Das Blinken Lichten

I called my last post ‘basic’ external access into the cluster because I didn’t get a chance to talk about the ingress object.  Ingress resources are interesting in that they allow you to use one object to load balance to different back-end objects.  This could be handy for several reasons and allows you a more fine-grained means to load balance traffic.  Let’s take a look at an example of using the Nginx ingress controller in our Kubernetes cluster.

To demonstrate this we’re going to continue using the same lab that we used in previous posts but for the sake of level setting we’re going to start by clearing the slate.  Let’s delete all of the objects in the cluster and then we’ll start by build them from scratch so you can see every step of the way how we setup and use the ingress.

Since this will kill our net-test pod, let’s start that again…

Recall that we used this pod as a testing endpoint so we could simulate traffic originating from a pod so it’s worth keeping around.

Alright – now that we have an empty cluster the first thing we need to do is build some things we want to load balance to. In this case, we’ll define two deployments. Save this definition as a file called back-end-deployment.yaml on your Kubernetes master…

Notice how we defined two deployments in the same file and separated the definitions with a ---. Next we want to define a service that can be used to reach each of these deployments. Save this service definition as back-end-service.yaml…

Notice how the service selectors are looking the specific labels that match each pod. In this case, we’re looking for app and version with the version differing between each deployment. We can now deploy these definitions and ensure everything is created as expected…

I created a new folder called ingress to store all these definitions in.

These deployments and services represent the pods and services that we’ll be doing the actual load balancing to. They appear to have deployed successfully so let’s move onto the next step.

Next we’ll start building the actual ingress. To start with we need to define what’s referred to as a default back-end. Default back-ends serve as the default endpoint that the ingress will send traffic to in the event it doesn’t match any other rules. In our case the default back-end will consist of a deployment and a service that matches the deployed pods to make them easily reachable. First define the default back-end deployment. Save this as a file called default-back-end-deployment.yaml…

Next lets define the service that will match the default back-end. save this file as default-back-end-service.yaml…

Now let’s deploy both the definition for the default back-end deployment as well as the service…

Great! This looks just like we’d expect but let’s do some extra validation from our net-test pod to make sure the pods and services are working as expected before we get too far into the ingress configuration…

If you aren’t comfortable with services see my post on them here.

As expected pods can resolve the services by DNS name and we can successfully reach each service.   In the case of the default back-end we get a 404. Since all of the back end pods are reachable we can move on to defining the ingress itself. The Nginx ingress controller comes in the form of a deployment. The deployment definition looks like this…

Go ahead and save this file as nginx-ingress-controller-deployment.yaml on your server. However – before we can deploy this definition we need to deploy a config-map. Config-maps are a Kubernetes construct that are used to handle non-private configuration information. Since the Nginx ingress controller above expects a config-map, we need to deploy that before we can deploy the ingress controller…

In this case, we’re using a config-map to pass service level parameters to the pod. In this case, we’re passing the enable-vts-status: 'true' parameter which is required for us to see the VTS page of the Nginx load balancer. Save this as nginx-ingress-controller-config-map.yaml on your server and then deploy both the config-map and the Nginx ingress controller deployment…

Alright – so we’re still looking good here. The pod generated from the deployment is running. If you want to perform another quick smoke test at this point you can try connecting to the Nginx controller pod directly from the net-test pod. Doing so should result in landing at the default back-end since we have not yet told the ingress what it should do…

Excellent!  Next we need to define the ingress policy or ingress object. Doing so is just like defining any other object in Kubernetes, we use a YAML definition like the one below…

The rules defined in the ingress will be read by the Nginx ingress controller and turned into Nginx load balancing rules. Pretty slick right? In this case, we define 3 rules. Let’s walk through the rules one at a time from top to bottom.

1. Is looking for http traffic headed to the host website8080.com.  If it receives traffic matching this host it will load balance it to the pods that match the selector for the service backend-svc-1
2. Is looking for http traffic headed to the host website9090.com.  If it receives traffic matching this host it will load balance it to the pods that match the selector for the service backend-svc-2
3. Is looking for traffic destined to the host website.com on multiple different paths…
   1. Is looking for http traffic matching a path of /eightyeighty.  If it receives traffic matching this path it will load balance it to the pods that match the selector for the service back-end-svc-1
   2. Is looking for http traffic matching a path of /ninetyninety.  If it receives traffic matching this path it will load balance it to the pods that match the selector for the service back-end-svc-2
   3. Is looking for http traffic matching a path of /nginx_status.  If it receives traffic matching this path it will load balance it to the pods that match the selector for the service nginx-ingress (not yet defined)

Those rules are pretty straight forward and things we’re used to dealing with on traditional load balancing platforms. Let’s go ahead and save this definition as nginx-ingress.yaml and deploy it to the cluster…

