NAME

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

SYNOPSIS

       sysctl net.inet.sctp.blackhole[={0 | 1 | 2}]
       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
       SCTP, TCP, or UDP ports where there is no socket listening.

       The blackhole behaviour is useful to slow down an attacker who is port-scanning a system in an attempt to
       detect vulnerable services.  It might also slow down an attempted denial of service attack.

   SCTP
       Setting the SCTP blackhole MIB to a numeric value of one will prevent sending an ABORT packet in response
       to an incoming INIT.  A MIB value of two will do the same, but will also prevent sending an ABORT  packet
       when unexpected packets are received.

   TCP
       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.

   UDP
       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.

WARNING

       The SCTP, 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), sctp(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.

       The SCTP blackhole MIB first appeared in FreeBSD 9.1.

AUTHORS

       Geoffrey M. Rehmet

Debian                                          September 6, 2015                                   BLACKHOLE(4)