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Debug Services

An issue that comes up rather frequently for new installations of Kubernetes is that a Service is not working properly. You’ve run your Deployment and created a Service, but you get no response when you try to access it. This document will hopefully help you to figure out what’s going wrong.

Conventions

Throughout this doc you will see various commands that you can run. Some commands need to be run within a Pod, others on a Kubernetes Node, and others can run anywhere you have kubectl and credentials for the cluster. To make it clear what is expected, this document will use the following conventions.

If the command “COMMAND” is expected to run in a Pod and produce “OUTPUT”:

u@pod$ COMMAND
OUTPUT

If the command “COMMAND” is expected to run on a Node and produce “OUTPUT”:

u@node$ COMMAND
OUTPUT

If the command is “kubectl ARGS”:

kubectl ARGS
OUTPUT

Running commands in a Pod

For many steps here you will want to see what a Pod running in the cluster sees. The simplest way to do this is to run an interactive alpine Pod:

kubectl run -it --rm --restart=Never alpine --image=alpine sh
/ #
Note: If you don’t see a command prompt, try pressing enter.

If you already have a running Pod that you prefer to use, you can run a command in it using:

kubectl exec <POD-NAME> -c <CONTAINER-NAME> -- <COMMAND>

Setup

For the purposes of this walk-through, let’s run some Pods. Since you’re probably debugging your own Service you can substitute your own details, or you can follow along and get a second data point.

kubectl run hostnames --image=k8s.gcr.io/serve_hostname \
                        --labels=app=hostnames \
                        --port=9376 \
                        --replicas=3
deployment.apps/hostnames created

kubectl commands will print the type and name of the resource created or mutated, which can then be used in subsequent commands.

Note:

This is the same as if you started the Deployment with the following YAML:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: hostnames
spec:
  selector:
    matchLabels:
      app: hostnames
  replicas: 3
  template:
    metadata:
      labels:
        app: hostnames
    spec:
      containers:
      - name: hostnames
        image: k8s.gcr.io/serve_hostname
        ports:
        - containerPort: 9376
          protocol: TCP

Confirm your Pods are running:

kubectl get pods -l app=hostnames
NAME                        READY     STATUS    RESTARTS   AGE
hostnames-632524106-bbpiw   1/1       Running   0          2m
hostnames-632524106-ly40y   1/1       Running   0          2m
hostnames-632524106-tlaok   1/1       Running   0          2m

Does the Service exist?

The astute reader will have noticed that we did not actually create a Service yet - that is intentional. This is a step that sometimes gets forgotten, and is the first thing to check.

So what would happen if I tried to access a non-existent Service? Assuming you have another Pod that consumes this Service by name you would get something like:

u@pod$ wget -O- hostnames
Resolving hostnames (hostnames)... failed: Name or service not known.
wget: unable to resolve host address 'hostnames'

So the first thing to check is whether that Service actually exists:

kubectl get svc hostnames
No resources found.
Error from server (NotFound): services "hostnames" not found

So we have a culprit, let’s create the Service. As before, this is for the walk-through - you can use your own Service’s details here.

kubectl expose deployment hostnames --port=80 --target-port=9376
service/hostnames exposed

And read it back, just to be sure:

kubectl get svc hostnames
NAME        TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)   AGE
hostnames   ClusterIP   10.0.1.175   <none>        80/TCP    5s

As before, this is the same as if you had started the Service with YAML:

apiVersion: v1
kind: Service
metadata:
  name: hostnames
spec:
  selector:
    app: hostnames
  ports:
  - name: default
    protocol: TCP
    port: 80
    targetPort: 9376

Now you can confirm that the Service exists.

Does the Service work by DNS?

From a Pod in the same Namespace:

u@pod$ nslookup hostnames
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local

Name:      hostnames
Address 1: 10.0.1.175 hostnames.default.svc.cluster.local

If this fails, perhaps your Pod and Service are in different Namespaces, try a namespace-qualified name:

u@pod$ nslookup hostnames.default
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local

Name:      hostnames.default
Address 1: 10.0.1.175 hostnames.default.svc.cluster.local

If this works, you’ll need to adjust your app to use a cross-namespace name, or run your app and Service in the same Namespace. If this still fails, try a fully-qualified name:

u@pod$ nslookup hostnames.default.svc.cluster.local
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local

Name:      hostnames.default.svc.cluster.local
Address 1: 10.0.1.175 hostnames.default.svc.cluster.local

Note the suffix here: “default.svc.cluster.local”. The “default” is the Namespace we’re operating in. The “svc” denotes that this is a Service. The “cluster.local” is your cluster domain, which COULD be different in your own cluster.

