Miscellaneous openVZ notes

From Notes_Wiki

Home > CentOS > CentOS 6.x > Virtualization tools > OpenvZ > Miscellaneous openVZ notes

Enabling iptables modules for container

Note use of --iptables is deprecated in latest openVZ. iptables can be enabled using:

vzctl set <ctid> --netfilter full

The new setup also disables connection tracking in base machine. This can be re-enabled by editing '/etc/modprobe.d/openvz.conf' and change the line to:

options nf_conntrack ip_conntrack_disable_ve0=0

Source NAT for containers using base machine

Source NAT for containers can be done using base machine to provide LAN/Internet access to container without requiring an additional IP or exposing the container to outside world. To NAT outgoing connections from container use:

iptables -t nat -I POSTROUTING -s <container-private-IP> -o <exit-interface> -j SNAT --to-source <container-ip>

Enabling tun/tap devices for container

To enable tun/tap devices for container (to use container as VPN server) use following steps:

  1. Use following commands with appropriate CID on base machine
    vzctl set <CID> --devnodes net/tun:rw --save
    vzctl set <CID> --devices c:10:200:rw --save
    vzctl set <CID> --capability net_admin:on --save
  2. Use following commands as root user inside container
    mkdir -p /dev/net
    mknod /dev/net/tun c 10 200
    chmod 600 /dev/net/tun
  3. Restart container

Finding container for a given base machine process

Command 'pstree -pun | less' on base machine can help in seeing process tree of all containers which includes all container inits and their childrens. This can help in finding process id of init of container under which given process has been started. Once init process id for given container is known, container ID can be determined using:

lsof 2>&1 | grep <init-pid> | grep -v lsof

so that various files used by that process in base machine can be listed. Now if the given process has opened /vz/root/<CID>/dev/null file then it means it is init process of <CID> container.

Detecting if current machine or VM or container is using openVZ

To check if current host is using openVZ use following command as root user:

cat /proc/1/status | grep envID

If value is present and is 0 then openVZ is being used and the command was run on base host. Any other envID indicates CTID of the container being used.

Learned from http://christian.hofstaedtler.name/blog/2008/10/detecting-openvz.html

Changing VE_LAYOUT from ploop to simfs

Latest openVZ installations have default VE_LAYOUT as ploop instead of older simfs. If this is not desired then edit '/etc/vz/vz.conf' and set


If ploop is desired then ploop package must be installed.

Change CTID of container

To change CTID of container use following steps:

  1. Stop container with <old-ctid>
  2. Rename /etc/vz/conf/<old-ctid>.conf to /etc/vz/conf/<new-ctid>.conf
  3. Edit /etc/vz/conf/<new-ctid>.conf and change virtual-interface name to new CTID if such name is present
  4. Rename /vz/private/<old-ctid> to /vz/private/<new-ctid>
  5. Rename /vz/root/<old-ctid> to /vz/root/<new-ctid>
  6. Start container with <new-ctid>

Home > CentOS > CentOS 6.x > Virtualization tools > OpenvZ > Miscellaneous openVZ notes