| Network Working Group | S. Cheshire |
| Internet-Draft | D. Schinazi |
| Updates: 7050 (if approved) | Apple Inc. |
| Intended status: Standards Track | October 30, 2017 |
| Expires: May 3, 2018 |
Special Use Domain Name 'ipv4only.arpa'
draft-cheshire-sudn-ipv4only-dot-arpa-08
The specification for how a client discovers its network's NAT64 prefix [RFC7050] defines the special name 'ipv4only.arpa' for this purpose, but declares it to be a non-special name in that specification's Domain Name Reservation Considerations section.
Consequently, despite the well articulated special purpose of the name, (at the time of writing) 'ipv4only.arpa' still does not appear as one of the names with special properties recorded in the Special-Use Domain Names registry.
As a result of this omission, in cases where software needs to give this name special treatment in order for it to work correctly, there is no clear mandate authorizing software authors to implement that special treatment. Software implementers are left with the choice between not implementing the special behavior necessary for the name queries to work correctly, or implementing the special behavior and being accused of being noncompliant with some RFC.
This document formally declares the actual special properties of the name, and adds similar declarations for the corresponding reverse mapping names.
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The specification for how a client discovers its network's NAT64 prefix defines the special name 'ipv4only.arpa' for this purpose, but declares it to be a non-special name in that specification's Domain Name Reservation Considerations section.
Consequently, despite the well articulated special purpose of the name, (at the time of writing) 'ipv4only.arpa' still does not appear as one of the names with special properties recorded in the Special-Use Domain Names registry.
As a result of this omission, in cases where software needs to give this name special treatment in order for it to work correctly, there is no clear mandate authorizing software authors to implement that special treatment. Software implementers are left with the choice between not implementing the special behavior necessary for the name queries to work correctly, or implementing the special behavior and being accused of being noncompliant with some RFC.
This document formally declares the actual special properties of the name, and 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. In normal operation, clients never query for the two records that do in fact exist; instead they query for records that are known to not exist, and then get 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, these clients have, in effect, co-opted the DNS protocol as an impromptu client-to-middlebox communication protocol, to communicate with the NAT64/DNS64 [RFC6146][RFC6147] 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 as an impromptu client-to-middlebox communication protocol that makes the name 'ipv4only.arpa' most definitely a special name, and one that should be listed in IANA's registry along with other DNS names that have special uses.
As a result of the original specification not formally declaring 'ipv4only.arpa' to have special properties, there was no mandate for any DNS 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.
A more serious problem occurs when a device is configured to use a recursive/caching DNS server other than the one it learned from the network. Typically a device joining a NAT64 network will learn the recursive/caching DNS server recommended for that network either via IPv6 Router Advertisement Options for DNS Configuration or via DNS Configuration options for DHCPv6. On a NAT64 network it is essential that the client use the recursive/caching DNS64 server recommended for that network, since only that DNS64 server can be relied upon to know the appropriate prefix(es) to use for synthesizing IPv6 addresses that will be acceptable to the NAT64 server.
However, it is not uncommon for users to manually override their default DNS configuration because they wish to use some other public recursive resolver on the Internet, perhaps because they perceive their preferred recursive resolver to be faster, more reliable, or more trustworthy.
Another common scenario is the use of corporate VPN client software, which overrides the default configuration to divert DNS requests to the company's own private internal recursive resolver, because the local network's recursive resolver will typically be unable to provide answers for the company's private internal host names. Similarly, the company's private internal recursive resolver may not be able to synthesize IPv6 addresses correctly for use with the local network's NAT64 server, because it is unlikely to be aware of the NAT64 prefix in use on the local network. It is clear that a single recursive resolver cannot meet both needs. The local network's recursive resolver cannot give answers for some company's private internal host names, and some company's private internal recursive resolver cannot give correctly synthesized IPv6 addresses suitable for the local network's NAT64 gateway.
The conflict here arises because DNS is being used for two unrelated purposes. The first purpose is retrieving data from a (nominally) global database -- generally retrieving the IP address(es) associated with a hostname. The second purpose is using the DNS protocol as a middlebox communication protocol, to interrogate the local network infrastructure to discover the IPv6 prefix(es) in use by the local NAT64 gateway for address synthesis.
(Possibly this problem could be solved if we could force all NAT64 gateways to use the same Well-Known Prefix for IPv6 address synthesis [RFC6052], but that would alleviate the need for 'ipv4only.arpa' altogether.)
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.
... 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.
Section 3 of the DNS64 specification discusses this:
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 does not 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 the
Special-Use Domain Names registry:
IANA has recorded the following IPv4 addresses in the
IPv4 Special-Purpose Address Registry:
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this section 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.
There are no subdomains of ipv4only.arpa. All names falling below 'ipv4only.arpa' are defined to be nonexistent (NXDOMAIN).
The name 'ipv4only.arpa' is special to
(a) client software wishing to perform DNS64 address synthesis,
(b) APIs responsible for retrieving the correct information, and
(c) the DNS64 recursive/caching server responding to such requests.
These three considerations are listed in items 2, 3 and 4 below:
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‑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'.
There are no subdomains of '170.0.0.192.in‑addr.arpa' or '171.0.0.192.in‑addr.arpa'. All names falling below these names are defined to be nonexistent (NXDOMAIN).
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:
For all IPv6 addresses synthesized by a DNS64 recursive resolver, the DNS64 recursive resolver server is responsible for synthesizing the appropriate 'ip6.arpa' reverse mapping PTR records too, if it chooses to provide reverse mapping PTR records. 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 PTR 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 reverse mapping queries for those IPv4 addresses, but instead, according to rule 3 above, immediately returns the answer 'ipv4only.arpa'.
In the case of a client that uses the 'ipv4only.arpa' query to discover the IPv6 prefixes in use by the local NAT64 gateway, and then proceeds to perform its own address synthesis locally (which has benefits such as allowing DNSSEC validation), that client MUST also synthesize 'ip6.arpa' reverse mapping PTR records for those discovered prefix(es), according to the rules above: When a client's name resolution APIs and libraries receive a request to look up an 'ip6.arpa' reverse mapping PTR record for an address that falls within one of the discovered NAT64 address synthesis prefixes, the software extracts the embedded IPv4 address and then, for IPv4 addresses 192.0.0.170 and 192.0.0.171, returns the fixed answer 'ipv4only.arpa', and for all other IPv4 addresses performs a reverse mapping PTR query for the IPv4 address, and then synthesizes a corresponding 'ip6.arpa' reverse mapping PTR record containing the same rdata.
Thanks to Jouni Korhonen, Teemu Savolainen, and Dan Wing, for devising the NAT64 Prefix Discovery mechanism, and for their feedback on this document. Thanks to Geoff Huston for his feedback on the draft, and to Erik Kline for pointing out that the in‑addr.arpa names are special too. Thanks particularly to Lorenzo Colitti for an especially spirited hallway discussion at IETF 96 in Berlin, which lead directly to significant improvements in how this document presents the issues.
| [SUDN] | "Special-Use Domain Names Registry" |
| [SUv4] | "IANA IPv4 Special-Purpose Address Registry" |
A BIND 9 recursive/caching DNS server can be configured to act as authoritative for the necessary DNS64 names as described below.
zone "ipv4only.arpa" { type master; file "ipv4only"; };
In /etc/named.conf the following line is added:
$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 ; If not using Well-Known Prefix
@ IN AAAA 64:ff9b::192.0.0.171 ; place actual NAT64 prefix here
The file /var/named/ipv4only is created with the following content: