to be accessed by all hosts. In other words, there cannot be
   an overlap of FQDNs between end nodes communicating with each other.
   The following diagram illustrates how a DNS packet traversing a NAT
   device (with DNS_ALG) is subject to header and payload translations.
   A DNS packet can be a TCP or UDP packet with the source or
   destination port set to 53. NAT would translate the IP and TCP/UDP
   headers of the DNS packet and notify DNS-ALG to perform DNS payload
   changes. DNS-ALG would interact with NAT and use NAT state
   information to modify payload, as necessary.

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                Original-IP
                 packet
                   ||
                   ||
                   vv
   +---------------------------------+    +-----------------------+
   |                                 |    |DNS Appl. Level Gateway|
   |Network Address Translation (NAT)|--->|     (DNS_ALG)         |
   |  -IP & Transport header mods    |<---|  -DNS payload mods    |
   |                                 |    |                       |
   +---------------------------------+    +-----------------------+
                   ||
                   ||
                   vv
              Translated-IP
                 packet

    Figure 2: NAT & DNS-ALG in the translation path of DNS packets


3.1. Address Binding considerations

   We will make a distinction between "Temporary Address Binding" and
   "Committed Address Binding" in NATs. This distinction becomes
   necessary because the DNS_ALG will allow external users to create
   state on NAT, and thus the potential for denial-of-service attacks.
   Temporary address binding is the phase in which an address binding
   is reserved without any NAT sessions using the binding. Committed
   address binding is the phase in which there exists at least one
   NAT session using the binding between the external and private
   addresses. Both types of bindings are used by DNS_ALG to modify
   DNS payloads. NAT uses only the committed address bindings to
   modify the IP and Transport headers of datagrams pertaining to
   NAT sessions.

   For statically mapped addresses, the above distinction is not
   relevant. For dynamically mapped addresses, temporary address
   binding often precedes committed binding. Temporary binding occurs
   when DMZ name server is queried for a name lookup. Name query is
   likely a pre-cursor to a real session between query originator
   and the queried host. The temporary binding becomes committed only
   when NAT sees the first packet of a session between query initiator
   and queried host.

   A configurable parameter, "Bind-holdout time" may be defined for
   dynamic address assignments as the maximum period of time for which
   a temporary address binding is held active without transitioning
   into a committed binding. With each use of temporary binding by

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   DNS_ALG (to modify DNS payload), this Bind-holdout period is
   renewed. A default Bind-holdout time of a couple of minutes might
   suffice for most DNS-ALG implementations. Note, it is possible for a
   committed address binding to occur without ever having to be
   preceded by a temporary binding. Lastly, when NAT is ready to unbind
   a committed address binding, the binding is transitioned into a
   temporary binding and kept in that phase for an additional
   Bind-holdout period. The binding is freed only upon expiry of
   Bind-holdout time. The Bind-holdout time preceding the
   committed-address-binding and the address-unbinding are required to
   ensure that end hosts have sufficient time in which to initiate a
   data session subsequent to a name lookup.

   For example, say a private network with address prefix 10/8 is
   mapped to 198.76.29/24. When an external hosts makes a DNS query
   to host7, bearing address 10.0.0.7, the DMZ name server within
   private network responds with an A type RR for host7 as:

       host7  A  10.0.0.7

   DNS_ALG would intercept the response packet and if 10.0.0.7 is not
   assigned an external address already, it would request NAT to create
   a temporary address binding with an external address and start
   Bind-holdout timer to age the binding. Say, the assigned external
   address is 198.76.29.1. DNS-ALG would use this temporary binding to
   modify the RR in DNS response, replacing 10.0.0.7 with its external
   address and reply with:

       host7  A  198.76.29.1

   When query initiator receives DNS response, only the assigned
   external address is seen. Within a short period (presumably before
   the bind-holdout time expires), the query initiator would
   initiate a session with host7. When NAT notices the start of new
   session directed to 198.76.29.1, NAT would terminate
   Bind-holdout timer and transition the temporary binding between
   198.76.29.1 and 10.0.0.7 into a committed binding.

   To minimize denial of service attacks, where a malicious user
   keeps attempting name resolutions, without ever initiating a
   connection, NAT would have to monitor temporary address bindings
   that have not transitioned into committed bindings. There could
   be a limit on the number of temporary bindings and attempts to
   generate additional temporary bindings could be simply rejected.
   There may be other heuristic solutions to counter this type
   of malicious attacks.

   We will consider bi-directional NAT to illustrate the use of

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   temporary binding by DNS_ALG in the following sub-sections, even
   though the concept is applicable to other flavors of NATs as well.

3.2. Incoming queries

   In  order  to initiate incoming sessions, an external host obtains
   the V4 address of the DMZ-host it is trying to connect to by making
   a  DNS request.  This  request  constitutes  prelude to the start of
   a potential new session.

   The external host resolver makes a name lookup  for the DMZ host
   through  its  DNS  server.  When the DNS server does not have a
   record of IPv4 address attached  to this  name,  the lookup query
   is redirected at some point to the Primary/Backup DNS server
   (i.e., in DMZ) of  the private stub domain.

