libvirt virsh not using a predefined target while attaching interface

Question

I am trying to automate the process of creating a Guest VM. I am using uvt-kvm to create the VM. I am also taking a network type as a parameter. This can be either nat or macvtap.

Now in the script I create the macvtap interface using these steps:

MACVTAP_INTERFACE="macvtap$VM_NAME"
ip link add link eno1 name $MACVTAP_INTERFACE type macvtap mode bridge
ip link set $MACVTAP_INTERFACE up
ip addr add 10.0.0.1/24 dev $MACVTAP_INTERFACE

The MACVTAP interface with the same name is created without issues. Then I try to attach it to the guest VM which is already created and shutdown in the script before this step. I tried two ways of attaching it:

Way 1:

virsh attach-interface $VM_NAME --type direct --source eno1 --mode bridge --model virtio --target $MACVTAP_INTERFACE --config

This does not actually attach the interface I created before but rather creates a new interface like macvtap11 and attaches it to the VM. Although source, mode and model are set, the target is still not set.

Way 2:

cat > interface-{$VM_NAME}.xml << EOF
  <interface type='direct'>
    <source dev="eno1" mode="bridge"/>
    <model type='virtio'/>
    <target dev="$MACVTAP_INTERFACE"/>
  </interface>
EOF

  virsh attach-device $VM_NAME interface-{$VM_NAME}.xml --persistent --config
  rm interface-{$VM_NAME}.xml

This also results in the same thing. Creates a new interface and attaches it to the VM and doesn't take <target dev="$MACVTAP_INTERFACE"/> into consideration.

This is the XML from a VM which I created using virt-manager UI:

<interface type="direct">
  <mac address="52:54:00:58:03:46"/>
  <source dev="eno1" mode="bridge"/>
  <target dev="macvtap9"/>
  <model type="virtio"/>
  <alias name="net0"/>
  <address type="pci" domain="0x0000" bus="0x01" slot="0x00" function="0x0"/>
</interface>

I made sure all parameters remain same when I attach my interface to the new VM.

Please help me out with how I can define a custom interface and attach it to the VM. The same solution should also work for NAT as well.

Also I am assigning a public IP address subnet while creating the MACVTAP interface. However, my guest VM is not getting a public IP when I use ip addr show. When I create the VM with a MACVTAP interface in virt-manager, it does assign the public IP. I think it is not a DHCP issue or else virt-manager should not have been able to assign the IP either.

Answer 1

https://libvirt.org/formatdomain.html#overriding-the-target-element

This name can be manually specified, however the name should not start with either 'vnet', 'vif', 'macvtap', or 'macvlan', which are prefixes reserved by libvirt and certain hypervisors. Manually specified targets using these prefixes may be ignored.