Internet Engineering Task Force L. Howard
Internet-Draft Time Warner Cable
Intended status: Informational A. Durand
Expires: September 5, 2010 Comcast
March 4, 2010
Reverse DNS in IPv6 for Internet Service Providers
draft-howard-isp-ip6rdns-02
Abstract
In IPv4, Internet Service Providers (ISPs) commonly provide IN-
ADDR.ARPA. information for their customers by prepopulating the zone
with one PTR record for every available address. This practice does
not scale in IPv6. This document analyzes different approaches for
ISPs to manage the ip6.arpa zone for IPv6 address space assigned to
many customers.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Reverse DNS in IPv4 . . . . . . . . . . . . . . . . . . . 3
1.2. Reverse DNS Considerations in IPv6 . . . . . . . . . . . . 4
2. Alternatives in IPv6 . . . . . . . . . . . . . . . . . . . . . 5
2.1. No Response . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Wildcard match . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Dynamic DNS . . . . . . . . . . . . . . . . . . . . . . . 6
2.3.1. Dynamic DNS from Individual Hosts . . . . . . . . . . 7
2.3.2. Dynamic DNS through Residential Gateways . . . . . . . 7
2.3.3. Dynamic DNS Delegations . . . . . . . . . . . . . . . 8
2.3.4. Generate Dynamic Records . . . . . . . . . . . . . . . 8
2.3.5. Populate from DHCP Server . . . . . . . . . . . . . . 8
2.4. Delegate DNS . . . . . . . . . . . . . . . . . . . . . . . 9
2.5. Dynamically Generate PTR When Queried ("On the Fly") . . . 9
3. Security Considerations . . . . . . . . . . . . . . . . . . . 10
3.1. Using Reverse DNS for Security . . . . . . . . . . . . . . 10
3.2. DNS Security with Dynamic DNS . . . . . . . . . . . . . . 10
3.3. Considerations for Other Uses of the DNS . . . . . . . . . 10
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1. Normative References . . . . . . . . . . . . . . . . . . . 11
5.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
Best practice [RFC1033] is that "Every Internet-reachable host should
have a name" [RFC1912] that is recorded with a PTR resource record in
the IN-ADDR.ARPA zone. Many network services perform a PTR lookup on
the source address of incoming packets before performing services.
Some of the most common uses for reverse DNS include:
o Building trust. An administrator who spends time and effort
properly maintaining DNS records might be assumed to spend time
and effort on other maintenance, so the network might be more
trustworthy.
o Validating other data. Information from reverse DNS may be
compared to information higher in the stack (for instance, mail
originator), with a lower trustworthiness if they are dissimilar.
o Some degree of location information can often be inferred, since
most administrators create reverse zones corresponding to
aggregation points, which often correspond with geographical
areas. This information is useful for geolocation services and
for law enforacement.
Individual Internet users in the residential or consumer scale,
including small and home businesses, are constantly joining or moving
on the Internet. For large Internet service providers who serve
residential users, maintenance of individual PTR records for is often
impractical. Administrators of ISPs should evaluate methods for
responding to reverse DNS queries in IPv6.
1.1. Reverse DNS in IPv4
Internet service providers (ISPs) that provide access to many
residential users typically assign one or a few IPv4 addresses to
each of those users, and populate an IN-ADDR.ARPA zone with one PTR
record for every IPv4 address. Some ISPs also configure forward
zones with matching A records, so that lookups match. For instance,
if an ISP Example.com aggregated 192.0.2.0/24 at a network hub in
Anytown in the province of AnyWhere, the reverse zone might look
like:
1.2.0.192.IN-ADDR-ARPA. IN PTR 1.user.anytown.AW.example.com.
2.2.0.192.IN-ADDR-ARPA. IN PTR 2.user.anytown.AW.example.com.
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3.2.0.192.IN-ADDR-ARPA. IN PTR 3.user.anytown.AW.example.com.
.
.
.
254.2.0.192.IN-ADDR-ARPA. IN PTR
254.user.anytown.AW.example.com.
The conscientious Example.com might then also have a zone:
1.user.anytown.AW.example.com. IN A 1.2.0.192.IN-ADDR-ARPA.
2.user.anytown.AW.example.com. IN A 2.2.0.192.IN-ADDR-ARPA.
3.user.anytown.AW.example.com. IN A 3.2.0.192.IN-ADDR-ARPA.
.
.
.
254.user.anytown.AW.example.com. IN A 254.2.0.192.IN-ADDR-
ARPA.
Many ISPs generate PTR records for all IP addresses used for
customers, and many create the matching A record.
