Hot attach and hot detach network interface to kvm guest

To hot attach a network interface to a kvm guest, please follow below steps. The command we'll be using is virsh:

1. Get to know the name of the guest, run below command on the kvm host: 
   

   foo@host:~$ sudo virsh list
   
    Id Name                 State
   ----------------------------------
     1 kvm-guest running

2. Check whether module acpiphp is loaded on the guest: 
   

   foo@guest:~$ sudo lsmod | grep -i acpiphp
   

3. If yes, proceed to step 4. If no, run below command:
   

   foo@guest:~$ sudo modprobe acpiphp

4. Hot attach the network interface:
   

   foo@host:~$ sudo virsh attach-interface kvm-guest network --model virtio --persistent
   Interface attached successfully

5. Run dmesg on guest to verify that the interface has been attached successfully:
   

   foo@guest:~$ dmesg | tail 
   [38613567.591261] virtio-pci 0000:00:04.0: using default PCI settings
   [38613567.591283] pci 0000:00:05.0: no hotplug settings from platform
   [38613567.591285] pci 0000:00:05.0: using default PCI settings
   [38613567.591741] virtio-pci 0000:00:05.0: enabling device (0000 -> 0003)
   [38613567.593361] ACPI: PCI Interrupt Link [LNKA] enabled at IRQ 10
   [38613567.601486] virtio-pci 0000:00:05.0: PCI INT A -> Link[LNKA] -> GSI 10 (level, high) -> IRQ 10
   [38613567.601524] virtio-pci 0000:00:05.0: setting latency timer to 64
   [38613567.602328] virtio-pci 0000:00:05.0: irq 43 for MSI/MSI-X
   [38613567.602343] virtio-pci 0000:00:05.0: irq 44 for MSI/MSI-X
   [38613567.602357] virtio-pci 0000:00:05.0: irq 45 for MSI/MSI-X

6. Set ipaddress for the new interface:
   

   foo@guest:~$ sudo touch /etc/sysconfig/network-scripts/ifcfg-eth1; sudo echo -e "DEVICE=eth1\nONBOOT=yes\nTYPE=Ethernet\nBOOTPROTO=static\nIPADDR=10.0.0.8\nNETMASK=255.255.255.0" > /etc/sysconfig/ifcfg-eth1

7. Bring up the interface:

   foo@guest:~$ sudo ifup eth1

8. Check the interface:
   

   foo@guest:~$ ifconfig eth1
   eth1      Link encap:Ethernet  HWaddr 52:54:00:D7:10:04
             inet addr:10.0.0.8  Bcast:10.0.0.255  Mask:255.255.255.0
             UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
             RX packets:178767965 errors:0 dropped:0 overruns:0 frame:0
             TX packets:58477452 errors:0 dropped:0 overruns:0 carrier:0
             collisions:0 txqueuelen:1000
             RX bytes:11949338417 (11.1 GiB)  TX bytes:498944480375 (464.6 GiB)

Done.

For hot detaching, the command in virsh is detach-interface, please follow below steps to detach the newly added interface in the above instruction:

1. Bring down the interface in the guest:

   foo@guest:~$ sudo ifdown eth1

2. Delete the interface config file:

   foo@guest:~$ sudo rm /etc/sysconfig/network-scripts/ifcfg-eth1

3. Detach the network interface in host:

   foo@host:~$ sudo detach-interface kvm-guest type network --mac 52:54:00:D7:10:04

4. Verify that the network has been removed, by running the dumpxml command, pipe to less, and search for interface:

   foo@host:~$ sudo virsh dumpxml kvm-guest | less

Done.

Hope you all will gain benefit from this post.