xref: /freebsd/share/man/man4/blackhole.4 (revision 7f3dea244c40159a41ab22da77a434d7c5b5e85a)
1.\"
2.\" blackhole - drop refused TCP or UDP connects
3.\"
4.\" Redistribution and use in source and binary forms, with or without
5.\" modification, are permitted provided that the following conditions
6.\" are met:
7.\" 1. Redistributions of source code must retain the above copyright
8.\"    notice, this list of conditions and the following disclaimer.
9.\" 2. Redistributions in binary form must reproduce the above copyright
10.\"    notice, this list of conditions and the following disclaimer in the
11.\"    documentation and/or other materials provided with the distribution.
12.\"
13.\"
14.\" $FreeBSD$
15.Dd August 17, 1999
16.Dt BLACKHOLE 4
17.Os FreeBSD
18.Sh NAME
19.Nm \&blackhole
20.Nd a
21.Xr sysctl 8
22MIB for manipulating behaviour in respect of refused TCP or UDP connection
23attempts.
24.Sh SYNOPSIS
25.Nm \&sysctl net.inet.tcp.blackhole
26.Nm \&sysctl net.inet.udp.blackhole
27.Pp
28.Nm \&sysctl -w net.inet.tcp.blackhole=[0 | 1 | 2]
29.Nm \&sysctl -w net.inet.udp.blackhole=[0 | 1]
30.Sh DESCRIPTION
31The
32.Nm
33.Xr sysctl 8
34MIB is used to control system behaviour when connection requests
35are received on TCP or UDP ports where there is no socket listening.
36.Pp
37Normal behaviour, when a TCP SYN segment is received on a port where
38there is no socket accepting connections, is for the system to return
39a RST segment, and drop the connection.  The connecting system will
40see this as a "Connection reset by peer".  By turning the TCP black
41hole MIB on to a numeric value of one, the incoming SYN segment
42is merely dropped, and no RST is sent, making the system appear
43as a blackhole.  By setting the MIB value to two, any segment arriving
44on a closed port is dropped without returning a RST.  This provides
45some degree of protection against stealth port scans.
46.Pp
47In the UDP instance, enabling blackhole behaviour turns off the sending
48of an ICMP port unreachable message in response to a UDP datagram which
49arrives on a port where there is no socket listening.  It must be noted
50that this behaviour will prevent remote systems from running
51.Xr traceroute 8
52to your system.
53.Pp
54The blackhole behaviour is useful to slow down anyone who is port scanning
55your system, in order to try and detect vulnerable services on your system.
56It could potentially also slow down someone who is attempting a denial
57of service against your system.
58.Pp
59.Sh WARNING
60The TCP and UDP blackhole features should not be regarded as a replacement
61for
62.Xr ipfw 8
63as a tool for firewalling your system.  In order to create a highly
64secure system, you should use
65.Xr ipfw 8
66to protect your system, and not the blackhole feature.
67.Pp
68This mechanism is not a substitute for securing your system,
69but should be used together with other security mechanisms.
70.Pp
71.Sh "SEE ALSO"
72.Xr ipfw 8
73.Xr sysctl 8
74.Xr ip 4
75.Xr tcp 4
76.Xr udp 4
77.Sh AUTHORS
78.An Geoffrey M. Rehmet
79.Sh HISTORY
80The TCP and UDP
81.Nm
82MIBs
83first appeared in
84.Fx 4.0
85