blackhole(4) a


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


The 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 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.


The TCP and UDP MIBs first appeared in Fx 4.0 .


An Geoffrey M. Rehmet