Routing traffic from VPN clients to internet (Tincdaemon+Ubuntu)

Question

I have a tinc daemon running on Ubuntu virtual machine. My routing table looks like this:

default via 10.0.2.2 dev enp0s3  proto static  metric 100 
10.0.0.0/24 dev vpnnet  scope link 
10.0.2.0/24 dev enp0s3  proto kernel  scope link  src 10.0.2.15  metric 100 
169.254.0.0/16 dev enp0s3  scope link  metric 1000 

Where - 10.0.2.0/24 - local network subnet, 10.0.0.0/24 - VPN subnet

Interfaces:

vpnnet    Link encap:Ethernet  HWaddr b2:e8:0b:22:22:f1  
          inet addr:10.0.0.1  Bcast:10.0.0.255  Mask:255.255.255.0
          inet6 addr: fe80::b0e8:bff:fe22:22f1/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:62 errors:0 dropped:0 overruns:0 frame:0
          TX packets:47 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:2736 (2.7 KB)  TX bytes:5760 (5.7 KB)

enp0s3    Link encap:Ethernet  HWaddr 08:00:27:12:d8:be  
          inet addr:10.0.2.15  Bcast:10.0.2.255  Mask:255.255.255.0
          inet6 addr: fe80::5333:40a8:822c:e6f9/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:6428 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2575 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:6131799 (6.1 MB)  TX bytes:200156 (200.1 KB)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:206 errors:0 dropped:0 overruns:0 frame:0
          TX packets:206 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:15740 (15.7 KB)  TX bytes:15740 (15.7 KB)

enp0s3 - local interface name, vpnnet - VPN interface name. 10.0.0.1 - VPN server, 10.0.0.131 - VPN Client.

A client has it's default gateway replaced, so it can only ping server (10.0.0.1) but nothing else.

Client's routing table:

0.0.0.0/1 via 10.0.0.254 dev tap0
server-static-ip via 192.168.1.1 dev wlan0
10.0.0.0/8 dev tap0  proto kernel  scope link  src 10.0.0.131
10.0.0.254 dev tap0  scope link
128.0.0.0/1 via 10.0.0.254 dev tap0
192.168.1.0/24 dev wlan0  proto kernel  scope link  src 192.168.1.46  metric 310
192.168.1.1 dev wlan0  scope link

I want client to be able to access the internet through my Ubuntu server. (Kinda like a proxy) As far as I understand, I need to route all traffic that comes in from my vpn client to internet through default gateway on enp0s3 and back. What changes do I need to make to my routing table/NAT to achieve this?

I looked at articles describing a way to resolve this using iptables POSTROUTING rules, but nothing seems to work for me.

Help!

Answer 1

You didn't mention which gateway address the client uses. Is it set to 10.0.0.1 (your IP)? If so, everything's fine – just follow the exact same instructions (tutorials) for making your own router. Roughly:

1. Enable IPv4 forwarding system-wide, using the net.ipv4.conf.all.forwarding sysctl;

2. Add an iptables/nft masquerade rule, so that your clients' packets will be SNATed.

Alternatively (cleaner technically, but more configuration):

1. Enable IPv4 forwarding;

2. Don't do NAT, but configure your upstream router to have a static route towards 10.0.0.0/24 via your IP 10.0.2.15.