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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.

Gavron [Page 2]  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]