Comment 4 for bug 1833175

hi yulong:
Before this test, I have modprobe ip_gre, ip_nat_pptp, ip_conntrack_pptp, in VMs and Hosts, then lsmod showed that the nf_conntrack_proto_gre is inuse:
# lsmod | grep pptp
nf_nat_pptp 13115 0
nf_nat_proto_gre 13009 1 nf_nat_pptp
nf_conntrack_pptp 19257 1 nf_nat_pptp
nf_conntrack_proto_gre 14434 1 nf_conntrack_pptp
nf_nat 26787 4 nf_nat_proto_gre,nf_nat_ipv4,nf_nat_ipv6,nf_nat_pptp
nf_conntrack 133053 9 nf_conntrack_proto_gre,nf_nat,nf_nat_ipv4,nf_nat_ipv6,nf_nat_pptp,xt_conntrack,nf_conntrack_ipv4,nf_conntrack_ipv6,nf_conntrack_pptp

And the gre tunnel worked fine, but pptp vpn tunnel was blocked, the difference between them is, the gre tunnel is based on Grev0, the pptp vpn is based on Grev1, seems the Grev1 tunnel connect status cannot be recognized normally.