1.2. Reverse DNS Considerations in IPv6
The length of individual addresses makes manual zone entries
cumbersome. A sample entry for 2001:db8:f00::0012:34ff:fe56:789a
might be:
a.9.8.7.6.5.e.f.f.f.4.3.2.1.0.0.0.0.0.0.0.0.f.0.8.b.d.0.1.0.0.2
.IP6.ARPA. IN PTR 1.user.anytown.AW.example.com.
Since 2^^96 possible addresses could be configured in the 2001:db8:
f00/48 zone alone, it is impractical to write a zone with every
possible address entered. If 1000 entries could be written per
second, the zone would still not be complete after two quintillion
years.
Furthermore, since the 64 bits in the host portion of the address are
frequently assigned using SLAAC [RFC4826] when the host comes online,
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it is not possible to know which addresses may be in use ahead of
time.
[RFC1912] is an informational document that says "PTR records must
point back to a valid A record" and further that the administrator
should "Make sure your PTR and A records match." [RFC1912] DNS
administrators of residential ISPs should consider how to follow this
advice for AAAA and PTR RRs in the residential ISP.
2. Alternatives in IPv6
Several options existing for providing reverse DNS in IPv6. All of
these options also exist for IPv4, but the scaling problem is much
less severe in IPv4. Each option should be evaluated for its scaling
ability, its compliance with existing standards and best practices,
and its availability in common systems.
2.1. No Response
Some ISP DNS administrators may choose to provide only a NXDOMAIN
response to PTR queries for subscriber addresses. Providing no data
in response to PTR queries does not satisfy the expectation in
[RFC1912] for entries to match. Users of services which are
dependent on a successful lookup will have a poor experience. DNS
administrators should consider the uses described above and the
number of services affecting the number of users when evaluating this
option.
2.2. Wildcard match
The use of wildcards in the DNS is described in [RFC4592], and their
use in IPv6 reverse DNS is described in [RFC4472]. Use of a wildcard
may lead to unpredictable results if other records exist in the zone.
Administrators considering use of wildcards for PTR records in IPv6
should determine whether the wildcard will match all records and
subdomains of the zone.
Consider a PTR lookup on 2001:db8:f00:1234:0012:34ff:fe56:789a
against the following entries:
*.4.3.2.1.0.0.f.0.8.b.d.f.0.1.0.0.2.ip6.arpa. IN PTR
1.user.anytown.AW.example.com.
a.9.8.7.6.5.e.f.f.f.4.3.2.1.0.0.4.3.2.1.0.0.f.0.8.b.d.0.1.0.0.2
.ip6.arpa. IN PTR mail.user.anytown.AW.example.com.
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2.1.0.0.4.3.2.1.0.0.f.0.8.b.d.0.1.0.0.2.ip6.arpa. IN NS
ns.user.anytown.AW.example.com.
The administrator should determine what response would be returned
for such a query. While recording all possible addresses is not
scalable, it may be possible to record a wildcard entry for each
prefix assigned to a customer. Consider also that "inclusion of
wildcard NS RRSets in a zone is discouraged, but not barred."
[RFC4035]
This solution generally scales well. However, since the response
will match any address in the wildcard range (/48, /56, /64, etc.), a
forward DNS lookup on that response given will not be able to return
the same hostname. This method therefore fails the expectation in
[RFC1912] for forward and reverse to match. DNSsec [RFC4035]
scalability is limited to signing the wildcard zone, which may be
satisfactory.
2.3. Dynamic DNS
One way to ensure forward and reverse records match is for hosts to
update DNS servers dynamically, once interface configuration (whether
SLAAC, DHCPv6, or other means) is complete, as described in
[RFC4472]. Hosts would need to provide both AAAA and PTR updates,
and would need to know which servers would accept the information.
This option should scale as well or as poorly as IPv4 dynamic DNS
does. Dynamic DNS may not scale effectively in large ISP networks
which have no single master name server. The ISP's DNS system may
provide a point for Denial of Service attacks, including many
attempted dDNS updates. Accepting updates only from authenticated
sources may mitigate this risk, but only if authentication itself
does not require excessive overhead. No authentication of dynamic
DNS updates is inherently provided; implementers should consider use
of DNSsec [RFC2535], or at least ingress filtering so updates are
only accepted from customer address space from internal network
interfaces. UDP is allowed per [RFC2136] so transmission control is
not assured, though the host should expect an ERROR or NOERROR
message from the server [RFC2136]; TCP provides transmission control,
but the updating host would need to be configured to use TCP.
Administrators should consider what domain will contain the records,
and who will provide the names. If subscribers provide hostnames,
they may provide inappropriate strings. Consider "ihate.example.com"
or "badword.customer.example.com" or
"celebrityname.committed.illegal.acts.example.com."
