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

In the last articles of our excursion on network namespaces, veth-devices and virtual networking

we studied virtual VLANs a bit. We saw that virtual VLANs can be defined just by applying certain configuration options to Linux bridge ports. In addition, virtual VLANs can be extended over several Linux bridges via veth sub-interfaces OR pure veth trunk connections. These possibilities support already a large variety of options for the configuration of virtual networks (e.g. for a bunch of containers). We discussed some simple illustrative test cases, in which containers were represented by simple network namespaces.

However, so far, four properties characterized our test configurations:

  • All network namespaces (or container hosts) connected to a Linux bridge belonged to exactly one of the involved VLANs.
  • All network namespaces (or container hosts) belonging to the involved VLANs were connected to a Linux bridge via ports which sent out untagged packets on egress from the bridge to the target namespaces and received untagged packets from the namespaces (or container hosts).
  • The VLANs (e.g. VLAN1, VLAN2) were completely defined by PVID/VID definition at Linux bridge ports, only. We eliminated in addition default PVID/VID values. Thus, the VLANs were completely isolated from each other: No host/namespace of a VLAN1 could communicate with a host/namespace belonging to a different VLAN2.
  • Different Linux bridges (which could reside on different hosts) were connected by (virtual or real) cables between trunk ports or sub-interface ports; the cables connecting the bridges transferred packets with different tags. We used this to keep up the isolation of the VLANs against each other even when we extended the VLANs over multiple bridges.

The third point may be good in the sense of security in many applications - but it is also restrictive. The first deficit may be that at least some hosts in a VLAN2 should be able to reach a certain server in VLAN1. This problem can be solved by establishing routing, forwarding and packet filtering outside the bridge. But there may be other requirements ....

New challenges

More interesting may be configurations

  • where you need to set up some containers/namespaces as common members of two ore more VLANs
  • or in which you need to establish network namespaces for gathering network packets from different VLANs and organize a common communication with further networks via specific interfaces.

In future posts of this series, we, therefore, introduce additional network namespaces (representing LXC or Docker containers) to test examples for such configurations. These new namespaces should at least be able to communicate with member namespaces/hosts of different VLANs and transfer packets from multiple VLANs to other network namespaces or routers.

In the present post I walk through some basic considerations of such configurations. For this purpose we restrict the number of involved VLANs to 2 (VLAN1: green tags / VLAN2: pink tags). Each VLAN shall be represented by one example member network namespace (VLAN1: netns1 / VLAN2: netns2). In addition, we introduce a third network namespace netns3, which shall be connected to the VLANs and which should fulfill the following requirements:

  • Requirement 1: netns3 shall be able to receive packets from members of both VLANs and send packets to destination targets in both VLANs. I.e., netns3 must be able to communicate with member systems of both VLANs.
  • Requirement 2: netns3 shall, however, not become a packet forwarder between the VLANs; the VLANs shall remain separated despite the fact that they have a common communication partner netns3.

After all we have learned in this article series, we would, of course, try to establish the connection between members of VLAN1 (represented by netns1) and members of VLAN2 (netns2) to netns3 with the help of an intermediate network namespace netnsX. If required we would equip netnsX with a Linux bridge. Thus, the requirements lead to a typical

"3 point connection problem":
Each of the VLANs is connected to netnX by 2 separate "connectors" (NICs or ports of a Linux bridge inside netnsX). A third "connector" attaches netns3 somehow. Schematically this is shown in the following graphics:

We associate VLAN1 with VLAN packet tags depicted in green color, VLAN2 with packets tags in pink. From "requirement 2" we conclude that we have to be careful with forwarding inside of BOTH netns3 AND netnsX.

Note:
We are not talking about reaching a member of VLAN2 from certain members of VLAN1. We shall touch this VLAN subject, too, but only as a side aspect. In the center of our analysis are instead network namespaces which can talk freely to members of two VLANs and which can receive and work with packets from two VLANs without destroying the communication isolation of members in VLAN1 against members in VLAN2.

What are real world applications for scenarios with network namespaces connected to two or more VLANs?

Two basic applications scenarios are the following:

  • A common administrative network namespace - or container host - for systems in both VLANs. This namespace/container shall operate without allowing for traffic between the VLANs.
  • A system which transfers packets from/to systems in both VLANs via a router to/from the external world or the Internet - without allowing for traffic between the VLANs.

The challenge is to find virtual network configurations for such scenarios. To make it a bit more challenging we assume that both VLANs are defined for systems of the same IP network class. (There is no requirement that limits different VLANs to different IP classes. A VLAN can cover several IP class networks; on the other side two different VLANs can each have members of the same IP class).

