Resolver Issues (non valid hostname characters)

Mark.Andrews at Mark.Andrews at
Wed Mar 26 05:23:35 PST 2003

> On Tuesday, March 25, 2003, at 09:53  PM, Mark.Andrews at wrote:
> > 	The current implementation fits this.  It handles (accepts)
> > 	garbage in and only returns (generates) clean respones to
> > 	the application.
> >
> Which I would say it not the intention of what being 'generous on what
> you accept' to mean.  IMHO, the maxim is to stop exactly what is
> happening.  We are being restrictive on what we return to the
> application so things are breaking.  I can't change the remote end so
> communication does not flow.  From my perspective, you advocating
> being restrict on what you will accept and what you will send.

	This is a security matter.  Sendmail was compromised due to
	lack of checking the results returned by gethostbyaddr().

	get*by*() and get*info() enforce RFC 952 so that every
	application written doesn't have to validate the results
	returned.  Allowing underscore (or IHN) is a API change
	and will potentially break applications that correctly
	depend upon get*by*() and get*info() filtering out the

	If you want to be liberal in what you accept bypass
	get*by*() and get*info() and call the resolver directly.

> > 	If the resolver died receiving underscore you would something
> > 	to complain about.  Currently it just filters out ALL illegal
> > 	responses.
> I can't talk to some hosts on the internet because FreeBSD will not
> resolve the host name which over 99% of the host on the Internet will.
> I guess that just doesn't matter.

	If the name contains a underscore it is not a hostname by
	definition.  Nothing stops you talking to the DNS directly
	and entering IP literals.

> DaveD
Mark Andrews, Internet Software Consortium
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark.Andrews at

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