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Disable SSH connections over IPv6

If a network interface offers IPv4 as well as IPv6, by default Ubuntu’s SSH server listens on both IP versions. In order to restrict access to protocol version 4 you have to comment in the line
#ListenAddress 0.0.0.0 in your /etc/ssh/sshd_config.

#ListenAddress ::
ListenAddress 0.0.0.0

Restart SSH service for the changes to take effect.

sudo service sshd restart

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