There are of course more application scenarios - but the two elementary ones named above cover most of the basic principles. We shall see that - depending on the solution approach - routing, packet filters and even forwarding must be addressed to realize the objectives of a certain scenario.

Ambiguities: Two different classes of packet transfer solutions

In netns3 we need to work with packets arriving from both VLANs. We also need to send back packets to destinations in both VLANs. But, there is a basic ambiguity related to the third connector and the connection line between netnsX and netns3. It is expressed by the following question:

Do we want to or can we afford to exchange tagged packets between netnsX and netns3?

This is not so trivial a question as it may seem to be! The answer depends on whether the network devices or applications inside netns3 know how to deal with and how to direct or transfer tagged packets.

In case we keep up VLAN tags until the inside of netns3 we must either provide a proper termination for the connection interface(s) or be able to pass tagged packets onward. If, however, netns3 does not know how to deal with tagged packets or if it makes no sense to keep up tagging we would rather send untagged packets from netnsX to netns3. One good reason why it may not make sense to keep up tagging could be that the tags would not survive a subsequent routing to the outside world anyway.

Thus we arrive at two rather different classes of connectivity solutions:

Let us first concentrate on termination solutions for tagged packets inside netns3 as depicted on the left side of the upper drawing:

As we have already seen in previous posts it is no problem to keep up tagging on the way from netns1 or netns2 to netns3. We know how to transfer tagged and untagged packets in and out of Linux bridges and thus we can be confident to find a suitable transfer solution based on a bridge inside netnsX. By the help of 2 sub-interfaces of e.g. a virtual veth device we could terminate the network transport properly inside netns3. So, it seems to be easy to make netns3 a member of both VLANs in this first class of connection approach. But, as we shall understand in a minute, we need a little more than just a bridge in netnsX and veth sub-interfaces to get a working configuration ....

A really different situations arises if we needed a configuration as presented on the right side of the graphics. The challenge there is not so much the creation of untagged packets going out of netnsX but the path of VLAN-ignorant packets coming in e.g. from the external world through netns3 and heading for members of either VLAN. Such packets must somehow then be directed to the right VLAN according to the IP address of the target. Such a targeting problem typically requires some kind of routing. So, on first sight a Linux bridge does not seem to be of much help in netnsX as there is no routing on a level 2 device! But, actually, we shall find that a Linux bridge in netnsX can lead to a working solution for untagged packets from/to netns3 - but such a solution comes with a prize.

Approaches with terminated VLAN connections in a common network namespace fit very well to the scenario of a common container host for the administration of systems in multiple VLANs. Solutions which instead use untagged packets entering and leaving netns3, instead fits very well to scenarios where multiple VLANs want to use a common connection (Ethernet card) or a common router to external networks.

Solutions which use packet tags and terminate VLAN traffic inside a common member of multiple VLANs

Let us assume that netns3 shall represent a host for the administration of netns1 in VLAN 1 (green) and netns2 in VLAN 2 (pink). Let us decide to keep up tagging all along the way from netns1 or netns2 to netns3. From the previous examples in this blog post series the following approaches for a netnsX-bridge-configuration look very plausible:

However, if you only configured the bridge, its ports and the veth devices properly and eventually tried pinging from netns1 to netns3 you would fail. (There are articles and questions on the Internet describing problems with such situations...). So, what is missing? The answer is as simple as it is instructive:

VLANs define a closed broadcast environment on TCP/IP network level 2. Why are broadcasts so important? Because we need a working ARP protocol to connect level2 to level 3, and ARP sends broadcast requests for the MAC address of a target, which has a given IP address AND which, hopefully, is a member of the VLAN.

With a proper bridge port configuration such a ARP request packet would travel all along from netns1 to netns3. BUT:
The real challenge is the way back of ARP answering packets - such answering packets must reach their targets before any other communication on level 3 can start to work properly. As we only are in the middle of an initial ARP communication: How can netns3 know where to direct the ARP answering packets to if there are two possible paths back? Without help it cannot. So, the proper answer is:

We need to establish routes inside netns3 when we keep up the separation of the VLANs up until to 2 different termination points inside netns3. These routes for outgoing packets must assign IP-targets located in each of the VLANs to one of the 2 network interfaces (termination points) inside netns3 in a unique way.

This is a trivial point, but often enough people forget this type of routing. Note in addition:
If the different VLANs have members with an IP of one and the same IP class, then you do not differentiate routes in the sense of "network class <=> interface" but in the sense "host IP <=> interface"; such routes must be defined for all members of each VLAN. I shall give examples for corresponding commands in my next blog post of this series.

Forwarding?

