Pascal's Digital Garden

Bridging an Ethernet-Only Device via a Computer

2 min read

Context

  • Some devices (e.g. NAS, lab hardware, controllers) don’t have WiFi.
  • Instead of running a long Ethernet cable to the router, a computer can act as a gateway/bridge.
  • Example here: a NAS, but could be any Ethernet device.

Network Setup

  • Main LAN: 192.168.8.0/24 (WiFi, router, laptops, cluster nodes)
  • Isolated subnet: 192.168.0.0/24 (device connected directly to PC via Ethernet)
  • The computer sits in between and provides:
    • DHCP (assign IP to device)
    • NAT (give device internet access)
    • Forwarding (route packets between interfaces)
    • Optional port forwards (make device reachable from main LAN)
graph LR
      Internet([Internet])
      Router[Router<br/>192.168.8.1]

      subgraph "Main LAN: 192.168.8.0/24"
          Node0[Gateway PC<br/>192.168.8.200<br/>WiFi: wlp2s0]
          Node1[Node1<br/>192.168.8.201]
          Laptop[Laptop<br/>192.168.8.***]
          Phone[Phone<br/>192.168.8.***]
      end

      subgraph "Isolated Subnet: 192.168.0.0/24"
          Device[Ethernet Device<br/>192.168.0.3<br/>example.local]
      end

      subgraph "Bridge Services"
          DHCP[dnsmasq DHCP]
          NAT[iptables NAT]
          PortForward[Port Forward<br/>9123 to 80]
      end

      Internet --> Router
      Router -.WiFi.- Node0
      Router -.WiFi.- Node1
      Router -.WiFi.- Laptop
      Router -.WiFi.- Phone

      Node0 ---|Ethernet eno1| Device
      Node0 --> DHCP
      Node0 --> NAT
      Node0 --> PortForward

      DHCP -.assigns IP.- Device
      NAT -.internet access.- Device

      Laptop -.port 9123.- PortForward
      Phone -.port 9123.- PortForward
      PortForward --> Device

      classDef device fill:#e1f5fe
      classDef computer fill:#f3e5f5
      classDef service fill:#fff3e0

      class Device device
      class Node0 computer
      class DHCP,NAT,PortForward service

Commands

Bring up Ethernet + assign IP

sudo ip link set eno1 up
sudo ip addr add 192.168.0.1/24 dev eno1

Enable IP forwarding

sudo sysctl net.ipv4.ip_forward=1

NAT + forwarding rules

sudo iptables -t nat -A POSTROUTING -s 192.168.0.0/24 -o wlp2s0 -j MASQUERADE
sudo iptables -A FORWARD -i eno1 -o wlp2s0 -j ACCEPT
sudo iptables -A FORWARD -i wlp2s0 -o eno1 -m state --state RELATED,ESTABLISHED -j ACCEPT

DHCP with dnsmasq

sudo apt install dnsmasq
 
echo "interface=eno1
dhcp-range=192.168.0.2,192.168.0.10,12h
dhcp-option=3,192.168.0.1
dhcp-option=6,8.8.8.8
port=0" | sudo tee /etc/dnsmasq.d/bridge.conf
 
sudo systemctl restart dnsmasq

Optional: port forwarding

sudo iptables -t nat -A PREROUTING -p tcp --dport 9123 -j DNAT --to-destination 192.168.0.3:80

Quick Checks

nmap -sn 192.168.0.0/24   # find device
ping 192.168.0.3          # check connectivity
curl -I http://192.168.0.3 # test web UI

Notes / Uses

  • From main LAN: reach device via http://192.168.8.200:9123 (or whichever port you forward).
  • From local subnet: use its direct IP (192.168.0.3).
  • Device gets internet via PC → WiFi.
  • Good for: NAS, smart-home controllers, dev boards, old lab equipment, etc.

Graph View