   Enroute to DMZ name server, DNS_ALG would intercept the datagram
   and modify the query as follows.

         a) For Host name to Host address query requests:
            Make no change to the DNS payload.

         b) For Host address to Host name queries:
            Replace the external V4 address octets (in reverse  order)
            preceding the string "IN-ADDR.ARPA"  with the corresponding
            private V4 address, if such an address binding exists
            already. However, if a binding does not exist, the DNS_ALG
            would simply respond (as a name server would) with a
            response code (RCODE) of 5 (REFUSED to respond due to policy
            reasons) and set ANCOUNT, NSCOUNT and ARCOUT to 0 in the
            header section of the response.

   In the opposite direction, as DNS response traverses from  the
   DNS  server in  private network, DNS_ALG would once again intercept
   the packet and modify as follows.

         a) For a host name to host address query requests, replace the
            private address sent by DMZ name server with a public
            address internally assigned by the NAT router. If a public
            address is not previously assigned to the host's private
            address, NAT would assign one at this time.

         b) For host address to host name queries, replace the private
            address octets preceding the string "IN-ADDR.ARPA" in
            response RRs with their external address assignments.
            There is a chance here that by the time the DMZ name server
            replies, the bind-holdout timer in NAT for the address in
            question has expired. In such a case, DNS_ALG would simply

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            drop the reply. The sender will have to resend the query
            (as would happen when a router enroute drops the response).

   For static address assignments, the TTL value supplied in the
   original RR will be left unchanged. For dynamic address assignments,
   DNS_ALG would modify the TTL value on DNS resource records (RRs) to
   be 0, implying that the RRs should only be used for transaction in
   progress, and not be cached. For compatibility with broken
   implementations, TTL of 1 might in practice work better.

   Clearly, setting TTL to be 0 will create more traffic than if the
   addresses were static, because name-to-address mapping is not cached.
   Specifically, network based applications will be required to use
   names rather than addresses for identifying peer nodes and must use
   DNS for every name resolution, as name-to-address mapping cannot be
   shared from the previously run applications.

   In addition, NAT would be requested to initiate a bind-holdout timer
   following the assignment. If no session is initiated to the private
   host within the Bind-holdout time period, NAT would terminate the
   temporary binding.

3.3. Outgoing Queries

   For Basic and bi-directional NATs, there is no need to distinguish
   between temporary and committed bindings for outgoing queries. This
   is because, DNS_ALG does not modify the DNS packets directed to or
   from external name servers (used during outbound sessions), unlike
   the inbound DNS sessions.

   Say, a private host needs to communicate with an external host.
   The  DNS query  goes  to  the internal name server (if there
   exists one) and from there to the appropriate authoritative/cache
   name server outside the private domain.  The  reply follows the
   same route but neither the query nor the response are subject to
   DNS_ALG translations.

   This however will not be the case with address isolated twice NAT
   private and external domains. In such a case, NAT would intercept
   all DNS packets and make address modifications to payload as
   discussed in the previous section. Temporary Private to external
   address bindings are created when responses are sent by private DNS
   servers and temporary external to private address bindings are
   created when responses are sent by external DNS servers.

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4. DNS payload modifications by DNS-ALG

   Typically, UDP is employed as the transport mechanism for DNS
   queries and responses and TCP for Zone refresh activities. In
   either case, name servers are accessed using a well-known DNS
   server port 53 (decimal) and all DNS payloads have the following
   format of data [Ref 4]. While NAT is responsible for the
   translation of IP and TCP/UDP headers of a DNS packet, DNS-ALG
   is responsible for updating the DNS payload.

   The header section within the DNS payload is always present and
   includes fields specifying which of the remaining sections are
   present. The header identifies if the message is a query or a
   response. No changes are required to be made by DNS-ALG to the
   Header section. DNS_ALG would parse only the DNS payloads whose
   QCLASS is set to IN (IP class).

    +---------------------+
    |        Header       |
    +---------------------+
    |       Question      | the question for the name server
    +---------------------+
    |        Answer       | RRs answering the question
    +---------------------+
    |      Authority      | RRs pointing toward an authority
    +---------------------+
    |      Additional     | RRs holding additional information
    +---------------------+

4.1. Question section

   The question section contains QDCOUNT (usually 1) entries, as
   specified in Header section, with each of the entries in the
   following format:

                                    1  1  1  1  1  1
      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                                               |
    /                     QNAME                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                     QTYPE                     |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                     QCLASS                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

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4.1.1. PTR type Queries

   DNS_ALG must identify all names, whose FQDNs (i.e., Fully Qualified
   Domain Names) fall within IN-ADDR.ARPA domain and replace the
   address octets (in reverse  order)  preceding the string
   "IN-ADDR.ARPA"  with the corresponding assigned address octets
   in reverse order, only if the address binding is active on
   the NAT router. If the address preceding the string
   "IN-ADDR.ARPA" falls within the NAT address map, but does not
   have at least a temporary address binding, DNS_ALG would simply
   simply respond back (as a DNS name server would) with a response
   code (RCODE) of 5 (REFUSED to respond due to policy reasons)
   and set ANCOUNT, NSCOUNT and ARCOUT to 0 in the header section
   of the response.