As we talk of routing: Do we need forwarding, too? Answer: No, not as long as netns3 is the final target or the origin of packet transport in a given application scenario. Why is this important? Because routing between interfaces connected to bridge ports of different VLANs would establish a communication connection between otherwise separated VLANs.

To enable packets to cross VLAN borders we either have to destroy the separation already on a bridge port level OR we must allow for routing and forwarding between NICs which are located outside the bridge but which are connected to ports of the bridge. E.g., let us assume that the sub-interfaces in netns3 are named veth33.10 (VLAN1 termination) and veth33.20 (VLAN2 termination). If we had not just set up routes like

route add 192.168.5.1 veth33.10
route add 192.168.5.4 veth33.20

but in addition had enabled forwarding with

echo 1 > /proc/sys/net/ipv4/conf/all/forwarding

inside netns3 we would have established a communication line between our two VLANs. Fortunately, in many cases, forwarding is not required in a common member of two VLANs. Most often only route definitions are necessary. In particular, we can set up a host which must perform administrative tasks in both VLANs without creating an open communication line between the VLANs. However, we would have to trust the administrator of netns3 not to enable forwarding. Personally, I would not rely on this; it is more secure to establish port and IP related packet filtering on the bridge inside netnsX. Especially rules in the sense:

Only packets for a certain IP address are allowed to leave the Linux bridge (which establishes the VLANs) across a certain egress port to a certain VLAN member.

Such rules for bridge ports can be set up e-g- with special iptables commands for bridged packets.

Intermediate conclusions for solutions with VLAN termination in a common network namespace

We summarize the results of our theoretical discussion for the first class of solutions:

  • VLAN termination inside a network namespace (or container host), which shall become a common member of several VLANs, can easily be achieved with sub-interfaces of a veth device. The other interface of the veth pair can be attached by sub-interfaces OR as a pure trunk port to a Linux bridge which is connected to the different VLANs or which establishes the VLANs itself by proper port configurations.
  • If we terminate VLANs inside a network namespace or container host, which shall become a member of two or more VLANs, then we need to define proper routes to IP targets behind each of the different VLAN related interfaces. However, we do NOT need to enable forwarding in this namespace or container host.

A three point netnX solution without packet tagging, but with forwarding to a common target network namespace

Now, let us consider solutions of the second class indicated above. If you think about it a bit you may come up with the following basic and simple approach regarding netnsX and netns3:

This solution is solid in the sense that it works on network level 3 and that it makes use of standard routing and forwarding. The required VLAN tagging at each of the lower connection points in netnsX can be achieved by a properly configured sub-interface of a veth device interface. We do not employ any bridge services in netnsX in this approach; packet distribution to VLAN members must be handled in other network namespaces behind the VLAN connection points in netnsX. (We know already how to do this ...).

This simple solution, however, has its prize:

We need to enable forwarding for the transfer of packets from the VLAN connection interfaces (attaching e.g. netns1 and netns2 to netnsX) to the the interface attaching netns3 to netnsX. But, unfortunately, this creates a communication line between VLAN1 and VLAN2, too! To compensate for this we must set up a packet filter, with rules disallowing packets to travel between the VLAN connection points inside netnsX. Furthermore, packets coming via/from netns3 shall only be allowed to pass through exactly one of the lower VLAN interfaces in netnsX if and when the target IP fits to a membership in the VLAN behind the NIC.

There is, by the way a second prize, we have to pay in such a router like solution for the connection of VLANs to an outside world without tags:

Level 3 routing costs a bit more computational time than packet transport on level 2.

But, if you (for whatever reason) only can provide one working Ethernet interface to the outside world, it is a small prize to pay!

Intermediate result:

An intermediate virtual network namespace (or virtual host) netnsX with conventional routing/forwarding AND appropriate packet filter rules on a firewall can be used to control the communication of members of two or more VLANs to the outside world via a third (common) interface attached to netnsX. We do not need to care for VLAN tags beyond this third interface as VLAN tags do not survive forwarding. Further routing, forwarding and required NAT configurations with respect to the Internet can afterward be done inside yet another virtual namespace "netns3" (with a bridge and an attached real Ethernet card) or even beyond netns3 in an external physical router.

A three point netnX solution without packet tagging - but based on a Linux bridge

Now, let us consider how a Linux bridge in netnsX could transfer packets even if we do not tag packets on their way between the bridge and netns3. I.e., if we want connect two VLANs to a VLAN-ignorant network namespace netns3 and a VLAN indifferent world beyond netns3. What is the problem with a configuration as indicated on the right side of the picture on different solution classes?

A port to netns3 which shall emit untagged packets from a VLAN-aware Linux bridge must be configured such

  • that it accepts tagged packets from both VLAN1 and VLAN2 on egress; i.e. we must apply two VID settings (for green and pink tagged pakets).
  • that it sends out packets on egress untagged; i.e. we must configure the port with the flag "untagged".

