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BLACKHOLE(4)		 BSD Kernel Interfaces Manual		  BLACKHOLE(4)

     blackhole -- a sysctl(8) MIB for manipulating behaviour in	respect	of re-
     fused TCP or UDP connection attempts

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

     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.

     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.

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

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

     Geoffrey M. Rehmet

BSD				January	1, 2007				   BSD


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