Tag Archives: ICMP

More fun with veth and Linux network namespaces – VII – two namespaces connected by a veth based VLAN

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With the last one of the previous posts in this series

we started to look at VLANs based on veths and Linux bridges. I presented multiple options to configure a veth endpoint in a network namespace for enabling communication with other namespaces via tagged Ethernet packets.

We saw that we can potentially start splitting network traffic already from within a multi-homed namespace which is connected to different VLAN-segments. However, a veth endpoint with multiple interfaces also poses a basic ambiguity problem for the direction of both ARP and ICMP requests into the right one of the various attached (V)LAN-segments. From the results of previous experiments we would assume that the Linux kernel solves this problem by following routes.

In this post we will study the most simple VLAN configuration one can think of: We connect two network namespaces directly with a veth based VLAN line. I.e. we use a veth connection to transmit tagged packets along it. As this would be a lit boring, we add some pepper to the scenario by allowing for untagged packets, too.

To achieve this we reduce “option 4” discussed in the last post to a one-armed solution: We allow for only one VLAN interface per veth endpoint (see a sketch of the scenario below). We start a series of experiments with assigning an IP to the veth’s main interface, only. The setting for the source validation kernel parameter rp_filter will be relaxed.

In the experiments of this post we focus on

  • some claimed aspects regarding the role of the main trunk interface of a veth endpoint,
  • the potential impact of routes on the ARP communication.
  • the fact that working ARP does not mean working ICMP for a variety of different reasons.

Regarding the first point we will see that tagged packets just traverse the trunk interfaces on their way from and to the VLAN interface. Regarding route settings we will look at 36 possible variants. We will see that under the given conditions asymmetric route settings may or may not disable communication – already on the ARP level. But even if ARP works and even if we had symmetric routes in the namespaces, ICMP may not function. I will try to isolate the causes of positive and negative results for ARP and ICMP requests.

I recommend that readers who want to perform these experiments on their own to watch the system log in parallel.

A most simple VLAN scenario: Connect two namespaces by a one VLAN-segment based on a veth connection

We define two namespaces netns1 and netns2. We connect them via a veth connection line. Each of the two veth endpoints gets just one VLAN interface. We assign the IP addresses to the main trunk interfaces of the veth endpoints.

Below netns1 will always be the namespace that sends ARP-requests and in some cases receives an ARP-reply. netns2 instead will be the namespace that potentially receives the ARP-request and in some cases will send an ARP-reply. We use the same sender and receiver roles for the exchange of ICMP for some interesting route settings. ICMP tests the functionality of IP communication for us. I will call the sender of requests also “requestor” and the sender of replies also “responder” below.

My scenario appears to be a very academic one. But already in this simple case

  1. we have multiple options to set up routes – and the various configuration options (36!) cause different effects regarding ARP,
  2. we have the alternative to address netns2 from netns1 via tagged or untagged packets
  3. and despite relaxed source validation we may get “martian sources”.

So, this scenario is not so trivial as it appears on first sight. It comes with many more surprises than one would guess at first sight …

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

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During the last two posts of this series

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

More fun with veth, network namespaces, VLANs – 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, 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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