# QEMU Networking

**QEMU can bridge guest networking to the host** via several backends: user-mode networking (simple, no root), tap devices (full performance, requires root), or socket-based for inter-VM communication.

```bash
# User-mode networking (default, no root needed)
qemu-system-x86_64 -nic user,hostfwd=tcp:127.0.0.1:2222-:22 disk.img

# Tap device (bridged, full speed, needs root)
sudo qemu-system-x86_64 -nic tap,ifname=tap0,script=no disk.img

# No networking
qemu-system-x86_64 -nic none disk.img
```

**User-mode networking** is the default and needs no setup. The guest gets a DHCP-assigned 10.0.2.x address and can reach the outside world and the host's loopback (127.0.0.1). Port forwarding with `hostfwd=` exposes guest services on the host: `hostfwd=tcp:127.0.0.1:2222-:22` maps host port 2222 to guest port 22 (SSH).

User-mode is slow (host does syscall translation for each packet) and doesn't support all protocols (UDP spoofing breaks some applications). Good for testing; use tap for production.

**Tap devices** are Linux virtual network interfaces that QEMU can attach to. Setup:

```bash
sudo ip tuntap add dev tap0 mode tap user $USER
sudo ip link set tap0 up
qemu-system-x86_64 -nic tap,ifname=tap0,script=no disk.img
```

`-nic tap` bypasses the QEMU network script system and uses a pre-configured tap device. The guest can reach the host's real network at full speed.

**Bridge networking** connects multiple VMs and the host to the same broadcast domain:

```bash
sudo brctl addbr br0
sudo brctl addif br0 eth0
sudo ip link set br0 up
sudo qemu-system-x86_64 -nic bridge,br=br0 disk.img
```

All VMs and the host share the same network segment. VMs get real IP addresses from your DHCP server or static assignment.

For embedded (non-networked) testing, use `-nic none` to skip the network entirely—faster and simpler.