Comment 13 for bug 1551351

For more data points on this, someone mentioned today that for him the leases get renewed (with dhclient started by network-manager). The command line is slightly different there:

/sbin/dhclient -d -q -sf <lease-script> -pf <pidfile> -lf <leasefile> -cf <conffile> <interface>

while on an ifupdown system where I see the stall it is:

/sbin/dhclient -1 -v -pf <pidfile> -lf <ipv4 leasefile> -I -df <ipv6 leasefile> <interface>

though it is the same command line used for ifupdown before. I got another Xenial/ifupdown based VM which was not recently updated (so leases get renewed still). I was able to attach to the daemon with "strace -p <pid> -ff" and could see that the main thread is in "select(7, [5 6], [], NULL, {276, 557923}". But there is other activity as well from time to time.
On the broken system the dhclient main thread is in "futex(0x7fb54906b0a4, FUTEX_WAIT_PRIVATE, 5, NULL" and there is no other activity. As a result the lease gets dropped which I can see on the server side as that is configured to do DNS updates when DHCP leases become active or inactive.