VPN on interface behind (not in routing path) FortiGate

[boneyard]

due to the VPN traffic possibly coming in or going out via one of three interfaces (due to BGP) i felt i should configure the VPN on another interface of the FortiGate.

only the VPN process doesn't want to start the VPN now, the debug logs shows:

2021-06-19 14:41:49.269526 ike 0:p1-01:91: could not send IKE Packet(SA_INIT):x.x.x.x:500->y.y.y.y:500, len=248: error 101:Network is unreachable

it set ups the VPN tunnel fine for incoming VPN requests, but it refuses to initiate a VPN itself. tried on both 6.0 and 6.4.

i can sort of understand it, but it feels not needed, if i do ping-options for the same source IP x.x.x.x i can ping y.y.y.y fine.

i can workaround it with a loopback interface, but that means no IPsec traffic offloading. another option is a second VDOM with the network used in the routing direction but that feels adding quite some config.

anyone encountered this and has an another solution to make it work in both directions?

[jps]

Did you ever find a solution for this?  I'm trying to do the exact same thing.

[Ysiak]

Any solution? I have a similar problem with two BGP interfaces. 

[jps]

The only answer I was given by TAC was to use a loopback interface.  I have the subnet configured on a VLAN interface and don't want to do this.

[gfleming]

I think the problem with using a VLAN (or other interface that's not a WAN interface) is the tunnel by default attempts to build it by egressing said interface.

It appears you may be able to use policy routing to dictate the outgoing interface and enabling "set ike-policy-route" in system settings:

https://docs.fortinet.com/document/fortigate/6.4.11/cli-reference/18620/config-system-settings

https://docs.fortinet.com/document/fortigate/6.4.11/cli-reference/535620/config-router-policy

[gfleming]

https://docs.fortinet.com/document/fortigate/6.4.11/cli-reference/18620/config-system-settings

https://docs.fortinet.com/document/fortigate/6.4.11/cli-reference/535620/config-router-policy

[gfleming]

Are the remote sites under your control? If so a possible solutions would be to create multiple VPN tunnels, one for each physical WAN interface IP (i.e. not using an IP in your block of routable IPs)?

Then you can either use SD-WAN to select the appropriate interface or, most likely preferable in your situation create an aggregate interface (if the other side has FGT as well).

[Ysiak]

Unfortunately not. I must configure many IPSec tunnels for different clients. I have 3 VLANs on wan1. Two are /30 for BGP connection from ISP with a default route, and the third has my public network /28, which I received from IPS. If I set this third VLAN as IPSec source interface, I get error 101:Network is unreachable.

[gfleming]

What version of FOS are you running?

[gfleming]

You might be able to use a policy route for this and enabling "set ike-policy-route" in system settings:

https://docs.fortinet.com/document/fortigate/6.4.11/cli-reference/18620/config-system-settings

https://docs.fortinet.com/document/fortigate/6.4.11/cli-reference/535620/config-router-policy

[Ysiak]

Thx. set ike-policy-route made a difference. P1 and P2 are up, but traffic doesn't go through the tunnel. The curious thing is that Tx counters grow up, but on the other side of the tunnel Rx is 0. package capture looks good, and logs look good but traffic is not flowing throw the tunnel.