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BLACKHOLE(4) Device Drivers Manual BLACKHOLE(4)


a sysctl(8) MIB for manipulating behaviour in respect of refused TCP or UDP connection attempts


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


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 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), 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
January 1, 2007 Debian