DragonFly On-Line Manual Pages

BLACKHOLE(4)          DragonFly Kernel Interfaces Manual          BLACKHOLE(4)

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 con-
     nection 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 reset by peer".  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 scan-
     ning 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 replace-
     ment for ipfw(8) as a tool for firewalling a system.  In order to create
     a highly secure system, ipfw(8) should be used for protection, not the
     blackhole feature.

     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), ipfw(8), sysctl(8)

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

AUTHORS
     Geoffrey M. Rehmet

DragonFly 3.5                   August 17, 1999                  DragonFly 3.5