You can also try this from a Node in the cluster:

Note: 10.0.0.10 is my DNS Service, yours might be different.
u@node$ nslookup hostnames.default.svc.cluster.local 10.0.0.10
Server:         10.0.0.10
Address:        10.0.0.10#53

Name:   hostnames.default.svc.cluster.local
Address: 10.0.1.175

If you are able to do a fully-qualified name lookup but not a relative one, you need to check that your /etc/resolv.conf file is correct.

u@pod$ cat /etc/resolv.conf
nameserver 10.0.0.10
search default.svc.cluster.local svc.cluster.local cluster.local example.com
options ndots:5

The nameserver line must indicate your cluster’s DNS Service. This is passed into kubelet with the --cluster-dns flag.

The search line must include an appropriate suffix for you to find the Service name. In this case it is looking for Services in the local Namespace (default.svc.cluster.local), Services in all Namespaces (svc.cluster.local), and the cluster (cluster.local). Depending on your own install you might have additional records after that (up to 6 total). The cluster suffix is passed into kubelet with the --cluster-domain flag. We assume that is “cluster.local” in this document, but yours might be different, in which case you should change that in all of the commands above.

The options line must set ndots high enough that your DNS client library considers search paths at all. Kubernetes sets this to 5 by default, which is high enough to cover all of the DNS names it generates.

Does any Service exist in DNS?

If the above still fails - DNS lookups are not working for your Service - we can take a step back and see what else is not working. The Kubernetes master Service should always work:

u@pod$ nslookup kubernetes.default
Server:    10.0.0.10
Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local

Name:      kubernetes.default
Address 1: 10.0.0.1 kubernetes.default.svc.cluster.local

If this fails, you might need to go to the kube-proxy section of this doc, or even go back to the top of this document and start over, but instead of debugging your own Service, debug DNS.

Does the Service work by IP?

Assuming we can confirm that DNS works, the next thing to test is whether your Service works at all. From a node in your cluster, access the Service’s IP (from kubectl get above).

u@node$ curl 10.0.1.175:80
hostnames-0uton

u@node$ curl 10.0.1.175:80
hostnames-yp2kp

u@node$ curl 10.0.1.175:80
hostnames-bvc05

If your Service is working, you should get correct responses. If not, there are a number of things that could be going wrong. Read on.

Is the Service correct?

It might sound silly, but you should really double and triple check that your Service is correct and matches your Pod’s port. Read back your Service and verify it:

kubectl get service hostnames -o json
{
    "kind": "Service",
    "apiVersion": "v1",
    "metadata": {
        "name": "hostnames",
        "namespace": "default",
        "selfLink": "/api/v1/namespaces/default/services/hostnames",
        "uid": "428c8b6c-24bc-11e5-936d-42010af0a9bc",
        "resourceVersion": "347189",
        "creationTimestamp": "2015-07-07T15:24:29Z",
        "labels": {
            "app": "hostnames"
        }
    },
    "spec": {
        "ports": [
            {
                "name": "default",
                "protocol": "TCP",
                "port": 80,
                "targetPort": 9376,
                "nodePort": 0
            }
        ],
        "selector": {
            "app": "hostnames"
        },
        "clusterIP": "10.0.1.175",
        "type": "ClusterIP",
        "sessionAffinity": "None"
    },
    "status": {
        "loadBalancer": {}
    }
}

Does the Service have any Endpoints?

If you got this far, we assume that you have confirmed that your Service exists and is resolved by DNS. Now let’s check that the Pods you ran are actually being selected by the Service.

Earlier we saw that the Pods were running. We can re-check that:

kubectl get pods -l app=hostnames
NAME              READY     STATUS    RESTARTS   AGE
hostnames-0uton   1/1       Running   0          1h
hostnames-bvc05   1/1       Running   0          1h
hostnames-yp2kp   1/1       Running   0          1h

The “AGE” column says that these Pods are about an hour old, which implies that they are running fine and not crashing.

The -l app=hostnames argument is a label selector - just like our Service has. Inside the Kubernetes system is a control loop which evaluates the selector of every Service and saves the results into an Endpoints object.

kubectl get endpoints hostnames
NAME        ENDPOINTS
hostnames   10.244.0.5:9376,10.244.0.6:9376,10.244.0.7:9376

This confirms that the endpoints controller has found the correct Pods for your Service. If the hostnames row is blank, you should check that the spec.selector field of your Service actually selects for metadata.labels values on your Pods. A common mistake is to have a typo or other error, such as the Service selecting for run=hostnames, but the Deployment specifying app=hostnames.

Are the Pods working?

At this point, we know that your Service exists and has selected your Pods. Let’s check that the Pods are actually working - we can bypass the Service mechanism and go straight to the Pods.

