Comment 14 for bug 1176046

Rectification, after more investigation here what is happening :

Trusty got the random port right away because NSUPDATE is enable at compile time in package "isc-dhcp-client"

In Xenial and late (including debian) with commit [b1d5bab630a29f2ca94c458f9402aa43cc5975c4], a new pkg binary package is introduced "isc-dhcp-client-ddns" set as optional.

#debian/control:
+Package: isc-dhcp-client-ddns
+Priority: optional
+Architecture: any
+Depends:
+ ${misc:Depends},
+ ${shlibs:Depends},
+ isc-dhcp-client (= ${binary:Version}),
+Enhances: isc-dhcp-client
+Description: Dynamic DNS (DDNS) enabled DHCP client
+ This a Dynamic DNS enabled version of the DHCP client.

So basically the random port still exist in Xenial and late, but it requires, the "isc-dhcp-client-ddns" package (optional) to be installed. So basically, it is split probably to accommodate the folks having this problem, and in the same time it allows peoples that really needs this feature to use it by installing this additionnal new package.

On Xenial :
** Without "isc-dhcp-client-ddns" **
# lsof -i udp
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
dhclient 140 root 6u IPv4 344477 0t0 UDP *:bootpc

** With "isc-dhcp-client-ddns" **
# lsof -i udp
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
dhclient 140 root 6u IPv4 336839 0t0 UDP *:bootpc
dhclient 140 root 20u IPv4 336827 0t0 UDP *:65239
dhclient 140 root 21u IPv6 336828 0t0 UDP *:12817

Eric