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

     blackhole -- a sysctl(8) MIB for manipulating behaviour in respect of
     refused 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 ipfw(8) as a tool for firewalling a system.  In order to create
     a highly secure system, ipfw(8) should be used for protection, not the
     blackhole feature.

     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), ipfw(8), sysctl(8)

     Geoffrey M. Rehmet

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

BSD                             August 17, 1999                            BSD