# Network emulation

QEMU can simulate several network cards (e.g. PCI or ISA cards on the PC target) and can connect them to a network backend on the host or an emulated hub. The various host network backends can either be used to connect the NIC of the guest to a real network (e.g. by using a TAP devices or the non-privileged user mode network stack), or to other guest instances running in another QEMU process (e.g. by using the socket host network backend).

## Using TAP network interfaces

This is the standard way to connect QEMU to a real network. QEMU adds a virtual network device on your host (called `tapN`), and you can then configure it as if it was a real ethernet card.

### Linux host

As an example, you can download the `linux-test-xxx.tar.gz` archive and copy the script `qemu-ifup` in `/etc` and configure properly `sudo` so that the command `ifconfig` contained in `qemu-ifup` can be executed as root. You must verify that your host kernel supports the TAP network interfaces: the device `/dev/net/tun` must be present.

See Invocation to have examples of command lines using the TAP network interfaces.

### Windows host

There is a virtual ethernet driver for Windows 2000/XP systems, called TAP-Win32. But it is not included in standard QEMU for Windows, so you will need to get it separately. It is part of OpenVPN package, so download OpenVPN from : https://openvpn.net/.

## Using the user mode network stack

By using the option `-net user` (default configuration if no `-net` option is specified), QEMU uses a completely user mode network stack (you don’t need root privilege to use the virtual network). The virtual network configuration is the following:

    guest (10.0.2.15)  <------>  Firewall/DHCP server <-----> Internet
                          |          (10.0.2.2)
                          |
                          ---->  DNS server (10.0.2.3)
                          |
                          ---->  SMB server (10.0.2.4)

The QEMU VM behaves as if it was behind a firewall which blocks all incoming connections. You can use a DHCP client to automatically configure the network in the QEMU VM. The DHCP server assign addresses to the hosts starting from 10.0.2.15.

In order to check that the user mode network is working, you can ping the address 10.0.2.2 and verify that you got an address in the range 10.0.2.x from the QEMU virtual DHCP server.

Note that ICMP traffic in general does not work with user mode networking. `ping`, aka. ICMP echo, to the local router (10.0.2.2) shall work, however. If you’re using QEMU on Linux >= 3.0, it can use unprivileged ICMP ping sockets to allow `ping` to the Internet. The host admin has to set the ping_group_range in order to grant access to those sockets. To allow ping for GID 100 (usually users group):

    echo 100 100 > /proc/sys/net/ipv4/ping_group_range

When using the built-in TFTP server, the router is also the TFTP server.

When using the `'-netdev user,hostfwd=...'` option, TCP or UDP connections can be redirected from the host to the guest. It allows for example to redirect X11, telnet or SSH connections.

## Using passt as the user mode network stack

passt can be used as a simple replacement for SLIRP (`-net user`). passt doesn’t require any capability or privilege. passt has better performance than `-net user`, full IPv6 support and better security as it’s a daemon that is not executed in QEMU context.

passt can be connected to QEMU either by using a socket (`-netdev stream`) or using the vhost-user interface (`-netdev vhost-user`). See passt(1) for more details on passt.

### To use socket based passt interface:

Start passt as a daemon:

    passt --socket ~/passt.socket

If `--socket` is not provided, passt will print the path of the UNIX domain socket QEMU can connect to (`/tmp/passt_1.socket`, `/tmp/passt_2.socket`, …). Then you can connect your QEMU instance to passt:

    qemu-system-x86_64 [...OPTIONS...] -device virtio-net-pci,netdev=netdev0 -netdev stream,id=netdev0,server=off,addr.type=unix,addr.path=~/passt.socket

Where `~/passt.socket` is the UNIX socket created by passt to communicate with QEMU.

### To use vhost-based interface:

Start passt with `--vhost-user`:

    passt --vhost-user --socket ~/passt.socket

Then to connect QEMU:

    qemu-system-x86_64 [...OPTIONS...] -m $RAMSIZE -chardev socket,id=chr0,path=~/passt.socket -netdev vhost-user,id=netdev0,chardev=chr0 -device virtio-net,netdev=netdev0 -object memory-backend-memfd,id=memfd0,share=on,size=$RAMSIZE -numa node,memdev=memfd0

Where `$RAMSIZE` is the memory size of your VM `-m` and `-object memory-backend-memfd,size=` must match.

### Migration of passt:

When passt is connected to QEMU using the vhost-user interface it can be migrated with QEMU and the network connections are not interrupted.

As passt runs with no privileges, it relies on passt-repair to save and load the TCP connections state, using the TCP_REPAIR socket option. The passt-repair helper needs to have the CAP_NET_ADMIN capability, or run as root. If passt-repair is not available, TCP connections will not be preserved.

#### Example of migration of a guest on the same host

Before being able to run passt-repair, the CAP_NET_ADMIN capability must be set on the file, run as root:

    setcap cap_net_admin+eip ./passt-repair

Start passt for the source side:

    passt --vhost-user --socket ~/passt_src.socket --repair-path ~/passt-repair_src.socket

Where `~/passt-repair_src.socket` is the UNIX socket created by passt to communicate with passt-repair. The default value is the `--socket` path appended with `.repair`.

Start passt-repair:

    passt-repair ~/passt-repair_src.socket

Start source side QEMU with a monitor to be able to send the migrate command:

    qemu-system-x86_64 [...OPTIONS...] [...VHOST USER OPTIONS...] -monitor stdio

Start passt for the destination side:

    passt --vhost-user --socket ~/passt_dst.socket --repair-path ~/passt-repair_dst.socket

Start passt-repair:

    passt-repair ~/passt-repair_dst.socket

Start QEMU with the `-incoming` parameter:

    qemu-system-x86_64 [...OPTIONS...] [...VHOST USER OPTIONS...] -incoming tcp:localhost:4444

Then in the source guest monitor the migration can be started:

    (qemu) migrate tcp:localhost:4444

A separate passt-repair instance must be started for every migration. In the case of a failed migration, passt-repair also needs to be restarted before trying again.

## Hubs

QEMU can simulate several hubs. A hub can be thought of as a virtual connection between several network devices. These devices can be for example QEMU virtual ethernet cards or virtual Host ethernet devices (TAP devices). You can connect guest NICs or host network backends to such a hub using the `-netdev hubport` or `-nic hubport` options. The legacy `-net` option also connects the given device to the emulated hub with ID 0 (i.e. the default hub) unless you specify a netdev with `-net nic,netdev=xxx` here.

## Connecting emulated networks between QEMU instances

Using the `-netdev socket` (or `-nic socket` or `-net socket`) option, it is possible to create emulated networks that span several QEMU instances. See the description of the `-netdev socket` option in Invocation to have a basic example.