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I have Internet access to my laptop, over Wi-Fi. I have connected a device with an Ethernet port (with an Ethernet cable from my laptop to the device), and set up a local area network between the two, so that they can ping each other. Now I want the device to access the Internet, i.e., to forward "Internet" from Wi-Fi on my laptop to the Ethernet port of the laptop.
How do I set this up if the laptop runs Ubuntu?
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You want to set up so-called Internet Connection Sharing (ICS). In general you'll need to assign a static address to the ethernet port on your laptop, then configure dnsmasq (DHCP/DNS server) and iptables (firewall) on it.
Fortunately Ubuntu has decent support for ICS with GUI (/usr/bin/nm-connection-editor), see this question at Ask Ubuntu which also just fits your situation. If you want to set up everything with your own configuration, refer to community wiki resources like this and this.
