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troubleshooting.md: add ethtool -K flannel.1 tx-checksum-ip-generic off for NAT #1929

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troubleshooting.md: add ethtool -K flannel.1 tx-checksum-ip-generic off for NAT #1929

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3 changes: 2 additions & 1 deletion Documentation/kubernetes.md
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Expand Up @@ -32,7 +32,8 @@ Other options include [Kyverno](https://kyverno.io/policies/pod-security/) and [
# Annotations

* `flannel.alpha.coreos.com/public-ip`, `flannel.alpha.coreos.com/public-ipv6`: Define the used public IP of the node. If configured when Flannel starts it'll be used as the `public-ip` and `public-ipv6` flag.
* `flannel.alpha.coreos.com/public-ip-overwrite`, `flannel.alpha.coreos.com/public-ipv6-overwrite`: Allows to overwrite the public IP of a node. Useful if the public IP can not determined from the node, e.G. because it is behind a NAT. It can be automatically set to a nodes `ExternalIP` using the [flannel-node-annotator](https://github.com/alvaroaleman/flannel-node-annotator)
* `flannel.alpha.coreos.com/public-ip-overwrite`, `flannel.alpha.coreos.com/public-ipv6-overwrite`: Allows to overwrite the public IP of a node. Useful if the public IP can not determined from the node, e.G. because it is behind a NAT. It can be automatically set to a nodes `ExternalIP` using the [flannel-node-annotator](https://github.com/alvaroaleman/flannel-node-annotator).
See also the "NAT" section in [troubleshooting](./troubleshooting.md) if UDP checksums seem corrupted.

## Older versions of Kubernetes

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21 changes: 21 additions & 0 deletions Documentation/troubleshooting.md
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Expand Up @@ -39,6 +39,27 @@ Vagrant typically assigns two interfaces to all VMs. The first, for which all ho

This may lead to problems with flannel. By default, flannel selects the first interface on a host. This leads to all hosts thinking they have the same public IP address. To prevent this issue, pass the `--iface=eth1` flag to flannel so that the second interface is chosen.

## NAT
When the public IP is behind NAT, the UDP checksum fields of the VXLAN packets can be corrupted.
In that case, try running the following commands to avoid corrupted checksums:

```bash
/usr/sbin/ethtool -K flannel.1 tx-checksum-ip-generic off
```

To automate the command above via udev, create `/etc/udev/rules.d/90-flannel.rules` as follows:

```
SUBSYSTEM=="net", ACTION=="add|change|move", ENV{INTERFACE}=="flannel.1", RUN+="/usr/sbin/ethtool -K flannel.1 tx-checksum-ip-generic off"
```

<!--
ref:
- https://github.com/flannel-io/flannel/issues/1279
- https://github.com/kubernetes/kops/pull/9074
- https://github.com/karmab/kcli/commit/b1a8eff658d17cf4e28162f0fa2c8b2b10e5ad00
-->

## Permissions
Depending on the backend being used, flannel may need to run with super user permissions. Examples include creating VXLAN devices or programming routes. If you see errors similar to the following, confirm that the user running flannel has the right permissions (or try running with `sudo)`.
* `Error adding route...`
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