Getting started with LXC using libvirt
For quite a while now, libvirt has had an LXC driver that uses Linux’s namespace + cgroups features to provide container based virtualization. Before continuing I should point out that the libvirt LXC driver does not have any direct need for the userspace tools from the LXC sf.net project, since it directly leverages APIs the Linux kernel exposes to userspace. There are in fact many other potential users of the kernel’s namespace APIs which have their own userspace, such as OpenVZ, Linux-VServer, Parallels. This blog post will just concern itself solely with the native libvirt LXC support.
Connecting to the LXC driver
At this point in time, there is only one URI available for connecting to the libvirt LXC driver, lxc:///, which gets you a privileged connection. There is not yet any support for unprivileged libvirtd instances using containers, due to restrictions of the kernel’s DAC security models. I’m hoping this may be refined in the future.
If you’re familiar with using libvirt in combination with KVM, then it is likely you are just relying on libvirt picking the right URI by default. Well each host can only have one default URI for libvirt, and KVM will usually take precedence over LXC. You can discover what libvirt has decided the default URI:
# virsh uri qemu:///system
So when using tools like virsh you’ll need to specify the LXC URI somehow. The first way is to use the ‘-c URI’ or ‘–connect URI’ arguments that most libvirt based applications have:
# virsh -c lxc:/// uri lxc:///
The second option is to explicitly override the default libvirt URI for your session using the LIBVIRT_DEFAULT_URI environment variable.
# export LIBVIRT_DEFAULT_URI=lxc:/// # virsh uri lxc:///
For the sake of brevity, all the examples that follow will presume that export LIBVIRT_DEFAULT_URI=lxc:/// has been set.
A simple “Hello World” LXC container
The Hello World equivalent for LXC is probably a container which just runs /bin/sh with the main root filesystem / network interfaces all still being visible. What you’re gaining here is not security, but a rather way to manage resource utilization of everything spawned from that initial process. The libvirt LXC driver currently does most of its resource controls using cgroups, but will also leverage the network traffic shaper directly for network controls which you want to be done per virtual network interface, not per cgroup.
Anyone familiar with libvirt will know that to create a new guest, requires an XML document specifying its configuration. Machine based virtualization requires either a kernel/initrd or a virtual BIOS to boot, and can create a fullyvirutalized (hvm) or paravirtualized machine (xen). Container virtualization by contrast, just wants to know the path to the binary to spawn as the container’s “init” (aka process with PID 1). The virtualization type for containers is thus referred to in libvirt as “exe”. Aside from the virtualization type & path of the initial process, the only other required XML parameters are the guest name, initial memory limit and a text console device. Putting this together, creating the “Hello World” container will require an XML configuration that looks like this:
# cat > helloworld.xml <<EOF
<domain type='lxc'>
<name>helloworld</name>
<memory>102400</memory>
<os>
<type>exe</type>
<init>/bin/sh</init>
</os>
<devices>
<console type='pty'/>
</devices>
</domain>
EOF
This configuration can be imported into libvirt in the normal manner
# virsh define helloworld.xml Domain helloworld defined from helloworld.xml
then started
# virsh start helloworld Domain helloworld started # virsh list Id Name State ---------------------------------- 31417 helloworld running
The ID values assigned by the libvirt LXC driver are in the process ID of the libvirt_lxc helper process libvirt launches. This helper is what actually creates the container, spawning the initial process, after which it just sits around handling console I/O. Speaking of the console, this can now be accessed with virsh
# virsh console helloworld Connected to domain helloworld Escape character is ^] sh-4.2#
That ‘sh’ prompt is the shell process inside the container. All the container processes are visible outside the container as regular proceses
# ps -axuwf ... root 31417 0.0 0.0 42868 1252 ? Ss 16:17 0:00 /usr/libexec/libvirt_lxc --name helloworld --console 27 --handshake 30 --background root 31418 0.0 0.0 13716 1692 pts/39 Ss+ 16:17 0:00 \_ /bin/sh ...
Inside the container, PID numbers are distinct, starting again from ‘1’.
# virsh console helloworld Connected to domain helloworld Escape character is ^] sh-4.2# ps -axuwf USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 1 0.0 0.0 13716 1692 pts/39 Ss 16:17 0:00 /bin/sh
The container will shutdown when the ‘init’ process exits, so in this example when ‘exit’ is run in the container’s bash shell. Alternatively issue the usual ‘virsh destroy’ to kill it off.