We can see that the ingress has been created successfully. If we would have been watching the logs of the Nginx ingress controller pod as we deployed the ingress we would have seen these log entries shortly after defining the ingress resource in the cluster…

The ingress controller is constantly watching the API server for ingress configuration. Directly after defining the ingress policy, the controller started building the configuration in the Nginx load balancer. Now that it’s defined, we should be able to do some preliminary testing within the cluster. Once again from our net-test pod we can run the following tests…

We can run tests from within the cluster by connecting directly to the pod IP address of the Nginx ingress controller which in this cases is 10.100.3.34. You’ll notice the first test fails and we end up at the default back-end. This is because we didnt pass a host header. In the second example we pass the website8080.com host header and get the correct response. In the third example we pass the website9090.com host header and also receive the response we’re expecting. In the fourth example we attempt to connect to website.com and receive once again the default back-end response of 404. If we then try the appropriate paths we’ll see we once again start getting the correct responses.

The last piece that’s missing is external access. In our case, we need to expose the ingress to the upstream network. Since we’re not running in a cloud environment, the best option for that would be with a nodePort type service like this…

I used a nodePort service here but you certainly could have also used the externalIP construct as well.  That would allow you to access the URLs on their normal port.  

Notice that this is looking for pods that match the selector nginx-ingress-lb and provides service functionality for two different ports. The first will be port 80 which we’re asking it provide on the host’s interface on port (nodePort) 30000. This will service the actual inbound requests to the websites. The second port is 18080 and we’re asking it to provide that on nodePort 32767. This will let us view the Nginx VTS monitoring page of the load balancer. Let’s save this definition as nginx-ingress-controller-service.yaml and deploy it to our cluster….

Now we should be able to reach all of our URLs from outside of the cluster either by passing the host header manually as we did above, or by creating DNS records to resolve the names to a Kubernetes node. If you want to access the Nginx VTS monitoring page through a web browser you’ll need to go the DNS route. I created local DNS zones for each domain to test and was then successful in reaching the website from my workstation’s browser…

If you added the DNS records you should also be able to reach the VTS monitoring page of the Nginx ingress controller as well…

When I first saw this, I was immediately surprised by something.  Does anything look strange to you?  I was surprised to see that the upstream pools listed the actual pod IP addresses.  Recall when we defined our ingress policy we listed the destinations as the Kubernetes services. My initial assumption was that the Nginx ingress controller would then simply be resolving the service name to an IP address and using the single service IP as it’s pool. That is – the ingress controller was just load balancing to a normal Kubernetes service.  Turns out that’s not the case. The ingress controller relies on the services to keep track of the pods but doesn’t actually use the service construct to get traffic to the pods. Since Kubernetes is keeping track of which pods are in a given service the ingress controller can just query the API server to get a list of pods that are currently alive and match the selector for the service. In this manner, traffic is load balanced directly to a pod rather than through a service construct. If we mapped out a request to one of our test websites through the Nginx ingress controller it would look something like this…

If you arent comfortable with how nodePort services work check out my last post.

In this case, I’ve pointed the DNS zones for website.com, website8080.com, and website9090.com to the host ubuntu-2. The diagram above shows a client session headed to website9090.com. Note that the client still believes that it’s TCP session (orange line) is with the host ubuntu-2 (10.20.30.72). The nodePort service is doing it’s job and sending the traffic over to the Nginx ingress controller. In doing so, the host hides the traffic behind it’s own source IP address to ensure the traffic returns successfully (blue line). This is entirely nodePort service functionality. What’s new is that when the Nginx pod talks to the back end pool, in this case 10.100.2.28, it does so directly pod to pod (green line).

As you can see – the ingress is allowing us to handle traffic to multiple different back ends now.  The ingress policy can be changed by editing the object using kubectl edit ingress nginx-ingress. So for instance, let’s say that we wanted to move the website8080.com to point to the pods that are selected by backend-svc-2 rather than backend-svc-1. Simply edit the ingress to look like this…

Then save the configuration and try to reconnect to website8080.com once again…

The Nginx ingress controller watches for changes in configuration on the API server and then implements those changes.  If we would have been watching the logs on the Nginx ingress controller container we would have seen something like this in the log entries after we made our change…

The ingress controller is also watching for changes to the service. For instance, if we now scaled our deploy-test-2 deployment we could see the Nginx pool size increase to account for the new pods. Here’s what VTS looks like before the change…

Then we can scale the deployment up with this command…

And after a couple of seconds VTS will show the newly deployed pods as part of the pool…

We can also modify properties of the controller itself by modifying the configMap that the controller is reading for it’s configuration. One of the more interesting options we can enable on the Nginx ingress controller is sticky sessions. Since the ingress is load balancing directly to the pods rather than to a service it’s possible for it to maintain sessions between the back-end pool members. We can enable this by editing the config map. kubectl edit configmap nginx-ingress-controller-conf and then add the highlighted line to the configMap…