   Note that the above form of host address to host name type queries
   will likely yield different results at different times, depending
   upon address bind status in NAT at a given time.

   For example, a resolver that wanted to find out the hostname
   corresponding to address 198.76.29.1 (externally)  would pursue a
   query of the form:
        QTYPE = PTR, QCLASS = IN, QNAME = 1.29.76.198.IN-ADDR.ARPA.

   DNS_ALG would intervene and if the address 198.76.29.1 is
   internally mapped to a private address of 10.0.0.1, modify the
   query as below and forward to DMZ name server within private
   network.

        QTYPE = PTR, QCLASS = IN, QNAME = 1.0.0.10.IN-ADDR.ARPA

   Presumably, the DMZ name server is the authoritative name server
   for 10.IN-ADDR.ARPA zone and will respond with an RR of the
   following form in answer section. DNS_ALG translations of the
   response RRs will be considered in a following section.

        1.0.0.10.IN-ADDR.ARPA  PTR  host1.fooboo_org.provider_domain

   An example of Inverse translation is e-mail programs using
   inverse translation to trace e-mail originating hosts for spam
   prevention. Verify if the address from which the e-mail was sent
   does indeed belong to the same domain name the sender claims in
   sender ID.

   Query modifications of this nature will likely change the length
   of DNS payload. As a result, the corresponding IP and TCP/UDP
   header checksums must be updated. In case of TCP based queries,
   the sequence number deltas must be tracked by NAT so that the

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   delta can be applied to subsequent sequence numbers in datagrams
   in the same direction and acknowledgement numbers in datagrams in
   the opposite direction. In case of UDP based queries, message
   sizes are restricted to 512 bytes (not counting the IP or UDP
   headers). Longer messages must be truncated and the TC bit should
   be set in the header.

   Lastly, any compressed domain names using pointers to represent
   common domain denominations must be updated to reflect new
   pointers with the right offset, if the original domain name had
   to be translated by NAT.

4.1.2. A, MX, NS and SOA type Queries

   For these queries, DNS_ALG would not modify any of the fields in
   the query section, not even the name field.

4.1.3. AXFR type Queries

   AXFR is a special zone transfer type query. Zone transfers from
   private address realm must be avoided for address assignments
   that are not static. Typically, TCP is used for AXFR requests.

   When changes are made to a zone, they must be distributed to all
   name servers.  The general model of automatic zone transfer or
   refreshing is that one of the name servers is the master or
   primary for the zone.  Changes are coordinated at the primary,
   typically by editing a master file for the zone.  After editing,
   the administrator signals the master server to load the new zone.
   The other non-master or secondary servers for the zone
   periodically check the SERIAL field of the SOA for the zone for
   changes (at some polling intervals) and obtain new zone copies
   when changes have been made.

   Zone transfer is usually from primary to backup name servers. In
   the case of NAT supported private networks, primary and backup
   servers are advised to be located in the same private domain
   (say, private.zone) so zone transfer is not across the domain
   and DNS_ALG support for zone transfer is not an issue. In
   the case a secondary name server is located outside the private
   domain, zone transfers must not be permitted for non-static
   address assignments. Primary and secondary servers are required
   to be within the same private domain because all references to
   data in the zone had to be captured. With dynamic address
   assignments and bindings, it is impossible to translate the
   axfr data to be up-to-date. Hence, if a secondary server for
   private.zone were to be located external to the domain, it
   would contain bad data. Note, however, the requirement outlined

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   here is not in confirmence with RFC 2182, which recommends
   primary and secondary servers to be placed at topologically and
   geographically dispersed locations on the Internet.

   During zone transfers, DNS_ALG must examine all A type records
   and replace the original address octets with their statically
   assigned address octets. DNS_ALG could also examine if there is
   an attempt to transfer records for hosts that are not assigned
   static addresses and drop those records alone or drop the whole
   transfer. This would minimize misconfiguration and human errors.

4.1.4. Dynamic Updates to the DNS.

   An authoritative name server can have dynamic updates from the
   nodes within the zone without intervention from NAT and DNS-ALG,
   so long as one avoids spreading a DNS zone across address
   realms. We recommend keeping a DNS zone within the same realm
   it is responsible for. By doing this, DNS update traffic will
   not cross address realms and hence will not be subject to
   consideration by DNS-ALG.

   Further, if dynamic updates do cross address realms, and the
   updates must always be secured via DNSSEC, then such updates are
   clearly out of scope for DNS-ALG (as described in section 7).

4.2. Resource records in all other sections

   The answer, authority, and additional sections all share the same
   format, with a variable number of resource records. The number of
   RRs specific to each of the sections may be found in the
   corresponding count fields in DNS header. Each resource record
   has the following format:

   The TTL value supplied in the original RRs will be left unchanged
   for static address assignments. For dynamic address assignments,
   DNS_ALG will modify the TTL to be 0, so the RRs are used just for
   the transaction in progress, and not cached.  RFC 2181 requires
   all RRs in an RRset (RRs with the same name, class and type, but
   with different RDATA) to have the same TTL. So if the TTL of an
   RR is set to 0, all other RRs within the same RRset will also
   be adjusted by the DNS-ALG to be 0.