# virsh destroy helloworld Domain helloworld destroyed
Finally remove its configuration
# virsh undefine helloworld Domain helloworld has been undefined
Adding custom mounts to the “Hello World” container
The “Hello World” container shared the same root filesystem as the primary (host) OS. What if the application inside the container requires custom data in certain locations. For example, using containers to sandbox apache servers, might require a custom /etc/httpd and /var/www. This can easily be achieved by specifying one or more filesystem devices in the initial XML configuration. Lets create some custom locations to pass to the “Hello World” container.
# mkdir /export/helloworld/config # touch /export/helloworld/config/hello.txt # mkdir /export/helloworld/data # touch /export/helloworld/data/world.txt
Now edit the helloworld.xml file and add in
<filesystem type='mount'> <source dir='/export/helloworld/config'/> <target dir='/etc/httpd'/> </filesystem> <filesystem type='mount'> <source dir='/export/helloworld/data'/> <target dir='/var/www'/> </filesystem>
Now after defining and starting the container again, it should see the custom mounts
# virsh define helloworld.xml Domain helloworld defined from helloworld.xml # virsh start helloworld Domain helloworld started # virsh console helloworld Connected to domain helloworld Escape character is ^] sh-4.2# ls /etc/httpd/ hello.txt sh-4.2# ls /var/www/ world.txt sh-4.2# exit # virsh undefine helloworld Domain helloworld has been undefined
A private root filesystem with busybox
So far the container has shared the root filesystem with the host OS. This may be OK if the application running in the container is going to an unprivileged user ID and you are careful not to mess up your host OS. If you want todo things like running DHCP inside the container, or have things running as root, then you almost certainly want a private root filesystem in the container. In this example, we’ll use the busybox tools to setup the simplest possible private root for “Hello World”. First create a new directory and copy the busybox binary into position
mkdir /export/helloworld cd /export/helloworld mkdir -p bin var/www etc/httpd cd bin cp /sbin/busybox busybox cd /root
Next step is to setup symlinks for all the busybox commands you intend to use. For example
for i in ls cat rm find ps echo date kill sleep \
true false test pwd sh which grep head wget
do
ln -s busybox /root/helloworld/bin/$i
done
Now all that is required, is to add another filesystem device to the XML configuration
<filesystem type='mount'> <source dir='/export/helloworld/root'/> <target dir='/'/> </filesystem>
With that added to the XML, follow the same steps to define and start the guest again
# virsh define helloworld.xml Domain helloworld defined from helloworld.xml # virsh start helloworld Domain helloworld started
Now when accessing the guest console a completely new filesystem should be visible
# virsh console helloworld Connected to domain helloworld Escape character is ^] # ls bin dev etc proc selinux sys var # ls bin/ busybox echo grep ls rm test which cat false head ps sh true date find kill pwd sleep wget # cat /proc/mounts rootfs / rootfs rw 0 0 devpts /dev/pts devpts rw,seclabel,relatime,gid=5,mode=620,ptmxmode=666 0 0 /dev/mapper/vg_t500wlan-lv_root / ext4 rw,seclabel,relatime,user_xattr,barrier=1,data=ordered 0 0 devpts /dev/pts devpts rw,seclabel,relatime,gid=5,mode=620,ptmxmode=666 0 0 devfs /dev tmpfs rw,seclabel,nosuid,relatime,mode=755 0 0 proc /proc proc rw,nosuid,nodev,noexec,relatime 0 0 proc /proc/sys proc ro,relatime 0 0 /sys /sys sysfs ro,seclabel,relatime 0 0 selinuxfs /selinux selinuxfs ro,relatime 0 0 devpts /dev/ptmx devpts rw,seclabel,relatime,gid=5,mode=620,ptmxmode=666 0 0 /dev/mapper/vg_t500wlan-lv_root /etc/httpd ext4 rw,seclabel,relatime,user_xattr,barrier=1,data=ordered 0 0 /dev/mapper/vg_t500wlan-lv_root /var/www ext4 rw,seclabel,relatime,user_xattr,barrier=1,data=ordered 0 0
Custom networking in the container
The examples thus far have all just inherited access to the host network interfaces. This may or may not be desirable. It is of course possible to configure private networking for the container. Conceptually this works in much the same way as with KVM. Currently it is possible to choose between libvirt’s bridge, network or direct networking modes, giving ethernet bridging, NAT/routing, or VEPA respectively. When configuring private networking, the host OS will get a ‘vethNNN’ device for each container NIC, and the container will see their own ‘ethNNN’ and ‘lo’ devices. The XML configuration additions are just the same as what’s required for KVM, for example