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2.3.1. Dynamic DNS from Individual Hosts
In the simplest case, a residential user will have a single host
connected to the ISP. Since the typical residential user cannot
configure IPv6 addresses and resolving name servers on their hosts,
the ISP should provide address information conventionally (i.e.,
their normal combination of RAs, SLAAC, DHCP, etc.), and should
provide a DNS Recursive Name Server and Domain Search List via DHCPv6
as described in [RFC3646]. In determining its Fully Qualified Domain
Name, hosts typically use a domain from the Domain Search List. This
is an overloading of the parameter; multiple domains could be listed,
since hosts may need to search for unqualified names in multiple
domains, without necessarily being a member of those domains.
Administrators should consider whether the domain search list
actually provides an appropriate DNS suffix(es) when considering use
of this option. For purposes of dynamic DNS, the host would
concatenate its local hostname (e.g., "hostname") plus the domain(s)
in the Domain Search List (e.g., "customer.example.com"), as in
"hostname.customer.example.com."
Once it learns its address, and has a resolving name server, the host
must perform an SOA lookup on the ip6.arpa record to be added, to
find the owner, which will lead to the NS record. Several recursive
lookups may be required to find the longest prefix which has been
delegated. The DNS administrator must designate the Primary Master
Server for the longest match required. Once found, the host sends
dynamic AAAA and PTR updates using the concatenation defined above
("hostname.customer.example.com").
In order to use this alternative, hosts must be configured to use
dynamic DNS. This is not default behavior for many hosts, which is
an inhibitor for the large ISP.
2.3.2. Dynamic DNS through Residential Gateways
Residential customers may have a gateway, which may provide DHCPv6
service to hosts from a delegated prefix. ISPs should provide a DNS
Recursive Name Server and Domain Search List to the gateway, as
described above and in [RFC3646]. There are two options for how the
gateway uses this information. The first option is for the gateway
to respond to DHCPv6 requests with the same DNS Recursive Name Server
and Domain Search List provided by the ISP. The alternate option is
for the gateway to relay dynamic DNS updates from hosts to the
servers and domain provided by the ISP. Host behavior is unchanged;
they should provide updates to the ISP's servers as described above.
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2.3.3. Dynamic DNS Delegations
An ISP may delegate authority for a subdomain such as
"customer12345.anytown.AW.customer.example.com" or
"customer12345.example.com" to the customer's gateway. Each domain
thus delegated must be unique within the DNS. The ISP may also then
delegate the ip6.arpa zone for the prefix delegated to the customer,
as in (for 2001:db8:f00::/48) "0.0.f.0.8.b.d.0.1.0.0.2.ip6.arpa."
Then the customer could provide updates to their own gateway, with
forward and reverse. However, individual hosts connected directly to
the ISP rarely have the capability to run DNS for themselves;
therefore, an ISP can only delegate to customers with gateways
capable of being authoritative name servers. If a device requests a
DHCPv6 Prefix Delegation, that may be considered a reasonably
reliable indicator that it is a gateway, rather than an individual
host. It is not necessarily an indicator that the gateway is capable
of providing DNS services, and therefore cannot be relied upon as a
way to test whether this option is feasible.
If the customer's gateway is the name server, it provides its own
information to hosts on the network, as normally done for enterprise
networks, and as described in [RFC2136].
An ISP may elect to provide authoritative responses as a secondary
server to the customer's primary server.
To implement this alternative, users' residential gateways must be
capable of acting as authoritative name servers capable of dynamic
DNS updates.
2.3.4. Generate Dynamic Records
An ISP's name server that receives a dynamic forward or reverse DNS
update may create a matching entry. Since a host capable of updating
one is generally capable of updating the other, this should not be
required, but redundant record creation will ensure a record exists.
ISPs implementing this method should check whether a record already
exists before accepting or creating updates.
This method is also dependent on hosts being capable of providing
dynamic DNS updates, which is not default behavior for many hosts.
2.3.5. Populate from DHCP Server
A ISP's DHCPv6 server may populate the forward and reverse zones when
the DHCP request is received, if the request contains enough
information.[RFC4702]
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However, this method will only work for a single host address; the
ISP's DHCP server would not have enough information to update all
records for a prefix delegation. If the zone authority is delegated
to a home gateway which used this method, the gateway could update
records for residential hosts. To implement this alternative, users'
residential gateways must support the FQDN DHCP option.
2.4. Delegate DNS
For customers who are able to run their own DNS servers, such as
commercial customers, often the best option is to delegate the
reverse DNS zone to them, as described in [RFC2317]. However, since
most residential users have neither the equipment nor the expertise
to run DNS servers, this method is unavailable to residential ISPs.
