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Network Working Group E. Gavron
Request for Comments: 1535 ACES Research Inc.
Category: Informational October 1993
A Security Problem and Proposed Correction
With Widely Deployed DNS Software
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.
Abstract
This document discusses a flaw in some of the currently distributed
name resolver clients. The flaw exposes a security weakness related
to the search heuristic invoked by these same resolvers when users
provide a partial domain name, and which is easy to exploit (although
not by the masses). This document points out the flaw, a case in
point, and a solution.
Background
Current Domain Name Server clients are designed to ease the burden of
remembering IP dotted quad addresses. As such they translate human-
readable names into addresses and other resource records. Part of
the translation process includes understanding and dealing with
hostnames that are not fully qualified domain names (FQDNs).
An absolute "rooted" FQDN is of the format {name}{.} A non "rooted"
domain name is of the format {name}
A domain name may have many parts and typically these include the
host, domain, and type. Example: foobar.company.com or
fooschool.university.edu.
Flaw
The problem with most widely distributed resolvers based on the BSD
BIND resolver is that they attempt to resolve a partial name by
processing a search list of partial domains to be added to portions
of the specified host name until a DNS record is found. This
"feature" is disabled by default in the official BIND 4.9.2 release.
Example: A TELNET attempt by User@Machine.Tech.ACES.COM
to UnivHost.University.EDU
Gavron [Page 1]
RFC 1535 DNS Software Enhancements October 1993
The resolver client will realize that since "UnivHost.University.EDU"
does not end with a ".", it is not an absolute "rooted" FQDN. It
will then try the following combinations until a resource record is
found:
UnivHost.University.EDU.Tech.ACES.COM.
UnivHost.University.EDU.ACES.COM.
UnivHost.University.EDU.COM.
UnivHost.University.EDU.
Security Issue
After registering the EDU.COM domain, it was discovered that an
unliberal application of one wildcard CNAME record would cause *all*
connects from any .COM site to any .EDU site to terminate at one
target machine in the private edu.com sub-domain.
Further, discussion reveals that specific hostnames registered in
this private subdomain, or any similarly named subdomain may be used
to spoof a host.
Example: harvard.edu.com. CNAME targethost
Thus all connects to Harvard.edu from all .com sites would end up at
targthost, a machine which could provide a Harvard.edu login banner.
This is clearly unacceptable. Further, it could only be made worse
with domains like COM.EDU, MIL.GOV, GOV.COM, etc.
Public vs. Local Name Space Administration
The specification of the Domain Name System and the software that
implements it provides an undifferentiated hierarchy which permits
delegation of administration for subordinate portions of the name
space. Actual administration of the name space is divided between
"public" and "local" portions. Public administration pertains to all
top-level domains, such as .COM and .EDU. For some domains, it also
pertains to some number of sub-domain levels. The multi-level nature
of the public administration is most evident for top-level domains
for countries. For example in the Fully Qualified Domain Name,
dbc.mtview.ca.us., the portion "mtview.ca.us" represents three levels
of public administration. Only the left-most portion is subject to
local administration.
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RFC 1535 DNS Software Enhancements October 1993
The danger of the heuristic search common in current practise is that
it it is possible to "intercept" the search by matching against an
unintended value while walking up the search list. While this is
potentially dangerous at any level, it is entirely unacceptable when
the error impacts users outside of a local administration.
When attempting to resolve a partial domain name, DNS resolvers use
the Domain Name of the searching host for deriving the search list.
Existing DNS resolvers do not distinguish the portion of that name
which is in the locally administered scope from the part that is
publically administered.
Solution(s)
At a minimum, DNS resolvers must honor the BOUNDARY between local and
public administration, by limiting any search lists to locally-
administered portions of the Domain Name space. This requires a
parameter which shows the scope of the name space controlled by the
local administrator.
This would permit progressive searches from the most qualified to
less qualified up through the locally controlled domain, but not
beyond.
For example, if the local user were trying to reach:
User@chief.admin.DESERTU.EDU from
starburst,astro.DESERTU.EDU,
it is reasonable to permit the user to enter just chief.admin, and
for the search to cover:
chief.admin.astro.DESERTU.EDU
chief.admin.DESERTU.EDU
but not
chief.admin.EDU
In this case, the value of "search" should be set to "DESERTU.EDU"
because that's the scope of the name space controlled by the local
DNS administrator.
This is more than a mere optimization hack. The local administrator
has control over the assignment of names within the locally
administered domain, so the administrator can make sure that
abbreviations result in the right thing. Outside of the local
control, users are necessarily at risk.
Gavron [Page 3]
RFC 1535 DNS Software Enhancements October 1993
A more stringent mechanism is implemented in BIND 4.9.2, to respond
to this problem:
The DNS Name resolver clients narrows its IMPLICIT search list IF ANY
to only try the first and the last of the examples shown.
Any additional search alternatives must be configured into the
resolver EXPLICITLY.
DNS Name resolver software SHOULD NOT use implicit search lists in
attempts to resolve partial names into absolute FQDNs other than the
hosts's immediate parent domain.
Resolvers which continue to use implicit search lists MUST limit
their scope to locally administered sub-domains.
DNS Name resolver software SHOULD NOT come pre-configured with
explicit search lists that perpetuate this problem.
Further, in any event where a "." exists in a specified name it
should be assumed to be a fully qualified domain name (FQDN) and
SHOULD be tried as a rooted name first.
Example: Given user@a.b.c.d connecting to e.f.g.h only two tries
should be attempted as a result of using an implicit
search list:
e.f.g.h. and e.f.g.h.b.c.d.
Given user@a.b.c.d. connecting to host those same two
tries would appear as:
x.b.c.d. and x.
Some organizations make regular use of multi-part, partially
qualified Domain Names. For example, host foo.loc1.org.city.state.us
might be used to making references to bar.loc2, or mumble.loc3, all
of which refer to whatever.locN.org.city.state.us
The stringent implicit search rules for BIND 4.9.2 will now cause
these searches to fail. To return the ability for them to succeed,
configuration of the client resolvers must be changed to include an
explicit search rule for org.city.state.us. That is, it must contain
an explicit rule for any -- and each -- portion of the locally-
administered sub-domain that it wishes to have as part of the search
list.
Gavron [Page 4]
RFC 1535 DNS Software Enhancements October 1993
References
[1] Mockapetris, P., "Domain Names Concepts and Facilities", STD 13,
RFC 1034, USC/Information Sciences Institute, November 1987.
[2] Mockapetris, P., "Domain Names Implementation and Specification",
STD 13, RFC 1035, USC/Information Sciences Institute, November
1987.
[3] Partridge, C., "Mail Routing and the Domain System", STD 14, RFC
974, CSNET CIC BBN, January 1986.
[4] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S. Miller,
"Common DNS Implementation Errors and Suggested Fixes", RFC 1536,
USC/Information Sciences Institute, USC, October 1993.
[5] Beertema, P., "Common DNS Data File Configuration Errors", RFC
1537, CWI, October 1993.
Security Considerations
This memo indicates vulnerabilities with all too-forgiving DNS
clients. It points out a correction that would eliminate the future
potential of the problem.
Author's Address
Ehud Gavron
ACES Research Inc.
PO Box 14546
Tucson, AZ 85711
Phone: (602) 743-9841
EMail: gavron@aces.com
Gavron [Page 5]
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