But VID settings also filter and drop incoming "ingress" packets at a port! E.g. untagged packets from netns3 are dropped on their way into the Linux bridge. See the post Fun with ... – IV for related rules on Linux bridge ports. This is a major problem:

Firstly, because we cannot send any ARP broadcast requests from netns3 to netns1 or netns2. And, equally bad, netns3 cannot answer to any ARP requests which it may receive from members of VLAN1 or VLAN2:

ARP broadcast requests from e.g. netns1 will pass the bridge port to netns3 and arrive there untagged. However, untagged ARP answer packets will not be allowed to enter the bridge at the port for netns3 because they do not fit to the VID settings at this port.

But, can't we use PVID settings? Hmm, remember: Only one PVID setting is allowed at a port! But in our case ARP broadcast and answering packets must be able to reach members of both VLANs! Are we stuck, then? No, a working solution is the following:

In the drawing above we have indicated PVID settings by squares with dotted, colored borders and VID settings by squares with solid borders. The configuration may look strange, but it eliminates the obstacles for ARP packet exchange! And it allows for packet transfer from netns3 to both VLANs.

Actually, the "blue" PVID/VID setting reflects the default PVID/VID settings (VID=1; PVID=1) which come up whenever we create a port in VLAN-aware bridge! Up to now, we have always deleted these default values to guarantee a complete VLAN isolation; but you may already have wondered why this default setting takes place at all. Now, you got a reason.

If you, in addition, take into account that a Linux bridge learns about port-MAC relations and that it - under normal conditions - forwards or filters packets during bridge internal forwarding between ports

  • according to MAC addresses located behind a port
  • AND tags matching VID values at a port,

you may rightfully assume that packets cannot move from VLAN1 to VLAN2 or vice versa under normal operation conditions. We shall test this in an example scenario in one of the coming blog posts.

HOWEVER ....virtual networks with level 2 bridges are endangered areas. The PVID/VID settings of our present bridge based approach weaken the separation between the VLANs significantly.

Security aspects

For all configurations discussed above, we must be careful with netns3: netns3 is in an excellent position to potentially transfer packets between VLAN1 and VLAN2 - either by direct forwarding/routing in some of the above scenarios or by capturing, manipulating and re-directing packets. Secondly, netns3 is in an excellent position for man-in-the-middle-attacks

  • regarding traffic between members of either VLAN
  • or regarding traffic between the VLANs and the outside world beyond netns3.

netns3 can capture, manipulate and redirect any packets passing it. As administrators we should, therefore, have full control over netns3.

In addition: If you ever worked on defense measures against bridge related attack vectors you know

  • that a Linux bridge can be forced into a HUB mode if flooded with wrong or disagreeing MAC information.
  • that man-in-the-middle-attacks are possible by flooding hosts attached to bridges with wrong MAC-IP-information; this leads to manipulated ARP tables at the attacked targets.

These points lead to potential risks especially in the last bridge based solution to our three point problem. Reason: The "blue" PVID/VID settings there eliminate the previously strict separation of the two VLANs for packets which come from netns3 and enter the bridge at a related port. We rely completely on correct entries in the bridge's MAC/port relation table for a safe VLAN separation.

But the bridge could be manipulated from any of the attached container hosts into a HUB mode. This in turn would e.g. allow a member of VLAN1 to see (e.g. answering) packets, which arrive from netns3 (or an origin located beyond netns3) and which are targeted to a member of VLAN2. Such packets may carry enough information for opening other attack vectors.

So, a fundamental conclusion of our discussion is the following:

It is essential that you apply packet filter rules on bridge based solutions that hinder packets to reach targets (containers) with the wrong IP/MAC-relation at egress ports! Such rules can be applied to bridge ports by the various means of Linux netfilter tools.

On a host level this may be a task which becomes relatively difficult if you apply flexible DHCP-based IP assignments to members of the VLANs. But, if you need to choose between flexibility and full control about which attached namespace/container gets which IP (and MAC) and your virtual networks are not too big : go for control - e.g via setup scripts.

Summary and outlook

Theoretically, there are several possibilities to establish virtual communication lines from a network namespace or container to members of multiple virtual VLANs. Solutions with tagged packet transfer require a proper termination inside the common member namespace and the definition of routes. As long as we do not enable forwarding outside the VLAN establishing Linux bridge the VLANs remain separated. Solutions where packets are transferred untagged from the VLANs to a target network namespace require special PVID/VID settings at the bridge port to enable a bidirectional communication. These settings weaken the VLAN separation and underline the importance of packet filter rules on the Linux bridge and for the various bridge ports.

