Using NAT for container with private IPs

Usually you supply public IP addresses to your containers. Sometimes you don't want to do it (lack of IPs, etc.). This article describes how to use private IP addresses for containers.

Prerequisites

Make sure that below prerequisites are met, otherwise it won't work for you!

IP forwarding

IP forwarding should be turned on on the hardware node in order for container networking to work. Make sure it is turned on:

$ cat /proc/sys/net/ipv4/ip_forward 
1

Output should be '1'. If it is '0', enable IP forwarding as it is described in Quick installation#sysctl.

NOTE: Ubuntu made some changes to the syntax for NAT. See this link if you are needing to enable NAT on an Ubuntu host :

Launchpad

The syntax of /etc/sysctl.conf has changed to :

net.ipv4.conf.default.forwarding=1
net.ipv4.conf.all.forwarding=1

IP conntracks

IP connection tracking should be enabled for CT0.

For recent OpenVZ kernels (2.6.9 and later) connection tracking for CT0 is enabled by default, but it can be disabled by vzctl (because it has a negative impact on venet performance). So, make sure there is no line like

options ip_conntrack ip_conntrack_disable_ve0=1

or

options nf_conntrack ip_conntrack_disable_ve0=1

in /etc/modules.conf, /etc/modprobe.conf, or any file under /etc/modprobe.d/. If there is such a line, please change =1 to =0 and reboot.

How to provide access for container to Internet

To enable the containers, which have only internal IP addresses, to access the Internet, SNAT (Source Network Address Translation, also known as IP masquerading) should be configured on the Hardware Node. This is ensured by the standard Linux iptables utility. To perform a simple SNAT setup, execute the following command on the Hardware Node:

# iptables -t nat -A POSTROUTING -s src_net -o eth0 -j SNAT --to ip_address

where src_net is a range of IP addresses of containers to be translated by SNAT, and ip_address is the external IP address of your Hardware Node. The format of src_net is xx.xx.xx.xx/xx (CIDR notation). For example:

# iptables -t nat -A POSTROUTING -s 192.168.2.0/24 -o eth0 -j SNAT --to ip_address

Multiple rules are allowed, for example, in case you wish to specify several ranges of IP addresses. If you are using a number of physical network interfaces on the Node, you may need to specify a different interface for outgoing connections, e.g. -o eth2.

To make all IP addresses to be translated by SNAT (not only the ones of containers with private addresses), you should type the following string:

# iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to ip_address

Firewall

For Debian hardware node, you may need to allow a forward rule. The table still being the default table (filter) but the chain is FORWARD:

# iptables -A FORWARD -s 192.168.2.0/24 -j ACCEPT
# iptables -A FORWARD -d 192.168.2.0/24 -j ACCEPT

For default RedHat/CentOS firewall, allow outgoing connections from your containers, for example:

# iptables -A RH-Firewall-1-INPUT -s 192.168.2.0/24 -j ACCEPT
# iptables-save > /etc/sysconfig/iptables
# service iptables restart

Nameserver

Make sure in-CT nameserver is set. The easiest way to do it is:

# vzctl set $CTID --nameserver inherit

Test

Now you should be able to reach internet from your container:

# vzctl exec $CTID ping openvz.org

How to provide access from Internet to a container

In addition, to make some services in container with private IP address be accessible from the Internet, DNAT (Destination Network Address Translation) should be configured on the Hardware Node. To perform a simple DNAT setup, execute the following command on the Hardware Node:

# iptables -t nat -A PREROUTING -p tcp -d ip_address --dport port_num \
  -i eth0 -j DNAT --to-destination ve_address:dst_port_num 

where ve_address is an IP address of the container, dst_port_num is a tcp port which requires service use, ip_address is the external (public) IP address of your Hardware Node, and port_num is a tcp port of Hardware Node, which will be used for Internet connections to private container service. Note that this setup makes the service which is using port_num on the Hardware Node be unaccessible from the Internet. Also note that SNAT translation is required too.

For example, if you need a web server in a container to be accessible from outside and, at the same time, keep a web server on the Hardware Node be accessible, use the following config:

# iptables -t nat -A PREROUTING -p tcp -d ip_address --dport 8080 \
  -i eth0 -j DNAT --to-destination ve_address:80
# iptables -t nat -A POSTROUTING -s ve_address -o eth0 -j SNAT --to ip_address

After applying this, you'll see container' web server at http://ip_address:8080/.

Note: this rule will only work for external clients, i.e. connections originating from a different host — so you can not test if it works locally.

Note: If you get any errors relating to: iptables: No chain/target/match by that name double check to see if you have all the iptables/netfilter modules loaded properly. I had to modprobe xt_tcpudp before getting it to work.

The iptables utility allows to set up more complex rules for Network Address Translation, involving various protocols and ports. If you wish to get more information on this, consult the numerous Internet sites (e.g. netfilter.org) and tutorials devoted to this issue.

External Links

• netfilter.org
• w:Private network