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Network Working Group                                          E. GavronRequest for Comments: 1535                            ACES Research Inc.Category: Informational                                     October 1993A Security Problem and Proposed CorrectionWith Widely Deployed DNS SoftwareStatus 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.EDUGavron                                                          [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 1993References   [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,RFC974, 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",RFC1537, 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.comGavron                                                          [Page 5]