In the next post of this series we will look at commands for setting up a test environment for 2 VLANs with a common communication target. And we will test the considerations discussed above.

In the meantime : Happy New Year - and stay tuned for more adventures with Linux, Linux virtual bridges and network namespaces ...

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

In the previous posts of this series

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

we studied network namespaces and related commands. We also started a series of experiments to deepen our understanding of virtual networking between network namespaces. For practical purposes you can imagine that our abstract network namespaces represent LXC containers in the test networks.

In the last post we have learned how to connect two network namespaces via veth devices and a Linux bridge in a third namespace. In coming experiments we will get more ambitious - and put our namespaces (or containers) into virtual VLANs. "V" in "VLAN" stands for "virtual". So, what are virtual VLANs? These are VLANs in a virtual network environment!

We shall create and define these VLANs essentially by configuring properties of Linux bridges. The topic of this article is an introduction into some elementary rules governing virtual VLAN setups based on virtual Linux bridges and veth devices.

I hope such a rule overview is useful as there are few articles on the Internet summarizing what happens at ports of virtual Linux bridges with respect to VLAN tagging of Ethernet packets. Actually, I found some of the rules by doing experiments with bridges for kernel 4.4. I was too lazy to study source codes. So, please, correct me and write me a mail if I made mistakes.

VLANs

VLANs define specific and very often isolated paths through a network for Ethernet packets. At some "junctions and crossings" only certain OUT paths are open for arriving packets, depending on how a packet is marked. Junctions and crossings are represented in a network by devices as real or virtual bridges. We can say: Ethernet packets are only allowed to move along only In/OUT directions in VLAN sensitive network devices. All decisions are made on the link layer level. IP addresses may influence entries into VLANs at routers - but once inside a VLAN criteria like "tags" of a packet and certain settings of connection ports open or close paths through the network:

VLANs are based on VLAN IDs (integer numbers) and a corresponding tagging of Ethernet packets - and on analyzing these tags at certain devices/interfaces or ports. In real and virtual Ethernet cards so called sub-interfaces associated with VLAN IDs typically send/receive tagged packets into/from VLANs. In (virtual) bridges ports can be associated with VLAN IDs and open only for packets with matching "tags". A VLAN ID assigned to a port is called a "VID". An Ethernet packet normally has one VLAN tag, identifying to which VLAN it belongs to. Such a tag can be set, changed or removed in certain VLAN aware devices.

A packet routed into a sub-interface gets a VLAN tag with the VLAN ID of the sub-interface. The rules at bridge ports are more complicated and device and/or vendor dependent. I list rules for Linux bridge ports in a paragraph below.

VLANs can be used to to isolate network communication paths and circuits between systems against each other. An important property is: Broadcast packets (e.g. required for ARP) are not allowed to cross the borders of VLANs. Thus the overall traffic can be reduced significantly in some network setups. VLANs can be set up in virtual networks on virtualization hosts, too; this is of major importance for the hosting of containers.

Whenever we use the word "trunk" in connection with VLANs we mean that an interface, port or a limited connection line behaves neutral with respect to multiple VLAN IDs and allows the transport of packets from different VLANs to some neighbor device - which then may differentiate again.

Note:

On a Linux system the kernel module "8021q" must be loaded to work with tagged packets. On some Linux distributions you may have to install additional packages to deal with VLANs and 802.1Q tags.

Veth devices support VLANs

As with real Ethernet cards we can define VLAN related sub-interfaces of one or of both Ethernet interfaces of a veth device pair. E.g., an interface vethx of a device pair may have two sub-interfaces, "vethx.10" and "vethx.20". The numbers represent different VLAN IDs. (Actually the sub-interface can have any name; but it is a reasonable convention to use the ".ID" notation.)

As a veth interface may or may not be splitted into a "mother" (trunk) interface and multiple sub-interfaces the following questions arise:

  • If we first define sub-interfaces for VLANs on one interface of a veth device, must we use sub-interfaces on the other veth side, too?
  • What about situations with sub-interfaces on one side of the veth device and a standard interface on the other?
  • Which type of interface can or should we connect to a virtual Linux bridge? If we can connect either: What are the resulting differences?

Connection of veth interfaces to Linux bridges

Actually, we have two possibilities when plugging veth interfaces into Linux bridges:

  • We can attach the sub-interfaces of a veth interface to a Linux bridge and create several respective ports, each of which receives tagged packets from the outside and emits tagged packets to the outside.
  • Or we can attach the neutral (unsplitted) "trunk" interface at one side of a veth device to a Linux bridge and create a respective port, which may transfer tagged and untagged packets into and out of the bridge. This is even possible if the other interface of the veth device has defined sub-interfaces.

