How do I resolve the error "Communication failed" from grbgetkey?

While running grbgetkey to retrieve a license file, you might receive the error:

Communication failed, check your network connection or set variable HTTPS_PROXY if you need a proxy

If your computer is connected to the internet but requires a proxy, you can specify a username and password in the environment variables HTTPS_PROXY and HTTP_PROXY by setting them to http://username:password@proxy-server:port.

For commercial users:

For the situations in which the computer is on a secure network without internet access or the network may have a firewall that prevents the computer from connecting to the license key server, Gurobi has a manual process:

1. Once Gurobi is installed on the target machine, run grbprobe to generate a report regarding your system.

   $ grbprobe 
   info  : grbprobe version 9.1.1, build v9.1.1rc0 
   HOSTNAME=my-MacBook-Pro-2018.local 
   HOSTID=980d6769 
   PLATFORM=mac64 
   USERNAME=butera 
   CORES=2 
   CPU=Intel(R) Core(TM) i5-7360U CPU @ 2.30GHz

2. Next, using any computer connected to the internet, log into your account at gurobi.com and navigate to "Downloads & Licenses > Your Gurobi Licenses". Select the correct license from the table of licenses. Scroll to the bottom of the "License Details" page to the link that says “Please click here for additional instructions”.
3. This will display additional instructions and a form where you can enter the information returned by grbprobe:
   
4. Once you press submit, you can download your license file and transfer it to the target machine.

Note: This process is only available for commercial customers; it is not available for free academic licenses or for free licenses for online courses.  

For Academic users:

Please check the following conditions for the computer where you want to install the free academic license:

• The computer can communicate with the Gurobi website (http://www.gurobi.com)
• The computer is not using a proxy server
• The grbgetkey program is not blocked by a personal firewall or an internet security program

If your computer is connected to the internet but requires a proxy, you can specify a username and password in the environment variables HTTPS_PROXY and HTTP_PROXY by setting them to http://username:password@proxy-server:port.

Further information 

• How do I put the license file in an alternative location? 
• What can I do to resolve an "ERROR 405" from grbgetkey?
• What can I do to resolve an "ERROR 303" from grbgetkey?