Sometimes you find out you need to register really ancient Red Hat Enterprise Linux systems into a new Satellite, but this poses a number of challenges, namely around support, and having the right tools in the system (subscription-manager, curl and others) available so the system can be successfully managed by Red Hat Satellite.

Supported Satellite client operating systems

Red Hat maintains a list of supported operating systems per version on Satellite here:

https://access.redhat.com/solutions/5607011 - What are the supported operating systems for hosts of Red Hat Satellite 6 ?

The reality is a bit more complex, as Red Hat only tests new Satellite versions with actively supported RHEL operating systems. For RHEL6, this means that the only tested combinations are those using RHEL 6.10 using the ELS (Extended Lifecycle Support) add-on.

For ancient RHEL 7.0 and RHEL 7.1 versions, it's even more complicated because curl lacks some cryptographic cyphers, which make connecting to Satellite impossible.

The workaround is to upgrade the relevant system packages to the latest version PRIOR to attempting to register the system into Satellite 6.x .

So here are some notes on how to accomplish that:

Registering RHEL 6 older than 6.10 are not supported with Satellite 6.10+. Registration does not work.

It is required to update the subscription-manager, rpm, and other components to those provided by RHEL 6.10 prior to onboarding the system into Satellite 6.12 .
This process might render the "subscription-manager" inoperable. It is recommended to perform a full backup of the system, and implement access to the RHEL 6.10 DVD prior to attempting this upgrade.
The RHEL 6.10 content can be made available via the RHEL ISO, or hosted in a web server or NFS server.
Red Hat Engineering does not certify a full list of packages that need to be upgraded. However, when testing the following list of packages seems to be sufficient to allow a successful registration:
yum upgrade -y "yum*" "subscription-manager*" "rpm" python-requests.noarch python-rhsm.x86_64 python-six.noarch python-urlgrabber.noarch python-urllib3.noarch openssl
More packages might be required depending on the installed packages in the managed system that needs to be migrated.

RHEL 7.0 and RHEL 7.1 cannot be registered with Satellite.

In a similar way to RHEL 6, RHEL 7.0 and RHEL 7.1 cannot connect to a new satellite because of openssl ciphers and because of subscription-manager components. Registration errors might include curl: (35) Peer reports incompatible or unsupported protocol version.
The problem can be solved by upgrading the following packages prior to registration. Packages should come from RHEL 7.9.
yum upgrade -y openssl curl "yum*" "subscription-manager*" "rpm" python-six python-rhsm
Again, this process might render the "subscription-manager" inoperable. It is recommended to perform a full backup of the system, and implement access to the RHEL 7.9 DVD prior to attempting this upgrade.
The RHEL 7.9 content can be made available via the RHEL ISO, or hosted in a web server or NFS server.

Happy Satellite-ing!