I'm trying to figure out exactly what is happening when I create a GRE tunnel.
My network looks like this (-> means directly connected):
- Computer A (eth0: 10.0.1.1) ->
- (eth0: 10.0.1.2) Router B (eth1: 10.0.2.1) ->
- (eth0: 10.0.2.2) Router C (eth1: 10.0.3.1) ->
- (eth0: 10.0.3.2) Router D (eth1: 10.0.4.1) ->
- (eth-: 10.0.4.2) Computer E
I've run the following commands on Router B:
ip tunnel add Tunnel5 mode gre local 10.0.2.1 remote 10.0.3.2 ifconfig Tunnel5 192.168.33.2 netmask 255.255.255.0 up ip route add 10.0.4.2/32 via 192.168.33.3
with the following connection information:
conn routerD_eth0 type=tunnel authby=secret left=10.0.2.1 leftsubnet=10.0.2.1/32 right=10.0.3.2 rightsubnet=10.0.3.2/32 auto=start
And the equivalent on Router D:
ip tunnel add Tunnel5 mode gre local 10.0.3.2 remote 10.0.2.1 ifconfig Tunnel5 192.168.33.3 netmask 255.255.255.0 up ip route add 10.0.1.1/32 via 192.168.33.2
conn routerb_eth1 type=tunnel authby=secret left=10.0.3.2 leftsubnet=10.0.3.2/32 right=10.0.2.1 rightsubnet=10.0.2.1/32 auto=start
This is what I can observe at Router A if I ping from Computer A to Computer B:
Traffic enters eth0 with the destination of 10.0.4.2.
Traffic is routed to the new Tunnel5 interface: Caused by the routing rule I added (ip route add 10.0.4.2/32 via 192.168.33.3)
??? Magic ??? Somehow the traffic is encapsulated and routed back to the router with the new destination address of 10.0.3.2
Normal OSPF routing rules cause the traffic to go out eth1 and on to its destination.
What happens at step 3?
Some commands and their output, all run at Router A:
$ ip tunnel show Tunnel5: gre/ip remote 10.0.3.2 local 10.0.2.1 $ setkey -DP 10.0.3.2[any] 10.0.2.1[any] 255 ... /esp/tunnel/10.0.3.2-10.0.2.1/unique:3 ... 10.0.2.1[any] 10.0.3.2[any] 255 ... /esp/tunnel/10.0.2.1-10.0.3.2/unique:3 ...
The router just knows, based on the information in "ip tunnel show", that traffic routed to Tunnel5 should be encapsulated with the new source and destination addresses.
The encapsulated packet should just be routed like normal. In this case, the IPSec Policies match up and encrypt the packet, preserving the source and destination addresses.
The packet is then routed, based on the routing table, to Router C.
Just a guess.