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      0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                                               |
    /                                               /
    /                      NAME                     /
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                      TYPE                     |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                     CLASS                     |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                      TTL                      |
    |                                               |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
    |                   RDLENGTH                    |
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
    /                     RDATA                     /
    /                                               /
    +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+


4.2.1. PTR type RRs

   The considerations specified in the Question section
   is equally valid with names for PTR type RRs. Private address
   preceding the string "IN-ADDR.ARPA" (in reverse order of
   octets) must be replaced by its external address assignment
   (in reverse order), if a binding exists. The remaining fields
   for this RR remain unchanged.

4.2.2. A type RRs

   The RDATA for A records  is a 4-byte IP address. DNS_ALG would
   simply replace the original address in RDATA with its externally
   assigned IP address, if it succeeded in finding an address
   binding. Successful address translation should cause no
   changes to payload length. Only the transport header checksum
   would need updating. In case of failure to find an address
   binding, DNS_ALG would have to drop the record and decrement
   the corresponding COUNT field in the header section.

4.2.3. CNAME, MX, NS and SOA type RRs

   No changes required to be made by DNS_ALG for these RRs, as the
   RDATA does not contain any IP addresses. The host names within
   the RDATA remain unchanged between realms.

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5. Illustration of DNS_ALG in conjunction with Bi-directional NAT

   The following diagram illustrates the operation of DNS_ALG in a
   a bi-directional NAT router. We will illustrate by walking
   through how name lookup and reverse name lookup queries are
   processed.

                                             .
                         ________________    .     External.com
                        (                )   .
                       (                  )  .   +-------------+
            +--+      (      Internet      )-.---|Border Router|
            |__|------ (                  )  .   +-------------+
           /____\       (________________)   .          |
            Root                 |           .          |
         DNS Server              |           .     ---------------
                         +---------------+   .       |         |
                         |Provider Router|   .     +--+       +--+
                         +---------------+   .     |__|       |__|
                                 |           .    /____\     /____\
                                 |           .  DNS Server   Host X
       External domain           |           .  171.68.1.1  171.68.10.1
     ............................|...............................
       Private domain            |
                                 |        Private.com
                                 |
                +--------------------------------------+
                |Bi-Directional NAT router with DNS_ALG|
                |                                      |
                | Private addresses:  172.19/16        |
                | External addresses: 131.108.1/24     |
                +--------------------------------------+
                              |      |
                      ----------    ----------
                        |                  |    DNS Server
                       +--+               +--+  Authoritative
                       |__|               |__|  for private.com
                      /____\             /____\
                      Host A           DNS Server
                   172.19.1.10        172.19.2.1
                                      (Mapped to 131.108.1.8)

    Figure 3: DNS-ALG operation in Bi-Directional NAT setup

   The above diagram depicts a scenario where a company private.com
   using private address space 172.19/16 connects to the Internet
   using bi-directional NAT. DNS_ALG is embedded in the NAT device

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   to make necessary DNS payload changes. NAT is configured to
   translate the private addresses space into an external address
   block of 131.108.1/24. NAT is also configured with a static
   translation for private.com's DNS server, so it can be referred
   in the external domain by a valid address.

   The company external.com is located in the external domain, using
   a registered address block of 171.68/16.  Also shown in the
   topology is a root DNS server.

   Following simplifications are made to the above configuration:

       * private.com is not multihomed and all traffic to the external
         space transits a single NAT.

       * The DNS server for private.com is authoritative for the
         private.com domain and points to the root server for all
         other DNS resolutions.  The same is true for the DNS server
         in external.com.

       * The internal name servers for private.com and external.com
         are same as their DMZ name servers. The DNS servers for these
         domains are configured with addresses private to the
         organization.

       * The name resolvers on host nodes do not have recursion
         available on them and desire recursive service from servers.
         All name servers are assumed to be able to provide
         recursive service.

5.1. Outgoing Name-lookup queries

   Say, Host A in private.com needs to perform a name lookup for
   host X in external.com to initiate a session with X.  This would
   proceed as follows.

   1. Host A sends a UDP based name lookup query (A record) for
      "X.External.Com" to its local DNS server.

   2. Local DNS server sends the query to the root server enroute
      NAT. NAT would change the IP and UDP headers to reflect DNS
      server's statically assigned external address.  DNS_ALG will
      make no changes to the payload.

   3. The root server, in turn, refers the local DNS server to query
      the DNS server for External.com. This referal transits the NAT
      enroute to the local DNS server.  NAT would  simply translate
      the IP and UDP headers of the incoming packet to reflect DNS

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      server's private address. No changes to the payload by DNS_ALG.

   4. Private.com DNS server will now send the query to the DNS server
      for external.com, once again, enroute NAT. Just as with the query
      to root, The NAT router would change the IP and UDP headers to
      reflect the DNS server's statically assigned external address.
      And, DNS_ALG will make no changes to the payload.

   5. The DNS server for external.com replies with the IP address
      171.68.10.1.  This reply also transits the NAT. NAT would
      translate the IP and UDP headers of the incoming packet to
      reflect DNS server's private address. Once again, no changes
      to the payload by DNS_ALG.

