Special Use Domain Name 'ipv4only.arpa'Apple Inc.1 Infinite LoopCupertinoCalifornia95014USA+1 408 974 3207cheshire@apple.comApple Inc.1 Infinite LoopCupertinoCalifornia95014USA+1 669 227 9921dschinazi@apple.comThe specification for how a client discovers its network's NAT64
prefix [RFC7050] defines the special name 'ipv4only.arpa' for this purpose,
but treats it as a non special name in
the document's Domain Name Reservation Considerations section.Consequently, despite the well articulated special purpose of the name,
as of July 2016 'ipv4only.arpa' still does not appear as one of the names with special
properties that are recorded in the Special-Use Domain Names registry.This document formally declares the actual
special properties of the name, and adds similar declarations
for the corresponding reverse mapping names.The specification for
how a client discovers its network's NAT64 prefix
defines the special name 'ipv4only.arpa' for this purpose,
but treats it as a non special name in
the document's Domain Name Reservation Considerations section.Consequently, despite the well articulated special purpose of the name,
as of July 2016 'ipv4only.arpa' still does not appear as one of the names with special
properties that are recorded in the Special-Use Domain Names registry.This document formally declares the actual
special properties of the name. This document also adds similar declarations
for the corresponding reverse mapping names.The hostname 'ipv4only.arpa' is peculiar in that it was never intended
to be treated like a normal hostname.A typical client never looks up the IPv4 address records for 'ipv4only.arpa',
because it is already known, by specification,
to have exactly two IPv4 address records, 192.0.0.170 and 192.0.0.171.
No client ever has to look the name in order to learn those two addresses.In contrast, clients often look up the IPv6 AAAA address records for
'ipv4only.arpa', which is contrary to general DNS expectations, given
that it is already known, by specification,
that no such IPv6 AAAA address records exist. And yet, clients expect to
receive, and do in fact receive, positive answers for these IPv6 AAAA
address records that are known to not exist.This is clearly not a typical DNS name. Clients never normally query for
the two records that do in fact exist, instead querying for records that
are known to not exist, and getting positive answers to those abnormal queries.
Clients are using DNS to perform queries for this name, but they are certainly not
using DNS to learn legitimate answers from the name's legitimate authoritative server.
Instead, clients are using these pseudo-DNS queries as an impromptu middlebox
communication protocol, to communicate with the
NAT64/DNS64 gateway,
if present, and request that it disclose the prefix it is using for IPv6 address synthesis.It is this use of specially-crafted DNS queries for 'ipv4only.arpa' as an
impromptu middlebox communication protocol that makes the name 'ipv4only.arpa'
noteworthy, and legitimately qualifying to be described as a 'special' name.As a result of the original specification
not formally declaring 'ipv4only.arpa' to have special properties,
there was no mandate for any server software to treat this name
specially. Consequently, queries for this name had to be handled normally,
resulting in unnecessary queries to the authoritative 'arpa' name servers.Having millions of devices around the world issue these queries generated
pointless additional load on the authoritative 'arpa' name servers, which was completely
unnecessary when the name 'ipv4only.arpa' is defined, by Internet Standard,
to have exactly two IPv4 address records, 192.0.0.170 and 192.0.0.171,
and no other records of any type.Also, at times, for reasons that are as yet
unclear, the authoritative 'arpa' name servers have been observed to be slow or unresponsive.
The failures of these 'ipv4only.arpa' queries result in unnecessary failures
of software that depends on them for DNS64 address synthesis.Even when the authoritative 'arpa' name servers are operating correctly,
having to perform an unnecessary query to obtain an answer that is already
known in advance can add precious milliseconds of delay for no reason.This document leverages this operational experience to update the
Domain Name Reservation Considerations section
of the earlier specification
with one that
accurately lists the actual special properties of the name 'ipv4only.arpa' so that
software can legitimately make appropriate performance and reliability optimizations.Hard-coding the known answers for 'ipv4only.arpa' queries in
recursive/caching DNS servers reduces the risk of malicious devices
intercepting those queries and returning incorrect answers, particularly in the
case of recursive/caching DNS servers that do not perform DNSSEC validation.One of the known concerns with DNS64 is that
it interferes with DNSSEC. DNSSEC may cryptographically assert that a name
has no IPv6 AAAA records, while at the same time DNS64 address synthesis
is contradicting this and claiming that IPv6 AAAA records do exist.Section 3 of the DNS64 specification
discusses this:
... DNS64 receives a query with the DO bit set and
the CD bit set. In this case, the DNS64 is supposed
to pass on all the data it gets to the query initiator.
