• Switching - how does Host A reach Host B? Through a switch.
  • The switch contains a network between the two systems.
  • Require an interface on each host.
    • Run ip link as a good way to check.
  • Add IPs to each host: 
    ip addr add 192.168.1.10/24 dev eth0
ip addr add 192.168.1.11/24 dev eth0
  • Routing - On one network with 192.168.1.0 and another network of 192.168.2.0, how do the two networks talk to each other?
    • Routers are therefore needed for this.
    • To bridge the two networks, one address is assigned to one side of the router 192.168.1.1 and 192.168.2.1
  • How does one Host on one Network know where to send its packet to on the second Network?
  • Gateways - Use the route command to show routing information such as Destination, Gateway, Genmask, Flags, Metric, Ref, Use, Iface
    • route displays the kernel’s routing table.
    • If there is nothing populated from the route command, Host A on 192.168.1.0 can’t see any hosts on 192.168.2.0 and vice versa.
    • To therefore be able to reach hosts on 192.168.2.0, need the ip route add command with ip route add 192.168.2.0/24 via 192.168.1.1
      • Need to select the interface on the router closest to the network.
    • Running the route command again will see the routing table populated.
    • Needs to be configured on all systems, not just ip route add 192.168.2.0/24 via 192.168.1.1, but also on the other side with ip route add 192.168.1.0/24 via 192.168.2.1
  • Default Gateway - ip route add default via 192.168.2.1, then you don’t need to add individual IPs for everything on the Internet
    • Can also write it as ip route add 0.0.0.0 via 192.168.2.1. 0.0.0.0 means all IPs.
    • For two separate routers, one for the outside Internet and one on the internal network.
      • For example, you have the router from above and also the secondary router with an interface of 192.168.2.2. To add this, run ip route add 192.168.1.0/24 via 192.168.2.2

        How to set up a Linux Host as a Router

  • Three hosts, Host A, Host B and Host C.
    • Hosts A and B are connected to network 192.168.1.0
    • Hosts B and C are connected to network 192.168.2.0
    • Host B has left-hand side IP of 192.168.1.6 and a right-hand side IP of 192.168.2.6.
    • Host A has 192.168.1.5 and Host C has 192.168.2.5
  • How does Host A talk to Host C?
  • Add the following to Host A: 
    ip route add 192.168.2.0/24 via 192.168.1.6
  • If the packages reach Host C, Host C needs to send a response to Host A.
    • We need to tell Host C that it can reach Host A through Host B.
  • We add the following to Host C: 
    ip route add 192.168.1.0/24 via 192.168.2.6
  • Pinging from either Host A or Host C at this point and no network unreachable messages should be present.
  • By default in Linux, packets are not forwarded from one interface to the next.
    • For example, packets received on Host B on Eth0 are not forwarded else here on Eth1.
      • This is for security reasons, if Eth0 is on the private network and Eth1 is on the public network, don’t want anyone from the public network to easily send messages to the private network.
    • Therefore need to make sure the cat /proc/sys/net/ipv4/ip_forward says 1
    • To quickly change this, run echo 1 > /proc/sys/net/ipv4/ip_forward
    • To persist between reboots: 
      /etc/sysctl.conf
net.ipv4.ip_forward = 1
  • Key commands:
  • ip link - list modifiable interfaces
  • ip addr - see IP addresses assigned.
  • ip route / route is to view the routing table.
  • ip route add - add routes into the routing table.
  • To persist changes, need to add them to the /etc/network/interfaces file.

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