#!/usr/bin/perl -w
use strict;
#white list
my @ALLOW_USERS = qw{
  test
  root
  lulu1
};
#the maximum number of ssh login
my $LOGIN_TIMES = 1;
sub main
{
  my @lines = `ps -eo user,pid,etime,cmd | grep sshd`;
  my $users;
  for my $line (@lines) {
    if(my ($user, $pid, $etime, $cmd) = $line =~ /^([^\s]+)\s+(\d+)\s+([^\s]+)\s+(sshd:.+)$/) {
      next if grep {$user eq $_} @ALLOW_USERS;
      my $proc = {'pid', $pid, 'etime', $etime, 'cmd', $cmd};
      push @{$users->{$user}}, $proc;
    }
  }
  for my $key(keys(%$users)) {
     my @sshs = sort {
      my ($lb, $la) = (length($b->{'etime'}), length($a->{'etime'}));
      if($lb == $la) {
        $b->{'etime'} cmp $a->{'etime'};
      } else {
        $lb <=> $la;
      }
    } @{$users->{$key}};
    $LOGIN_TIMES = 1 if $LOGIN_TIMES < 1;
    for (1 .. $LOGIN_TIMES) { pop @sshs; };
    for my $ssh (@sshs) {
      kill 9, $ssh->{'pid'};
    }
  }
}
while(1) {
  main;
  sleep 3;
}

echo "/root/limit.pl &" >> /etc/rc.d/rc.local （加入開機啟動）
/root/limit.pl & （運行腳本）