[Bug network/13935] getaddrinfo NXDOMAIN hijack exploit for hosts with two-component hostnames

["nagle at sitetruth dot com" <sourceware-bugzilla@sourceware.org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=13935

--- Comment #2 from John Nagle <nagle at sitetruth dot com> 2012-04-04 02:50:46 UTC ---
SOME HISTORY

This problem was addressed back in 1993 in RFC 1535, "A Security Problem and
Proposed Correction With Widely Deployed DNS Software".  Back then, DNS
resolution involved chopping off one element at a time of the local hosts'
domain and appending that to the query until a match was found.  This allowed
attacks on domains with more than two elements.  That's where the current
restrictions come from.   

RFC 1535 says "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."

It then goes on:

"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."

Those two statement are in conflict when a host has a name such as 
"example.com".  The "immediate parent domain" is "com", but it is not
a "locally administered subdomain".  Since the second sentence is a MUST,
while the first sentence is a SHOULD, the second sentence controls.
Thus, to conform to RFC 1535, the search path should never default to a TLD.
glibc is not in compliance.

That RFC was written in 1993, before domains were ever offered for sale. A
world in which most domains were second level was not envisioned at the time. 
That's probably why the authors of the RFC didn't think of this. 

WORKAROUNDS, FAILURE OF

This is a tough problem to work around without changing host names.  Adding
commands to /etc/resolv.conf tends to cause problems, because, in modern Linux
systems, that file is usually generated by system administration software.  

Editing /etc/resolv.conf is not particularly helpful, anyway. 
Adding a blank "search" command does not delete the implicit search path.
(That behavior comes from lines 267-268 in res_init.c, which, for a blank 
"search" line, does nothing.  The code in that area seems to be set up so that
there is always an alternate search path of some kind, either from SEARCH
statement in resolv.conf, a DOMAIN statement, an environment variable, or
the host name of the host.

Nor does setting "ndots:0" have that effect.  Setting "domain" in
/etc/resolv.conf to a value with an invalid TLD might work, but could confuse
other parts of the system.  "no_tld_query" only applies to inputs with no dots
(I think) so that doesn't help, either. 

EFFECT

The effect of all this is that you can't trust "getaddrinfo" or "gethostbyname"
to do an honest DNS lookup.

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