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From Jeff Trawick <>
Subject IPv6 capability and name lookup cost
Date Thu, 05 Sep 2002 15:43:52 GMT
(this discussion assumes Apache has IPv6 capability)

There are some situations during normal web server operation where
resolver calls are made to find addresses associated with names and we
have no clue whether or not the name has an IPv6 address associated with
it.  Forward proxy is a good example of this.  apr_sockaddr_info_get()
will return all addresses associated with the name, and some existing
resolvers first do IPv4 name lookup followed by IPv6 name lookup in
order to implement this.  The lack of overlap results in higher elapsed
time; the inability to bundle the queries in a single flow results in
higher kernel/library CPU.  A resolver cache can help with this.

In most cases, there is no IPv6 address.  In some cases where there is
an IPv6 address, using the IPv4 address would be sufficient.

A useful performance improvement can be achieved by allowing the
administrator to select the following algorithm:

  lookup IPv4
  if at least one IPv4 address was found, we're done
  lookup IPv6

The benefit is clear.  The drawback is that if IPv4 addresses were found
but those addresses are not usable (e.g., IPv4 addresses disabled on
that machine but admin didn't remove from DNS), we wouldn't then be able
to try the IPv6 addresses (without much more work).  (But the admin
chose the behavior so what do I care :) )

The bulk of the implementation would be in APR (new flag to
apr_sockaddr_info_get()), but Apache would have a configuration
mechanism to allow the administrator to turn on the flag.  Up for
discussion would be if there is an Apache-wide preference or whether
different components/modules (core, mod_proxy_foo, etc.) should have
knobs.  Personally I'd prefer an Apache-wide preference which would be
respected by core and by any modules
distributed by us.  Any 3rd-party modules could/should respect the
configuration too.


(If APR has no IPv6 capability, the new processing flag would be ignored
since we're only going to do the IPv4
lookup anyway.)

Jeff Trawick

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