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Setup additional interface

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I have a dedicated server with Ubuntu Server 18.04. I need an additional IP-address so I leased a cheap VDS, then I've setup pptpd on VDS and pptp on dedicated server. Then I configured a VPN-connection on both machines. Then I've setup a static local IP-address for my dedicated server as a PPTP-client. Local IP-address of PPTP-server is 192.168.0.1, local IP-address of PPTP-client is 192.168.0.10. Also I forwarded some ports from VDS to my PPTP-client (dedicated server) via iptables:

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT --to-destination 192.168.0.10
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 443 -j DNAT --to-destination 192.168.0.10
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 8080 -j DNAT --to-destination 192.168.0.10

Dedicated server:

root@cencored:~# pon pptp
root@cencored:~# ifconfig
enp1s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 10.0.1.10  netmask 255.255.255.0  broadcast 10.0.1.255
        inet6 fe80::dabb:c1ff:fe9d:f29a  prefixlen 64  scopeid 0x20<link>
        ether d8:bb:c1:9d:f2:9a  txqueuelen 1000  (Ethernet)
        RX packets 5143110  bytes 3266730706 (3.2 GB)
        RX errors 0  dropped 420968  overruns 0  frame 0
        TX packets 5426143  bytes 3358034990 (3.3 GB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 24290274  bytes 4438508584 (4.4 GB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 24290274  bytes 4438508584 (4.4 GB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

ppp0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST>  mtu 1496
        inet 192.168.0.10  netmask 255.255.255.255  destination 192.168.0.1
        ppp  txqueuelen 3  (Point-to-Point Protocol)
        RX packets 11  bytes 322 (322.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 6  bytes 102 (102.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

The thing with forwarded port didn't work, so I tried to add a gateway:

ip route add default via 192.168.0.1

Now it works, but any applications like web-server, ssh, etc. is unreachable from enp1s0.

Then I deleted gateway and tried to configure netplan:

# This file describes the network interfaces available on your system
# For more information, see netplan(5).
network:
  version: 2
  renderer: networkd
  ethernets:
    enp1s0:
      addresses:
      - 10.0.1.10/24
      gateway4: 10.0.1.1
      nameservers:
        addresses:
        - 8.8.8.8
        - 8.8.4.4
        search: []
      optional: true
    ppp0:
      addresses:
      - 192.168.0.10/32
      gateway4: 192.168.0.1
      nameservers:
        addresses:
        - 8.8.8.8
        - 8.8.4.4
        search: []
      optional: true

and netplan apply

Now route looks like:

root@cencored:~# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         10.0.1.1        0.0.0.0         UG    0      0        0 enp1s0
default         192.168.0.1     0.0.0.0         UG    0      0        0 ppp0
10.0.1.0        0.0.0.0         255.255.255.0   U     0      0        0 enp1s0
192.168.0.1     0.0.0.0         255.255.255.255 UH    0      0        0 ppp0

But ports 80, 8080, 443 still unreachable from internet

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xRef avatar Ubuntu
in flag
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I've found solution on my own. It was enough to setup netplan and iproute2 correctly.

01-netcfg.yaml (netplan)

network:
  version: 2
  ethernets:
    ppp0: 
      addresses: 
      - 192.168.0.10/32
      gateway4: 192.168.0.1
      mtu: 1496
      nameservers: 
        addresses: 
        - 8.8.8.8 
        - 8.8.4.4 
        search: [] 
      routes: 
        - to: 0.0.0.0/0 
          via: 192.168.0.1
          metric: 50
          table: 800 
      routing-policy: 
        - from: 192.168.0.10 
          table: 800 
          priority: 100
    enp1s0:
      addresses:
      - 10.0.1.10/24
      mtu: 1500
      gateway4: 10.0.1.1
      nameservers:
        addresses:
        - 8.8.8.8
        - 8.8.4.4
        search: []
      routes:
        - to: 0.0.0.0/0
          via: 10.0.1.1
          metric: 100
          table: 801
      routing-policy:
        - from: 10.0.1.10
          table: 801
          priority: 200

rt_tables (iproute2)

#
# reserved values
#
255 local
254 main
253 default
0   unspec
#
# local
#
#1  inr.ruhep
801 801
800 800
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