Networking vserver guests

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Setting up network access to and from your vserver guests.



Lets imagine, you have only one external IP -- $EXTIP.

You want to have several vservers running without worrying about port overlapping.


Two vservers run a default webserver, running on port 80. If each "guest" vserver shares an IP with the host, then the two webservers will conflict.

One solution is:

  • All vservers are contained in a "virtual lan", say 192.168.1.x
  • Each vserver has its own IP
  • Control port forwarding on "parent" host. That is, run a router.



Optional: Load 'dummy' device(s)


This driver allows you to associate vServer guests with a virtual network device rather than the host's real network interface, which functions to hide packet counters from vServer guests. There is no other use for this step, so feel free to skip it if you are not worried about sharing packet counters. (Packet counts may be useful to an attacker with control of a vServer guest who wishes to perform side-channel attacks during cryptanalysis, or network traffic analysis against your host or other vServer guests. If you did not understand the last sentence and your installation is not particularly security sensitive, then the chances that you will encounter a skilled attacker and this will actually matter are slim to none and you should feel free to skip this section.)


First you need to load the dummy interface driver (requires CONFIG_DUMMY=m in your kernel configuration)

# modprobe dummy

To have it automatically loaded on startup, add "loopback" to /etc/modules

Set up dummy0 interface on the parent host

# /etc/network/interfaces on a Debian box, 
# configure on other distros with your preferred way
auto dummy0
iface dummy0 inet static

Optional Extra Step

It is possible to facilitate further segregation (such that even dummy packet counters are never shared between guests). To do so, use:

# modprobe dummy numdummies=<number-of-devices-required>

... and associate each vServer with a unique dummy device.


Set up each guest vserver. If you skipped the host configuration above, you will need to replace dummy0 with the name of your host's primary network interface, for example eth0.

cd /etc/vservers/$VSERVER/interfaces/0
echo dummy0 > dev
echo > ip
echo 1 > name
echo 24 > prefix

Consider using a value of name equal to the last digit of the IP for easy separation.

Host as router

Configure the host to act as a router.

For internal packets going outside, pretend each packet came from our external IP (put it in one line without backslash):

# iptables -t nat -A POSTROUTING -s \ 
  ! -d -j SNAT --to-source $EXTIP

For each service that runs on a vserver, map it to an external port. (For each ?) Vserver local address $VHOST and port $INTPORT, you select one external port $EXTPORT and run the following (put it in one line without backslash):

# iptables -t nat -A PREROUTING ! -s \
  -m tcp -p tcp --dport $EXTPORT 
  -j DNAT --to-destination $VHOST:$INTPORT

That's all!


Try ping from your vserver -- it should ping fine.

Try to connect to your $EXTIP:$EXTPORT (from another external host) -- you will successfully connect to service running on a guest vserver.

See also

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