1999-08-17 13:46:38 +00:00
|
|
|
.\"
|
|
|
|
|
.\" blackhole - drop refused TCP or UDP connects
|
|
|
|
|
.\"
|
|
|
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
|
|
|
.\" 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.
|
|
|
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
|
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
|
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
|
|
|
.\"
|
|
|
|
|
.\"
|
1999-08-28 00:22:10 +00:00
|
|
|
.\" $FreeBSD$
|
1999-08-17 13:46:38 +00:00
|
|
|
.Dd August 17, 1999
|
|
|
|
|
.Dt BLACKHOLE 4
|
|
|
|
|
.Os FreeBSD
|
|
|
|
|
.Sh NAME
|
|
|
|
|
.Nm \&blackhole
|
|
|
|
|
.Nd a
|
|
|
|
|
.Xr sysctl 8
|
|
|
|
|
MIB for manipulating behaviour in respect of refused TCP or UDP connection
|
|
|
|
|
attempts.
|
|
|
|
|
.Sh SYNOPSIS
|
|
|
|
|
.Nm \&sysctl net.inet.tcp.blackhole
|
|
|
|
|
.Nm \&sysctl net.inet.udp.blackhole
|
|
|
|
|
.Pp
|
1999-08-20 05:47:05 +00:00
|
|
|
.Nm \&sysctl -w net.inet.tcp.blackhole=[0 | 1 | 2]
|
|
|
|
|
.Nm \&sysctl -w net.inet.udp.blackhole=[0 | 1]
|
1999-08-17 13:46:38 +00:00
|
|
|
.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
|
1999-08-20 05:47:05 +00:00
|
|
|
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.
|
1999-08-17 13:46:38 +00:00
|
|
|
.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.
|
|
|
|
|
.Pp
|
|
|
|
|
.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.
|
|
|
|
|
.Pp
|
|
|
|
|
.Sh "SEE ALSO"
|
|
|
|
|
.Xr ipfw 8
|
|
|
|
|
.Xr sysctl 8
|
|
|
|
|
.Xr ip 4
|
|
|
|
|
.Xr tcp 4
|
|
|
|
|
.Xr udp 4
|
|
|
|
|
.Sh AUTHORS
|
|
|
|
|
.An Geoffrey M. Rehmet
|
|
|
|
|
.Sh HISTORY
|
|
|
|
|
The TCP and UDP
|
|
|
|
|
.Nm
|
|
|
|
|
MIBs
|
|
|
|
|
first appeared in
|
|
|
|
|
.Fx 4.0
|
