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I configured vpn with openswan. To allow L2TP vpn connections new interface got created: ppp0. Here's what ifconfig shows:

ppp0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST>  mtu 1410
        inet MY_VPN_GATEWAY  netmask 255.255.255.255  destination MY_VPN_DEST
        ppp  txqueuelen 3  (Point-to-Point Protocol)
        RX packets 13  bytes 1528 (1.4 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 14  bytes 1236 (1.2 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

Why new interface is needed to support vpn connections?

Can't I just add routes to required hosts using default interface eth0?

What's the difference between having default interface eth0 and ppp0? (ip route add default via SOME_IP)

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Why new interface is needed to support vpn connections?

Can't I just add routes to required hosts using default interface eth0?

Basically VPN server will not route just any connection through gateway. Connections first need to be secured to avoid viruses, trojans etc. So all outgoing packets to VPN network without tunneling protocol like PPTP most likely will be dropped.

What's the difference between having default interface eth0 and ppp0? (ip route add default via SOME_IP)

Default route is used when all other routes have not worked. In my case however I only needed setting up routes to VPN network. I added following

sudo ip route add 10.10.1.0/24 dev ppp0

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