Gfrktyl

I have an AWS Lightsail instance

• to which I'd like route all traffic from my network directed to a specific (i.e.: 192.168.1.225) IP address


• and from which I'd like to route all traffic directed to 192.168.1.0/24 via my public IP

I've been trying via iptables NAT but this doesn't seem to work either way:

# on my router

iptables -t nat -A  PREROUTING -d 192.168.1.225 -j DNAT --to-destination $AWS_ADDRESS

iptables -t nat -A POSTROUTING -s $AWS_ADDRESS  -j SNAT --to-source 192.168.1.225

and

# on lightsail

iptables -t nat -A  PREROUTING -d 192.168.1.144 -j DNAT --to-destination $PUBLIC_IP

iptables -t nat -A POSTROUTING -s $PUBLIC_IP -j SNAT --to-source 192.168.1.144

What am I doing wrong here?

Have you considered/tried using ssh with port forwarding? I think you can only expect to forward a few ports, but this seems like exactly what it would do for you.

First problem: You are trying to use an address which is local to your subnet. Same-subnet traffic does not go through the router and is not affected by the router's firewall rules. When other computers in your LAN want to reach 192.168.1.225, they instead immediately send local ARP queries to learn its MAC address.

This can be avoided by using a different subnet for the AWS host, or by configuring Proxy-ARP on the router, making it spoof the ARP responses for that address.

Second problem: IP packets can only carry one destination, so DNAT loses information – the recipient (your local router) no longer knows what the original destination was supposed to be. (Only the same host which performed the translation has this information.) So the AWS host can DNAT a single destination to your local public address, but cannot DNAT a whole /24.

To avoid this, you need to tunnel the packets – instead of translating them, encapsulate the original packet within a second IP header. (Also called a VPN.) There are many options for configuring a tunnel on Linux, from plain IPIP or GRE to IPsec to WireGuard or Tinc or OpenVPN.

Most tunnel/VPN types are L3 (they carry IP packets inside), so you will still need to use a different subnet number from the regular LAN subnet, and have the router actually route between LAN interface and VPN/tunnel interface.

Some tunnels support L2 mode (carry Ethernet frames inside), e.g. EoIP, gretap, OpenVPN --dev-type tap, ZeroTier. These L2 tunnel interfaces can be bridged with your LAN, allowing the same subnet to be used on both sides (with fully working ARP), which is what you originally asked for.