Port forwarding with UFW
Solution 1:
Add before filter section in /etc/ufw/before.rules(top of file):
*nat
:PREROUTING ACCEPT [0:0]
-A PREROUTING -p tcp --dport 1724 -j REDIRECT --to-port 1723
COMMIT
These rules will then need to be reloaded; simply disabling and re-enabling ufw or running ufw reload is not sufficient. The simplest solution is to reboot, otherwise you can use one of the workarounds mentioned in Launchpad bug #881137.