In both cases bridge specific VLAN settings for the bridge ports may have different impacts on the tagging of forwarded IN or OUT packets. We come back to this point in a minute. The following drawing illustrates some principles:

We have symbolized packets by diamonds. Different colors correspond to different tag numbers (VLAN IDs).

The virtual cable of a veth device can transport Ethernet packets with different VLAN tags. However, packet processing at certain targets like a network namespace or a bridge requires a termination with a suitable Ethernet device, i.e. an interface which can handle the specific tag of packet. This termination device is:

  • either a veth sub-interface located in a specific network namespace
  • or veth sub-interface inside a bridge ( => this creates a bridge port, which requires at least a matching VID)
  • or a veth trunk interface inside a Linux bridge (=> this creates a trunk bridge port, which may or may not require VIDs, but no PVID.)

Both variants can also be combined as shown in the lower part of the drawing: One interface ends in a bridge in one namespace, whereas the other interface is located in another namespace and splits up into sub-interfaces for different VLAN IDs.

Untagged packets may be handled by the standard trunk interfaces of a veth device.

Note: In the sketch below the blue packet "x" would never be available in the target namespace for further processing on higher network layers.

So, do not forget to terminate a trunk line with all required sub-interfaces in network namespaces!

A reasonably working setup of course requires measures and adequate settings on the bridge's side, too. This is especially important for trunk interfaces at a bridge and trunk connection lines used to transport packets of various VLANs over a limited connection path to an external device. We come to back to relevant rules for tagging and filtering inside the bridge later on.

Below we call a veth interface port of a bridge, which is based on the the standard trunk interface a trunk port.

The importance of route definitions in network namespaces

Inside network namespaces where multiple VLANs terminate, we need properly defined routes for outgoing packets:

Situations where it is unclear through which sub-interface a packet shall be transported to certain target IP addresses, must always be avoided! A packet to a certain destination must be routed into an appropriate VLAN sub-interface! Note that defining such routes is not equivalent to enable routing in the sense of IP forwarding!

Forgetting routes in network namespaces with devices for different VLANs is a classical cause of defunct virtual network connections!

Commands to set up veth sub-interfaces for VLANs

Commands to define sub-interfaces of a veth interface and to associate a VLAN ID with each interface typically have the form:

ip link add link vethx name vethx.10 type vlan id 10
ip link add link vethx name vethx.20 type vlan id 20
ip link set vethx up
ip link set vethx.10 up
ip link set vethx.20 up

Sub-interfaces must be set into an active UP status! Inside and outside of bridges.

Setup of VLANs via a Linux bridge

Some years ago one could read articles and forum posts on the Internet in which the authors expressed their opinion that VLANs and bridging are different technologies which should be separated. I take a different point of view:

We regard a virtual bridge not as some additional tool which we somehow plant into an already existing VLAN landscape. Instead, we set up VLANs by configuring the virtual bridge.

A Linux bridge today can establish a common "heart" of multiple virtual VLANs - with closing and opening "valves" to separate the traffic of different circulation paths. From a bridge/switch that defines a VLAN we expect

  • the ability to assign VLAN tags to Ethernet packets
  • and the ability to filter packets at certain ports according to the packets' VLAN tags and defined port/tag relations.
  • and the ability to emit untagged packets at certain ports.

In many cases, when a bridge is at the core of simple separated VLANs, we do not need to tag outgoing packets to clients or network namespaces at all. All junction settings for the packets' paths are defined inside the bridge!

Tagging, PVIDs and VIDs - VLAN rules at Linux bridge ports

What happens at a bridge port with respect to VLANs and packet tags? Almost the same as for real switches. An important point is:

We must distinguish the treatment of incoming packets from the handling of outgoing packets at one and the same port.

As far as I understand the present working of virtual Linux bridges, the relevant rules for tagging and filtering at bridge ports are the following:

