The problem lies hardcoded within /sbin/ifup:
# strings /sbin/ifup | grep /var/run
dhclient3 -pf /var/run/dhclient.%iface%.pid -lf
/var/run/dhclient.%iface%.leases %iface%
dhclient -e -pf /var/run/dhclient.%iface%.pid -lf
/var/run/dhclient.%iface%.leases %iface%
udhcpc -n -p /var/run/udhcpc.%iface%.pid -i %iface% [[-H %hostname%]]
[[-c %clientid%]]
dhclient3 -r -pf /var/run/dhclient.%iface%.pid -lf
/var/run/dhclient.%iface%.leases %iface%# dpkg -S /sbin/ifup ifupdown: /sbin/ifup
