If you have installed docker using the vagrant method. That is, if you had cloned docker from github, you would have along with the some cool docker code also got a Vagrantfile.
This Vagrantfile could be used with vagrant to spin a docker host. Once you had done that using…
# vagrant up
That would have spun up a virtualbox vm with all the need components of Docker installed. Great! Or is it?
From within the vm i ran a docker container
# docker run -i -t ubuntu /bin/bash
And when tried to ping it just wouldn’t. Well then I figured out that packet forwarding was disabled in the host vm, so went ahead and enabled it by setting the net.ipv4.ip_forward flag in the /etc/sysctl.conf file on virtualbox vm.
# Uncomment the next line to enable packet forwarding for IPv4
Then the rebooted the vm. Started a docker container and tried to ping and it work flawlessly.