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SSH Diffie-Hellman

moduli of 2048 bits or greater​

Take a copy of /etc/ssh/moduli and /etc/ssh/sshd_config file first:

cp /etc/ssh/moduli /etc/ssh/moduli.backup
cp /etc/ssh/sshd_config /etc/ssh/sshd_config.backup

Delete lines from /etc/ssh/moduli where the 5th column is less than 2000:

awk '$5 > 2000' /etc/ssh/moduli > /usr/src/moduli
mv "/usr/src/moduli" /etc/ssh/moduli

Diffie-Hellman​

Add the following KexAlgorithms in /etc/ssh/sshd_config

KexAlgorithms ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1,diffie-hellman-group-exchange-sha256

Restart SSH service on the server​

systemctl restart sshd