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я использую модуль iptables под названием hashlimit))) это для защиты от брутфорса.
Также можно сделать защиту с помощью SSHD самого же:

Код:

MaxAuthTries,MaxSessions,  MaxStartups:
             Specifies the maximum number of concurrent unauthenticated con-
             nections to the SSH daemon.  Additional connections will be
             dropped until authentication succeeds or the LoginGraceTime
             expires for a connection.  The default is 10.

             Alternatively, random early drop can be enabled by specifying the
             three colon separated values ``start:rate:full'' (e.g.
             "10:30:60").  sshd(8) will refuse connection attempts with a
             probability of ``rate/100'' (30%) if there are currently
             ``start'' (10) unauthenticated connections.  The probability
             increases linearly and all connection attempts are refused if the
             number of unauthenticated connections reaches ``full'' (60).