"RepoMDError: Cannot access repository" when syncing Red Hat (native) channels through CDN in SUSE Manager

SUSE Manager 4.2
SUSE Manager 4.1

Following the documentation to set up Red Hat channels, populating them with packages coming from their CDN (Content Delivery Network), the sync process will fail. Logs in /var/log/rhn/reposync will show the following relevant lines:

File 'repomd.xml' from repository 'rhel-x86_64-server-7-os' is unsigned, continue? [yes/no] (no): no error]
10:38:23 RepoMDError: Cannot access repository. Repository 'rhel-x86_64-server-7-os' is invalid. ...
Valid metadata not found at specified URL

The usual problem here is that when setting up the repository (needed in order to feed the custom channel created afterwards), there is an option called "Has signed metadata?" which is checked by default. Even if this is stated in the documentation, as it is a long procedure, sometimes it can be accidentally overlooked. Most of the time, simply unchecking this option in the web-UI ("Software - Manage - Repositories - Click on affected Repositories - Uncheck 'has signed metadata?' - Update repository") will solve the issue. After changing the option, the sync can be tested again in the web-UI: "Software - Manage - Channels - Click on affected channel associated to the repository - Repositories tab - Sync subtab - Click on 'Sync now' ", and the error should be gone.

Other conflicts could be regarding networking: connections to the CDN URL (usually cdn.redhat.com) should be bypassed in any proxy or firewall on the network, and obviously the proxy (in case one needs to be used) should be properly set up in the SUSE Manager, in both Yast configuration ("yast proxy") and the SUSE Manager web-UI (admin menu, setup wizard, http proxy). Both configurations have an option to be tested.

The access to the repository is already set up through the certificates, so enabling the option "Has signed metadata" when setting up the repository will not work.
As for the other possible problems, a network connection is always needed. If there's a firewall blocking connections, or a proxy that has not been properly set up, it will not be possible to download anything from the CDN.

See related documentation:
https://documentation.suse.com/suma/4.2/en/suse-manager/client-configuration/clients-rh-cdn.html
https://documentation.suse.com/external-tree/en-us/suma/4.1/suse-manager/client-configuration/clients-rh-cdn.html

Please notice that this article is not relevant for customers using SUSE Linux Enterprise Server with Expanded Support.