linux-kernel - Re: [PATCH 2/2] Define ENONAMESERVICE and ENAMEUNKNOWN to indicate name service errors

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Date:	Thu, 9 Feb 2012 08:44:29 -0500
From:	Jim Rees <rees@...ch.edu>
To:	David Howells <dhowells@...hat.com>
Cc:	jmorris@...ei.org, keyrings@...ux-nfs.org,
	linux-nfs@...r.kernel.org, linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-cifs@...r.kernel.org,
	linux-api@...r.kernel.org, libc-alpha@...rceware.org
Subject: Re: [PATCH 2/2] Define ENONAMESERVICE and ENAMEUNKNOWN to indicate
 name service errors

David Howells wrote:

  Jim Rees <rees@...ch.edu> wrote:
  
  >   Define ENAMEUNKNOWN to indicate "Network name unknown".  This can be used to
  >   indicate, for example, that an attempt was made by dns_query() to make a query,
  >   but the name server (e.g. a DNS server) replied indicating that it had no
  >   matching records.
  > 
  > Would this be the same as NXDOMAIN?  That is, does it mean the name server
  > couldn't find a record, or does it mean that the record doesn't exist?
  
  Is there a way to tell the difference?  Can you store a negative record in the
  DNS?  Or is it that the DNS has records for the name, just not records of the
  type you're looking for (eg. NO_ADDRESS/NO_DATA from gethostbyname())?

It's an important distinction to the resolver if you want to avoid dns
hijacking.  See rfc2308.  There doesn't seem to be a way to tell the
difference from the gethostbyname call, which was designed before this was a
problem.  The on-the-wire dns query protocol does make the distinction.

I suspect kernel dns clients won't need to know the difference, but I think
it's useful if we decide on and document the meaning of the error codes.
Maybe the answer is that ENAMEUNKNOWN means the same as a HOST_NOT_FOUND
from gethostbyname().
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