More fun with veth and Linux network namespace – 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 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 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 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. ARP-requests created by the Linux-kernel to support the execution of ICMP-requests.

Therefore, it is still unclear

  • whether routes do have an impact on ARP-replies;
  • 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).

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

In my first post of my new series about virtual networking

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

I collected some questions which had remained open in an older post series of 2016 about veths, unnamed network namespaces and virtual VLANs. In the present series I will try to answer at least some of these questions.

In the present post we will study a scenario with the following basic properties:

Two L2-segments (based on two Linux bridges), each with two attached NICs in separate network namespaces, will be connected by veths to a further common network namespace. The IPs of all NICs and relevant veth end-points will be members of one and the same IP subnet class (a class C net).

No VLANs or firewalls will be set up. So, this is a very plain and seemingly simple scenario: Two otherwise separate L2-segments terminate with border NICs in a common network namespace.

But note that our scenario is different from the typical situation of a router or routing namespace. The reason is that our L2-segments and their respective NICs do not belong to different logical IP networks with different IP broadcast regions. We have just one common C-class IP-net and not two different ones.

In this second post we will first try to find out by theoretical reasoning what is required to enable communication between the common namespace and each of the segments. Meaning separate communication; no forwarding between the segments. We shall also study ARP requests and replies under these conditions. In a forthcoming third post we will verify our ideas by concrete experiments.

In future posts we will afterward try to describe what may be required in addition to enabling forwarding in the common network namespace to establish a cross-segment communication. I.e., we want to find out what we must do to ensure that a namespace (host) in one segment can talk to namespaces (hosts) in the other segment.

I will use the abbreviation “netns” for network namespaces throughout this post.

Scenario SC-1: Two L2-segments coupled by a common and routing namespace

Let us first look at a graphical drawing showing our scenario:

A simple way to build a virtual L2-segment is the following:

We set up a Linux bridge (e.g. brB1) in a dedicated network namespace (e.g. netnsB1). Via veth devices we attach two further network namespaces (netns11 and netns12) to the bridge. You may associate the latter namespace with virtual hosts reduced to elementary networking abilities. As I have shown in my post series of the year 2016 we can enter such a namespace and execute commands there. The veth endpoints in netns11 and netns12 get IP addresses.

We build two of such segments, S1 and S2, with each of the bridges located in its own namespace (netnsB1 and netnsB2). Then we use veth devices again to connect the two bridges (= segments) to a common namespace (netnsR).

The graphics shows that we all in all have 7 network namespaces:

  • netns11, netns12, netns21, netns22 represent hosts with NICs and IPs that want to communicate with other hosts.
  • netnsB1 and netnsB2 host the bridges.
  • netnsR is a namespace where both segments, S1 and S2, terminate – each via a border NIC (veth-endpoint).

The sketch makes it clear that netns11 will certainly be able to communicate with netns12. The same holds for netns21 and netns22.

netnsR is the namespace which is most interesting in our scenario: Without special measures packets from netns11 will not reach netns21 or netns22. So, we have indeed realized two separated L2-segments S1 and S2 attached to a common network namespace.

But we cannot be so sure what will happen with ARP and ICMP request and/or answering packets send from netnsR to one of the four namespaces netns11, netns12, netns21, netns22. I come back to this point in a minute.

Regarding IP addresses: Outside the bridges we must assign IP addresses to the respective veth-endpoints. As said: During the setup of the devices I use IP-addresses of one and the same class C network. All IPs belong to the same IP-subnet: The bridges themselves do not need assigned IPs. In our scenario they could have been replaced by an Ethernet bus cable with outtakes.

Side remark: Do not forget that a Linux bridge can in principle get an IP address itself and work as a special NIC connected to the bridge ports. We do, however, not need or use this capability in our scenario.

Theoretical analysis of the situation of and in netnsR

Both L2-segments terminate in netnsR. The role of netnsR is basically similar to that of a router, but with more ambiguity and uncertainty because both border NICs belong to the same IP-net.

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