Как настроить Ubuntu 12.04 в качестве VPN-сервера, который разрешает подключения из OS X и iOS?

Я пытаюсь настроить VPN с использованием PPTP и подключиться с помощью iPhone (iOS), но это не удается.

May 25 22:43:38 zork pptpd[28793]: CTRL: Client x.x.x.x control connection finished
May 25 22:44:02 zork pptpd[28810]: MGR: connections limit (100) reached, extra IP addresses ignored
May 25 22:44:02 zork pptpd[28811]: MGR: Manager process started
May 25 22:44:02 zork pptpd[28811]: MGR: Maximum of 100 connections available
May 25 22:44:08 zork pptpd[28812]: CTRL: Client x.x.x.x control connection started
May 25 22:44:09 zork pptpd[28812]: CTRL: Starting call (launching pppd, opening GRE)
May 25 22:44:09 zork pppd[28813]: Plugin /usr/lib/pptpd/pptpd-logwtmp.so loaded.
May 25 22:44:09 zork pppd[28813]: pppd 2.4.5 started by root, uid 0
May 25 22:44:09 zork pppd[28813]: Using interface ppp0
May 25 22:44:09 zork pppd[28813]: Connect: ppp0  /dev/pts/5
May 25 22:44:09 zork pptpd[28812]: GRE: Bad checksum from pppd.