Tag Archives: Ethernet broadcasts

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

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

we have studied a somewhat academic network scenario:

Two L2-segments were attached to a common network namespace, with all IPs of both segments belonging to one and the same IP subnet class. The network traffic was analyzed for different route configurations. Only detailed routes ensured a symmetric handling of packets in both segments.

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, network namespaces, VLANs – II – two L2-segments attached to a common network namespace

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In the first post of this series about virtual networking

More fun with veth, network namespaces, VLANs – I – open questions

I have collected some questions which had remained open in an older post series of 2017 about veths, unnamed network namespaces and virtual VLANs. In the course of the present series I will try to answer at least some of these questions.

The topic of the present and the next two post will be a special network namespace which we create with an artificial ambiguity regarding the path that ICMP and even ARP packets could take. We will study a scenario with the following basic properties:

We set up two L2-segments, each based on a Linux bridge to which we attach two separate network namespaces by veth devices. These L2-segments will be connected (by further veths) to yet another common, but not forwarding network namespace “netnsR”. The IPs of all veth end-points will be members of one and the same IP-subnet (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.

Note, however, that our scenario is different from the typical situation of a router or routing namespace: One 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-subnet and not two different ones. The other other reason is that we will not enable “forwarding” in the coupling namespace “netnsR”.

In this post we will first try to find out by theoretical reasoning what pitfalls may await us and what may be required to enable a symmetric communication between the common namespace netnsR and each of the L2-segments. We will try to identify critical issues which we have to check out in detail by experiments.

One interesting aspect is that the setup basically is totally symmetric. But therefore it is also somewhat ambiguous regarding the possible position of IPs in one of the networks. Naively set routes in netnsR may break or reflect this symmetry on the IP layer. But we shall also consider ARP requests and replies on the Link layer under the conditions of our scenario.

In the next forthcoming post we will verify our ideas and clarify open points by concrete experiments. As long as forwarding is disabled in the coupling namespace netnsR we do not expect any cross-segment transfer of packets. In yet another post we will use our gathered results to establish a symmetric cross-segment communication and study how we must set up routes to achieve this. All these experiments will prepare us for a later investigation of virtual VLANs.

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 such 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 previous post series of the year 2017 we can enter such a network namespace and execute shell commands there; see here. The veth endpoints in netns11 and netns12 get IP addresses.

We build two of such segments, S1 and S2, with each of the respective bridges located in its own namespace (netnsB1 and netnsB2). Then we use further veth devices to connect the two bridges (= segments) to a common namespace (netnsR). Despite setting default routes we do not enable forwarding in 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 Linux bridges.
  • netnsR is a non-forwarding namespace where both segments, S1 and S2, terminate – each via a border NIC (veth-endpoint).

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.

The sketch makes it clear that communication within each of the segments is possible: netns11 will certainly be able to communicate with netns12. The same holds for netns21 and netns22.

But we cannot be so sure what will happen with e.g. ARP and ICMP request and/or answering packets send from netnsR to one of the four namespaces netns11, netns12, netns21, netns22. You may guess that this might depend on route definitions. 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: 192.168.5.0/24. The bridges themselves do not need assigned IPs. In our scenario the bridges could at least in principle have been replaced by a Hub or even 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.

Regarding routes: Of course we need to define routes in all of the namespaces. Otherwise the NICs with IPs would not become operative. In a first naive approach we will just rely on the routes which are automatically generated when we create the NICs. We will see that this leads to a somewhat artificial situation in netnsR. Both routes will point to the same IP-subnet 192.168.5.0/24 – but via different NICs.

Theoretical analysis of the situation of and within netnsR

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

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