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From: jbvb@FTP.COM (James B. Van Bokkelen)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: name handling in DNS resolvers
Message-ID: <9105171735.AA25294@ftp.com>
Date: 17 May 91 17:35:20 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: jbvb@ftp.com
Organization: The Internet
Lines: 56


    BIND (and derivations - most UN*Xes) build a search list of
    progressively wider domains to successively append to the user
    provided name...
    
    However some implementations do not provide such user friendly
    procedures. e.g. FTP Inc's PC/TCP uses the algorithm that if the user
    supplied name contains a '.' (period) it is used "as-is" in the DNS
    resolution, otherwise a default domain is appended. (FTP Inc. support
    staff don't think there is any need for change.)
    
The Internet's experience with defaulting of partial domains is poor.
Perhaps my response to another person who recently asked for the same thing
will explain.
    
    He said:

    Machine "opus" in domain "eng.hou.xyz.com" wishes to establish a
    connection with machine "milo" in "se.hou.xyz.com".  Currently "opus"
    user must use the fully qualified domain name in his command, i.e. tn
    "milo.se.hou.xyz.com".

    Under other TCP/IP implementations, i.e Lachmann, BSD, SUNOS, the
    "opus" user only has to specify the portion of the name different
    from his own domain, i.e. telnet "milo.se".

The problem is that domain names are hierarchical, and the Internet is big.
In an Internet environment, given your example, the best 'opus' can do is to
issue the following DNS queries to translate "milo.se":

	milo.se.eng.hou.xyz.com			fails at eng.hou.xyz.com
	milo.se.hou.xyz.com			works

However, the host 'opus' will never be able to reach a machine named "milo"
in the "se" domain (Sweden).  This is perplexing to the user and frustrating
to explain if you're a Tech Support person.  If you reverse the order of
search in order to allow access to all top-level domains, 'opus' will send:

	milo.se					goes to the DNS root & fails
	milo.se.com				goes to the DNS root & fails
	milo.se.xyz.com				goes to xyz root & fails
	milo.se.hou.xyz.com			goes to hou.xyz.com, works.

Two failed queries to the DNS root for most lookups is a major price to pay,
and the people who run the DNS root know where I live; I'd suffer severely
if I did this to them...

The Host Requirements RFCs (1123 in this case) address this issue (pp 83, 84),
and their view of "search lists" is that they should not be implemented unless
the host is capable of caching "negative responses", which DOS PC/TCP's TSR
kernel cannot do at this time.  Such a cache might use a significant amount
of DOS memory; would you consider 8Kb or 16Kb a reasonable price to pay for
this feature?

James B. VanBokkelen		26 Princess St., Wakefield, MA  01880
FTP Software Inc.		voice: (617) 246-0900  fax: (617) 246-0901