This case will not work with DNS64, unless the
validating resolver is prepared to do DNS64 itself.The NAT64 Prefix Discovery specification
provides the mechanism for the query initiator to learn the NAT64 prefix
so that it can do its own validation and DNS64 synthesis as described above.
With this mechanism the client can
(i) interrogate the local NAT64/DNS64 gateway with an 'ipv4only.arpa'
query to learn the IPv6 address synthesis prefix,
(ii) query for the (signed) IPv4 address records itself, and then
(iii) perform its own IPv6 address synthesis locally,
combining the IPv6 address synthesis prefix learned from the local NAT64/DNS64 gateway
with the secure DNSSEC-signed data learned from the global Domain Name System.It is conceivable that over time, if DNSSEC is successful, the
majority of clients could move to this validate-and-synthesize-locally
model, which reduces the DNS64 machinery to the vestigial role of
simply responding to the 'ipv4only.arpa' query to report the local
IPv6 address synthesis prefix. In no case does the client care what
answer(s) the authoritative 'arpa' name servers might give for that query.
The 'ipv4only.arpa' query is being used purely as a local
client-to-middlebox communication message.This approach is even more attractive if it doesn't create
an additional dependency on the authoritative 'arpa' name
servers to answer a query that is unnecessary
because the NAT64/DNS64 gateway already knows the answer
before it even issues the query. Avoiding this unnecessary
query improves performance and reliability for the client,
and reduces unnecessary load for the authoritative 'arpa' name servers.[Once published, this should say]IANA has recorded the following names in theSpecial-Use Domain Names registry:
ipv4only.arpa.170.0.0.192.in-addr.arpa.171.0.0.192.in-addr.arpa.IANA has recorded the following IPv4 addresses in theIPv4 Special-Purpose Address Registry:
192.0.0.170192.0.0.171The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL"
in this document are to be interpreted as described in "Key words for use
in RFCs to Indicate Requirement Levels" .The name 'ipv4only.arpa' is defined, by Internet Standard, to have
two IPv4 address records with rdata 192.0.0.170 and 192.0.0.171.When queried via a DNS64 recursive/caching server, the name
'ipv4only.arpa' is also defined to have IPv6 AAAA records,
with rdata synthesized from a combination of the NAT64 IPv6 prefix(es),
and the IPv4 addresses 192.0.0.170 and 192.0.0.171.
This can return more than one pair of IPv6 addresses
if there are multiple NAT64 prefixes.The name 'ipv4only.arpa' has no other DNS records of any type.The name 'ipv4only.arpa' is special only to
(a) client software wishing to perform DNS64 address synthesis, and
(b) the DNS64 recursive/caching server responding to such requests.
These two considerations are listed in items 2 and 4 below:
Normal users should never have reason to encounter the 'ipv4only.arpa' domain name.
If they do, they should expect queries for 'ipv4only.arpa' to result in
the answers required by the specification.
Normal users have no need to know that 'ipv4only.arpa' is special.Application software may explicitly use the name 'ipv4only.arpa' for NAT64/DNS64
address synthesis, and expect to get
the answers required by the specification.
If application software encounters the name 'ipv4only.arpa' in the normal
course of handling user input, the application software should resolve
that name as usual and need not treat it in any special way.Name resolution APIs and libraries SHOULD NOT recognize
'ipv4only.arpa' as special and SHOULD NOT treat it differently.