  1. The bridge receives incoming packets at a port and identifies the address information for the packet's destination (IP => MAC of a target). The bridge then forwards the packet to a suitable port (target port; or sometimes to all ports) for further transport to the destination.
  2. The bridge learns about the right target ports for certain destinations (having an IP- and a MAC-address) by analyzing the entry of ARP protocol packets (answer packets) into the bridge at certain ports.
  3. For setting up VLANs based on a Linux bridge we must explicitly activate "VLAN filtering" on the bridge (commands are given below).
  4. We can assign one or more VIDs to a bridge port. A VID (VLAN ID) is an integer number; the default value is 1. At a port with one or more VIDs both incoming tagged packets from the bride's outside and outgoing tagged packets forwarded from the bridge's inside are filtered with respect to their tag number and the port VID(s): Only, if the packet's tag number is equal to one of the VIDs of the ports the packet is allowed to pass.
  5. Among the VIDs of a port we can choose exactly one to be a so called PVID (Port VLAN ID). The PVID number is used to tag (untagged) incoming packets. The new tag is then used for filtering inside the bridge at target ports. A port with a PVID is also called "access port".
  6. Handling of incoming tagged packets at a port based on a veth sub-interface:
    If you attached a sub-interface (for a defined VLAN ID number) to a bridge and assigned a PVID to the resulting port then the tag of the incoming packets is removed and replaced by the PVID before forwarding happens inside the bridge.
  7. Incoming packets at a standard trunk veth interface inside a bridge:
    If you attached a standard (trunk) veth interface to a bridge (trunk interface => trunk port) and packets with different VLAN tags enter the bridge through this port, then only incoming packets with a tag fitting one of the port's VIDs enter the bridge and are forwarded and later filtered again.
  8. Untagged outgoing packets: Outgoing packets get their tag number removed, if we configure the bride port accordingly: We must mark its egress behavior with a flag "untagged" (via a command option; see below). If the standard veth (trunk) interface constitutes the port the packet leaves the bridge untagged.
  9. Retagging of outgoing untagged packets at ports based on veth sub-interfaces:
    If a sub-interface of a veth interface constitutes the port, an outgoing packet gets tagged with VLAN ID associated with the sub-interface - even if we marked the port with the "untagged" flag.
  10. Carry tags from the inside of a bridge to its outside:
    Alternatively, we can configure ports for outgoing packets such that the packet's tag, which the packet had inside the bridge, is left unchanged. The port must be configured with a flag "tagged" to achieve this. An outgoing packet leaves a trunk port with the tag it got/had inside the bridge. However, if a veth sub-interface constituted the port the tag of the outgoing packet must match the subinterface's VLAN ID to get transported at all. /li>
  11. A port with multiple assigned VIDs and the flag "tagged" is called a "trunk" port. Packets with different tags can be carried along the outgoing virtual cable line. In case of a veth device interface the standard (trunk) interface and not a sub-interface must constitute such a port.

Note that point 2 opens the door for attacking a bridge by flooding it with wrong IP/MAC information (ARP spoofing). Really separated VLANs make such attacks more difficult, if not impossible. But often you have hosts or namespaces which are part of two or more VLANs, or you may have routers somewhere which do not filter packet transport sufficiently. Then spoofing attack vectors are possible again - and you need packet filters/firewalls with appropriate rules to prevent such attacks.

Note rule 6 and the stripping of previous tags of incoming packets at a PVID access port based on a veth sub-interface! Some older bridge versions did not work like this. In my opinion this is, however, a very reasonable feature of a virtual bridge/switch:

Stripping tags of packets entering at ports based on veth sub-interfaces allows the bridge to overwrite any external and maybe forged tags. This helps to keep up the integrity of VLAN definitions just by internal bridge settings!

The last three points of our rule list are of major importance if you need to distinguish packets in terms of VLAN IDs outside the bridge! The rules mean that you can achieve a separation of the bridge's outgoing traffic according to VLAN IDs with two different methods :

  • Trunk interface connection to the bridge and sub-interfaces at the other side of an veth cable.
  • Ports based on veth sub-interfaces at the bridge and veth sub-interfaces at the other side of the cable, too.

We discuss these alternatives some of our next experiments in more detail.

Illustration of packet transport and filtering

The following graphics illustrates packet transport and filtering inside a virtual Linux bridge with a few examples. Packets are symbolized by diamonds. VLAN tags are expressed by colors. PVIDs and VIDS at a port (see below) by dotted squares and normal squares, respectively. The blue circles have no special meaning; here some paths just cross.

The main purpose of this drawing is to visualize our bunch of rules at configured ports and not so much reasonable VLANs; the coming blog posts will discuss simple multiple examples of separated and also coupled VLANs. In the drawing only the left side displays two really separated VLANs. Ports C and D, however, illustrate special rules for specially configured ports. Note that not all possible port configurations are covered by the graphics.

With the rules above you can now follow the paths of different packets through the drawing. This is simple for packet "5". It gets a pink tag at its entry through the lowest port "D". Its target port is the port "C" where it passes due to the fact that the VID is matching. Packet "2" follows an analogous story.

All ports on the left (A, B, C, D) have gotten the flag "untagged". Packets 5 and 2,6,7, therefore, leave the bridge untagged. Note that no pink packets are allowed to leave ports A, B and D. Vice versa, no green packets are allowed to leave target ports C and D.

