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Another FAQ for VServer 2.0

Andreas John's self-FAQ developed from a dialogue with myself and answers from the mailing-list/Bertl and others (Thx to all!)

Q: What is a 'Guest'?

A: To talk about stuff, we need some naming. The physical machine is called 'Host' and the 'main' context running the Host Distro is called 'Host Context'. The virtual machine/distro is called 'Guest' and basically is a Distribution (Userspace) running inside a 'Guest Context'.

Q: What kind of Operating System (OS) can I run as guest?

A: With VServer you can only run Linux guests. The trick is that a guest does not run a kernel on its own (as XEN and UML do), it merely uses a virtualized host kernel-interface. VServer offers so called security contexts which make it possible to seperate one guest from each other, i.e. they cannot get data from each other. Imagine it as a chroot environment with much more security and features.

Q: Which distributions did you test?

A: Some. Check out the wiki for ready-made guest images. But you can easily build own guest images, e.g. with Debian's debootstrap. Checkout step-by-step Guide 2.6 how to do that.

Q: Is VServer comparable to XEN/UML/QEMU?

A: Nope. XEN/UML/QEMU and VServer are just good friends. Because you ask, you probably know what XEN/UML/QEMU are. VServer in contrary to XEN/UML/QEMU not "emulate" any hardware you run a kernel on. You can run a VServer kernel in a XEN/UML/QEMU guest. This is confirmed to work at least with Linux 2.6/vs2.0.

Q: Is VServer secure?

A: We hope so. It should be as least as secure as Linux is. We consider it much much more secure though.

Q: Performance?

A: For a single guest, we basically have native performance. Some tests showed insignificant overhead (about 1-2%) others ran faster than on an unpatched kernel. This is IMVHO significantly less than other solutions waste, especially if you have more than a single guest (because of the resource sharing).

Q: Is SMP Supported?

A: Yes, on all SMP capable kernel architectures.

Q: Resource sharing?

A: Yes ....

Q: Resource limiting?

A: Yes, you can set maximum limits per guest, but you can only offer guaranteed resource availability with some ticks at the time. There is the possibility to ulimit and to rlimit. Rlimit is a new feature of kernel 2.6/vs2.0.

Q: Disk I/O limiting? Is that possible?

A: Well, since vs2.1.1 linux-vserver supports a mechanism called 'I/O scheduling', which appeared in the 2.6 mainline some time ago. The mainline kernel offers several I/O schedulers:

# cat /sys/block/hdc/queue/scheduler
noop [anticipatory] deadline cfq

The default is anticipatory a.k.a. "AS". When running several guests on a host you probably want the I/O performance shared in a fair way among the different guests. The kernel comes with a "completely fair queueing" scheduler, CFQ, which can do that. (More on schedulers can be found at http://lwn.net/Articles/114770/)

This is how to set the scheduler to "cfq" manually:

root# echo "cfq" > /sys/block/hdc/queue/scheduler
root# cat /sys/block/hdc/queue/scheduler
noop anticipatory deadline [cfq]

Keep in mind that you have to do it on all physical discs. So if you run an md-softraid, do it to all physical /dev/hdXYZ discs!

If you run Debian there is a predefined way to set the /sys values at boot-time:

# apt-get install sysfsutils

# cat /etc/sysfs.conf  | grep cfq
block/sda/queue/scheduler = cfq
block/sdc/queue/scheduler = cfq

# /etc/init.d/sysfsutils restart

For non-vserver processes and CFQ you can set by which key the kernel decides about the fairness:

cat /sys/block/hdc/queue/iosched/key_type
pgid [tgid] uid gid

Hint: The 'key_type'-feature has been removed in the mainline kernel recently. Don't look for it any longer :(

The default is tgid, which means to share fairly among process groups. Think every guest is treated like a own process group. It's not possible to set a scheduler strategy within a guest. All processes belonging to the same guest are treated like "noop" within the guest. So: If you run apache and some ftp-server within the _same_ guest, there is no fair scheduling between them, but there is fair scheduling between the whole guest and all other guests.

And: It's possible to tune the scheduler parameters in several ways. Have a look at /sys/block/hdc/queue/....

You need a very recent Version of VS devel, e.g. the 2.1.1-rc18 can do it. Some older version have that feature too, then it got lost and was reinvented. So: Go and get a rc18 - only in 'devel', not stable!

Q: Why isn't there a device /dev/bla? within a guest

A: Device nodes allow Userspace to access hardware (or virtual resources). Creating a device node inside the guest's namespace will give access to that device, so for security reasons, the number of 'given' devices is small.

