Design misfeature of dhclient for multiple interfaces

["Steven M. Bellovin" <smb%cs.columbia.edu@localhost>]

The other day, my daughter ran into a design misfeature of dhclient
that I suspect generalizes.  Her laptop (running 4.0, but that isn't
relevant) had wm and ath interfaces configured.  The wm interface got a
lease right away, and set up a default route and
modified /etc/resolv.conf.  When ath0 got a lease, the attempt to set a
default route of course failed, but it overwrote resolv.conf with the
new data.  However -- the wireless net was in 1918 space, and packets
sent via the default route couldn't reach the name servers.  Her
machine was thus left without working name resolution.

The underlying problem is that an entire group of attributes -- source
address, default route, name servers, time servers, etc., are all bound
together.  If there's any sort of routing or access control barrier
between the two different nets -- access restrictions on recursive name
servers, for example -- the fact that they don't share fate, and that
there's one obvious place where one will fail and the others succeed
can cause trouble.

                --Steve Bellovin, http://www.cs.columbia.edu/~smb