Once again, the Nginx ingress controller will detect the change and reload it’s configuration. Now if we access website8080.com repeatedly from the same host we should see the load is sent to the same pod.  In this case the pod 10.100.0.25…

The point of this post was just to show you basics of how the ingress worked from a network perspective.  There are many other uses cases for them and many other ingress controllers to choose from besides Nginx. Take a look at the official documentation for them as well as these other posts that I found helpful during my research…

Jay Gorrell – Kubernetes Ingress

Daemonza – Kubernetes nginx-ingress-controller

In our last post we talked about an important Kubernetes networking construct – the service.  Services provide a means for pods running within the cluster to find other pods and also provide rudimentary load balancing capabilities.  We saw that services can create DNS entries within Kube-DNS which makes the service accessible by name as well.  So now that we know how you can use services to access pods within the cluster it seems prudent to talk about how things outside of the cluster can access these same services.  It might make sense to use the same service construct to provide this functionality, but recall that the services are assigned IP addresses that are only known to the cluster.  In reality, the service CIDR isnt actually routed anywhere but the Kubernetes nodes know how to interact with service IPs because of the netfilter rules programmed by the kube-proxy.  The service network just needs to be unique so that the containers running in the pod will follow their default route out to the host where the netfilter rules will come into play.  So really the service network is sort of non-existent from a routing perspective as it’s only locally significant to each host.  This means that it can’t really be used by external clients since they wont know how to route to it either.  That being said, we have a few other options we can use most of which still rely on the service construct.  Let’s look at them one at a time…

ExternalIP

In this mode – you are essentially just assigning a service an external IP address.  This IP address can be anything you want it to be but the catch is that you’re on the hook to make sure that the external network knows to send that traffic to the Kubernetes cluster nodes.  In other words – you have to ensure that traffic destined to the assigned external IP makes it to a Kubernetes node.  From there, the service construct will take care of getting it where it needs to be.  To demonstrate this, let’s take a look at our lab where we left it after our last post…

We had two different pod deployments running in the cluster in addition to the ‘net-test’ deployment but we wont need that for this example.  We had also defined a service called ‘svc-test-1’ that is currently targeting the pods of the ‘deploy-test-2’ deployment matching the selectors app=web-front-end and version=v2.  As we did when we changed the service selector, let’s once again edit the service and add another parameter.  To edit the service use this command…

In the editor, add the ‘externalIPs:’ list parameter followed by the IP address of 169.10.10.1 as shown in the highlighted field below…

When done, save the service definition by closing the file as you typically would (ESC, :wq, ENTER). You should get confirmation that the service was edited when you return to the console. What we just did was told Kubernetes to answer on the IP address of 169.10.10.1 for the service ‘svc-test-1’. To test this out, let’s point a route on our gateway for 169.10.10.0/24 at the host ubuntu-2. Note that ubuntu-2 is the only host that is not currently running a pod that will match the service selector…

This makes ubuntu-2 a strange choice to point the route at but highlights how the external traffic gets handled within the cluster.  With our route in place, let’s try to access the 169.10.10.1 IP address from a remote host…

Awesome, so it works!  Let’s now dig into how it works.  We can assume that since services use netfilter rules, and that the IP was assigned as part of the service, that the externalIP configuration likely also uses it.  So let’s start there.  For the sake of easily pointing out how the exteralIPs were implemented, I took a ‘iptables-save’ before and after the modification of the service.  Afterwards, I diffed the files and these three lines were added to the iptables-save output after the externalIPs were implemented in the service…

So what do these rules do? The first rule is looking for traffic that is TCP and destined to the IP address of 169.10.10.1 on port 80. This has a target of jump and points to a chain called ‘KUBE-MARK-MASQ’. This chain has the following rules…

This rule matches all traffic and has a target of ‘MARK’ which is a non-terminating target. The traffic in this case will be marked with a value of ‘0x4000/0x400’. So what do I mean by marked? In this case ‘–set-xmarl’ is setting a marking on the packet that is internal to the host. That is – this marking is only locally significant. Since the MARK target is non-terminating we jump back to the KUBE-SERVICES chain after the marking has occurred. The next line is looking for traffic that is…

• Destined to 169.10.10.1
• Has a protocol of TCP
• Has a destination port of 80
• Has not entered through a bridge interface (! –physdev-is-in)
• Source of the traffic is not a local interface (! –src-type LOCAL)

The last two rules ensure that this is not traffic that is originated from a POD or the host itself destined to a service.

If the last two rules are foreign to you I suggest you take a look at the MAN page for the IPTables extensions.  It’s definitely worth bookmarking.  

