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This document contains a number of observations on the implications of
the use of wildcards in DNS zones, and makes some recommendations
concerning their use.
The contact person for the IAB on this statement is
<mailto:harald@xxxxxxxxxxxxx>Harald Alvestrand
19 September 2003
IAB Commentary:
Architectural Concerns on the use of DNS Wildcards
There are many architectural assumptions regarding DNS behavior that are
not specified in the IETF standards documents describing DNS, but which
are deeply embedded in the behavior of Internet protocols and
applications. These assumptions are inherent parts of the network
architecture of which the DNS is one component.
It has long been known that it is possible to use DNS wildcards in ways
that violate these assumptions.
Recent deployments of DNS wildcards with A records at high levels in the
DNS tree have shown by experience that the cost of violating these
assumptions is significant. In this document we provide an explanation of
how DNS wildcards function, and many examples of how their injudicious
use negatively impacts both individual Internet applications and indeed
the Internet architecture itself.
In particular, we recommend that DNS wildcards should not be used in a
zone unless the zone operator has a clear understanding of the risks, and
that they should not be used without the informed consent of those
entities which have been delegated below the zone.
----------
A brief primer on DNS wildcards
The DNS "wildcard" mechanism has been part of the DNS protocol since the
original specifications were written twenty years ago, but the
capabilities and limitations of wildcards are sufficiently tricky that
discussions of both the protocol details of precisely how wildcards
should be implemented and the operational details of how wildcards should
or should not be used continue to the present day. This section attempts
to explain the essential details of how wildcards work, but readers
should refer to the DNS specifications
([<http://www.ietf.org/rfc/rfc1034.txt>RFC 1034] et sequentia) for the
full details.
In essence, DNS wildcards are rules which enable an authoritative name
server to synthesize DNS resource records on the fly. The basic mechanism
is quite simple, the complexity is in the details and implications.
The most basic and by far the most common operation in the DNS protocols
is a simple query for all resource records matching a given query name,
query class, and query type. Assuming (for simplicity) that all the
software and networks involved are working correctly, such a query will
produce one of three possible results: success
If the system finds a match for all three parameters, it returns the
matching set of resource records;
no data
If the system finds a match for the query name and query class but not
for the query type, it returns an indication that the name exists but no
data matching the given query type is present.
no such name
If the system fails to find a match for the given query name and query
class, it returns an indication that the name does not exist.
Ordinarily, matches for all three parameters must be exact. This is where
wildcards come into the picture.
A wildcard record is an otherwise ordinary DNS resource record whose
leftmost (least significant) label consists of a single asterisk ("*")
character, such as "*.bar.example". Conceptually, the asterisk matches
one or more labels at the left (least significant) end of the DNS name.
When wildcard records are present, the rules become more complicated.
Specifically, if the query class matches, there is no exact match for the
query name, and the closest match for the query name is a wildcard, the
system in effect synthesizes a set of resource records matching the query
name on the fly by treating the resource records present at the wildcard
name as if they had been present at the query name. Thus, if the wildcard
name has records matching the desired query type, the system will return
those records, precisely as in the "success" case above; otherwise, the
system will return an indication that the name exists but no data
matching the given query type is present, precisely as in the "no data"
case above. The response is identical to that of a normal "success"
response for the query name, so the resolver which issued the query can
not tell that the results it got back were the result of wildcard expansion.
Note that, in the case of a wildcard match, the "no such name" case
cannot occur; the wildcard match eliminates this possibility. Note also
that only the query name and query class matter for purposes of
determining whether a wildcard matches: any record type can produce a
wildcard match, regardless of whether or not the record type happens to
match the query type.
----------
Problems with Wildcard Records
One of the main known weaknesses and dangers of wildcard records is that
they interact poorly with any use of the DNS which depends on "no such
name" responses. The list of such uses turns out to be quite large, and
will be discussed in some detail in a later section.
