provide access for container using private ips to Internet[NAT]

January 27, 2015

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 theHardware 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

[edit]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

[edit]Nameserver

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

# vzctl set $CTID --nameserver inherit

[edit]Test

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

# vzctl exec $CTID ping openvz.org

[edit]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/.

Reference: http://openvz.org/Using_NAT_for_container_with_private_IPs

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