apt-get update error - W: Failed to fetch mirrors / Some index files failed to download. They have been ignored, or old ones used instead

Hello there,

When I log in to my droplet and execute the following command:

apt-get update

I get the following:

Err http://repo.mysql.com trusty InRelease

Err http://ppa.launchpad.net trusty InRelease

Err http://repo.mysql.com trusty Release.gpg
  Could not resolve 'repo.mysql.com'
Err http://ppa.launchpad.net trusty Release.gpg
  Could not resolve 'ppa.launchpad.net'
Err http://security.ubuntu.com trusty-security InRelease

Err http://security.ubuntu.com trusty-security Release.gpg
  Could not resolve 'security.ubuntu.com'
Err http://mirrors.digitalocean.com trusty InRelease

Err http://mirrors.digitalocean.com trusty-updates InRelease

Err http://mirrors.digitalocean.com trusty Release.gpg
  Could not resolve 'mirrors.digitalocean.com'
Err http://mirrors.digitalocean.com trusty-updates Release.gpg
  Could not resolve 'mirrors.digitalocean.com'
Reading package lists... Done
W: Failed to fetch http://mirrors.digitalocean.com/ubuntu/dists/trusty/InRelease

W: Failed to fetch http://mirrors.digitalocean.com/ubuntu/dists/trusty-updates/InRelease

W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/trusty-security/InRelease

W: Failed to fetch http://repo.mysql.com/apt/ubuntu/dists/trusty/InRelease

W: Failed to fetch http://ppa.launchpad.net/ondrej/php5-5.6/ubuntu/dists/trusty/InRelease

W: Failed to fetch http://repo.mysql.com/apt/ubuntu/dists/trusty/Release.gpg  Could not resolve 'repo.mysql.com'

W: Failed to fetch http://ppa.launchpad.net/ondrej/php5-5.6/ubuntu/dists/trusty/Release.gpg  Could not resolve 'ppa.launchpad.net'

W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/trusty-security/Release.gpg  Could not resolve 'security.ubuntu.com'

W: Failed to fetch http://mirrors.digitalocean.com/ubuntu/dists/trusty/Release.gpg  Could not resolve 'mirrors.digitalocean.com'

W: Failed to fetch http://mirrors.digitalocean.com/ubuntu/dists/trusty-updates/Release.gpg  Could not resolve 'mirrors.digitalocean.com'

W: Some index files failed to download. They have been ignored, or old ones used instead.

How can I fix this?

I don’t know if it is related but I will mention it anyway. Yesterday I did a few changes on my machine. I followed the tutorials from here:

https://www.digitalocean.com/community/tutorials/how-to-set-up-a-firewall-using-iptables-on-ubuntu-14-04

and

https://www.digitalocean.com/community/tutorials/how-to-implement-a-basic-firewall-template-with-iptables-on-ubuntu-14-04

Please help!