Another known weakness and danger of wildcard records stems from the fact
that the wildcard label will match anything at all, so long as no
non-wildcard name within the zone is a closer match to the query name
than the wildcard is. This doesn't sound like a major problem until one
considers the number of conventions and, in some cases, protocols, which
use labels at the left (least significant) ends of the names of resource
records to distinguish between records associated with different
services, rather than using different types of records. That is, in these
cases, otherwise unrelated services use the same type of record and
clients (or users) are expected to use the name corresponding to the
particular service desired. This applies both to the ad-hoc naming
conventions described in [<http://www.ietf.org/rfc/rfc2219.txt>RFC 2219]
such as www.foo.example and also to mechanisms such as the SRV record
type [<http://www.ietf.org/rfc/rfc2782.txt>RFC 2782] in which the naming
scheme is part of the formal protocol. When names of this type are
covered by wildcards such as an address record named *.bar.example, such
a wildcard would hand back the same address record regardless of the
service name encoded in the query name, thus ftp.foo.bar.example,
mail.foo.bar.example, ntp.foo.bar.example and so forth would all end up
with the same synthesized address record. This problem is even worse in
the SRV case, both because names such as _finger._tcp.foo.bar.example are
part of the protocol and because SRV records include TCP and UDP port
numbers, so the client will be confused not only about which host it
should contact but also about the port on which it should contact that
host. The only way to avoid these problems with names of this type is to
add explicit records for such names to the DNS.
Finally, the two factors listed above ("match anything" behavior, and
poor interaction with anything that depends on "no such name" responses)
interact with normal and predictable human errors to allow wildcards to
have effects far beyond their intended scope. Properly speaking, a
wildcard record's scope is limited to a single zone, since, by
definition, a wildcard record never matches any name that really does
exist in the zone, and thus will not match any (non-wildcard) delegation
of a portion of the namespace from a parent zone to its child. (Wildcard
NS records, while theoretically possible, have sufficiently bizarre
semantics that it is probably best to limit their use to torture-tests of
DNS software.) So, at first blush, it would seem that the administrator
of a zone is free to use wildcards without worrying about effects which
this might have on the zone's delegated children. Unfortunately, this
turns out not to be the case, because DNS names are heavily exposed in
user interfaces, and users, being humans, make mistakes. So, while
delegating the bar.example zone will prevent a wildcard record *.example
from affecting a user who typed foo.bar.example as foi.bar.example, it
will not prevent the same wildcard record from affecting the same user
when the error is foo.bat.example. Thus, from the users' point of view,
some of the effects of wildcards do leak from a parent zone to its
children. This is not a big deal if the parent and child zones are
associated with a single organization, but it can become a real problem
if the parent and child zones are associated with different organizations
whose interests are not perfectly aligned.
The above is probably not an exhaustive list. Even after twenty years of
experience with the DNS, the effects of unexpected uses of wildcards can
still be quite surprising, because the small but fundamental way in which
they change the record lookup rules has a nasty way of violating implicit
(or, sometimes, explicit) assumptions in deployed DNS-using software.
For these reasons, almost all use of DNS wildcards has been limited to a
relatively small number of reasonably well-understood roles, and most
wildcard use has been limited to a single role: the MX records used in
mail delivery.
Since MX records are only used for electronic mail delivery, wildcard MX
records are relatively safe, and since electronic mail for any particular
DNS name is generally handed by the organization that is furthest down
the delegation tree, wildcard MX records are most likely to appear in
zones where their effects will not cross organizational boundaries. While
the latter is not universally true, the primary use of wildcard records
has been and remains wildcard MX records for handling an organization's
own mail.
Given these issues, it seems clear that the use of wildcards with record
types that affect more than one protocol should be approached with
caution, that the use of wildcards in situations where their effects
cross organizational boundaries should also be approached with caution,
and that the use of wildcards with record types that affect more than one
protocol in situations where the effects cross organizational boundaries
should be approached with extreme caution, if at all.
----------
Principles To Keep In Mind
In reading the rest of this document, it may be helpful to bear in mind
two basic principles of architectural design which have served the
Internet well for many years:
* The Robustness Principle: "Be conservative in what you do, be
liberal in what you accept from others." [Jon Postel,
<http://www.ietf.org/rfc/rfc793.txt>RFC 793]
* The Principle Of Least Astonishment: A program should always
respond in the way that is least likely to astonish the user.
[Traditional, original source unknown]
We will come back to these points after the next section.
----------
Problems encountered in a recent experiment with wildcards
We have recently had the opportunity to observe the results of an
experiment on use of wildcards in large top-level domains, with some
rather undesirable and unintended consequences. This section attempts to
detail some of the problems that network users and operators around the
world encountered as a result of this deployment.
We must emphasize that, technically, this was a legitimate use of
wildcard records that did not in any way violate the DNS specifications
themselves. One of our main points here is that simply complying with the
letter of the protocol specification is not sufficient to ensure the
operational stability of the applications which depend on the DNS: there
are protocol features which simply are not safe to use in some circumstances.
