Improved DNS resolving for laptops?

[Petter Reinholdtsen]

My laptop moves from network to network.  The network configuration is
updated using DHCP, but the DNS resolver info is not as easy to
update.  The file /etc/resolv.conf is updated by the DHCP client, but
the programs do not re-read this file once they are started.  This is
a problem for long-lived programs, and for my laptop this means it is
a problem for most programs (hibernate works. :).

Here is an idea on how to make it possible to solve this, without
affecting all users of the resolver functions.  The idea consist of
two parts:

 - Every time a DNS lookup fail, (or only when a specificed time
   period has elapsed since the last check), stat /etc/resolv.conf,
   and if this file changed, reread it by calling res_init().

 - Make the new behaviour optional using a flag in /etc/host.conf.
   This way only the hosts in need of this dynamic rebinding would
   enable it, and the others would keep working as before.

There could also be a flag to make the resolver check the timestamp of
resolv.conf ever X seconds.  This would make sure programs start using
the new content even if the old DNS server is still replying to
queries.

Do you think this would work?

There is a similar problem with /etc/localtime when moving my laptop
from timezone to timezone, but I'm not sure how that could be solved
without stat()ing the file every time localtime() is called.  And I
don't want that.