   6. The DNS server in Private.com replies to host A.

   When Host A finds the address of Host X, A initiates a session with
   host X, using a destination IP address of 171.68.10.1. This datagram
   and any others that follow in this session will be translated as
   usual by NAT.

   Note, DNS_ALG does not change the payload for DNS packets in
   either direction.

5.2. Reverse name lookups originated from private domain

   This scenario builds on the previous case by having host A in
   Private.com perform a reverse name lookup on 171.68.10.1, which
   is host X's global address. Following is a sequence of events.

   1. Host A sends a UDP based inverse name lookup query (PTR record)
      for "1.10.68.171.IN-ADDR.ARPA." to its local DNS server.

   2. Local DNS server sends the query to the root server enroute
      NAT. As before, NAT would change the IP and UDP headers to
      reflect DNS server's statically assigned external address.
      DNS_ALG will make no changes to the payload.

   3. The root server, in turn, refers the local DNS server to query
      the DNS server for External.com. This referal transits the NAT
      enroute to the local DNS server.  NAT would  simply translate
      the IP and UDP headers of the incoming packet to reflect DNS
      server's private address. No changes to the payload by DNS_ALG.

   4. Private.com DNS server will now send the query to the DNS server
      for external.com, once again, enroute NAT. Just as with the query
      to root, The NAT router would change the IP and UDP headers to
      reflect the DNS server's statically assigned external address.

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      And, DNS_ALG will make no changes to the payload.

   5. The DNS server for external.com replies with the host name
      of "X.External.Com.". This reply also transits the NAT. NAT would
      translate the IP and UDP headers of the incoming packet to
      reflect DNS server's private address. Once again, no changes
      to the payload by DNS_ALG.

   6. The DNS server in Private.com replies to host A.

   Note, DNS_ALG does not change the payload in either direction.

5.3. Incoming Name-lookup queries

   This time, host X in external.com wishes to initiate a session with
   host A in Private.com. Below are the sequence of events that take
   place.

   1. Host X sends a UDP based name lookup query  (A record) for
      "A.Private.com" to its local DNS server.

   2. Local DNS server in External.com sends the query to root server.

   3. The root server, in turn, refers the DNS server in External.com
      to query the DNS server for private.com,

   4. External.com DNS server will now send the query to the DNS server
      for Private.com. This query traverses the NAT router. NAT would
      change the IP and UDP headers of the packet to reflect the DNS
      server's private address. DNS_ALG will make no changes to the
      payload.

   5. The DNS server for Private.com replies with the IP address
      172.19.1.10 for host A.  This reply also transits the NAT. NAT
      would translate the IP and UDP headers of the outgoing packet
      from the DNS server.

      DNS_ALG will request NAT to (a) setup a temporary binding for
      Host A (172.19.1.10) with an external address and (b) initiate
      Bind-holdout timer. When NAT successfully sets up a temporary
      binding with an external address (say, 131.108.1.12), DNS_ALG
      would modify the payload to replace A's private address with
      its external assigned address and set the Cache timeout to 0.

   6. The server in External.com replies to host X

   When Host X finds the address of Host A, X initiates a session with
   A, using a destination IP address of 131.108.1.12. This datagram and

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   any others that follow in this session will be translated as usual
   by the NAT.

   Note, DNS_ALG changes only the response packets from the DNS server
   for Private domain.

5.4. Reverse name lookups originated from external domain

   This scenario builds on the previous case (section 5.3) by having
   host X in External.com perform a reverse name lookup on 131.108.1.12,
   which is host A's assigned external address. The following sequence
   of events take place.

   1. Host X sends a UDP based inverse name lookup query (PTR record)
      for "12.1.108.131.IN-ADDR.ARPA." to its local DNS server.

   2. Local DNS server in External.com sends the query to the root
      server.

   3. The root server, in turn, refers the local DNS server to query
      the DNS server for Private.com.

   4. External.com DNS server will now send the query to the DNS server
      for Private.com. This query traverses the NAT router. NAT would
      change the IP and UDP headers to reflect the DNS server's private
      address.

      DNS_ALG will enquire NAT for the private address associated
      with the external address of 131.108.1.12 and modify the payload,
      replacing 131.108.1.12 with the private address of 172.19.1.10.

   5. The DNS server for Private.com replies with the host name
      of "A.Private.Com.". This reply also transits the NAT. NAT would
      translate the IP and UDP headers of the incoming packet to
      reflect DNS server's private address.

      Once again, DNS_ALG will enquire NAT for the assigned external
      address associated with the private address of 172.19.1.10 and
      modify the payload, replacing 172.19.1.10 with the assigned
      external address of 131.108.1.12.

   6. The DNS server in External.com replies to host X.

   Note, DNS_ALG changes the query as well as response packets from DNS
   server for Private domain.

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6. Illustration of DNS_ALG in conjunction with Twice-NAT

   The following diagram illustrates the operation of DNS_ALG in a
   Twice NAT router. As before, we will illustrate by walking through
   how name lookup and reverse name lookup queries are processed.

