Based on the situation, I need to connect to a VPN via/through another VPN. Both of them are running on my Ubuntu machine. I connected to the first VPN . tun0 interface created and my routing table with ip r as follows:
0.0.0.0/1 via 100.64.100.5 dev tun0
default via 192.168.1.1 dev wlp2s0 proto dhcp metric 600
10.0.0.0/8 via 192.168.1.1 dev wlp2s0
100.64.100.1 via 100.64.100.5 dev tun0
100.64.100.5 dev tun0 proto kernel scope link src 100.64.100.6
128.0.0.0/1 via 100.64.100.5 dev tun0
169.254.0.0/16 dev wlp2s0 scope link metric 1000
172.16.0.0/12 via 192.168.1.1 dev wlp2s0
185.208.9.98 via 192.168.1.1 dev wlp2s0
192.168.0.0/16 via 192.168.1.1 dev wlp2s0
192.168.1.0/24 dev wlp2s0 proto kernel scope link src 192.168.1.68 metric 600
I removed the first and 6th rule to prevent transfer traffic to tun0 by default:
default via 192.168.1.1 dev wlp2s0 proto dhcp metric 600
10.0.0.0/8 via 192.168.1.1 dev wlp2s0
100.64.100.1 via 100.64.100.5 dev tun0
100.64.100.5 dev tun0 proto kernel scope link src 100.64.100.6
169.254.0.0/16 dev wlp2s0 scope link metric 1000
172.16.0.0/12 via 192.168.1.1 dev wlp2s0
185.208.9.98 via 192.168.1.1 dev wlp2s0
192.168.0.0/16 via 192.168.1.1 dev wlp2s0
192.168.1.0/24 dev wlp2s0 proto kernel scope link src 192.168.1.68 metric 600
Then I added the second VPN server's IP to go through the first VPN. Consider the second VPN server's IP is 200.200.200.200
ip r add 200.200.200.200 via 100.64.100.5 dev tun0
Then I tried to connect the second VPN - which is an L2TP type, but it was not successful. I tried to debug the L2TP connection with the following command:
/usr/lib/NetworkManager/nm-l2tp-service --debug
I found these lines in the log:
** Message: 17:29:56.476: Check port 1701
** Message: 17:29:56.476: Can't bind to port 1701
nm-l2tp[81418] <warn> L2TP port 1701 is busy, using ephemeral.
nm-l2tp[81418] <info> starting ipsec
Stopping strongSwan IPsec failed: starter is not running
Starting strongSwan 5.9.5 IPsec [starter]...
Loading config setup
Loading conn '3cf99dbd-577c-4c5b-85f7-b081483bd849'
nm-l2tp[81418] <info> Spawned ipsec up script with PID 81797.
initiating IKE_SA 3cf99dbd-577c-4c5b-85f7-b081483bd849[1] to 200.200.200.200
generating IKE_SA_INIT request 0 [ SA KE No N(NATD_S_IP) N(NATD_D_IP) N(FRAG_SUP) N(HASH_ALG) N(REDIR_SUP) ]
sending packet: from 100.64.100.6[500] to 200.200.200.200[500] (904 bytes)
retransmit 1 of request with message ID 0
sending packet: from 100.64.100.6[500] to 200.200.200.200[500] (904 bytes)
nm-l2tp[81418] <warn> Timeout trying to establish IPsec connection
nm-l2tp[81418] <info> Terminating ipsec script with PID 81797.
Stopping strongSwan IPsec...
destroying IKE_SA in state CONNECTING without notification
establishing connection '3cf99dbd-577c-4c5b-85f7-b081483bd849' failed
** Message: 17:30:09.608: Could not establish IPsec connection.
