Linux – How to configure a Linux VPN Client to get into a network through a Fortigate firewall

without (left,right)protoport=17/1701 you've configured the openswan side for tunnel mode ipsec, not transport mode to encrypt l2tp.

what does main mode debugging say on the linux end? i bet it's a protocol mismatch.

you can also turn on RAS debugging on the winXP side

My word, you do have your work cut out. OK; here's a sort of outline primer, because at the moment you've got the fundamentals so wrong that the details don't matter.

Before anything else, the fact that you haven't got static public addresses for each of your internet connections is a problem. IPSec doesn't easily support tunnels in such configurations [1], so you're going to end up editing your ipsec.conf each time either of your addresses changes. OK?

Now, when I asked you if each OpenSWAN endpoint had a public IP address, and you confidently said "yes", it turns out - as I suspected - that you were wrong. Your ifconfig output shows me that one end has the address 192.168.1.78 and the other has the address 10.0.2.15. You also tell me that one end is (currently) behind the public IP address 212.251.112.115 and the other is behind 79.103.7.114. You don't say which is which, so I'll assume 192.168.1.78 is behind 212.251.112.115 and 10.0.2.15 is behind 79.103.7.114. If that's wrong, just reverse the correspondence. I'll also call the former pair left and the latter pair right. It makes no difference which is which, but it'll help us keep our thinking straight, which'd be a really good idea right about now.

You'll need to set up the public routers at both ends to forward UDP/500 and protocols 50 and 51 (just for completeness) to the OpenSWAN endpoints inside each public address. If you can't manage the two protocol punchthroughs, then investigate the doco on NAT traversal and forward UDP/4500 as well.

Firstly, it is a requirement that each end find its own IP address in the config, so that each end can know which of left and right it is when it starts. So left needs to have an ipsec.conf that contains

conn net-to-net  
    authby=secret
    left=192.168.1.78
    leftsubnet=192.168.1.0/24
    leftnexthop=%defaultroute
    right=79.103.7.114
    rightsubnet=10.0.2.0/24

and an ipsec.secrets that says

192.168.1.78 79.103.7.114: PSK "123"

while right must have an ipsec.conf that contains

conn net-to-net  
    authby=secret
    left=212.251.112.115
    leftsubnet=192.168.1.0/24
    right=10.0.2.15
    rightsubnet=10.0.2.0/24 
    rightnexthop=%defaultroute

and an ipsec.secrets that says

10.0.2.15 212.251.112.115: PSK "123"

Each end really needs to know who it really is, whilst it can pretend that it doesn't care that the remote end is behind a NAT. Do you see?

Furthermore, you'll need to config all the clients on each end so that they have a route to the remote RFC1918 network via the local OpenSWAN endpoint. You'll need to check that /proc/sys/net/ipv4/ip_forward is set to 1 on each endpoint. And it would be a really good idea to turn off any firewalling on the two endpoints, at least for now. You may also need to activate some config variables that tell each endpoint not to care that the remote endpoint thinks it has a different IP address from what the local endpoint thinks it has; from memory, these are leftid= and rightid=, but you're going to have to work that out for yourself.

So that's the basics. If you get the fundamental topology and concepts right, the rest is just debugging the details. Good luck with this.

[1] This isn't quite true. The SWAN implementations support opportunistic IPSec encryption, but this requires that you control your reverse DNS at both ends, and I'm guessing that you don't. Again, read the man pages if you want to know more about this.