Virtual Network interfaces can be attached or detached from a guest virtual machine (domU) while the virtual machine is up and running. The below Xen networking steps gives details on those.

xm network-list” is the command to display the virtual network interfaces attached to a the guest Xen virtual machine. In this example, the command lists 3 NICs for the virtual machine “vm455”. We’ll try to remove and add back the third vif.

[root@host203 ~]# xm network-list vm455
Idx BE MAC Addr. handle state evt-ch tx-/rx-ring-ref BE-path
0 0 00:16:3E:47:AA:FD 0 4 12 2304 /2305 /local/domain/0/backend/vif/1/0
1 0 00:16:3E:22:33:44 1 4 13 2306 /2307 /local/domain/0/backend/vif/1/1
2 0 00:16:3E:40:22:12 2 4 14 2308 /2309 /local/domain/0/backend/vif/1/2
[root@host203 ~]#

To remove a NIC card from the virtual machine, use “xm network-detach”, with the corresponding options. In this example, the interface 3 is removed from the virtual machine. This command has to be executed on the Xen server.

[root@host203 ~]# xm network-detach vm455 2
[root@host203 ~]# xm network-list vm455
Idx BE MAC Addr. handle state evt-ch tx-/rx-ring-ref BE-path
0 0 00:16:3E:47:AA:FD 0 4 12 2304 /2305 /local/domain/0/backend/vif/1/0
1 0 00:16:3E:22:33:44 1 4 13 2306 /2307 /local/domain/0/backend/vif/1/1
[root@host203 ~]#

Once the network is detached, you can only 2 virtual NICs using “xm network-list” on the Xen server. If you check the virtual machine, it will have only 2 NICS. Use ifconfig to check on the guest virtual machine.

To add a NIC to the virtual machine, use the command “xm network-attach”. Pass the MAC-id , bridge name and the script to create the VIF as arguments. The default vif creation script is “vif-bridge”.  This is defined in /etc/xen/xend-config.sxp .

[root@host203 ~]# xm network-attach vm455 bridge=xenbr1 script=vif-bridge mac=00:16:3E:40:22:12
[root@host203 ~]# xm network-list vm455
Idx BE MAC Addr. handle state evt-ch tx-/rx-ring-ref BE-path
0 0 00:16:3E:47:AA:FD 0 4 12 2304 /2305 /local/domain/0/backend/vif/1/0
1 0 00:16:3E:22:33:44 1 4 13 2306 /2307 /local/domain/0/backend/vif/1/1
2 0 00:16:3E:40:22:12 2 4 14 2308 /2309 /local/domain/0/backend/vif/1/2
[root@host203 ~]#

The xm network-attach/detach command adds and removes NIC on a live machine. To make the changes permanent, edit the virtual machine configuration file in /etc/xen and edit the vifs. The same steps apply to Oracle Virtual machine (OVM) also.

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