Updates – Fix ‘sudo apt-get update’ Failure

updates

I am not able to sudo apt-get update successfully. I am using Ubuntu 16.04.1 LTS

Error message:

W: GPG error: http://security.ubuntu.com/ubuntu xenial-security InRelease: At least one invalid signature was encountered.

More error messages attached in the screenshot.

There are lots of "not signed" and "can't be authenticated" error messages.

How could I fix it?

You should use the --allow-unauthenticated option to skip it:

sudo apt-get --allow-unauthenticated update
sudo apt-get --allow-unauthenticated upgrade

Or if you have have the id of the missing key (from the output of apt-get), you can add it to the apt key manager to be authenticate :

 sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys put_the missing_key_here

This requires a manual edit, due to a bug in a settings file for the fwupd package:

sudo apt install expat
xmlwf /var/cache/app-info/xmls/fwupd.xml
sudo vim /var/cache/app-info/xmls/fwupd.xml

Go to the line number given by xmlwf In my case I got

/var/cache/app-info/xmls/fwupd.xml:268:72: not well-formed (invalid token)

Replace the & character (on the line 268 in my case) with &

Then you can run update again:

sudo apt update

The port you are trying to connect to is not open on the IP address that you are trying to connect to for one of the following reasons.

You are connecting to the wrong IP address. Use nmap on the server with hostname of localhost (nmap localhost) to check if the server is running properly in the port you want, and on the client with your server's public IP address to check if the destination port is open.
You did not open the port on your firewall(s) successfully.
Your system is configured to use a local HTTP proxy for apt connections, and no such proxy is running (included in this answer for the sake of completeness although you commented that no proxy is running). Many companies and higher education institutions have restrictions in place which includes a policy to control the types of websites you can access.