Command Section
BLACKHOLE(4)           FreeBSD Kernel Interfaces Manual           BLACKHOLE(4)

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

     sysctl net.inet.sctp.blackhole[={0 | 1 | 2}]
     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 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.

     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.

     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.

     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 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.

     ip(4), sctp(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.

     The SCTP blackhole MIB first appeared in FreeBSD 9.1.

     Geoffrey M. Rehmet

FreeBSD 11.1-RELEASE-p4        September 6, 2015       FreeBSD 11.1-RELEASE-p4
Command Section