# Troubleshooting Network Issues

### DNS <a href="#dns" id="dns"></a>

Kubernetes has a guide on [Debugging DNS Resolution](https://kubernetes.io/docs/tasks/administer-cluster/dns-debugging-resolution/) which is quite extensive and covers most of the general troubleshooting with relation to resolving domain names from within the cluster. In particular, though, the following sections would be the most pertinent.

* [Check Local DNS Resolution](https://kubernetes.io/docs/tasks/administer-cluster/dns-debugging-resolution/#check-the-local-dns-configuration-first)
* [Ensure CoreDNS is Running](https://kubernetes.io/docs/tasks/administer-cluster/dns-debugging-resolution/#check-if-the-dns-pod-is-running)
* [Enable Query Logging](https://kubernetes.io/docs/tasks/administer-cluster/dns-debugging-resolution/#are-dns-queries-being-received-processed)

Additionally, see [Known Issues](https://kubernetes.io/docs/tasks/administer-cluster/dns-debugging-resolution/#known-issues) for any other documented edge case scenarios.

### Calico <a href="#calico" id="calico"></a>

#### Pod Networking Broken if Kernel IP Forwarding Not Enabled <a href="#pod-networking-broken-if-kernel-ip-forwarding-not-enabled" id="pod-networking-broken-if-kernel-ip-forwarding-not-enabled"></a>

If `net.ipv4.ip_forward` is set to `0`, meaning that kernel IP forwarding is disabled, pod networking will be broken. IP forwarding needs to be enabled (set to `1`) to allow traffic to be forwarded from one interface to the other.

Pods may enter into a CrashLoopBackOff state as a result of being unable to reach the cluster VIP or otherwise and constantly failing their health checks.

The enabled value should be set by default on most installations. Otherwise, though, ensure it is not manually defined, or, if it cannot be undefined, set to enabled explicitly.

{% tabs %}
{% tab title="/etc/sysctl.d/90-ip-forward.conf" %}

```javascript
net.ipv4.ip_forward = 1
```

{% endtab %}
{% endtabs %}


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