Comment 5 for bug 1503680

I can confirm this behavior on my systems.

from syslog (identifying information removed):
Mar 23 19:10:41 HOSTNAME dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 3 (xid=0x3e38a8c2)
Mar 23 19:10:44 HOSTNAME dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 8 (xid=0x3e38a8c2)
Mar 23 19:10:52 HOSTNAME dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 20 (xid=0x3e38a8c2)
Mar 23 19:11:12 HOSTNAME dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 8 (xid=0x3e38a8c2)
Mar 23 19:11:20 HOSTNAME dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 13 (xid=0x3e38a8c2)
Mar 23 19:11:33 HOSTNAME dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 9 (xid=0x3e38a8c2)
Mar 23 19:11:42 HOSTNAME dhclient: No DHCPOFFERS received.
Mar 23 19:11:42 HOSTNAME dhclient: No working leases in persistent database - sleeping.
Mar 23 19:13:12 HOSTNAME dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 3 (xid=0x49984e3f)
Mar 23 19:13:15 HOSTNAME dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 5 (xid=0x49984e3f)
Mar 23 19:13:20 HOSTNAME dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 10 (xid=0x49984e3f)
Mar 23 19:13:30 HOSTNAME dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 11 (xid=0x49984e3f)
Mar 23 19:13:41 HOSTNAME dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 17 (xid=0x49984e3f)
Mar 23 19:13:58 HOSTNAME dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 10 (xid=0x49984e3f)
Mar 23 19:14:08 HOSTNAME dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 5 (xid=0x49984e3f)
Mar 23 19:14:13 HOSTNAME dhclient: No DHCPOFFERS received.
Mar 23 19:14:13 HOSTNAME dhclient: No working leases in persistent database - sleeping.

This repeats forever.

dhclient was launched by a script thusly: /sbin/dhclient -1 -nw eth0_20

/etc/dhcp/dhclient.conf:
option rfc3442-classless-static-routes code 121 = array of unsigned integer 8;

send host-name = gethostname();
request subnet-mask, broadcast-address, time-offset, routers,
 domain-name, domain-name-servers, domain-search, host-name,
 dhcp6.name-servers, dhcp6.domain-search,
 netbios-name-servers, netbios-scope, interface-mtu,
 rfc3442-classless-static-routes, ntp-servers,
 dhcp6.fqdn, dhcp6.sntp-servers;
timeout 60;
retry 60;
initial-interval 3;

package isc-dhcp-client version 4.2.4-7ubuntu8

Is this fixed in the newer versions?