• 1Configuring Guest Networking

Guest (VM) networking in kvm is the same as in qemu, so it is possible to refer to other documentation about networking in qemu. This page will try to explain how to configure the most frequent types of networking needed.

• Download the latest drivers for your network adapter. Locate and install the downloaded drivers. Your network adapter can now detect your WiFi 6 network. If updating your drivers does not fix the issue, you might still be able to detect your WiFi network if you disable the AX WiFi feature of your router.
• Note that by default, RethinkDB only opens connections bound to localhost in order to prevent unauthorized clients on the network from connecting to the server. The -bind all option allows connections from anywhere on the network. It works well if the network is protected. If your network is open to the internet, you should take additional.

User Networking

Use case:

• You want a simple way for your virtual machine to access to the host, to the internet or to resources available on your local network.
• You don't need to access your guest from the network or from another guest.
• You are ready to take a huge performance hit.
• Warning: User networking does not support a number of networking features like ICMP. Certain applications (like ping) may not function properly.

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Prerequisites:

• You need kvm up and running
• If you don't want to run as root, then the user needs to have rw access to /dev/kvm
• In order for the guest to be able to access the internet or a local network, the host system must be able to access these resources as well

Solution:

• Simply run your guest without specifying network parameters, which by default will create user-level (a.k.a slirp) networking:

Notes:

• The IP address can be automatically assigned to the guest thanks to the DHCP service integrated in QEMU
• If you run multiple guests on the host, you don't need to specify a different MAC address for each guest
• The default is equivalent to this explicit setup:

• The user.0 identifier above is just to connect the two halves into one. You may use any identifier you wish, such as 'n' or 'net0'.
• Use rtl8139 instead of e1000 to get an rtl8139-based network interface.
• You can still access one specific port on the guest using the 'hostfwd' option. This means e.g. if you want to transport a file with scp from host to guest, start the guest with '-device e1000,netdev=user.0 -netdev user,id=user.0,hostfwd=tcp::5555-:22'. Now you are forwarding the host port 5555 to the guest port 22. After starting up the guest, you can transport a file with e.g. 'scp -P 5555 file.txt root@localhost:/tmp' from host to guest. Or you can also use the other address of the host to connect to.

Private Virtual Bridge

Use case:

• You want to set up a private network between 2 or more virtual machines. This network won't be seen from the other virtual machines nor from the real network.

Prerequisites:

• You need kvm up and running
• If you don't want to run as root, then the user needs to have rw access to /dev/kvm
• The following commands must be installed on the host system and executed as root:

Solution:

• You need to create a bridge, e-g:

• You need a qemu-ifup script containing the following (run as root):

• Generate a MAC address, either manually or using:

• Run each guest with the following, replacing $macaddress with the value from the previous step

Notes:

• If you don't want to run qemu-ifup as root, then consider using sudo
• You can either create a system-wide qemu-ifup in /etc/qemu-ifup or use another one. In the latter case, run

• Each guest on the private virtual network must have a different MAC address

Public Bridge

WARNING: The method shown here will not work with all wireless drivers as they might not support bridging.

Use case:

• You want to assign IP addresses to your virtual machines and make them accessible from your local network
• You also want performance out of your virtual machine

Prerequisites:

• You need kvm up and running
• If you don't want to run kvm as root, then the user must have rw access to /dev/kvm
• The following commands must be installed on the host system and executed as root:

• Your host system must be able to access the internet or the local network

Solution 1: Using Distribution-Specific Scripts

• /etc/init.d/networking restart
• The bridge br0 should get the IP address (either static/dhcp) while the physical eth0 is left without an IP address.

VLANs

Please note that the rtl8139 virtual network interface driver does not support VLANs. If you want to use VLANs with your virtual machine, you must use another virtual network interface like virtio.

When using VLANs on a setup like this and no traffic is getting through to your guest(s), you might want to do:

Solution 2: Manual Configuration

• You need to create a bridge, e-g:

• Add one of your physical interface to the bridge, e-g for eth0:

• You need a qemu-ifup script containing the following (run as root):

• Generate a MAC address, either manually or using:

• Run each guest with the following, replacing $macaddress with the value from the previous step

Notes:

• If you don't want to run qemu-ifup as root, then consider using sudo
• Each guest on the network must have a different MAC address
• You can either create a system-wide qemu-ifup in /etc/qemu-ifup or use another one. In the latter case, run

Routing with iptables

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With this method, you can connect your guest vm to a tap device in your host. Then you can set iptables rules in your host so that it acts as a router and firewall for your guest.

Routing is done simply by setting the default route on the client to the IP address of the host, allowing IP forwarding, and setting a route to the tap device of the client on the host.

• Host-side: Allow IPv4 forwarding and add a route to the guest (could be put in a script, but the route has to be added after the guest has started):

• Guest-side: Set the default gateway to the IP address of the host (make sure the host and guest IP addresses are in the same subnet):

• Note: If the host is not on the same subnet as the guest, then you must manually add the route to the host before you create the default route:

VDE

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Another option is using VDE (Virtual Distributed Ethernet).

More information will be provided later.

Performance

Data on benchmarking results should go in here.There's now a page dedicated to ideas for improvingNetworking Performance.

Some 10G NIC performance comparisons between VFIO passthrough and virtio are discussed in VFIO vs virtio.

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Compatibility

There's another, old and obsolete syntax of specifying network for virtual machines. Above examples uses -netdev..-device model, old way used -net..-net pairs. For example,

is about the same as old

(note mac => macaddr parameter change as well; vlan=0 is the default).

Old way used the notion of 'VLANs' - these are QEMU VLANS, which has nothing to do with 802.1q VLANs. Qemu VLANs are numbered starting with 0, and it's possible to connect one or more devices (either host side, like -net tap, or guest side, like -net nic) to each VLAN, and, in particular, it's possible to connect more than 2 devices to a VLAN. Each device in a VLAN gets all traffic received by every device in it. This model was very confusing for the user (especially when a guest has more than one NIC).

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In new model, each host side correspond to just one guest side, forming a pair of devices based on -netdev id= and -device netdev= parameters. It is less confusing, it is faster (because it's always 1:1 pair), and it supports more parameters than old -net..-net way.

However, -net..-net is still supported, used widely, and mentioned in lots of various HOWTOs and guides around the world. It is also a bit shorter and so faster to type.