I am using Ubuntu 12.04 Desktop. I am trying to establish VPN PPTP connection to Ubuntu 12.04 Server. VPN connection available when PC is connected to LAN - ADSL modem - Internet - VPN Server. No VPN connection available when using 3G modem Vodafone K3773 HSPA USB Stick - Internet - VPN Server. Internet is connected. VPN is not. How to establish the VPN Connection? Server side log: GRE: Bad checksum from pppd.