5.2.1.1. Host-Routed Mode for Containers

By default, a new container starts operating in the host-routed mode. In this mode, the container uses a special network adapter, venet0, to communicate with the server where it resides, with the other containers on the server, and with computers on external networks. The figure below demonstrates an example network configuration where all containers are set to work in the host-routed mode.

images/container_venet_mode.png

In this configuration:

  • Containers #1, #2, and #3 use the venet0 adapter as the default gateway to send and receive data to/from other networks. They also use this adapter to exchange the traffic between themselves.

  • When containers #1, #2, and #3 start, the server creates ARP and routing entries for them in its ARP and routing tables. You can view the current ARP and routing entries on a server using the arp -n and route -n commands. For example: 

    # arp -n
Address              HWtype   HWaddress            Flags Mask      Iface
10.30.0.4            ether    00:1a:e2:c7:17:c1    C               enp0s5
10.30.23.162         ether    70:71:bc:42:f6:a0    C               enp0s5
192.168.200.101      *        *                    MP              enp0s5
192.168.200.102      *        *                    MP              enp0s5
192.168.200.103      *        *                    MP              enp0s5
# route -n
Kernel IP routing table
Destination       Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.200.101   *               255.255.255.255 UH    1000   0        0 venet0
192.168.200.102   *               255.255.255.255 UH    1000   0        0 venet0
192.168.200.103   *               255.255.255.255 UH    1000   0        0 venet0
10.30.0.0         *               255.255.0.0     U     0      0        0 enp0s5
default           virtuozzo.com   0.0.0.0         UG    0      0        0 enp0s5

    As you can see, the ARP and routing tables contain entries about IP addresses 192.168.200.101, 192.168.200.102, and 192.168.200.103 that belong to containers 1, #2, and 3.

  • All container outgoing network traffic goes to the venet0 adapter and is forwarded via the enp0s5 physical adapter to the destination, according to the routing table of the server.

  • All container incoming network traffic is also processed by the venet0 adapter. Consider the following situation:

    1. Computer X on the local network wants to send a data packet to container #1 with IP address 192.168.200.101, so it issues an ARP request which computer has this IP address.
    2. The server hosting container #1 replies with its MAC address.
    3. Computer X sends the data packet to the indicated MAC address.
    4. The server receives the packet and transmits it to venet0 that forwards the packet to container #1.
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