Q: What is Unification (vunify)?

A: Unification is Hard Links on Steroids. Guests can 'share' common files (usually binaries and libraries) in a secure way, by creating hard links with special properties (immutable but unlinkable (removable)). The tool to identify common files and to unify them is called vunify.

Q: What is vhashify?

A: The successor of vunify, a tool which does unification based on hash values (which allows to find common files in arbitrary paths.)

Q: How do I manage a multi-guest setup with vhashify?

A: For 'vhashify', just do these once:

mkdir /etc/vservers/.defaults/apps/vunify/hash /vservers/.hash
ln -0s /vservers/.hash /etc/vservers/.defaults/apps/vunify/hash/root

Then, do this one line per vserver:

mkdir /etc/vservers/<vservername>/apps/vunify   # vhashify reuses vunify configuration

The command 'ln' creates a link between two files. "ln -s" creates a symbolic link -- two files are linked by name. "ln -0s" uses a Vserver extention to create a unified link.

Q: With which VS version should I begin?

A: If you are new to VServer I recommend to try 2.0.+. Take "alpha utils" Version 0.30.210. In Debian Sid there appeared well running version of it recently. (It's a .210 at the time of writing).

Q: is there a way to implement "user/group quota" per VServer?

A: Yes, but not on a shared partition for now. You need to put the guest on a separate partition, setup a vroot device (to make the quota access secure), copy that into the guest, and adjust the mtab line inside the guest.

Q: what about "Quota" for a context?

A: Context quotas are now called Disk Limits (so that we can tell them apart from the user/group quotas :). They are supported out of the box (with vs2.0) for all major filesystems (Ext2/3, ReiserFS, XFS, JFS)

Q: Does it support IPv6?

A: Currently not. Some developer has to move his ... to reimplement this functionality from the V4 code (I read that on the ML ;)). Will probably be superseded by the ngnet (next generation networking) soon. There is a Wiki page regarding this: http://linux-vserver.org/IPv6

Q: I can't do all I want with the network interfaces inside the guest?

A: For now the networking is 'Host Business' -- the host is a router, and each guest is a server. You can set the capability ICMP_RAW in the context of the guest, or even the capability CAP_NET_RAW (which would even allow to sniff interfaces of other guests!). Likely to change with ngnet.

Q: Is there a web-based interface for vserver that will allow creation/deletion/configuration etc. of vserver guests?

A. [Update] Errrh, there is http://OpenVPS.org which is a set of scripts with a web-interface for webhosters/ISPs.

A. [Update] Errrh, there is http://Openvcp.org which is a distributed system (agent!) with a web-interface, with which you can build/remove guests! cool stuff! beta, try out!

Q: What is old-style and new-style config?

A. Old-style config refers to a single text-file that contains all the configuration settings. With new-style config the configuration is split into several directories and files. You should probably go for new-style config if you are asking.

Q: What is the "great flower page"?

A. Well, this page contains all configation options for vserver in version > 1.9 (I think .. I joined Linux-VServer in version 2, so I don't know for sure). The name of the page is derivived from the stylesheet(s) it contains: It displays background pictures of a very great flower, so regard it as highly optimized. It was designed by a non-designer, who asks us to create a better one. I played with the thought of creating a complete new theme for that page - but actually we all got used to the name "great flower page", so we stick to it. If you are unable to read it clearly, feel invited to join the IRC channel #vserver, we may tell you how to ;)

Q: How do I add several IPs to a vserver?

A: First of all a single guest vserver only supports up to 16 IPs (There is a 64-IP patch available, which is in "derjohn's kernel", you need extra util-vserver anyway).

Here is a little helper-script that adds a list of IPs defined in a text file, one per line.

for i in `cat myiplist`; do
        mkdir $j
        echo $i > $j/ip
        echo $i > $j/ip-old
        echo "24" > $j/prefix

Q: If my host has only one a single public IP, can I use RFC1918 IP (e.g. 192.168.foo.bar) for the guest vservers?

A: Yes, use iptables with SNAT to masquerade it.

iptables -t nat -I POSTROUTING -s $VSERVER_NETZ  ! -d $VSERVER_NETZ -j SNAT --to $EXT_IP

See: HowtoPrivateNetworking and

http://www.tgunkel.de/it/software/doc/linux_server#h3-Vserver_Masquerading_SNAT (THX, [MUPPETS]Gonzo)

Q: If I shut down my vserver guest, the whole Internet interface ethX on the host is shut down. What happened?

