Provided by: freebsd-manpages_8.2-1_all
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
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
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
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
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