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BLACKHOLE(4)	       FreeBSD Kernel Interfaces Manual		  BLACKHOLE(4)

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

     sysctl net.inet.sctp.blackhole[={0	| 1 | 2}]
     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 SCTP, TCP, or UDP	ports where there is
     no	socket listening.

     The blackhole behaviour is	useful to slow down an attacker	who is port-
     scanning a	system in an attempt to	detect vulnerable services.  It	might
     also slow down an attempted denial	of service attack.

     Setting the SCTP blackhole	MIB to a numeric value of one will prevent
     sending an	ABORT packet in	response to an incoming	INIT.  A MIB value of
     two will do the same, but will also prevent sending an ABORT packet when
     unexpected	packets	are received.

     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.

     Enabling blackhole	behaviour turns	off the	sending	of an ICMP port	un-
     reachable 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 SCTP, TCP, and	UDP blackhole features should not be regarded as a re-
     placement for firewall solutions.	Better security	would consist of the
     blackhole sysctl(8) MIB used in conjunction 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), sctp(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.

     The SCTP blackhole	MIB first appeared in FreeBSD 9.1.

     Geoffrey M. Rehmet

FreeBSD	13.0		       September 6, 2015		  FreeBSD 13.0


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