A: When you shut down a guest (i.e. vserver foo stop), the IP is brought down on the host also. If this IP happens to be the primary IP of the host, the kernel will not only bring down the primary IP, but also all secondary IP addresses. But in very recent kernels, there is an option settable which prevents that nasty feature. It's called "alias promotion". You may set it via sysctl by adding net.ipv4.conf.all.promote_secondaries=1 in /etc/sysctl.conf or via sysctl command line.

Q: On Debian Sarge (stable) only util-vserver is 0.30-204 available, which has been reported to be buggy (I didnt check the version for longer time) How do I compile a local version of alpha util-vserver .210 on Debian?


apt-get build-dep util-vserver

./configure --prefix=/usr/local/ --enable-release \
--mandir=/usr/local/share/man \
--infodir=/usr/local/share/info \
--sysconfdir=/etc --enable-dietlibc \
--localstatedir=/var \


make install-distribution
(Which does a make install + setting a symlink ln -s /usr/local/lib/util-vserver/vshelper /sbin/vshelper )

To test which version you are running:

# which vserver

This should point to ..local...

Q: I use derjohn's kernel or a differnet kernel with a more-than-16-IPs-per-guest-patch and can't use more than 16 IPs. Why?

A: You need to patch util-vserver, too. So you obviously need to recompile util-vserver (see above). In the util-vserver directory there are header files in the ./kernel/ directory. Patch like this:

kernel/network.h:#define NB_IPV4ROOT    64

BTW: The initial patches can be found here: http://vserver.13thfloor.at/Experimental/VARIOUS/util-vserver-0.30.196-net64.diff.bz2 and http://vserver.13thfloor.at/Experimental/VARIOUS/delta-2.6.9-vs1.9.3-net64.diff

Q: I run a Debian host and want to build an Ubuntu guest. Howto?

A: Simple ;) Assume you want to build a breezy guest on a sid host with IP and hostname vubuntu, then do:

vserver vubuntu build --force -m debootstrap --hostname vubuntu.myvservers.net --netdev eth0 --interface \
--context 42 -- -d breezy -m http://de.archive.ubuntu.com/ubuntu

[UPDATE] Currently there are problems in building breezy under unclear circumstances, which seems to have to do with udev. If the above didnt work, try:

vserver vubuntu build --force -m debootstrap --hostname vubuntu.myvservers.net --netdev eth0 --interface \
--context 42 -- -d breezy -m http://de.archive.ubuntu.com/ubuntu -- --exclude=udev

In very recent versions of the utils, the problem should not occur anymore (it has to do with the 'secure-mount' if you look in the MLs)

Well, sid's debootstrap knows how to bootstrap Ubuntu linux. Make sure to have a current debootstrap package:

apt-get update
apt-get install debootstrap

The knowledge how to build ubuntu 'breezy badger' (which you probably want to be your guest at the time of writing) has been added recently.

Q: How do I make a vserver guest start by default?

A: At least on Debian, I can tell you how to do it with the new-style config. If your guest is called "derjohn" and you want it to be started somewhere at the of your bootstrap process, then do:

echo "default" > /etc/vservers/derjohn/apps/init/mark

If you want to start it earlier, please read the init script "/etc/init.d/vserver-default" to find out how to do it. In most cases you don't need to change this. On Debian the vservers are started at "90", so after most other stuff is up (networking etc.).

Besides that I created a small helper script for managing the autostart foo: vserver-autostart

Q: My host works, but when I start a guest it says that it has a problem with chbind.

A: You are probably using util-vserver <= 0.30.209, which does use dynamic network contexts internally (With 0.30.210 this fact changed). So if you compiled your kernel without dynamic contexts, you may start guests, but you can't use the network context.The solution is either to switch to .210 util (or Hollow's toolset) or compile the kernel with dynamic network contexts.

SE Keyword: invalid option `nid' testme.sh

Q: When I try to ssh to the guest, I log into the host, even if I installed sshd on the guest. What's wrong here?

A: Look at /etc/ssh/sshd_config of the host:

Port 22
# Use these options to restrict which interfaces/protocols sshd will bind to
#ListenAddress ::

And now change the setting to

Port 22
# Use these options to restrict which interfaces/protocols sshd will bind to
ListenAddress your.hosts.ip.here  # not the guests IP! 

Then '/etc/init.d/ssh restart' on the host, after that on the guest (if you did apt-get install ssh on the guest already.)

Do I have to explain more? If the hosts sshd binds all available IP addresses on port 22 (The hosts 'sees' even all addresses of the guests!). So if the guest starts its sshd, it cant bind to port 22 any more. You need to change that setting only on the host.