Note: These commands use the Pod port (9376), rather than the Service port (80).
u@pod$ wget -qO- 10.244.0.5:9376
hostnames-0uton

pod $ wget -qO- 10.244.0.6:9376
hostnames-bvc05

u@pod$ wget -qO- 10.244.0.7:9376
hostnames-yp2kp

We expect each Pod in the Endpoints list to return its own hostname. If this is not what happens (or whatever the correct behavior is for your own Pods), you should investigate what’s happening there. You might find kubectl logs to be useful or kubectl exec directly to your Pods and check service from there.

Another thing to check is that your Pods are not crashing or being restarted. Frequent restarts could lead to intermittent connectivity issues.

kubectl get pods -l app=hostnames
NAME                        READY     STATUS    RESTARTS   AGE
hostnames-632524106-bbpiw   1/1       Running   0          2m
hostnames-632524106-ly40y   1/1       Running   0          2m
hostnames-632524106-tlaok   1/1       Running   0          2m

If the restart count is high, read more about how to debug pods.

Is the kube-proxy working?

If you get here, your Service is running, has Endpoints, and your Pods are actually serving. At this point, the whole Service proxy mechanism is suspect. Let’s confirm it, piece by piece.

Is kube-proxy running?

Confirm that kube-proxy is running on your Nodes. You should get something like the below:

u@node$ ps auxw | grep kube-proxy
root  4194  0.4  0.1 101864 17696 ?    Sl Jul04  25:43 /usr/local/bin/kube-proxy --master=https://kubernetes-master --kubeconfig=/var/lib/kube-proxy/kubeconfig --v=2

Next, confirm that it is not failing something obvious, like contacting the master. To do this, you’ll have to look at the logs. Accessing the logs depends on your Node OS. On some OSes it is a file, such as /var/log/kube-proxy.log, while other OSes use journalctl to access logs. You should see something like:

I1027 22:14:53.995134    5063 server.go:200] Running in resource-only container "/kube-proxy"
I1027 22:14:53.998163    5063 server.go:247] Using iptables Proxier.
I1027 22:14:53.999055    5063 server.go:255] Tearing down userspace rules. Errors here are acceptable.
I1027 22:14:54.038140    5063 proxier.go:352] Setting endpoints for "kube-system/kube-dns:dns-tcp" to [10.244.1.3:53]
I1027 22:14:54.038164    5063 proxier.go:352] Setting endpoints for "kube-system/kube-dns:dns" to [10.244.1.3:53]
I1027 22:14:54.038209    5063 proxier.go:352] Setting endpoints for "default/kubernetes:https" to [10.240.0.2:443]
I1027 22:14:54.038238    5063 proxier.go:429] Not syncing iptables until Services and Endpoints have been received from master
I1027 22:14:54.040048    5063 proxier.go:294] Adding new service "default/kubernetes:https" at 10.0.0.1:443/TCP
I1027 22:14:54.040154    5063 proxier.go:294] Adding new service "kube-system/kube-dns:dns" at 10.0.0.10:53/UDP
I1027 22:14:54.040223    5063 proxier.go:294] Adding new service "kube-system/kube-dns:dns-tcp" at 10.0.0.10:53/TCP

If you see error messages about not being able to contact the master, you should double-check your Node configuration and installation steps.

One of the possible reasons that kube-proxy cannot run correctly is that the required conntrack binary cannot be found. This may happen on some Linux systems, depending on how you are installing the cluster, for example, you are installing Kubernetes from scratch. If this is the case, you need to manually install the conntrack package (e.g. sudo apt install conntrack on Ubuntu) and then retry.

Is kube-proxy writing iptables rules?

One of the main responsibilities of kube-proxy is to write the iptables rules which implement Services. Let’s check that those rules are getting written.

The kube-proxy can run in “userspace” mode, “iptables” mode or “ipvs” mode. Hopefully you are using the “iptables” mode or “ipvs” mode. You should see one of the following cases.

Userspace

u@node$ iptables-save | grep hostnames
-A KUBE-PORTALS-CONTAINER -d 10.0.1.175/32 -p tcp -m comment --comment "default/hostnames:default" -m tcp --dport 80 -j REDIRECT --to-ports 48577
-A KUBE-PORTALS-HOST -d 10.0.1.175/32 -p tcp -m comment --comment "default/hostnames:default" -m tcp --dport 80 -j DNAT --to-destination 10.240.115.247:48577

There should be 2 rules for each port on your Service (just one in this example) - a “KUBE-PORTALS-CONTAINER” and a “KUBE-PORTALS-HOST”. If you do not see these, try restarting kube-proxy with the -v flag set to 4, and then look at the logs again.

Almost nobody should be using the “userspace” mode any more, so we won’t spend more time on it here.