                                             .
                         ________________    .     External.com
                        (                )   .
                       (                  )  .   +-------------+
            +--+      (      Internet      )-.---|Border Router|
            |__|------ (                  )  .   +-------------+
           /____\       (________________)   .          |
            Root                 |           .          |
         DNS Server              |           .     ---------------
                         +---------------+   .       |         |
                         |Provider Router|   .     +--+       +--+
                         +---------------+   .     |__|       |__|
                                 |           .    /____\     /____\
                                 |           .  DNS Server   Host X
       External domain           |           .  171.68.1.1  171.68.10.1
     ............................|...............................
       Private domain            |
                                 |        Private.com
                                 |
                +-------------------------------------------+
                | Twice-NAT router with DNS_ALG             |
                |                                           |
                | Private addresses:  171.68/16             |
                | Assigned External addresses: 131.108.1/24 |
                |                                           |
                | External addresses:  171.68/16            |
                | Assigned Private addresses: 10/8          |
                +-------------------------------------------+
                              |      |
                      ----------    ----------
                        |                  |    DNS Server
                       +--+               +--+  Authoritative
                       |__|               |__|  for private.com
                      /____\             /____\
                      Host A           DNS Server
                   171.68.1.10        171.68.2.1
                                      (Mapped to 131.108.1.8)

    Figure 4: DNS-ALG operation in Twice-NAT setup

   In this scenario, hosts in private.com were not numbered from the
   RFC 1918 reserved 172.19/16 space, but rather were numbered with the

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   globally-routable 171.68/16 network, the same as external.com.  Not
   only does private.com need translation service for its own host
   addresses, but it also needs translation service if any of those
   hosts are to be able to exchange datagrams with hosts in
   external.com. Twice-NAT accommodates the transition by translating
   the overlapping address space used in external.com with a unique
   address block (10/8) from RFC 1918 address space. Routes are set up
   within the private domain to direct datagrams destined for the
   address block 10/8 through Twice-NAT device to the external global
   network space.

   Simplifications and assumptions made in section 5.0 will be valid
   here as well.

6.1. Outgoing Name-lookup queries

   Say, Host A in private.com needs to perform a name lookup for
   host X in external.com (host X has a FQDN of X.external.com),
   to find its address.  This would would proceed as follows.

   1. Host A sends a UDP based name lookup query (A record) for
      "X.External.Com" to its local DNS server.

   2. Local DNS server sends the query to the root server enroute
      NAT. NAT would change the IP and UDP headers to reflect DNS
      server's statically assigned external address.  DNS_ALG will
      make no changes to the payload.

   3. The root server, in turn, refers the local DNS server to query
      the DNS server for External.com. This referal transits the NAT
      enroute to the local DNS server.  NAT would  simply translate
      the IP and UDP headers of the incoming packet to reflect DNS
      server's private address.

      DNS_ALG will request NAT for an assigned private address for
      the referral server and replace the external address with its
      assigned private address in the payload.

   4. Private.com DNS server will now send the query to the DNS server
      for external.com, using its assigned private address, via NAT.
      This time, NAT would change the IP and UDP headers to reflect the
      External addresses of the DNS servers. I.e., Private.com DNS
      server's IP address is changed to its assigned external address
      and External.Com DNS server's assigned Private address is
      changed to its external address.

      DNS_ALG will make no changes to the payload.


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   5. The DNS server for external.com replies with the IP address
      171.68.10.1.  This reply also transits the NAT. NAT would
      once again translate the IP and UDP headers of the incoming
      to reflect the private addresses of the DNS servers.
      I.e., Private.com DNS server's IP address is changed to its
      private address and External.Com DNS server's external
      address is changed to its assigned Private address.

      DNS_ALG will request NAT to (a) set up a temporary binding for
      Host X (171.68.10.1) with a private address and (b) initiate
      Bind-holdout timer. When NAT successfully sets up temporary
      binding with a private address (say, 10.0.0.254), DNS_ALG would
      modify the payload to replace X's external address with its
      assigned private address and set the Cache timeout to 0.

   6. The DNS server in Private.com replies to host A.

   When Host A finds the address of Host X, A initiates a session with
   host X, using a destination IP address of 10.0.0.254. This datagram
   and any others that follow in this session will be translated as
   usual by Twice NAT.

   Note, the DNS_ALG has had to change payload in both directions.

6.2. Reverse name lookups originated from private domain

   This scenario builds on the previous case by having host A in
   Private.com perform a reverse name lookup on 10.0.0.254, which
   is host X's assigned private address. Following is a sequence
   of events.

   1. Host A sends a UDP based inverse name lookup query (PTR record)
      for "254.0.0.10.IN-ADDR.ARPA." to its local DNS server.

   2. Local DNS server sends the query to the root server enroute
      NAT. As before, NAT would change the IP and UDP headers to
      reflect DNS server's statically assigned external address.

      DNS_ALG will translate the private assigned address 10.0.0.254
      with its external address 171.68.10.1.

   3. The root server, in turn, refers the local DNS server to query
      the DNS server for External.com. This referal transits the NAT
      enroute to the local DNS server.  NAT would  simply translate
      the IP and UDP headers of the incoming packet to reflect DNS
      server's private address.

