More fun with veth and Linux network namespaces – V – link two L2-segments of the same IP-subnet by a routing network namespace

During the last two posts of this series

More fun with veth and Linux network namespace – IV – L2-segments, same IP-subnet, ARP and routes

More fun with veth and Linux network namespace – III – L2-segments of the same IP-subnet and routes in coupling network namespaces

we have studied a Linux network namespace with two attached L2-segments. All IPs were members of one and the same IP-subnet. Forwarding and Proxy ARP had been deactivated in this namespace.

So far, we have understood that routes have a decisive impact on the choice of the destination segment when ICMP- and ARP-requests are sent from a network namespace with multiple NICs – independent of forwarding being enabled or not. Insufficiently detailed routes can lead to problems and asymmetric arrival of replies from the segments – already on the ARP-level!

The obvious impact of routes on ARP-requests in our special scenario has surprised at least some readers, but I think remaining open questions have been answered in detail by the experiments discussed in the preceding post. We can now move on, on sufficiently solid ground.

We have also seen that even with detailed routes ARP- and ICMP-traffic paths to and from the L2-segments remain separated in our scenario (see the graphics below). The reason, of course, was that we had deactivated forwarding in the coupling namespace.

In this post we will study what happens when we activate forwarding. We will watch results of experiments both on the ICMP- and the ARP-level. Our objective is to link our otherwise separate L2-segments (with all their IPs in the same IP-subnet) seamlessly by a forwarding network namespace – and thus form some kind of larger segment. And we will test in what way Proxy ARP will help us to achieve this objective.

Not just fun …

Now, you could argue that no reasonable admin would link two virtual segments with IPs in the same IP-subnet by a routing namespace. One would use a virtual bridge. First answer: We perform virtual network experiments here for fun … Second answer: Its not just fun ..

Our eventual objective is the configuration of virtual VLAN configurations and related security measures. Of particular interest are routing namespaces where two tagging VLANs terminate and communicate with a third LAN-segment, the latter leading to an Internet connection. The present experiments with standard segments are only a first step in this direction.

When we imagine a replacement of the standard segments by tagged VLAN segments we already get the impression that we could use a common namespace for the administration of VLANs without accidentally mixing or transferring ICMP- and ARP-traffic between the VLANs. But the results in the last two previous posts also gave us a clear warning to distinguish carefully between routing and forwarding in namespaces.

The modified scenario – linking two L2-segments by a forwarding namespace

Let us have a look at a sketch of our scenario first:

We see our segments S1 and S2 again. All IPs are memebers of 192.168.5.0/24. The segments are attached to a common network namespace netnsR. The difference to previous scenarios in this post series lies in the activated forwarding and the definition of detailed routes in netnsR for the NICs with IPs of the same C-class IP-subnet.

Our experiments below will look at the effect of default gateway definitions and at the requirement of detailed routes in the L2-segments’ namespaces. In addition we will also test in what way enabling Proxy ARP in netnsR can help to achieve seamless segment coupling in an efficient centralized way.

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More fun with veth and Linux network namespaces – IV – L2-segments, same IP-subnet, ARP and routes

In the course of my present series on veths, network namespaces and virtual VLANs

More fun with veth and Linux network namespace – III – L2-segments of the same IP-subnet and routes in coupling network namespaces

More fun with veth and Linux network namespace – II – two L2-segments attached to a common network namespace

More fun with veth and Linux network namespace – I – open questions

we have started to study a somewhat academic network configuration:

We have attached two L2-segments to a common network namespace, with all IPs of both segments belonging to one and the same IP-subnet class.

The IP-setup makes the scenario a bit peculiar. It is not a situation an admin would normally create. Instead the IPs of each segment would be members of one of two distinct and different IP-subnets.

However, our scenario already taught us some important lessons to keep in mind when we approach our eventual objective:

Sooner or later we want to answer the question how to configure virtual VLAN configurations, in which veth-subdevices for tagged VLAN lines terminate in a common and routing namespace.

By our academic scenario we found out that we need to set up much more specific routes than the standard ones which are automatically created in the wake of “ip addr add” commands. Despite the fact that forwarding was disabled in the coupling network namespace! Only with clear and fitting routes we got a reasonable behavior of our artificial network with respect to ICMP- and ARP-requests in the last post (III).

