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    Command:

    blackhole

    
         sysctl net.inet.tcp.blackhole[=[0 | 1 | 2]]
         sysctl net.inet.udp.blackhole[=[0 | 1]]
    
    
    

    DESCRIPTION

         The blackhole sysctl(8) MIB is used to control system behaviour when con-
         nection requests are received on TCP or UDP ports where there is no
         socket listening.
    
         Normal behaviour, when a TCP SYN segment is received on a port where
         there is no socket accepting connections, is for the system to return a
         RST segment, and drop the connection.  The connecting system will see
         this as a "Connection refused".  By setting the TCP blackhole MIB to a
         numeric value of one, the incoming SYN segment is merely dropped, and no
         RST is sent, making the system appear as a blackhole.  By setting the MIB
         value to two, any segment arriving on a closed port is dropped without
         returning a RST.  This provides some degree of protection against stealth
         port scans.
    
         In the UDP instance, enabling blackhole behaviour turns off the sending
         of an ICMP port unreachable message in response to a UDP datagram which
         arrives on a port where there is no socket listening.  It must be noted
         that this behaviour will prevent remote systems from running
         traceroute(8) to a system.
    
         The blackhole behaviour is useful to slow down anyone who is port scan-
         ning a system, attempting to detect vulnerable services on a system.  It
         could potentially also slow down someone who is attempting a denial of
         service attack.
    
    
    

    WARNING

         The TCP and UDP blackhole features should not be regarded as a replace-
         ment for firewall solutions.  Better security would consist of the
         blackhole sysctl(8) MIB used in conjuction with one of the available
         firewall packages.
    
         This mechanism is not a substitute for securing a system.  It should be
         used together with other security mechanisms.
    
    
    

    SEE ALSO

         ip(4), tcp(4), udp(4), ipf(8), ipfw(8), pfctl(8), sysctl(8)
    
    
    

    HISTORY

         The TCP and UDP blackhole MIBs first appeared in FreeBSD 4.0.
    
    
    

    AUTHORS

         Geoffrey M. Rehmet
    
    
    

    BSD January 1, 2007 BSD

    
    
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