Name resolution APIs SHOULD send queries for this name to
their configured recursive/caching DNS server(s).Recursive/caching DNS servers SHOULD recognize 'ipv4only.arpa' as special
and SHOULD NOT, by default, attempt to look up NS records for it, or
otherwise query authoritative DNS servers in an attempt to resolve this name.
Instead, recursive/caching DNS servers SHOULD, by default, act as
authoritative and generate immediate responses for all such queries.
Traditional recursive/caching DNS servers that act as authoritative for this
name MUST generate only the 192.0.0.170 and 192.0.0.171 responses for
IPv4 address queries (DNS qtype "A"),
and a "no error no answer" response for all other query types.
All DNS64 recursive/caching DNS servers MUST generate
the 192.0.0.170 and 192.0.0.171 responses for IPv4 address queries (DNS qtype "A"),
the appropriate synthesized IPv6 address record responses for IPv6 address queries (DNS qtype "AAAA"),
and a "no error no answer" response for all other query types.
This local self-contained generation of these responses is
to avoid placing unnecessary load on the authoritative 'arpa' name servers.
Example configurations for BIND 9 showing how to achieve these results
are given in Appendix A.Traditional authoritative DNS server software need not recognize
'ipv4only.arpa' as special or handle it in any special way.
Recursive/caching DNS servers SHOULD routinely act as
authoritative for this name and return the results described
above. Only the administrators of the 'arpa' namespace need to
explicitly configure their actual authoritative name servers to be
authoritative for this name
and to generate the appropriate answers; all other authoritative
name servers will not be configured to know anything about this name
and will reject queries for it as they would reject queries for any
other name about which they have no information.Generally speaking, operators of authoritative DNS servers need
not know anything about the name 'ipv4only.arpa', just as they don't need
to know anything about any other names they are not responsible for.
Operators of authoritative DNS servers who are configuring their name servers
to be authoritative for this name MUST understand that 'ipv4only.arpa' is
a special name, with records rigidly specified by Internet Standard
(generally this applies only to the administrators of the 'arpa' namespace).DNS Registries/Registrars need not know anything about the
name 'ipv4only.arpa', just as they don't need to know
anything about any other name they are not responsible for.
Only the administrators of the 'arpa' namespace need to be aware
of this name's purpose and how it should be configured.Since the IPv4 addresses 192.0.0.170 and 192.0.0.171 are defined
to be special, and are listed in the
IPv4 Special-Purpose Address Registry,
the corresponding reverse mapping names in the in&nbhy;addr.arpa domain
are similarly special.The name '170.0.0.192.in-addr.arpa' is defined, by Internet Standard,
to have only a single DNS record, type PTR, with rdata 'ipv4only.arpa'.The name '171.0.0.192.in-addr.arpa' is defined, by Internet Standard,
to have only a single DNS record, type PTR, with rdata 'ipv4only.arpa'.Practically speaking these two names are rarely used, but to the extent that
they may be, they are special only to recursive/caching DNS servers as
described in item 4 below:
Normal users should never have reason to encounter these two reverse mapping names.
However, if they do, queries for these reverse mapping names should
return the expected answer 'ipv4only.arpa'.
Normal users have no need to know that these reverse mapping names are special.Application software SHOULD NOT recognize these two reverse mapping
names as special, and SHOULD NOT treat them differently.
For example, if the user were to issue the Unix command "host 192.0.0.170"
then the "host" command should issue the query as usual and display the
result that is returned.Name resolution APIs and libraries SHOULD NOT recognize
these two reverse mapping names as special and SHOULD NOT treat them differently.
Name resolution APIs SHOULD send queries for these names to
their configured recursive/caching DNS server(s).Recursive/caching DNS servers SHOULD recognize these two reverse mapping
names as special and SHOULD NOT, by default, attempt to look up NS records
for them, or otherwise query authoritative DNS servers in an attempt to
resolve them. Instead, recursive/caching DNS servers SHOULD, by default,
act as authoritative and generate immediate responses for all such queries.