Well, this is, in a way, a platitude. Whenever you have a special network you must adapt the routes to your network layout. This, of course, includes the resolution of ambiguities which our scenario introduced. The automatically defined routes were insufficient and caused an asymmetry in the ICMP-replies in the otherwise very symmetric configuration (see the drawing below).

Nevertheless, the last post and also older posts on veths and virtual VLANs triggered an intense discussion of two of my readers with me concerning ARP. In this post I, therefore, want to look at the impact of routes on ARP-requests and ARP-replies between the namespaces in more detail.

The key question is whether and to what extend the creation and emission of ARP-packets via a particular network interface may become route-dependent in namespaces (or on hosts) with multiple network interfaces.

While we saw a clear and route dependent asymmetry in the reaction of the network to ICMP-requests, we did not fully analyze how this asymmetry showed up on the ARP-level.

In my opinion we have already seen that ARP-requests triggered within ICMP-requests showed some asymmetry regarding the NIC used and the segment addressed – depending on the defined routes. But the experimental data of the last post did not show the flow of ARP-replies in detail. And we only regarded ARP-requests triggered by ICMP-requests. I.e. we watched ARP-requests created by the Linux-kernel to support the execution of ICMP-requests, but not pure and standalone ARP-requests.

Therefore, it is still unclear

  • whether routes have an impact on pure standalone ARP-requests, i.e. ARP-requests not caused by ICMP-requests (or by other requests of higher layer protocols),
  • whether routes do have an impact on ARP-replies.

The experiments below will deliver detailed information to clarify these points.

When I speak of ARP-tables below, I am referring to the ARP-caches of the various network namespaces in our scenario.

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More fun with veth and Linux network namespaces – III – L2-segments of the same IP-subnet and routes in coupling network namespaces

Linux network namespaces allow for experiments in virtual networks without setting up (virtual) hosts. This includes the study of packet propagation through virtual devices as bridges/switches and between network namespaces. With or without packet filters or firewall rules in place. In the last precedent post of this series

More fun with veth and Linux network namespace – II – two L2-segments attached to a common network namespace

I have posed a scenario with two L2-segments connected to a common network namespace. Such a scenario appears to be nothing special; we all are used to situations that require routing and forwarding because LAN-segments belong to different IP-subnets.

However, in my scenario all IPs are members of one and the same IP-subnet. An other interesting point is that the assignment of IPs to (virtual) NICs and bridges by the command “ip addr add” causes an automatic setup of elementary routes. We will see that such routes are insufficient to resolve the ambiguities of packet transport from the segment-coupling namespace to the attached segments.

To keep the conditions as plain and simple as possible in our scenario we do not mix in VLANs or packet filters or firewalls.

In the previous post we have already come to some conclusions about the posed scenario and its elements. However, these conclusions were based on theoretical considerations. In this post and an additional one we will verify or falsify these conclusions by performing concrete experiments on a Linux host.

The host I used for the experiments below was a laptop with an Opensuse Leap 15.5 OS, based on packets of SuSE’s enterprise server SLES. The namespaces are set up as unnamed network namespaces. This requires some special commands.

The detailed analysis of the scenario will help us to better understand more complex configurations including virtual VLANs in forthcoming posts.

If you notice varying MACs in some images below: You may ignore this safely. It is due to repeating the setup in the course of the experiments.

The scenario

The scenario is visualized by the following drawing:

How to setup the network namespaces and required devices?

I have discussed all commands required to configure such scenarios with unnamed Linux network namespaces in the first post of another older post series. See

Fun with veth-devices, Linux bridges and VLANs in unnamed Linux network namespaces – I

If you are new to experiments with unnamed Linux network namespaces please read this post first. Of particular importance are the commands

  • unshare …
  • nsenter …
  • ip link …
  • ip address …

I will not discuss details of the chain of commands required for setting up the elements of our scenario. At the end of this post you will find three PDFs with contents you can copy to bash shell scripts. All commands in their given form are meant to be executed by the user root in a safe test environment. I am very confident that readers interested in this post will understand the structure of the commands without further explanation.

Due to the fact that the scripts set and export intermediate variables as environment variables for sub-shells you should run the scripts via the “source“-command if you want these variables to be available in your original shell (probably in a terminal window), too.

The first script (create_netns) will then execute the following commands (you may need to scroll to see all):

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