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Lab 11.2 Why are both addresses added to the static route configuration?

I'm finding this lab particularly difficult because I'm unclear of which IP addresses to use at each step.

  1. The first point of confusion is whether this command is required?

Where X and Y are unique to the lab/classroom.
# ip addr add ADDRESS/24 dev eth0

The lab does not state why this is necessary so I've made an assumption that it's not required if we use the existing IP addr of the device.

Using the following known properties for devices on separate networks (and assuming VLAN has been configured for the interface)
Device X
Interface wlan0
IP addr
External IP

Device Y
Interface eth0
IP addr
External IP

  1. Configuration of the static route
    The lab solution shows two entries. Are both entries required on each environment?
# /etc/network/interfaces
# Device X - Debian
up route add -net gw dev wlan0
# /etc/sysconfig/network-scripts/route-INTERFACE
# Device  Y - Fedora via dev eth0
  1. Restart networking services and test

systemctl restart network

Debian - this is different to what's specified in the lab
systemctl restart network-manager

I'm not in the position to be able to test this because of a lack of access to devices, some validation would be great :smile:

Best Answer


  • lee42x
    lee42x Posts: 380

    Gro 108, Thank you for your input,

    1/ (11.1) Additional addresses are used in this lab to demonstrate VLAN tagging and static routes while not disturbing the active network configuration. This avoids typing errors that make systems unreachable.

    2/ (11.2) This exercise creates routes from two networks to another system, so yes there needs to be a route for each subnet.
    Looking at your routes:

    up route add -net gw dev wlan0
    Says: When packets for the subnet arrive route them via the system accessible via eth0. via dev eth0
    Says: When packets arrive from network route them to system on eth0.
    /3 The systemctl command will be corrected, thank you for noticing.

    Using two systems is a little easier than making sure it works with one system. Here is a two system example.


  • GRO 108
    GRO 108 Posts: 46
    edited September 2020

    Thanks Lee, that's a lot clearer to me now.

    I was so focused on outbound traffic that I had completely overlooked the need for the inbound routing (in fact the inbound direction doesn't get much of a mention in any documentation, it's common sense sure but when you're swimming in new information it's easy to miss :blush: )

    Regarding my question about the route definitions

    The lab solution shows two entries. Are both entries required on each environment?

    By this I meant for does each config file require two entries - one for ingress and one for egress traffic?
    Your answer and the diagram show only one but the Lab solution shows two.
    Just so I am absolutely clear - when defining the route do we need to separate the ingress and egress traffic? Or does a single address work for both ways?

    Do I need to update my solution to be the following so it includes the egress route?

    # /etc/network/interfaces
    # Device X - Debian
    up route add -net gw dev wlan0 # ingress traffic
    up route add -net gw dev wlan0 # egress traffic? - is this required?
    # /etc/sysconfig/network-scripts/route-INTERFACE
    # Device  Y - Fedora via dev eth0.7 # ingress traffic via dev eth0.7 # egress traffic? - is this required?

    Thanks again

  • lee42x
    lee42x Posts: 380

    One common trap is not being specific when setting up static routes, so making sure you define clear paths is important. What I see happening is we define 1/2 of the routing pair, ingress/egress as you mentioned. So, from home network to target network and target network to home network there must be a pair for each network. Now if there is only 1/2 of the pair then the default route will take over (if one is defined) and unexpected results will occur.

    Note: This OPTIONAL challenge that goes way beyond the default setup for class and is presented here for the curious.
    As a challenge, add another machine that is only connected to (my example) the network, no other adapters. The "challenge system" should be accessible from "your system" and the "challenge system" should be able to browse the internet through the network provided by Hypervisor. My hypervisor is KVM using the network.

  • Thanks @lee42x, the realisation that the route definitions handle inbound/outbound traffic was a big one for me. Thanks for guiding me there. It also makes me realise how great DHCP is :smile:

    Am I right to interpret "1/2" as "1 of 2"?

    Re: the challenge - is the use of a VM/hypervisor required or only mentioned to provide a possible implementation when only one system is available?


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