      As with the original query, DNS_ALG will translate the private

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      assigned address 10.0.0.254 with its external address
      171.68.10.1. In addition, DNS_ALG will replace the external
      address of the referal server (i.e., the DNS server for
      External.com) with its assigned private address in the payload.

   4. Private.com DNS server will now send the query to the DNS server
      for external.com, using its statically assigned private address,
      via NAT. This time, NAT would change the IP and UDP headers to
      reflect the External addresses of the DNS servers. I.e.,
      Private.com DNS server's IP address is changed to its assigned
      external address and External.Com DNS server's assigned Private
      address is changed to its external address.

      As with the original query, DNS_ALG will translate the private
      assigned address 10.0.0.254 with its external address
      171.68.10.1.

   5. The DNS server for external.com replies with the FQDN of
      "X.External.Com.".  This reply also transits the NAT. NAT would
      once again translate the IP and UDP headers of the incoming
      to reflect the private addresses of the DNS servers.
      I.e., Private.com DNS server's IP address is changed to its
      private address and External.Com DNS server's external
      address is changed to its assigned Private address.

      Once again, DNS_ALG will translate the query section, replacing
      the external address 171.68.10.1 with its assigned private
      address of 10.0.0.254

   6. The DNS server in Private.com replies to host A.

   Note, the DNS_ALG has had to change payload in both directions.

6.3. Incoming Name-lookup queries

   This time, host X in external.com wishes to initiate a session with
   host A in Private.com. Below are the sequence of events that take
   place.

   1. Host X sends a UDP based name lookup query  (A record) for
      "A.Private.com" to its local DNS server.

   2. Local DNS server in External.com sends the query to root server.

   3. The root server, in turn, refers the DNS server in External.com
      to query the DNS server for private.com,

   4. External.com DNS server will now send the query to the DNS server

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      for Private.com. This query traverses the NAT router. NAT would
      change the IP and UDP headers to reflect the private addresses of
      the DNS servers. I.e., Private.com DNS server's IP address is
      changed to its  private address and External.Com DNS server's
      external address is changed to assigned Private address.

      DNS_ALG will make no changes to the payload.

   5. The DNS server for Private.com replies with the IP address
      171.68.1.10 for host A.  This reply also transits the NAT. NAT
      would once again translate the IP and UDP headers of the incoming
      to reflect the external addresses of the DNS servers.  I.e.,
      Private.com DNS server's IP address is changed to its
      assigned external address and External.Com DNS server's
      assigned private address is changed to its external address.

      DNS_ALG will request NAT to (a) set up temporary binding for
      Host A (171.68.1.10) with an external address and (b) initiate
      Bind-holdout timer. When NAT succeeds in finding an external
      address (say, 131.108.1.12) to temporarily bind to host A,
      DNS_ALG would modify the payload to replace A's private address
      with its external assigned address and set the Cache timeout to 0.

   6. The server in External.com replies to host X

   When Host X finds the address of Host A, X initiates a session with
   A, using a destination IP address of 131.108.1.12. This datagram and
   any others that follow in this session will be translated as usual
   by the NAT.

   Note, DNS_ALG changes only the response packets from the DNS server
   for Private domain.

6.4. Reverse name lookups originated from external domain

   This scenario builds on the previous case (section 6.3) by having
   host X in External.com perform a reverse name lookup on 131.108.1.12,
   which is host A's assigned external address. The following sequence
   of events take place.

   1. Host X sends a UDP based inverse name lookup query (PTR record)
      for "12.1.108.131.IN-ADDR.ARPA." to its local DNS server.

   2. Local DNS server in External.com sends the query to the root
      server.

   3. The root server, in turn, refers the local DNS server to query
      the DNS server for Private.com.

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   4. External.com DNS server will now send the query to the DNS server
      for Private.com. This query traverses the NAT router. NAT would
      change the IP and UDP headers to reflect the private addresses of
      the DNS servers. I.e., Private.com DNS server's IP address is
      changed to its  private address and External.Com DNS server's
      external address is changed to assigned Private address.

      DNS_ALG will enquire NAT for the private address associated
      with the external address of 131.108.1.12 and modify the payload,
      replacing 131.108.1.12 with the private address of 171.68.1.10.

   5. The DNS server for Private.com replies with the host name
      of "A.Private.Com.". This reply also transits the NAT. NAT would
      once again translate the IP and UDP headers of the incoming
      to reflect the external addresses of the DNS servers.  I.e.,
      Private.com DNS server's IP address is changed to its
      assigned external address and External.Com DNS server's
      assigned private address is changed to its external address.

      Once again, DNS_ALG will enquire NAT for the assigned external
      address associated with the private address of 172.19.1.10 and
      modify the payload, replacing 171.68.1.10 with the assigned
      external address of 131.108.1.12.

   6. The DNS server in External.com replies to host X.

   Note, DNS_ALG changes the query as well as response packets from DNS
   server for Private domain.

7. DNS-ALG limitations and Future Work

   NAT increases the probability of mis-addressing. For example,
   same local address may be bound to different public address at
   different times and vice versa. As a result, hosts that cache
   the name to address mapping for longer periods than the NAT
   router is configured to hold the map are likely to misaddress
   their sessions. Note, this is mainly an issue with bad host
   implementations that hold DNS records longer than the TTL
   in them allows and is not directly attributable to the
   mechanism described here.

