r/useproxy: Configure system-wide proxy
The *useproxy* role configures the `http_proxy` et al. environmet variables for systemd services and interactive shells. Additionally, it configures Yum repositories to use a single mirror via the `baseurl` setting, rather than a list of mirrors via `metalink`, since the proxy a) the proxy only allows access to _dl.fedoraproject.org_ and b) the proxy caches RPM files, but this is only effective if all clients use the same mirror all the time. The `useproxy.yml` playbook applies this role to servers in the *needproxy* group.
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