I'd like to ask you a question about tun interfaces, but without OpenVPN.
I have two applications that tunnel traffic: "sgsn userspace app" reads IP packets from the tun interface "tun sgsn", sends them to "ggsn userspace app", this app writes them to "tun ggsn". It works the other way too, so I can have a working ssh session, copy files, etc.
So, the packets go this way:
scp client --> sgsn tun interface ---> tunneling through my app -----> ggsn tun interface (dropped here) ----------> sshd
Now, I've tried it with IPv4 and it works, but when I switch to IPv6 the IP packet is written to "tun ggsn" and the IP stack discards it. I've seen it with dropwatch, which says the discard is in ip_rcv+c0.
The routes must be correct because otherwise the packet wouldn't have made it into tun_sgsn in the first place.
I've tried this in both CentOS 6.7 and 7.1 with same result. However, in the 7.1 I only have a single host, so I've done a few tricks with ip6tables to NAT addresses, so the traffic from each user space app (scp and sshd) goes through the tun interfaces. I'm saying this just so it's clear from the beginning, but with the 6.7 I have two machines and no NAT (I'm aware that NAT doesn't work in CentOS 6.7 for IPv6). I've used tcpdump in the tun ggsn interface and the IPv6 datagram seems all right.
Another hint: If I scp to the tun_ggsn interface directly, it works, but I don't think this discards anything with the tun driver, since being in the same machine that means the IP packets don't go down the TCP/IP stack to reach the tun driver (in fact, tcpdump does not capture the packet if I do this).
Any help would be appreciated, thank you very much in advance.