The specific change which this operator chose to make was to add a single
wildcard address record at the zone apex of each of the affected zones.
As a direct result of this change, two things happened:
* the authoritative servers for these two zones no longer give out
"no such name" responses for any possible name in these zones, and
* every possible name rooted in one of these zones which, until this
change, did not exist at all, now has a synthesized address record
pointing at a "redirection server" run by the operator of this zone.
The implications of this simple change were many and varied. The list
below is almost certainly incomplete:
Web Browsing
Web browsers all over the world stopped displaying "page not found" in
the local language and character set of the users when given incorrect
URLs rooted under these TLDs. Instead, these browsers now display an
English language search page from a web server run by the zone operator.
It should be noted that the language tags in the HTTP protocol do not
always match the locale used in the local browser. So, even though the
global search page is dynamic and uses the information in the HTTP
request to guess what language and script is to be used -- it will never
be able to emulate what the user expected. There is, in short, not enough
context in the HTTP protocol for the engine which generates the search page.
In many situations, web browsers have been written to provide some
assistance to the user, often based on local conventions, directories,
and language, when a DNS lookup fails. All such systems are now disabled
for URLs rooted under these TLDs, since DNS lookups no longer fail, even
when the specified destination does not exist.
Even if these were acceptable changes, the new mechanism has poor scaling
properties, and unless the operator chooses to invest significant
resources in maintaining a large, robust web server setup, the user
experience is going to get even worse: instead of either a local language
error message or an English search page, the user is going to get
"attempting to connect..." followed by a long wait.
Email
All mail to non-existent hostnames under these TLDs now flows to the
registry operator's server, where the registry operator bounces it. Some
operators find this intolerable and have changed their mail system
configurations to bypass this "bounce service", but the vast majority of
mail servers undoubtedly now route mail for nonexistent names under these
TLDs to the bounce server rather than just bouncing it directly. This has
a number of ramifications:
* If operators choose to allow their mail to go to the bounce server,
they now have an increased mail load handling additional routing of
messages to the bounce server; if operators choose not to allow this to
happen, they have an additional development and maintenance burden
configuring their servers to prevent it.
* Operators who allow mail to go to the bounce server are now
dependent on the performance of the bounce server. If the bounce server
ever slows or fails, mail that previously would bounce will now queue at
the SMTP relay for that relay's queue time before bouncing back to the
user. This creates a very poor user experience, since typographical
errors that in the past would have bounced immediately may now go
unnoticed for several days.
* Operators who allow mail to go to the bounce server are also
dependent on the correct operation of the bounce server. If the bounce
server is buggy (which happened to be the case with this rollout), mail
may not bounce at all: it may be reported to the user as having been
delivered correctly while actually vanishing without a trace. This also
creates a very poor user experience.
* In some cases where the set of MX records associated with a
particular DNS name included a misconfigured record pointing to a
nonexistent hostname, installing these wildcard records was the last
straw that broke a misconfigured-but-functional mail configuration:
previously, the nonexistent hostname would have failed to resolve and
been ignored, now it bounces.
* The normal flow of data from a client in SMTP when one address has
a typo is as follows:
* The client looks up the IP address of his outgoing SMTP proxy
in DNS.
* The client opens a TCP connection to his outgoing SMTP proxy.
* The client sends information about himself to the SMTP proxy.
* The proxy accepts or rejects the client.
* The client sends information about the recipient to the SMTP proxy.
* The proxy look up the destination in DNS, and gets "no such
name" back.
* The proxy sends information to the client that the address is
wrong.
With a wildcard for mistyped domain, the following happens:
* The client looks up the IP address of his outgoing SMTP proxy
in DNS.
* The client opens a TCP connection to his outgoing SMTP proxy.
* The client sends information about himself to the SMTP proxy.
* The proxy accepts or rejects the client.
* The client sends information about the recipient to the SMTP proxy.
* The proxy looks up the destination in DNS, and gets "success" back.
* The proxy accepts the message and closes the connection to the
client.
* The proxy opens a TCP connection to the bounce server.
* The proxy present himself to the bounce server.
* The bounce server indicates that the recipient address is not
acceptable.
* The proxy generates an error message which is sent back to the
sender's email address.