Port "E" would be a typical example for a trunk port. Incoming and outgoing green, pink and blue packets keep their tags! Packet 8 and packet 9, which both are forwarded to their target port "E", therefore, move out with their respective green and pink tags. The incoming green packet "7" is allowed to pass due to the green VID at this port.

Port "D", however, is a strange guy: Here, the PVID (blue) differs from the only VID (green)! Packet "6" can enter the bridge and leave it via target port "B", which has two VIDs. Note, however, that there is no way back! And the blue packet "3" entering the bridge via trunk port "E" for target port "D" is not allowed to leave the bridge there. Shit happens ...

The example of port "D" illustrates that VLAN settings can look different for outgoing and incoming packets at one and the same port.
But also ports like "D" can be used for reasonable configurations - if applied in a certain way (see coming blog posts).

Commands to set up the VLANs via port configuration of virtual Linux bridges

We first need to make the bridge "VLAN aware". This is done by explicitly activating VLAN filtering. On a normal system (in the root namespaces) and for a bridge "brx" we could enter

echo 1 > /sys/class/net/brx/bridge/vlan_filtering

But in artificially constructed network namespaces we will not find such a file. Therefore, we have to use a variant of the "ip" command:

ip link set brx type bridge vlan_filtering 1

For adding/removing a VID or PVID to/from a bridge port - more precisely a device interface for which the bridge is a master - we use the "bridge vlan" command. E.g., in the network namespace where the bridge is defined:

bridge vlan add vid 10 pvid untagged dev veth53

bridge vlan add vid 20 untagged dev veth53

bridge vlan del vid 20 dev veth53

See the man page for more details!

Note: We can only choose exactly one VID to be a PVID. As already explained above, the "untagged" option means that we want outgoing packets to leave the port untagged (on egress).

Data transfer between VLANs?

Sometimes you may need to allow for certain clients in one VLAN (with ID x) to access specific services of a server in another VLAN (with ID y). Note that for network traffic to cross VLAN borders you must use routing in the sense of IP forwarding, e.g. in a special network namespace that has connections to both VLANs. In addition you must apply firewall rules to limit the packet exchange to exactly the services you want to allow and eliminate general traffic.

There is one noteworthy and interesting exception:

With the rules above and a suitable PVID, VID setting you can isolate and control traffic by a VLAN from a sender in the direction of certain receivers, but you can allow answering packets to reach several VLANs if the answering sender (i.e. the former receiver) has connections to multiple VLANs - e.g. via a line which transports untagged packets (see below). Again: VLAN regulations can be different for outgoing and incoming packets at a port!

An example is illustrated below:

Intentionally or by accident - the bridge will do what you ask her to do at a port in IN and OUT directions. A setup as in the graphic breaks isolation, of course! So, regarding security this may be harmful. On the other side it allows for some interesting possibilities with respect to broadcast messages - as with ARP. We shall explore this in some of the coming posts.

Note that we always can involve firewall rules to allow or disallow packet travel across a certain OUT port according to the IP destination addresses expected behind a port!

The importance of a working ARP communication

Broadcast packets are not allowed to leave a VLAN, if no router bridges the VLANs. The ARP protocol requires that broadcast messages from a sender, who wants to know the MAC address of an IP destination, reach their target. But your VID and PVID settings must also allow the returning answer to reach the original sender of the broadcast. Among other things this requires special settings at trunk ports which send untagged packets from different VLANs to a target and receive untagged packets from this target. Without a working ARP communication to and from a memeber of a VLAN to other members unmanipulated traffic on higher network protocol layers will fail!

Conclusion

Veth devices and virtual Linux bridges support VLANs, VLAN IDs and a tagging of Ethernet packets. Tagging at pure veth interfaces outside a bridge requires the definition of sub-interfaces with associated VLAN IDs. The cable between a veth interface pair can be seen as a trunk cable; it can transport packets with different VLAN tags.

A virtual Linux bridge can become master of standard interfaces and/or sub-interfaces of veth devices - resulting in different port rules with respect to VLAN tagging. Similar to real switches we can assign VIDs and PVIDs to the ports. VIDs allow for filtering and thus VIDs are essential for VLAN definitions via a bridge. PVIDs allow for a tagging of incoming untagged packets or a retagging of packets entering through a port based on sub-interfaces. We can also define whether packets shall leave a port outwards of the bridge untagged or tagged.

Separated VLANs can, therefore, be set up with pure settings for ports inside a bridge without necessarily requiring any package tagging outside.

We now have a toolset for building reasonable VLANs with the help of one or more virtual bridges. In the next blog post

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

we shall apply what we have learned for the setup of two separated VLANs in our experimental network namespace environment.