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Re: correct definition of localhost?

On Sun, Jul 06, 2008 at 03:09:09PM -0700, Steve Langasek wrote:
> Hi folks,
> I've run across an ipv4/ipv6 configuration issue which I think needs to have
> light cast on it so we can try to resolve this in time for lenny (whatever
> the right resolution actually is), in order to avoid a pile-up of
> /etc/hosts-related kludges as has been known to happen before...
> In response to bug #427067, the netbase maintainer made a change that adds
> localhost as an alias for ::1 on new installs.  In April of this year, the
> Debian Installer team followed suit, adding this line in the netcfg udeb.
> The result of these changes is that since July 2007, any new lenny or sid
> chroots have had two addresses listed for localhost, and since April of this
> year, any new installs of lenny done using d-i have had it as well.
> Now, the problem I ran into is that when I enabled the test suite in the
> openldap2.3 package, the build failed mysteriously on a seemingly random set
> of architectures.  The reason?  The test suite configures slapd to run on a
> particular port on localhost, and the glibc "files" NSS backend
> special-cases the ::1 IPv6 loopback address, so that when you request an
> IPv4 address, it will map any ::1 entries to for you.  But of
> course we already have an entry for localhost as, so now we end up
> with duplicate addresses returned, and slapd tries to bind twice to the same
> address and port!

You don't seem to request ipv4 addresses, you request AF_UNSPEC, which
should get you both ipv4 and ipv6.  You get twice, and ::1 one

> A test program showing this behavior is attached - compile and run it on a
> system with '::1 localhost' set in /etc/hosts, and you'll see
> returned twice.  An alternate test case, which also works on systems with
> older /etc/hosts and which I think shows the counterintuitiveness of the
> nss_files special-casing, is to run "getent ahostsv4 ip6-localhost".
> I don't think it's the responsibility of callers such as slapd to check that
> getaddrinfo() hasn't returned duplicate entries, so I see a couple of
> solutions here:
> - the ::1 address should *not* be special-cased by nss_files.  I really
>   can't perceive any reason why it should be special-cased in the first
>   place; i.e., why should the files backend behave differently than the DNS
>   backend, and why would we want names that were specifically assigned to
>   ::1, including names like "ip6-loopback", to be automatically mapped to

I can't find any good reason why it should be changing ::1 to
So I think that atleast glibc should stop doing that.  In any case, it
shouldn't return twice when it's not configured to return
it twice.

> - we should only set up a single 'localhost' entry in /etc/hosts, pointing
>   at ::1, and let nss_files handle the mapping to automatically.

- You could also argue that openldap should get fixed to deal with cases
  where it tries to bind to the same ip/port twice.  On the other hand,
  I don't think it a normal case, and I think it's unlikely that people
  would set up dns to have 2 times the same IP address and then try
  to bind to that hostname.


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