Iptables

u@node$ iptables-save | grep hostnames
-A KUBE-SEP-57KPRZ3JQVENLNBR -s 10.244.3.6/32 -m comment --comment "default/hostnames:" -j MARK --set-xmark 0x00004000/0x00004000
-A KUBE-SEP-57KPRZ3JQVENLNBR -p tcp -m comment --comment "default/hostnames:" -m tcp -j DNAT --to-destination 10.244.3.6:9376
-A KUBE-SEP-WNBA2IHDGP2BOBGZ -s 10.244.1.7/32 -m comment --comment "default/hostnames:" -j MARK --set-xmark 0x00004000/0x00004000
-A KUBE-SEP-WNBA2IHDGP2BOBGZ -p tcp -m comment --comment "default/hostnames:" -m tcp -j DNAT --to-destination 10.244.1.7:9376
-A KUBE-SEP-X3P2623AGDH6CDF3 -s 10.244.2.3/32 -m comment --comment "default/hostnames:" -j MARK --set-xmark 0x00004000/0x00004000
-A KUBE-SEP-X3P2623AGDH6CDF3 -p tcp -m comment --comment "default/hostnames:" -m tcp -j DNAT --to-destination 10.244.2.3:9376
-A KUBE-SERVICES -d 10.0.1.175/32 -p tcp -m comment --comment "default/hostnames: cluster IP" -m tcp --dport 80 -j KUBE-SVC-NWV5X2332I4OT4T3
-A KUBE-SVC-NWV5X2332I4OT4T3 -m comment --comment "default/hostnames:" -m statistic --mode random --probability 0.33332999982 -j KUBE-SEP-WNBA2IHDGP2BOBGZ
-A KUBE-SVC-NWV5X2332I4OT4T3 -m comment --comment "default/hostnames:" -m statistic --mode random --probability 0.50000000000 -j KUBE-SEP-X3P2623AGDH6CDF3
-A KUBE-SVC-NWV5X2332I4OT4T3 -m comment --comment "default/hostnames:" -j KUBE-SEP-57KPRZ3JQVENLNBR

There should be 1 rule in KUBE-SERVICES, 1 or 2 rules per endpoint in KUBE-SVC-(hash) (depending on SessionAffinity), one KUBE-SEP-(hash) chain per endpoint, and a few rules in each KUBE-SEP-(hash) chain. The exact rules will vary based on your exact config (including node-ports and load-balancers).

IPVS

u@node$ ipvsadm -ln
Prot LocalAddress:Port Scheduler Flags
  -> RemoteAddress:Port           Forward Weight ActiveConn InActConn
...
TCP  10.0.1.175:80 rr
  -> 10.244.0.5:9376               Masq    1      0          0
  -> 10.244.0.6:9376               Masq    1      0          0
  -> 10.244.0.7:9376               Masq    1      0          0
...

IPVS proxy will create a virtual server for each service address(e.g. Cluster IP, External IP, NodePort IP, Load Balancer IP etc.) and some corresponding real servers for endpoints of the service, if any. In this example, service hostnames(10.0.1.175:80) has 3 endpoints(10.244.0.5:9376, 10.244.0.6:9376, 10.244.0.7:9376) and you’ll get results similar to above.

Is kube-proxy proxying?

Assuming you do see the above rules, try again to access your Service by IP:

u@node$ curl 10.0.1.175:80
hostnames-0uton

If this fails and you are using the userspace proxy, you can try accessing the proxy directly. If you are using the iptables proxy, skip this section.

Look back at the iptables-save output above, and extract the port number that kube-proxy is using for your Service. In the above examples it is “48577”. Now connect to that:

u@node$ curl localhost:48577
hostnames-yp2kp

If this still fails, look at the kube-proxy logs for specific lines like:

Setting endpoints for default/hostnames:default to [10.244.0.5:9376 10.244.0.6:9376 10.244.0.7:9376]

If you don’t see those, try restarting kube-proxy with the -v flag set to 4, and then look at the logs again.

A Pod cannot reach itself via Service IP

This can happen when the network is not properly configured for “hairpin” traffic, usually when kube-proxy is running in iptables mode and Pods are connected with bridge network. The Kubelet exposes a hairpin-mode flag that allows endpoints of a Service to loadbalance back to themselves if they try to access their own Service VIP. The hairpin-mode flag must either be set to hairpin-veth or promiscuous-bridge.

The common steps to trouble shoot this are as follows:

Seek help

If you get this far, something very strange is happening. Your Service is running, has Endpoints, and your Pods are actually serving. You have DNS working, iptables rules installed, and kube-proxy does not seem to be misbehaving. And yet your Service is not working. You should probably let us know, so we can help investigate!

Contact us on Slack or Forum or GitHub.

What's next

Visit troubleshooting document for more information.

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