Can't install Kubernetes on Vagrant

In order to set /proc/sys/net/bridge/bridge-nf-call-iptables by editing /etc/sysctl.conf. There you can add [1]

net.bridge.bridge-nf-call-iptables = 1

Then execute

sudo sysctl -p

And the changes will be applied. With this the pre-flight check should pass.

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[1] http://wiki.libvirt.org/page/Net.bridge.bridge-nf-call_and_sysctl.conf

Update @2019/09/02

Sometimes modprobe br_netfilter is unreliable, you may need to redo it after relogin, so use the following instead when on a systemd sytem:

echo br_netfilter > /etc/modules-load.d/br_netfilter.conf
systemctl restart systemd-modules-load.service
echo 1 > /proc/sys/net/bridge/bridge-nf-call-iptables
echo 1 > /proc/sys/net/bridge/bridge-nf-call-ip6tables

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YES, the accepted answer is right, but I faced with

cannot stat /proc/sys/net/bridge/bridge-nf-call-ip6tables: No such file or directory

So I did

modprobe br_netfilter

echo 1 > /proc/sys/net/bridge/bridge-nf-call-iptables
echo 1 > /proc/sys/net/bridge/bridge-nf-call-ip6tables
sudo sysctl -p

Then solved.

On Ubuntu 16.04 I just had to:

modprobe br_netfilter

Default value in /proc/sys/net/bridge/bridge-nf-call-iptables is already 1.

Then I added br_netfilter to /etc/modules to load the module automatically on next boot.