.\"
.\" blackhole - drop refused TCP or UDP connects
.\"
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.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
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.\"
.\"
.\" $FreeBSD$
.Dd August 17, 1999
.Dt BLACKHOLE 4
.Os
.Sh NAME
.Nm blackhole
.Nd a
.Xr sysctl 8
MIB for manipulating behaviour in respect of refused TCP or UDP connection
attempts
.Sh SYNOPSIS
.Cd sysctl net.inet.tcp.blackhole
.Cd sysctl net.inet.udp.blackhole
.Pp
.Cd sysctl -w net.inet.tcp.blackhole=[0 | 1 | 2]
.Cd sysctl -w net.inet.udp.blackhole=[0 | 1]
.Sh DESCRIPTION
The
.Nm
.Xr 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.
.Pp
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 turning the TCP black
hole MIB on 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.
.Pp
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
.Xr traceroute 8
to your system.
.Pp
The blackhole behaviour is useful to slow down anyone who is port scanning
your system, in order to try and detect vulnerable services on your system.
It could potentially also slow down someone who is attempting a denial
of service against your system.
.Sh WARNING
The TCP and UDP blackhole features should not be regarded as a replacement
for
.Xr ipfw 8
as a tool for firewalling your system.  In order to create a highly
secure system, you should use
.Xr ipfw 8
to protect your system, and not the blackhole feature.
.Pp
This mechanism is not a substitute for securing your system,
but should be used together with other security mechanisms.
.Sh SEE ALSO
.Xr ip 4 ,
.Xr tcp 4 ,
.Xr udp 4 ,
.Xr ipfw 8 ,
.Xr sysctl 8
.Sh AUTHORS
.An Geoffrey M. Rehmet
.Sh HISTORY
The TCP and UDP
.Nm
MIBs
first appeared in
.Fx 4.0 .