Recursive/caching DNS servers that act as authoritative for these
names MUST generate only the 'ipv4only.arpa' response for PTR queries,
and a "no error no answer" response for all other query types.
This local self-contained generation of these responses is to avoid
placing unnecessary load on the authoritative 'in&nbhy;addr.arpa' name servers.Traditional authoritative DNS server software need not
recognize these two reverse mapping names as special or
handle them in any special way.
As a practical matter, only the administrators of the 'in&nbhy;addr.arpa' namespace
will configure their name servers to be authoritative for these names
and to generate the appropriate answers; all other authoritative
name servers will not be configured to know anything about these names
and will reject queries for them as they would reject queries for any
other name about which they have no information.Generally speaking, operators of authoritative DNS servers need
not know anything about these two reverse mapping names, just as they don't need
to know anything about any other names they are not responsible for.
Operators of authoritative DNS servers who are configuring their name servers
to be authoritative for this name MUST understand that these two reverse
mapping names are special, with answers specified by Internet Standard
(generally this applies only to the administrators of the 'in&nbhy;addr.arpa' namespace).DNS Registries/Registrars need not know anything about
these two reverse mapping names, just as they don't need to know
anything about any other name they are not responsible for.
Only the administrators of the 'in&nbhy;addr.arpa' namespace need
to be aware of the purpose of these two names.For all IPv6 addresses synthesized by the NAT64 gateway,
the DNS64 recursive/caching server is responsible for
synthesizing the appropriate ip6.arpa reverse mapping PTR records,
if it chooses to do so.
The same applies to the synthesized IPv6 addresses corresponding
to the IPv4 addresses 192.0.0.170 and 192.0.0.171.Generally a DNS64 recursive/caching server synthesizes
appropriate ip6.arpa reverse mapping PTR records by extracting
the embedded IPv4 address from the encoded IPv6 address,
performing a reverse mapping query for that IPv4 address,
and then synthesizing a corresponding ip6.arpa reverse mapping
PTR record containing the same rdata.In the case of synthesized IPv6 addresses corresponding
to the IPv4 addresses 192.0.0.170 and 192.0.0.171,
the DNS64 recursive/caching server does not issue mapping queries
for those IPv4 addresses, but instead, according to rule 3 above,
immediately returns the answer 'ipv4only.arpa'.Special-Use Domain Names RegistryIANA IPv4 Special-Purpose Address RegistryA BIND 9 recursive/caching DNS server could be configured to
act as authoritative for the appropriate names as follows.
In /etc/named.conf the following lines are added:
zone "ipv4only.arpa" { type master; file "ipv4only"; };
zone "170.0.0.192.in-addr.arpa" { type master; file "ipv4only-r"; };
zone "171.0.0.192.in-addr.arpa" { type master; file "ipv4only-r"; };
The file /var/named/ipv4only is created with the following content.
(The lines marked "Only for DNS64 server" are omitted on a standard
recursive/caching DNS server.)
$TTL 86400 ; Default TTL 24 hours
@ IN SOA nameserver.example. admin.nameserver.example. (
2016052400 ; Serial
7200 ; Refresh ( 7200 = 2 hours)
3600 ; Retry ( 3600 = 1 hour)
15724800 ; Expire (15724800 = 6 months)
60 ; Minimum
)
@ IN NS nameserver.example.
@ IN A 192.0.0.170
@ IN A 192.0.0.171
@ IN AAAA 64:ff9b::192.0.0.170 ; Only for DNS64 server
@ IN AAAA 64:ff9b::192.0.0.171 ; Only for DNS64 server
The file /var/named/ipv4only-r is created with the following content:
$TTL 86400 ; Default TTL 24 hours
@ IN SOA nameserver.example. admin.nameserver.example. (
2016052400 ; Serial
7200 ; Refresh ( 7200 = 2 hours)
3600 ; Retry ( 3600 = 1 hour)
15724800 ; Expire (15724800 = 6 months)
60 ; Minimum
)
@ IN NS nameserver.example.
@ IN PTR ipv4only.arpa.