Creating NAT for Use in NetBSD QEMU

I always forget how to connect to the internet in QEMU. Usually, I don’t perform any special configurations and simply use QEMU’s user mode networking. In this setup, I can’t ping 1.1.1.1, although I can ping google.com. When I was using NetBSD in QEMU, the internet was very slow, so I looked for a way to speed it up. The first method I considered was using a bridge, but bridging to a wireless Wi-Fi isn’t feasible. Eventually, I found that using NAT could be a solution¹.

The operating system I am using is Alpine Linux. Before I can create the tap0 interface, I need to install iproute2:

doas apk add iproute2

After that, I created tap0.

doas ip tuntap add tap0 mode tap
doas ip addr add 192.168.100.1/24 dev tap0

Next, I enabled IP forwarding and set up the routing table:

doas sysctl net.ipv4.ip_forward=1
doas iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE

i usualy create shell script for qemu

#!/bin/sh

set -xe

ISO="NetBSD-10.1-amd64.iso"
DISK="NetBSD.qcow2"

qemu-system-x86_64   \
    -enable-kvm  \
    -cpu host    \
    -m 4G        \
    -smp 2       \
    -hda $DISK   \
    -boot b      \ # b: boot to hda, d: boot to cdroom
    -cdrom $ISO  \
    -netdev tap,id=net0,ifname=tap0,script=no,downscript=no \
    -device virtio-net-pci,netdev=net0 \
    -object rng-random,filename=/dev/urandom,id=rng0 \
    -device virtio-rng-pci,rng=rng0 \
    -vga std -display sdl,gl=on

Don’t forget to create disk

qemu-img create -f qcow2 NetBSD.qcow2 20G

Set network like this in NetBSD

IP Address: 192.168.100.2
Netmask: 255.255.255.0
Gateway: 192.168.100.1
DNS: 1.1.1.1

To do this, you can run the following commands in NetBSD:

ifconfig vioif0 inet 192.168.1.100 netmask 255.255.255.0 up
route add default 192.168.1.1
echo "nameserver 1.1.1.1" > /etc/resolv.conf