Google is showing me some returns on PPTP/GRE NAT through iptables/netfilter that will allow multiple connections.
However, I am not finding an example of using this pptp helper module that allows for this. Anyone done this before?
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This changed radically in kernel 2.6.14, and that accounts for the various random documentation examples you're finding (many with the pre-2.6.14 syntax). I don't have a machine handy to test on, but you should be fine with loading
ip_nat_pptp to get PPTP connection tracking.
I am assuming you are talking about being behind a NAT, and trying to have multiple users connect to a remote VPN server, then I just managed to fix this on DD-WRT v24-sp2 (04/16/10):
Apparently the kernel modules were missing which do the NAT translation for the GRE protocol - inserting them proved enough. I have added the following lines to the firewall script:
# Insert modules for PPTP over NAT: insmod nf_nat_proto_gre insmod nf_conntrack_proto_gre insmod nf_conntrack_pptp insmod nf_nat_pptp
The rest of iptables/netfilter is already set up correctly in DD-WRT apparently :)
If wanted I can dig up the rest of the NAT table entries relating to this.