1.11.1 User mode networking

User mode networking emulates a built-in IP router and DHCP server, and routes TCP and UDP traffic between the guest and host. It uses the user mode socket layer of the host to communicate with other hosts.

This allows the use of a significant number of IP network services without requiring administrative privileges, or the installation of a separate driver on the host on which the model is running. Fast Models supports the following kinds of Ethernet device models:

This is paired with an external HostBridge component. The user mode networking specification is set on the external HostBridge.
This has a built-in HostBridge sub-component. The user mode networking specification is set on the internal HostBridge.


  • You can use TCP and UDP over IP, but not ICMP (ping).
  • You can only use DHCP within the private network.
  • You can only make inward connections by mapping TCP ports on the host to the model. This is common to all implementations that provide host connectivity using NAT.
  • Operations that require privileged source ports, for example NFS in its default configuration, do not work.
  • If setup fails, or the parameter syntax is incorrect, there is no error reporting.

To set up user mode networking, run the model with CADI parameters that activate the components:

-C motherboard.hostbridge.userNetworking=true
-C motherboard.smsc_91c111.enabled=true
-C motherboard.virtio_net.hostbridge.userNetworking=true
-C motherboard.virtio_net.enabled=true

To map a host TCP port to a model port, run the model with userNetPorts:

-C motherboard.hostbridge.userNetPorts=8022=22
-C motherboard.virtio_net.hostbridge.userNetPorts=8022=22

This parameter maps port 8022 on the host to port 22 on the model.


User mode networking does not support forwarding UDP ports on the host to the model.
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