* A different scenario happens if the SMTP client has been
misconfigured with the incorrect name of the outgoing SMTP proxy. As the
domain name resolves using a wildcard, the client will connect to the
bounce server, and start to send mail to it. The result is that the
bounce server (at the IP address of the wildcard) says that the
recipient address is wrong even though it is in fact correct. The error
presented to the user is incorrect, as it is the name of the outgoing
proxy which was wrong and not the name of the recipient.
Informing Users of Errors
Many application GUIs check domain names for validity before allowing the
user to progress to the next step. Examples include email clients that
directly check the domain of the email addresses resolves before sending,
and network printer configuration tools that check that the print spooler
name is valid before accepting the configuration. Previously the user
would be prompted early that they had made an error in the domain name.
In the case of email, the error may now not be noticed at the time of
sending, but only when email later bounces. In the case of the printer
configuration, the error may not be noticed during configuration, but
only afterwards when printing fails to work, where the problem diagnosis
is more difficult.
Spam Filters
Installing these wildcard records broke several simple spam filters
commonly used to front end inbound mail servers, as well as more complex
filtering that checks for the existence of a sending domain in order to
screen out obviously bogus senders. This technique for spam has
diminished as this filtering mechanism has increased, but one sample
operator reports that it still equals about 10% of inbound mail attempts
on their large shared MX cluster. ISPs who are aware of this problem will
probably extend their filtering rules to have special knowledge of the
address returned by these wildcard records, but will have to carry the
cost of doing so, both in terms of code maintenance and increased
execution time for their filtering.
Interactions with Other Protocols
The wildcard address records trap lookups for any network service, but
the number of protocols somewhere in use on the Internet (including
private protocols used between two or more parties on ports which they
may or may not have registered with IANA) is large enough that it simply
is not possible for the zone operator (or anyone) to provide a
redirection service for every protocol. In this particular example, the
zone operator only provided handlers for HTTP (which they directed to a
search page) and SMTP (which they attempted to bounce). All other
protocols received at best TCP resets, or, in some cases, simply had
their packets dropped. Any application that uses the DNS has (or should
have) some way of handling "no such name" errors; in almost all cases the
error message is sufficiently clear to an experienced user that it is
immediately obvious when the application has failed because it was given
an incorrect DNS name. With these wildcard records in place, however,
incorrect DNS names which are matched by the wildcard record will not
show up as DNS name errors at all, but instead will show up as mysterious
connection failures or as unreachable destinations for all services that
the zone operator does not redirect. Depending on the details of the
application protocol and implementation involved, this change may also
convert an obvious "hard failure" (incorrect name) into a soft failure
which the application thinks it should retry, as seen above in the email
case. This may result in very long delays, perhaps of days or weeks,
before even trivial errors are brought to the user's attention. Transport
protocols using UDP may also retry until the transport protocol retry
limit is reached (especially if ICMP messages are being filtered at a
firewall), which may be very considerably longer than the time it would
have taken to return an error to the user indicating they mistyped the
destination.
Automated Tools
Automated or embedded tools which use HTTP but which do not have a user
interface may also be confused by this change, since such tools may
expect configuration failures to show up as DNS errors and may not
realize that the HTTP response they have received from the zone
operator's search page is not the page which the tool expected to reach.
Such tools may fail in unpredictable ways, and may not be easy to upgrade.
Charging
The current response from the service in question is just over 17 KBytes
of data because the client has to open a TCP connection and receive a not
insignificant amount of data. A "no such data" response would have fitted
in one packet. In the case of volume-based charging for Internet Access
(as with most cellular data services) the recipient will have to pay
additional charges.
Single Point of Failure
Even for cases in which the redirection service works as intended, such a
service creates a very large single point of failure. Single points of
failure are obvious targets both for deliberate attacks and for the sort
of accidental "attacks" caused by bugs and configuration errors which
already generate much of the traffic at the DNS name servers for the root
zone. Furthermore, the IP address associated with this single point of
failure is a likely target both for routing attacks intended to redirect
the IP address to some other server.
Privacy
An interception service with this kind of scope raises significant
privacy concerns, since traffic received by the interception service is,
pretty much by definition, not going where its sender originally
intended. The potential for abuse in this situation is very high, and
makes the interception service an even more attractive target, this time
for attackers who wish to gain control of it in order to practice such abuse.
