Provided by: freebsd-manpages_8.2-1_all bug


     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
     connection 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
     scanning 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
     replacement 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