dhclient: ISP's server won't answer unicast DHCPREQUEST anymore

[Jordan Russell]

A bizarre problem has appeared after upgrading from Fedora 14 to 15,
which brought dhclient from 4.2.0-P2 to 4.2.1-P1.

Since the upgrade (weeks ago), my ISP's DHCP server no longer responds
to any of the unicast DHCPREQUEST packets sent by dhclient when it
enters the RENEWING state. However, hours later, when dhclient reaches
the REBINDING state and broadcasts instead, the server responds
immediately, and the renewal succeeds.

  DHCPREQUEST on eth1 to 10.72.0.1 port 67
  DHCPREQUEST on eth1 to 10.72.0.1 port 67
  DHCPREQUEST on eth1 to 10.72.0.1 port 67
  [...]
  DHCPREQUEST on eth1 to 255.255.255.255 port 67
  DHCPACK from 10.72.0.1
  bound to 76.x.x.x -- renewal in 14685 seconds.

tcpdump confirms that the unicast DHCPREQUEST packets are making it out
(with correct IP and MAC addresses), but no corresponding responses are
coming in.

Was there any change in dhclient 4.2.1 that might explain this? I'm
using an empty dhclient.conf file just as I was prior to the upgrade.
(Initially, Fedora 15 inserted dhcp-client-identifier and hostname
settings into the config, but I've since removed both of those and
restarted dhclient numerous times.)

Also, as an interim workaround to cut back on the syslog spam, is there
some way to force dhclient to broadcast all DHCPREQUEST packets (i.e.
skip the RENEWING state)?

Thanks.
-- 
Jordan Russell