Since the second rule is a match for our external traffic we follow that JUMP target into the KUBE-SVC-SWP62QIEGFZNLQE7 chain.  At that point – the load balancing works just like an internal service.  It’s worth pointing out that the masquerade rule is crucial to all of this working.  Let’s look at an example of what this might look like if we didn’t have the masquerade rule…

Let’s walk through what will happen without the masquerade rule shown above…

• An external user, in this case 192.168.127.50, makes a request to the external IP of 169.10.10.1.
• The request reaches the gateway, does a route lookup, and sees that there’s a route for 169.10.10.0/24 pointing at ubuntu-2 (10.20.30.72)
• The request reaches the host ubuntu-2 and hits the above mentioned IPTables rules.  Without the masquerade rule, the only rule that gets hit is the one for passing the traffic to the service chain KUBE-SVC-SWP62QIEGFZNLQE7.  Normal service processing occurs as explained in our last post and a pod out of the service gets selected, in this case the pod 10.100.3.8 on host ubuntu-5.
• Traffic is destination NAT’d to 10.100.3.8 and makes it way to the host ubuntu-5.
• The pod receives the traffic and attempts to reply to the TCP connection based on the source IP address of the request.  The source in this case is unchanged and the host ubuntu-5 attempts to reply directly to the user at 192.168.127.50.
• The user making the request receives the reply from 10.100.3.8 and drops the packet since it hasn’t initiated a session to that IP address.

So as you can see – this just wont work.  This is why we need the masquerade rule.  When we use the rule, the processing looks like this…

This looks much better.  In this example the flows specify the correct source and destination since the host ubuntu-2 is now hiding it’s request to the pod behind it’s own interface.  This ensures that the reply from the pod 10.100.3.8 will come back to the hose ubuntu-2.  This is an important step because this is the host which performed the initial service DNAT.   If the request does not come back to this host, the DNAT to the pod can not be reversed.  Reversing the DNAT in this case means changing the source of the packet back to the original pre-DNAT source of 169.10.10.1.   So as you can see – the masquerade rule is quite important to ensuring that the externalIP construct works.

NodePort

If you’re not interested in dealing with routing a new subnet to the hosts your other option would be what’s referred to as nodeport.  Nodeport works a lot like the original Docker bridge mode for publishing ports.  It makes a service available externally on the nodes through a different port.  That port is by default in the range of 30000-32767 but can be modified by changing the ‘–service-node-port-range’ configuration flag on the API server.  To change out service to nodeport we simply delete the externalips definition we inserted during the previous example and change the service type to nodeport as shown below…

After we save the change, we can view the services again with kubectl…

Notice that our port column for the service now lists more than one port. The first port is that of the internal service. The second port (30487) is the nodeport, or the port that we can use externally to reach the service. One point about nodeport that I’d like to mention is that it’s an overlay on top of a typical clusterip service.  In the externalip example above, notice that we didnt change the type of the service, we just added the externalips to the spec.  In the case of nodeport, you need to change the service type.  If you’re using a service within the cluster you might be concerned that making this change would remove the clusterip configuration and prevent pods from accessing the service.  That is not the case.  Nodeport works ontop of the clusterip service configuration so you will always have a clusterip when you configure nodeport.

At this point, we should be able to reach the service by connecting to any Kubernetes node on that given port…

Let’s now do a similar stare and compare with the iptables rules on each host.  Once again, I’ll compare the rules in place after the configuration to a copy of the rules I had before we started our work.  These are the only lines that were added to get the nodeport functionality working…

These lines are pretty straight forward and perform similar tasks to what we saw above with the externalip functionality. The first line is looking for traffic destined to port 30487 which it will then pass to the KUBE-MARK-MASQ chain so that the traffic will be masqueraded. We have to do this for the same reason we explained above. The second line is also looking for traffic destined to port 30487 and when matched will pass the traffic to the specific chain for the service to handle the load balancing. But how do we get to this chain? If we look at the KUBE-SERVICES chain we will see this entry at the bottom of the chain…

This rule has always been present in the ruleset the chain it references (KUBE-NODEPORTS) just never existed.

Nodeport offers a couple of advantages over externalip. Namely, we can have more than one load balancing target. With externalip, all of the traffic is headed to the same IP address. While you could certainly route that IP to more than one host, letting the network load balance for you, you’d need to worry about how to update the routing when the host failed. With nodeport, it’s reasonable to think about using an external load balancer that referenced a back-end pool of all of the Kubernetes nodes or minions…

The pool could reference a specific port for each service which would be front-ended on the load balancer by a single VIP.  In this model, if a node went away the load balancer could have the intelligence (in the form of a health check) to automatically pull that node from the pool.  Keep in mind that the destination the load balancer is sending traffic to will not necessarily host the pod that is answering the client request.  However – that’s just the nature of Kubernetes services so that’s pretty much table stakes at this point.

Lastly – if it’s more convenient, you can also specify manually the nodeport you wish to use.  In this instance, I edited the spec to specify a nodeport of 30000…