Introducing Linux Network Namespaces

Introducing Linux Network Namespaces
4 September 2013
In this post, I’m going to introduce you to the concept of Linux network namespaces. While it might seem a bit esoteric right now, trust me that there is a reason why I’m introducing you to network namespaces―if you, like me, are on a journey to better understand OpenStack, you’ll almost certainly run into network namespaces again.
So what are network namespaces? Generally speaking, an installation of Linux shares a single set of network interfaces and routing table entries. You can modify the routing table entries using policy routing (here’s an introduction I wrote and here’s a write-up on a potential use case for policy routing), but that doesn’t fundamentally change the fact that the set of network interfaces and routing tables/entries are shared across the entire OS. Network namespaces change that fundamental assumption. With network namespaces, you can have different and separate instances of network interfaces and routing tables that operate independent of each other.
This concept is probably best illustrated through some examples. Along the way, I’ll introduce a few new ideas as well. First, though, I need to provide some assumptions.
Assumptions
Throughout these examples, I’m using Ubuntu Server 12.04.3 LTS. Please note that support for network namespaces varies between Linux distributions; Ubuntu supports them but Red Hat doesn’t. (I’m not sure about Fedora. If you know, speak up in the comments.) If you’re thinking about using network namespaces, be sure your Linux distribution includes support.
Further, I’ll assume that you’re either running as root, or that you will prepend  sudo  to the commands listed here as necessary.
Creating and Listing Network Namespaces
Creating a network namespace is actually quite easy. Just use this command:

ip netns add <new namespace name>

For example, let’s say you wanted to create a namespace called “blue”. You’d use this command:

ip netns add blue

To verify that the network namespace has been created, use this command:

ip netns list

You should see your network namespace listed there, ready for you to use.
Assigning Interfaces to Network Namespaces
Creating the network namespace is only the beginning; the next part is to assign interfaces to the namespaces, and then configure those interfaces for network connectivity. One thing that threw me off early in my exploration of network namespaces was that you couldn’t assign physical interfaces to a namespace. How in the world were you supposed to use them, then?
It turns out you can only assign virtual Ethernet (veth) interfaces to a network namespace. Virtual Ethernet interfaces are an interesting construct; they always come in pairs, and they are connected like a tube―whatever comes in one veth interface will come out the other peer veth interface. As a result, you can use veth interfaces to connect a network namespace to the outside world via the “default” or “global” namespace where physical interfaces exist.
Let’s see how that’s done. First, you’d create the veth pair:

ip link add veth0 type veth peer name veth1

I found a few sites that repeated this command to create  veth1  and link it to  veth0 , but my tests showed that both interfaces were created and linked automatically using this command listed above. Naturally, you could substitute other names for  veth0  and  veth1 , if you wanted.
You can verify that the veth pair was created using this command:

ip link list

You should see a pair of veth interfaces (using the names you assigned in the command above) listed there. Right now, they both belong to the “default” or “global” namespace, along with the physical interfaces.
Let’s say that you want to connect the global namespace to the blue namespace. To do that, you’ll need to move one of the veth interfaces to the blue namespace using this command:

ip link set veth1 netns blue

If you then run the  ip link list  command again, you’ll see that the veth1 interface has disappeared from the list. It’s now in the blue namespace, so to see it you’d need to run this command:

ip netns exec blue ip link list

Whoa! That’s a bit of a complicated command. Let’s break it down:

The first part,  ip netns exec , is how you execute commands in a different network namespace.

Next is the specific namespace in which the command should be run (in this case, the blue namespace).

Finally, you have the actual command to be executed in the remote namespace. In this case, you want to see the interfaces in the blue namespace, so you run  ip link list .

When you run that command, you should see a loopback interface and the veth1 interface you moved over earlier.
Configuring Interfaces in Network Namespaces
Now that veth1 has been moved to the blue namespace, we need to actually configure that interface. Once again, we’ll use the  ip netns exec  command, this time to configure the veth1 interface in the blue namespace:

ip netns exec blue ifconfig veth1 10.1.1.1/24 up

As before, the format this command follows is:

ip netns exec <network namespace> <command to run against that namespace>

In this case, you’re using  ifconfig  to assign an IP address to the veth1 interface and bring that interface up. (Note: you could use the  ip addr ,  ip route , and  ip link  commands to accomplish the same thing.)
Once the veth1 interface is up, you can verify that the network configuration of the blue namespace is completely separate by just using a few different commands. For example, let’s assume that your “global” namespace has physical interfaces in the 172.16.1.0/24 range, and your veth1 interface is in a separate namespace and assigned something from the 10.1.1.0/24 range. You could verify how network namespaces keep the network configuration separate using these commands:

ip addr list  in the global namespace will not show any 10.1.1.0/24-related interfaces or addresses.

ip netns exec blue ip addr list  will show only the 10.1.1.0/24-related interfaces and addresses, and will not show any interfaces or addresses from the global namespace.

Similarly,  ip route list  in each namespace will show different routing table entries, including different default gateways.

Connecting Network Namespaces to the Physical Network
This part of it threw me for a while. I can’t really explain why, but it did. Once I’d figured it out, it was obvious. To connect a network namespace to the physical network, just use a bridge. In my case, I used an Open vSwitch (OVS) bridge, but a standard Linux bridge would work as well. Place one or more physical interfaces as well as one of the veth interfaces in the bridge, and―bam!―there you go. Naturally, if you had different namespaces, you’d probably want/need to connect them to different physical networks or different VLANs on the physical network.
So there you go―an introduction to Linux network namespaces. It’s quite likely I’ll build on this content later, so while it seems a bit obscure right now just hang on to this knowledge. In the meantime, if you have questions, clarifications, or other information worth sharing with other readers, please feel free to speak up in the comments.
Tags: CLI ・ Linux ・ Networking  Previous Post: Technology Short Take #35 Next Post: Vendor Meetings at VMworld 2013 
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