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I'm using a fresh raspbian stretch SD card.

Sorry if I can't get my meaning across well, I'm new to this and still learning.

I'm trying to set up my Pi as a sort of VPN gateway/router. I'd love to know which of the two would be a more apt description. Ultimately devices connected to the pi through eth1 (via USB) should be able to access both the internet and the VPN connection.

The pi access the interent through a local network on eth0 (ip 10.10.0.126, netmask 255.255.255.0) and gets it's address from a DHCP server.

The pi builds a connection to the SSL VPN server using openfortivpn (using PPP). Works like a charm. I'll worry about VPN later, right now I'm stuck at networking.

I would like to create a subnet on eth1 (via USB). I'm trying to wrap my head around dhcpcd.conf and whether I need to set up isc-dhcpd-server to create the subnet, but honestly I'm at a loss. Any help would be appreciated.

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I would say the easiest solution for this setup is to use a bridge because there is already a DHCP server running so we don't have to worry about it. You tagged systemd-networkd so I will use it. First switch over to it with:

# disable classic networking
rpi ~$ sudo -Es
rpi ~# systemctl mask networking.service dhcpcd.service
rpi ~# mv /etc/network/interfaces /etc/network/interfaces~
rpi ~# sed -i '1i resolvconf=NO' /etc/resolvconf.conf

# enable systemd-networkd
rpi ~# systemctl enable systemd-networkd.service systemd-resolved.service
rpi ~# ln -sf /run/systemd/resolve/resolv.conf /etc/resolv.conf

Then create these files to configure the interfaces:

rpi ~# cat > /etc/systemd/network/02-br0.netdev <<EOF
[NetDev]
Name=br0
Kind=bridge
EOF

rpi ~# cat > /etc/systemd/network/04-br0_add-eth.network <<EOF
[Match]
Name=eth*
[Network]
Bridge=br0
EOF

rpi ~# cat > /etc/systemd/network/12-br0_up.network <<EOF
[Match]
Name=br0
[Network]
DHCP=yes
EOF

Reboot. That's it.

Now all devices, also that are connected to eth1, will get an ip address from the existing DHCP server from subnet 10.10.0.0/24 and are member of that subnet with access to all its resources, also to the internet router.

As you wrote the next step would be to use the VPN interface. With an existing VPN connection it is only a routing issue but I don't know openfortivpn and how it can be setup together with systemd-networkd and with a bridge. But lets have a look at it later with a new question from you about it.

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