In ifup-post, there appears to be a scope issue with the construct:

eval `/bin/ipcalc --silent --hostname ${IPADDR}` && set_hostname $HOSTNAME

The set_hostname function does not get handed the value of $HOSTNAME as is
set by the eval, instead uses $HOSTNAME as it exists before the eval.
In my case, I have HOSTNAME set to "localhost" to force hostname to be
set to the fqdn of the IP the DHCP server hands it. As a result of the
above mentioned problem, the hostname never gets set. I can provide bash -x
output of this if you need.

I have resolved this on my setup via (lame but easy):

eval `/bin/ipcalc --silent --hostname ${IPADDR}` && HOSTNAME=`/bin/ipcalc
--silent --hostname ${IPADDR}|sed s/HOSTNAME=//` && set_hostname $HOSTNAME

- Jim Collins (jim_collins)