Linux Box as Router

In continuation of my previous posts where i’m create a distributed cloud infrastructure i need to connect VM’s on multiple Host Machines

Pre knowledge of routing (IPTables) and networking is required for information below. Technically this is what containers like docker or software routers do internally when they need to connect 2 different network’s

distributedhostrouting

Lets assume

HostMachine1 has VM’s on network 10.0.0.1/24
HostMachine2 has VM’s on network 192.168.0.1/24

Now our Gateway1 and Gateway2 have 2 Network Interfaces/NIC cards.

Gateway1 and Gateway2 are connected by switch hence on same network as well as their respective VM networks as they have 2 NIC cards connected.

Let assume Gateway1 has IP 10.0.0.2
Let assume Gateway2 has IP 192.168.0.2

Both Gateway1 and Gateway2 can connect to each other as they are directly connected.

My Current Configuration on Gateway1 which is our target router and i’ve below Network Interfaces on that


enp0s9: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 08:00:27:9d:08:3f brd ff:ff:ff:ff:ff:ff
    inet 10.0.0.169/24 brd 10.0.0.255 scope global enp0s9
       valid_lft forever preferred_lft forever
    inet6 fe80::a00:27ff:fe9d:83f/64 scope link
       valid_lft forever preferred_lft forever
 enp0s10: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 08:00:27:7b:12:89 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.120/16 brd 192.168.255.255 scope global enp0s10
       valid_lft forever preferred_lft forever
    inet6 fe80::a00:27ff:fe7b:1289/64 scope link 

Now we need to add routing table configurations on either gateway1 or gateway2 to forward packets from one network to another.

We can do this 2 ways.

Either by creating network bridge


# Install brige utils that gives us easy commands to creatr bridge network
apt-get install bridge-utils
or
yum install bridge-utils

# Create new Bridge
brctl addbr br0

# Enable Spanning Tree Support if you need
brctl stp br0 on

# Make sure you get your Devices down before creating bridge and explicity assign 0.0.0.0 to make sure they loose ip
ifconfig enp0s9 0.0.0.0 down
ifconfig enp0s10 0.0.0.0 down

# Add them to our newly created bridge network
brctl addif br0 enp0s9
brctl addif br0 enp0s10

# Finally get all interfaces up.
ifconfig enp0s9 up
ifconfig enp0s10 up
ifconfig br0 up

or

by modifying routing table. I’m explaining second concept here

Enable forwarding in the kernel:


echo 1 >> /proc/sys/net/ipv4/ip_forward

To set this value on boot uncomment this line in/etc/sysctl.conf

#net.ipv4.ip_forward=1

Now i need to route traffic from one Interface to another using routing tables
Below are statements that can do that


# Always accept loopback traffic
iptables -A INPUT -i lo -j ACCEPT

# We allow traffic from the HostMachine1 side
iptables -A INPUT -i enp0s9  -j ACCEPT

# We allow traffic from the HostMachine2 side
iptables -A INPUT -i enp0s10  -j ACCEPT

######################################################################
#
#                         ROUTING
#
######################################################################

# enp0s9 is HostMachine1 Network
# enp0s10 is HostMachine2 Network

# Allow established connections
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Masquerade.
iptables -t nat -A POSTROUTING -o enp0s10  -j MASQUERADE

# fowarding
iptables -A FORWARD -i enp0s9 -o enp0s10  -m state --state RELATED,ESTABLISHED -j ACCEPT

# Allow outgoing connections from the HostMachine1 side.
iptables -A FORWARD -i enp0s10 -o enp0s9  -j ACCEPT


# Repeat same steps on reverse route
iptables -t nat -A POSTROUTING -o enp0s9   -j MASQUERADE

iptables -A FORWARD -i enp0s10 -o enp0s9  -m state --state RELATED,ESTABLISHED -j ACCEPT

iptables -A FORWARD -i enp0s10 -o enp0s9  -j ACCEPT

Finally on HostMachine1 route all traffic on subnet 192.168.0.1/24 to Gateway1

I use Mac as one my host machine hence below command

sudo route -n add -net 192.168.1.1/24 10.0.0.193

If you are using any *nix systems command would be

ip route add 192.168.1.1/24 via 10.0.0.169 dev

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