Module 22 - Networking - Bridged Adapters and NAT Networks¶

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Introduction¶

In common virtualization software, there is this concept of a Bridged Adapter and a NAT Network. You can create both of these topologies in minimega.

Creating a Bridge¶

First you will need to create a bridge.

This should only be done from a machine you have physical access to as networking will fail temporarily. Depending on a number of factors it may not come back.

$ su -
# ovs-vsctl add-br mega_bridge
# ovs-vsctl set bridge mega_bridge stp_enable=false
# cat > createbridge.sh << EOF
#!/bin/sh
ovs-vsctl add-port mega_bridge eth0
dhclient -r eth0
dhclient mega_bridge
EOF
# chmod +x createbridge.sh
# tmux
# ./createbridge.sh

The server will now move the eth0 interface to the newly created mega_bridge bridge, remove DHCP lease from eth0, and acquire a DHCP lease for mega_bridge.

Note: This will cause the network to disconnect and if things go wrong you will have to connect with keyboard and mouse manually to fix networking.

On reboot mega_bridge will not get an IP address by default and you will have to run dhclient mega_bridge, unless it is added to /etc/network/interfaces.

Do not run nuke until the bridge is removed as this will kill the mega_bridge interface and disable networking.

Troubleshooting¶

Sometimes eth0 won't want to freely give up the IP and it takes some forcing.

ifconfig eth0 down
ifconfig eth0 up
ip addr del 192.168.1.100 dev eth0
dhclient -r eth0
dhclient -r mega_bridge
dhclient mega_bridge

Bridged Adapters¶

Bridged Adapters function as if you had a networking interface that was directly connected to your network.

Cleanup¶

$ vm kill all
$ vm flush

Now you can start some VMs and test your Bridged Adapters by placing VMs on VLAN 0 of mega_bridge.

Boot¶

# minimega -attach
vm config disk /home/ubuntu/tinycore.qcow
vm config memory 128
vm config net 0
vm launch kvm linux[1-5]
vm start all

Your VMs should now be able to reach your network as if they were directly connected, enabling your VMs to access the internet directly.

To undo the change you made you will need to delete the bridge and use dhclient to request an IP address again for your Ethernet adapter.

Nat Network¶

NAT Networks have a DHCP server that is running that also acts like a router forwarding traffic onto the internet for them. This can be accomplished by combining a dnsmasq service and iptables rules.

Cleanup¶

$ vm kill all
$ vm flush

Now you can start some VMs and create a NAT Network.

Boot¶

# minimega -attach
vm config disk /home/ubuntu/tinycore.qcow
vm config memory 128
vm config net 100
tap create 100 ip 10.0.0.1/24
shell sleep 5
dnsmasq start 10.0.0.1 10.0.0.2 10.0.0.254
shell sleep 5
vm launch kvm linux[1-5]
shell sleep 5
vm start all
disconnect

Now let's create some iptables rules:

WAN=mega_bridge
sysctl -w net.ipv4.ip_foward=1
iptables -t nat -A POSTROUTING -o $WAN -j MASQUERADE
iptables -A FORWARD -i $WAN -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -o $WAN -j ACCEPT

Your VMs should receive a 10.0.0.x IP address from dnsmasq and iptables will forward network traffic to and from the network. Enabling your VMs to have internet access through the server's connection.

If need be you can delete all the existing iptables rules and stop the VM connection to the internet with

iptables -P INPUT ACCEPT
iptables -P OUTPUT ACCEPT
iptables -P FORWARD ACCEPT
iptables -F
iptables -F -t mangle
iptables -F -t nat

Special Note¶

With a tap interface VMs on the VLAN will be able to ssh into 10.0.0.1 and connect to the outside.

Removing the Bridge¶

If you wish to undo the setup steps that created the bridge, you can do the following.

Note: These steps will disable network access if done incorrectly; proceed with caution.

$ su -
# cat > fixeth.sh << EOF
ovs-vsctl del-br mega_bridge
ifconfig eth0 down
ifconfig eth0 up
dhclient -r eth0
dhclient eth0
EOF
# chmod +x fixeth.sh
# tmux
# ./fixeth.sh