This is a general case of the specific case described in
Section 2.3.3. All of the same considerations still apply.
2.5. Dynamically Generate PTR When Queried ("On the Fly")
Common practice in IPv4 is to provide PTR records for all addresses,
regardless of whether a host is actually using the address. In IPv6,
ISPs may generate PTR records for all IPv6 addresses as the records
are requested. Configuring records "on the fly" may consume more
processor resource than other methods, but only on demand. A denial
of service is therefore possible, but with rate-limiting and normal
countermeasures, this risk is no higher than with other options.
An ISP using this option should generate a PTR record on demand, and
cache or prepopulate the forward (AAAA) entry for the duration of the
time-to-live of the PTR. This option has the advantage of assuring
matching forward and reverse entries, while being simpler than
dynamic DNS. Administrators should consider whether the lack of
user-specified hostnames is a drawback.
This method may not scale well in conjunction with DNSsec [RFC4035],
because of the additional load, but since keys may be pregenerated
for zones, and not for each record, the risk is moderate.
Another consideration is that the algorithm used for generating the
record must be the same on all servers for a zone. In other words,
any server for the zone must produce the same response for a given
query. Administrators managing a variety of rules within a zone
might find it difficult to keep those rules synchronized on all
servers.
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3. Security Considerations
3.1. Using Reverse DNS for Security
Some people think the existence of reverse DNS records, or matching
forward and reverse DNS records, provides useful information about
the hosts with those records. For example, one might infer that the
administrator of a network with properly configured DNS records was
better-informed, and by further inference more responsible, than the
administrator of a less-thoroughly configured network. For instance,
most email providers will not accept incoming connections on port 25
unless forward and reverse DNS entries match. If they match, but
information higher in the stack (for instance, mail source) is
inconsistent, the packet is questionable. These records may be
easily forged though, unless DNSsec or other measures are taken. The
string of inferences is questionable, and may become unneeded if
other means for evaluating trustworthiness (such as positive
reputations) become predominant in IPv6.
Providing location information in PTR records is useful for
troubleshooting, law enforcement, and geolocation services, but for
the same reasons can be considered sensitive information.
3.2. DNS Security with Dynamic DNS
Security considerations of using dynamic DNS are described in
[RFC3007]. DNS Security Extensions are documented inRFC2535
[RFC2535].
Interactions with DNSsec are described throughout this document.
3.3. Considerations for Other Uses of the DNS
Several methods exist for providing encryption keys in the DNS. Any
of the options presented here may interfere with these key
techniques.
4. IANA Considerations
There are no IANA considerations or implications that arise from this
document.
5. References
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5.1. Normative References
[RFC1033] Lottor, M., "Domain Administrators Operators Guide",
November 1987.
[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
November 1987.
[RFC1912] Barr, D., "Common DNS Operational and Configuration
Errors", February 1996.
[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
April 1917.
[RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", November 2000.
[RFC3646] Droms, R., Ed., "DNS Configuration options for Dynamic
Host Configuration Protocol for IPv6 (DHCPv6)",
December 2003.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", March 2005.
[RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name
System", July 2006.
[RFC4702] Stapp, M., Volz, Y., and Y. Rekhter, "The Dynamic Host
Configuration Protocol (DHCP) Client Fully Qualified
Domain Name (FQDN) Option".
[RFC4826] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", September 2007.
[RFC883] Mockapetris, P., "Domain names: Implementation
specification", November 1983,
<http://tools.ietf.org/html/rfc883>.
5.2. Informative References
[RFC2317] Eidnes, H., de Groot, G., and P. Vixie, "Classless IN-
ADDR.ARPA delegation", March 1998.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
March 1999.
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[RFC2672] Crawford, M., "Non-Terminal DNS Name Redirection",
August 1999.
[RFC4339] Jeong, J., Ed., "IPv6 Host Configuration of DNS Server
Information Approaches", February 2006.
[RFC4472] Durand, A., Ihren, J., and P. Savola, "Operational
Considerations and Issues with IPv6 DNS", April 2006.
[inaddr-reqd]
Senie, D., "draft-ietf-dnsop-inaddr-required-07",
August 2005.
[rmap-consider]
Senie, D. and A. Sullivan,
"draft-ietf-dnsop-reverse-mapping-considerations-06",
March 2008.
Authors' Addresses
Lee Howard
Time Warner Cable
13820 Sunrise Valley Drive
Herndon, VA 20171
US
Email: lee.howard@twcable.com
Alain Durand
Comcast
1 Comcast center
Philadelphia, PA 19103
US
Phone:
Email: alain_durand@cable.comcast.com
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