(BTW: A similar approach has to be done for a lot of daemons, e.g. Apache. If the daemon does not support an explicit bind, you may use the chbind command to 'hide' IP addresses from the daemon before starting.)

Q: I did everything right, but the application foo does not start. What's up there?

A: Before asking on the IRC channel, please check out the 'problematic programs' page:


Q: Bind9 does not like to start in my guest.

A: Check out the 'problematic programs' page:

ProblematicPrograms and/or get my [ProblematicPrograms and/or get my [vserver-guest-ready Debian package] for Debian Sid guests from that URL: http://linux-vserver.derjohn.de/bind9-packages/bind9-capacheck_9.3.2-2_i386.deb and check out the [readme]. (Hint: This is fresh stuff. The give me Feedback)

[UPDATE] Since VServer Devel 2.1.1-rc18 you do not need to patch the userland tools anymore. The capabilities are masked.

Q: Which guest vservers are running?

A: vserver-stat. Example output:

0       77 965.1M 334.6M  14m14s18   2m28s69   1h33m46 root server
49152    7    14M   5.2M   0m00s40   0m00s30   1h30m15 chiffon

Q: How can I reboot/halt guests?

A: It depends.

For vserver with legacy-interfaces support, you have to replace /sbin/halt in guests with vreboot and start rebootmgr in host. You also need to have a dummy <guest>.conf file in /etc/vservers for each guest. Please have a look at /etc/init.d/rebootmgr.

Vserver with native interface utilizes /dev/initctl. No changes are needed in guests. Just make sure that REBOOT capability is adjusted in guests.

Q: Do I really need the legacy-interfaces? What are these legacy-interfaces?

A: Since vserver is an ongoing project, new features might replace old ones, some might still on development. Legacy-interfaces are available for backward compability (which might be removed someday). See Q: How can I reboot/halt guests?

Q: I have a vserver running on a Linux kernel with preemption. Is VServer "preempt" safe?

A: There are no known issues about running vserver on a preemption enabled kernel. I would like to add, that the vserver kernelhackers would probably exclude that option in 'make menuconfig' if there would be an incompatibility. Just my $.02 :)

Q: Is this a new project? When was it started?

A: The first public occurance of linux-vserver was Oct 2001. The initial mail can be found here: http://www.cs.helsinki.fi/linux/linux-kernel/2001-40/1065.html

So you can expect a mature software product wich does it's magic quite well (And hey, we have a version > 2.0 ! )

Q: Can I run an OpenVPN Server in a guest?

A: Yes. I don't want to provide an in-depth OpenVPN tutorial, but want to show how I made OpenVPN work in a guest as server. I was not able to run it with a tun devive, due to a buglet in util-vserver and kernel when it comes to settings a an ip address a point to point link: If you add "ip addr add <ip> peer <mypeer> dev tun0" there is no way to map the tun0 interface into a guest, even not with a 'nodev' option. (bug confirned to be reproducible by daniel_hoczac)

First of all you have to prepare the host with a persistent tuntap interface in tap-mode. The tools we need come from the uml-utilities.

Then you need to create a device /dev/net/tun, which the OpenVPN userspace daemon reads. Well assume is the server IP, and is the client ip - to be cool be choose a /31 netmask (, so we have a net without broadcast and don't waste IPs :)

On the host do:

# apt-get install uml-utilities
# cd /var/lib/vserver/<myopenvpnserver>/dev/
# ./MAKEDEV tun
  (creates the dev/net/tun device accessible by te guest - even a tap interface need /dev/net/tun !)
# tunctl -t tap0
  (creates the network device 'tap0' persistently)

Then add the ip to the guest:

# cat /etc/vservers/<myopenvpnserver>/interfaces/1/ip
# cat /etc/vservers/<myopenvpnserver>/interfaces/1/prefix
# cat /etc/vservers/<myopenvpnserver>/interfaces/1/dev
(This kind of config brings the ip when the vserver is started - only the tap0 interface has to exist already, see above!)