<interface type='network'> <mac address='52:54:00:4d:2b:cd'/> <source network='default'/> </interface>
Define and start the container as before, then compare the network interfaces in the container to what is in the host
# virsh console helloworld Connected to domain helloworld Escape character is ^] # ifconfig eth0 Link encap:Ethernet HWaddr 52:54:00:16:61:DA inet6 addr: fe80::5054:ff:fe16:61da/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:93 errors:0 dropped:0 overruns:0 frame:0 TX packets:6 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:5076 (4.9 KiB) TX bytes:468 (468.0 B) lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:0 (0.0 B) TX bytes:0 (0.0 B) # route -n Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface
We have a choice of configuring the guest eth0 manually, or just launching a DHCP client. To do manual configuration try
# virsh console helloworld Connected to domain helloworld Escape character is ^] # ifconfig eth0 192.168.122.50 # route add 0.0.0.0 gw 192.168.122.1 eth0 # route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface default 192.168.122.1 255.255.255.255 UGH 0 0 0 eth0 192.168.122.0 * 255.255.255.0 U 0 0 0 eth0 # ping 192.168.122.1 PING 192.168.122.1 (192.168.122.1): 56 data bytes 64 bytes from 192.168.122.1: seq=0 ttl=64 time=0.786 ms 64 bytes from 192.168.122.1: seq=1 ttl=64 time=0.157 ms ^C --- 192.168.122.1 ping statistics --- 2 packets transmitted, 2 packets received, 0% packet loss round-trip min/avg/max = 0.157/0.471/0.786 ms
Am I running in an LXC container?
Some programs may wish to know if they have been launched inside a libvirt container. To assist them, the initial process is given two environment variables, LIBVIRT_LXC_NAME and LIBVIRT_LXC_UUID
# echo $LIBVIRT_LXC_NAME helloworld # echo $LIBVIRT_LXC_UUID a099376e-a803-ca94-f99c-d9a8f9a30088
An aside about CGroups and LXC
Every libvirt LXC container gets placed inside a dedicated cgroup, $CGROUPROOT/libvirt/lxc/$CONTAINER-NAME. Libvirt expects the memory, devices, freezer, cpu and cpuacct cgroups controllers to be mounted on the host OS. Work on leveraging cgroups inside LXC with libvirt is still ongoing, but there are already APIs to set/get memory and CPU limits, with networking to follow soon. This is could be a topic for a blog post on its own, so won’t be discussed further here.
An aside about LXC security, or lack thereof
You might think that since we can create a private root filesystem, it’d be cool to run an entire Fedora/RHEL OS in the container. I strongly caution against doing this. The DAC (discretionary access control) system on which LXC currently relies for all security is known to be incomplete and so it is entirely possible to accidentally/intentionally break out of the container and/or impose a DOS attack on the host OS. Repeat after me “LXC is not yet secure. If I want real security I will use KVM”. There is a plan to make LXC DAC more secure, but that is no where near finished. We also plan to integrate sVirt with LXC to so that MAC will mitigate holes in the DAC security model.
An aside about Fedora >= 15, SystemD and autofs
If you are attempting to try any of this on Fedora 16 or later, there is currently an unresolved problem with autofs that breaks much use of containers. The root problem is that we are unable to unmount autofs mount points after switching into the private filesystem namespace. Unfortunately SystemD uses autofs in its default configuration, for several type mounts. So if you find containers fail to start, then as a temporary hack you can try disabling all SystemD’s autofs mount points
for i in `systemctl --full | grep automount | awk '{print $1}'`
do
systemctl stop $i
done
We hope to resolve this in a more satisfactory way in the near future.
The complete final example XML configuration
# cat helloworld.xml
<domain type='lxc'>
<name>helloworld</name>
<memory>102400</memory>
<os>
<type>exe</type>
<init>/bin/sh</init>
</os>
<devices>
<console type='pty'/>
<filesystem type='mount'>
<source dir='/export/helloworld/root'/>
<target dir='/'/>
</filesystem>
<filesystem type='mount'>
<source dir='/export/helloworld/config'/>
<target dir='/etc/httpd'/>
</filesystem>
<filesystem type='mount'>
<source dir='/export/helloworld/data'/>
<target dir='/var/www'/>
</filesystem>
<interface type='network'>
<source network='default'/>
</interface>
</devices>
</domain>