My build fails because it can't connect or download from an external URL

If your build process requires connecting to an external site, for instance, because a package needs to be downloaded as part of the build, it will fail. The reason is that the build daemon machines don't allow outgoing connections to the Internet; this is done for security reasons.

If the licensing of the external package allows redistribution, you can address this by doing two things:

 1. Package the external resource yourself, and either include it in the package itself or as a dependency.
 2. Change the build process to fetch the resource locally

If the licensing of the external package does not allow redistribution, you will need to delay the actual download/build process to be done after the user installs the package. One package which does something similar is Ubuntu's flashplugin-nonfree.