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RHEL includes several cryptographic components who's security doesn't remain constant over time. Algorithms such as (cryptographic) hashing and encryption typically have a lifetime after which they are considered either too risky to use or plain insecure. That would mean we need to phase out such algorithms from the default settings, or completely disable if they could cause irreparable issue. 

This bug is about disabling any TLS connections which utilize less than 1024-bit Diffie-Hellman parameters, while at the same hand providing a configuration method for these parameters to be allowed when needed.

For future extensibility in RHEL-7 it is recommended for any introduced configuration method to be re-usable for future algorithm or parameter deprecation (e.g., SHA1 or less than 1024-bit RSA/DH parameters), and ideally part of upstream.

The system administrator can reenable shorter DH support with:

echo 'MinimumDHBits 768' >> /etc/pki/tls/legacy-settings

Any value between 512 and 10000 is supported so this can be used to even raise the minimum to 2048 if needed by the system administrator.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2017-0660.html