[chef] Error when downloading from cookbook site

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Good day to all!

I am very new to Chef server and Chef client and also to Ruby. Our setup has 1
Chef, 1 Chef client node and 1 Chef client (workstation where we execute
knife).

When I download from cookbook site
# knife cookbook site download revealcloud

I keep getting this error

ERROR: Network Error: Connection refused - Connection refused connecting to
cookbooks.opscode.com:80 for /api/v1/cookbooks/revealcloud, giving up
Check your knife configuration and network settings

Is this because of proxy? firewall?

Additional question:
Our project requires us to monitor the Chef server's performance. So I am
trying to use revealcloud but I cannot download it. Is there any other way to
monitor the Chef server's network traffic and performance?

I have been looking for a solution to this in the web but cannot find 
anything.
Thank you very much. 

Sincerely,
Ella