   DNS_ALG cannot support secure DNS name servers in the private
   domain. I.e., Signed replies from an authoritative DNS name server
   in the DMZ to queries originating from the external world will be
   broken by the DNS-ALG. At best, DNS-ALG would be able to transform
   secure dnssec data into unprotected data. End-node demanding DNS
   replies to be signed may reject replies that have been tampered with

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   by DNS_ALG. Since, the DNS server does not have a way to find
   where the queries come from (i.e., internal or external), it will
   most likely have to resort to the common denomination of today's
   insecure DNS. Both are serious limitations to DNS_ALG. Zone
   transfers between DNS-SEC servers  is also impacted the same way,
   if the transfer crosses address realms.

   The good news, however, is that only end-nodes in DMZ pay the
   price for the above limitation in a traditional NAT (or, a
   bi-directional NAT), as external end-nodes may not access internal
   hosts due to DNS replies not being secure. However, for outgoing
   sessions (from private network) in a bi-directional NAT setup,
   the DNS queries can be signed and securely accepted by DMZ and
   other internal hosts since DNS_ALG does not intercept outgoing
   DNS queries and incoming replies. Lastly, zone transfers between
   DNS-SEC servers  within the same private network are not impacted.

   Clearly, with DNS SEC deployment in DNS servers and end-host
   resolvers, the scheme suggested in this document will not work.

8. Security considerations.

   If DNS packets are encrypted/authenticated per DNSSEC, then DNS_ALG
   will fail because it won't be able to perform payload modifications.
   Alternately, if packets must be preserved in an address realm,
   DNS_ALG will need to hold the secret key to decrypt/verify payload
   before forwarding packets to a different realm. For example, if
   DNS-ALG, NAT and IPsec gateway (providing secure tunneling service)
   are resident on the same device, DNS-ALG will have access to the
   IPsec security association keys.  The preceding section, "DNS-ALG
   limitations and Future Work" has coverage on DNS_ALG security
   considerations.

   Further, with DNS-ALG, there is a possibility of denial of service
   attack from a malicious user, as outlined in section 3.1.
   Section 3.1 suggests some ways to counter this attack.

REFERENCES

   [1] P. Srisuresh, M. Holdrege, "The IP  Network  Address
       Translator (NAT) terminology and considerations",
       <draft-ietf-nat-terminology-03.txt> - Work in progress.

   [2] K. Egevang, P. Francis, "The IP Network Address Translator
       (NAT)", RFC 1631.

   [3] Rekhter, Y., Moskowitz, B., Karrenberg, D., G. de Groot, and,

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Internet Draft            DNS extensions to NAT                June 1999

       Lear, E. "Address Allocation for Private Internets", RFC 1918

   [4] P. Mockapetris, "Domain Names - Concepts and facilities",
       RFC 1034.

   [5] P. Mockapetris, "Domain Names - Implementation and
       Specification", RFC 1035.

   [6] J. Reynolds and J. Postel, "Assigned Numbers", RFC 1700.

   [7] R. Braden, "Requirements for Internet Hosts -- Communication
       Layers", RFC 1122.

   [8] R. Braden, "Requirements for Internet Hosts -- Application
       and Support", RFC 1123.

   [9] F. Baker, "Requirements for IP Version 4 Routers",  RFC 1812.

   [10] Brian carpenter, Jon Crowcroft, Yakov Rekhter, "IPv4 Address
        Behaviour Today", RFC 2101.

   [11] Donald E. Eastlake, "Domain Name System Security Extensions",
        RFC 2535

   [12] P. Vixie, S. Thompson, Y. Rekhter and J. Bound, "Dynamic
        Updates in the Domain Name System (DNS UPDATE)", RFC 2136

   [13] D. Eastlake, "Secure Domain Name System Dynamic Update",
        RFC 2137

   [14] R. Elz and R. Bush, "Clarifications to the DNS
        specification", RFC 2181

   [15] R. Elz, R. Bush, S. Bradner and M. Patton, "Selection and
        Operation of Secondary DNS Servers", RFC 2182

Authors' Addresses

   Pyda Srisuresh
   Lucent technologies
   Pleasanton, CA 94588-8519
   U.S.A.

   Phone: +1 (925) 737-2153
   Fax:   +1 (925) 737-2110
   e-mail: suresh@ra.lucent.com

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Internet Draft            DNS extensions to NAT                June 1999

   George Tsirtsis
   Internet Transport Group
   B29 Room 129
   BT Laboratories
   Martlesham Heath
   IPSWICH
   Suffolk IP5 3RE
   England

   Phone: +44 1473 640756
   Fax:   +44 1473 640709
   e-mail: george@gideon.bt.co.uk

   Praveen Akkiraju
   cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134  USA

   Phone:  +1 (408) 526-5066
   e-mail: spa@cisco.com

   Andy Heffernan
   Juniper Networks, Inc.
   385 Ravensdale Drive.
   Mountain View, CA  94043  USA

   Phone:  +1 (650) 526-8037
   Fax:    +1 (650) 526-8001
   e-mail: ahh@juniper.net

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