PPTP / GRE pass through issue

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http://www.linuxquestions.org – Hi all How do i open GRE pass through in linux rhel 6.0 ? currently the linux rhel 6 is function as NAT router, which is one of the vm guest in vm esxi 5 server, 2 physical network card on the esxi server, another window 2008 r2 standard is a vm guest also having RRAS vpn server configured in this window server, when i try to dial in from remote network, error 806 appeared (see attached screenshot) , in the linux rhel 6 firewall port 1723 already opened, just wonder how do i open GRE protocol 47 in linux for the window vpn server ? http://filedb.experts-exchange.com/i...672542/vpn.jpg (HowTos)