Here is a sample config for the guest (which is acting as a server):

Install OpenVPN package on server and client, in the Debian case:

# apt-get install openvpn

The server's conf looks like that:

# port and interface specs

# behave like a ssl-webserver
port 443
proto tcp-server

# tap device? (keep in mind you need /dev/net/tun !)
dev tap0

# now the ips we will use for the tunnel

# the server part

# Keep VPN connections, even if the client IP changes

# use compression (may also even obfuscate content filters)

# use a static key - create it with 'openvpn --genkey --secret static.key'
secret static.key

# dont reload the key after a SIGUSR1

# check alive all 10 secs
keepalive 10 60

# verbosity level (from 1 to 9, 9 is max log level)
verb 4
status openvpn-status.log

The client's conf may look like that (This example even makes the tunnel the clients default address):

# cat /etc/openvpn/client.conf
# port and interface specs

# the following is not necessary, if you bring up openvpn via Debian's init script:
daemon ovpn-my-clients-name

# behave like a ssl-webserver
port 443
proto tcp-client
remote %%%<insert-the-guest-primary-public-ip-here>%%%%
# what device tun ot tap?
dev tap

# now the ips we will use for the tunnel

# Keep VPN connections, even if the client IP changes

# use compression (may also even obfuscate content filters)

# use a static key
secret static.key

# dont reload the key after a SIGUSR1

# check alive all 10 secs
keepalive 10 60

# verbosity level (from 1 to 9, 9 is max log level)
verb 4

# set the default route
redirect-gateway def1
# to add special routes you can do it wihtin the openvpn client conf:
# route <dest> <mask> <gateway>

# if you need to connect via proxy (like squid)
# http-proxy s p [up] [auth] : Connect to remote host through an HTTP proxy at
#                  address s and port p.  If proxy authentication is required,
#                  up is a file containing username/password on 2 lines, or
#                  'stdin' to prompt from console.  Add auth='ntlm' if
#                  the proxy requires NTLM authentication.

# http-proxy s p [up] [auth]

# http-proxy-option type [parm] : Set extended HTTP proxy options.
#                                  Repeat to set multiple options.
#                  VERSION version (default=1.0)
#                  AGENT user-agent

# http-proxy-option type [parm]

In the next lesson I will talk about OpenVPN's server mode, which can deal with with multiple clients connecting to one ip and one port (i.e. you only need one guest for tons or 'roadwarriros'), tls connections and pki.

Contributions welcome. :)

Q: 32 vs 64 Bit? What should I take?

A: If you have the choice make the host a 64 bit one. You can run a guest as 32 bit or as 64 bit on a 64 bit host. To run it as 32 bit, you need to compile the x86_64 (a.k.a. AMD64) with the following options:

[*] Kernel support for ELF binaries
<M> Kernel support for MISC binaries
[*] IA32 Emulation <---- without that, the entire 32bit API is not present
<M>   IA32 a.out support  

You can force the guest to behave like a 32 environment like this:

echo linux_32bit > /etc/vservers/$NAME/personality
echo i686 > /etc/vservers/$NAME/uts/machine

(thanks cehteh for the hint!)

But you can force debootstrap to but 32 bit binaries into the guest by 'export ARCH=i386';

export ARCH=i386 ; vserver build .... 

Q: I want to (re)mount a partition in a running guest ... but the guest has no rights (capability) to (re)mount?

A: I'll explain. I take as example your /tmp partition within the guest is too small, what will be likely the case if you stay with the 16MB default (vserver build mounts /tmp as 16 MB tempfs!).

# vnamespace -e XID  mount -t tmpfs -o remount,size=256m,mode=1777 none /var/lib/vservers/<guest>/tmp/

Be warned that the guest will not recognize the change, as the /etc/mtab file is not updated when you mount like this. To permanently change the mount, edit /etc/vserver/<guest>/fstab on the host.

Q: How do I limit a guests RAM? I want to prevent OOM situations on the host!

A: First you can read [1].

If you want a recipe, do that:

1. Check the size of memory pages. On x86 and x86_64 is usually 4 KB per page.

2. Create /etc/vserver/<guest>/rlimits/

3. Check your physical memory size on the host, e.g. with "free -m". maxram = kilobytes/pagesize.

4. Limit the guests physical RAM to value smaller then maxram:

echo %%insertYourPagesHereSmallerThanMaxram%% > /etc/vserver/<guest>/rlimits/rss

5. Check your swapspace, e.g. with 'swapon -s'. maxswap = swapkilobytes/pagesize.

6. Limit the guest's maximum number of as pages to a value smaller than (maxram+maxswap):

echo %%desiredvalue%% > /etc/vserver/<guest>/rlimits/as

It should be clear this can still lead to OOM situations. Example: You have two guests and your as limit per guest is greater than 50% of (maxram+maxswap). If both guests request their maximum at the same point in time, there will be not enough mem .....

Q: Were can I get newer versions of VServer as ready made packages for Debian?

A: Here you go: http://linux-vserver.derjohn.de/ . There is also some stuff on backports.org, but my kernels are always 'devel' branch.