NAME

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

SYNOPSIS

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

DESCRIPTION

     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.

WARNING

     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.

SEE ALSO

     ip(4), tcp(4), udp(4), ipf(8), ipfw(8), pfctl(8), sysctl(8)

HISTORY

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

AUTHORS

     Geoffrey M. Rehmet