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.

Creating a persistent live cd (ubuntu or linux mint) using a loopback file

This technique will require the use of the live cd, and a usb stick, as a vessel for the persistent file. Two requirements: the partition of the live cd must be in a format that can be mounted automatically, for example: vfat, ext3 or ext4 and the loopback file must be in the root of the partition

1. Boot your machine using live cd
2. Once boot up, insert your usb stick
3. Check whether your usb stick is already mounted

   $ df -lh
   Filesystem      Size  Used Avail Use% Mounted on
   /cow            3.9G   59M  3.9G   2% /
   udev            3.9G  4.0K  3.9G   1% /dev
   tmpfs           798M  1.4M  796M   1% /run
   /dev/sr0        1.2G  1.2G     0 100% /cdrom
   /dev/loop0      1.2G  1.2G     0 100% /rofs
   none            4.0K     0  4.0K   0% /sys/fs/cgroup
   tmpfs           3.9G   24K  3.9G   1% /tmp
   none            5.0M     0  5.0M   0% /run/lock
   none            3.9G  700K  3.9G   1% /run/shm
   none            100M   12K  100M   1% /run/user
   /dev/sdb1         8G    1G    7G  12% /media/mint/backup 

4. In this case, your usb stick is already mounted to /media/mint/backup
5. Create a 1GB loopback file (or whatever size you desire)

   $ dd if=/dev/zero of=/media/mint/backup/casper-rw count=1000 bs=1M

6. Format the file as ext3:

   $ mkfs.ext3 -L casper-rw /media/mint/backup/casper-rw 
   mke2fs 1.42.9 (4-Feb-2014)
   casper-rw is not a block special device.
   Proceed anyway? (y,n) y
   Discarding device blocks: done                            
   Filesystem label=casper-rw
   OS type: Linux
   Block size=4096 (log=2)
   Fragment size=4096 (log=2)
   Stride=0 blocks, Stripe width=0 blocks
   64000 inodes, 256000 blocks
   12800 blocks (5.00%) reserved for the super user
   First data block=0
   Maximum filesystem blocks=264241152
   8 block groups
   32768 blocks per group, 32768 fragments per group
   8000 inodes per group
   Superblock backups stored on blocks: 
       32768, 98304, 163840, 229376
   
   Allocating group tables: done                            
   Writing inode tables: done                            
   Creating journal (4096 blocks): done
   Writing superblocks and filesystem accounting information: done

7. The file is done
8. To set your live cd to use the loopback file, after your live cd booted up, add a space followed by word "persistent" without the quotes, at the end of your kernel parameter. This can be achieved in linux mint by pressing tab in the kernel list menu, add the parameter, and enter to boot from that kernel. 
   For ubuntu, when the Live CD menu gets displayed hit the key to enter “Other Options”. This will display the arguments that the Live CD passes to the kernel. At the end of this argument list just add a space and add the word "persistent". This will instruct the Live CD to maintain and use persistence. 
9. That's all folks, test your persistent live cd by saving a few files in your home, and restart to see whether the files survived a reboot.