Reserved Names
This sort of wildcard usage is incompatible with any use of DNS which
relies on reserving names in a registry with the express intent of not
adding them to the DNS zone itself. An example of such a use is the
JET-derived IDN approach of "registry restrictions" and "reserved names",
which depends on the existence of names that are reserved and can be
registered only by the holder of some related name, but which do not
appear in the DNS. By some readings of the current ICANN IDN policy,
support for that "reserved name" approach is required. To accomplish the
goal of reduced consumer confusion, the reserved names must not be
resolvable at all. This reserved name approach appears to be completely
incompatible with this sort of wildcard usage: since the wildcard will
always cause a result to be returned, even for a reserved name which does
not appear in the zone, one can support either one or the other, but not both.
Undesirable Workarounds
ISPs have responded to the deployment of these wildcards in a number of
ways, all of which are both understandable and worrisome. Some ISPs have
contemplated modifying their routing systems to drop all packets destined
to the zone operator's redirection server into a black hole. Others have
deployed patches to their DNS resolvers which attempt to reverse the
effects of these wildcard records. Still other ISPs have considered using
this as an opportunity to play the same game that the zone operator is
playing, but for the ISP's own benefit. All of these responses are both
understandable and predictable, but none of them are good. Even more
worrisome is that different ISPs are taking different approaches to
dealing with this, which may lead to a balkanization problem and create
an ongoing headache for anyone having to deal with cross-network DNS or
application debugging.
----------
Principles, Conclusions, and Recommendations
The Robustness principle tells us that in some (not all) of the problems
detailed above, both parties could be construed as being at fault. In
some cases this is hardly surprising: spam filtering in particular, by
its nature, tends to be extremely ad hoc and somewhat fragile. No doubt
there are lessons here for all parties involved.
The Principle of Least Astonishment suggests that the deployment of
wildcards was disastrous for the users. It had widesweeping effects on
other users of the Internet far beyond those enumerated by the zone
operator, created several brand new problems, and caused other internet
entities to make hasty, possibly mutually incompatible and possibly
deleterious (to the internet as a whole) changes to their own operations
in an attempt to react to the change.
Note that these considerations apply to any wildcard deployment of this
type. The list of problems encountered in this case clearly demonstrates
that, although wildcard records are part of the base DNS protocol, there
are situations in which it simply is not safe to use them. As noted in an
earlier section, two warning flags suggesting that this type of wildcard
deployment is dangerous were that
* it affected more than one protocol, and
* it was done high enough up in the DNS hierarchy that its effects
were not limited to the organization that chose to deploy these wildcard
records.
Note also that a significant component of some of the listed problems was
not precisely the wildcard-induced behavior per se so much as it was the
abrupt change in the behavior of a long established infrastructure mechanism.
In conclusion, we would like to propose a guideline for when wildcard
records should be considered too risky to deploy, and make a few
recommendations on how to proceed from here.
Proposed guideline: If you want to use wildcards in your zone and
understand the risks, go ahead, but only do so with the informed consent
of the entities that are delegated within your zone.
Generally, we do not recommend the use of wildcards for record types that
affect more than one application protocol. At the present time, the only
record types that do not affect more than one application protocol are MX
records.
For zones which do delegations, we do not recommend even wildcard MX
records. If they are used, the owners of zones delegated from that zone
must be made aware of that policy and must be given assistance to ensure
appropriate behavior for MX names within the delegated zone. In other
words, the parent zone operator must not reroute mail destined for the
child zone without the child zone's permission.
We hesitate to recommend a flat prohibition against wildcards in
"registry"-class zones, but strongly suggest that the burden of proof in
such cases should be on the registry to demonstrate that their intended
use of wildcards will not pose a threat to stable operation of the DNS or
predictable behavior for applications and users.
We recommend that any and all TLDs which use wildcards in a manner
inconsistent with this guideline remove such wildcards at the earliest
opportunity.
----------
Acknowledgements
The IAB gratefully acknowledges the kind assistance of David Schairer,
John Curran, John Klensin, and Steve Bellovin for helpful suggestions
and, in some cases, significant chunks of text. None of these
contributors bear any responsibility for what the IAB has done with their
contributions. We note that Leslie Daigle recused herself from the
process of producing this document.
----------
IAB Contact for this Document
The contact person for the IAB on this statement is
<mailto:harald@xxxxxxxxxxxxx>Harald Alvestrand.
This page is maintained by the <mailto:execd@xxxxxxx>IAB Executive
Director for the IAB.
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