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: src/share/man/man4/blackhole.4,v 1.4.2.4 2001/12/17 11:30:11 ru Exp $ 15.\" $DragonFly: src/share/man/man4/blackhole.4,v 1.3 2006/02/17 19:37:09 swildner Exp $ 16.Dd August 17, 1999 17.Dt BLACKHOLE 4 18.Os 19.Sh NAME 20.Nm blackhole 21.Nd a 22.Xr sysctl 8 23MIB for manipulating behaviour in respect of refused TCP or UDP connection 24attempts 25.Sh SYNOPSIS 26.Cd sysctl net.inet.tcp.blackhole[=[0 | 1 | 2]] 27.Cd sysctl net.inet.udp.blackhole[=[0 | 1]] 28.Sh DESCRIPTION 29The 30.Nm 31.Xr sysctl 8 32MIB is used to control system behaviour when connection requests 33are received on TCP or UDP ports where there is no socket listening. 34.Pp 35Normal behaviour, when a TCP SYN segment is received on a port where 36there is no socket accepting connections, is for the system to return 37a RST segment, and drop the connection. The connecting system will 38see this as a "Connection reset by peer". By setting the TCP blackhole 39MIB to a numeric value of one, the incoming SYN segment 40is merely dropped, and no RST is sent, making the system appear 41as a blackhole. By setting the MIB value to two, any segment arriving 42on a closed port is dropped without returning a RST. This provides 43some degree of protection against stealth port scans. 44.Pp 45In the UDP instance, enabling blackhole behaviour turns off the sending 46of an ICMP port unreachable message in response to a UDP datagram which 47arrives on a port where there is no socket listening. It must be noted 48that this behaviour will prevent remote systems from running 49.Xr traceroute 8 50to a system. 51.Pp 52The blackhole behaviour is useful to slow down anyone who is port scanning 53a system, attempting to detect vulnerable services on a system. 54It could potentially also slow down someone who is attempting a denial 55of service attack. 56.Sh WARNING 57The TCP and UDP blackhole features should not be regarded as a replacement 58for 59.Xr ipfw 8 60as a tool for firewalling a system. In order to create a highly 61secure system, 62.Xr ipfw 8 63should be used for protection, not the blackhole feature. 64.Pp 65This mechanism is not a substitute for securing a system. 66It should be used together with other security mechanisms. 67.Sh SEE ALSO 68.Xr ip 4 , 69.Xr tcp 4 , 70.Xr udp 4 , 71.Xr ipfw 8 , 72.Xr sysctl 8 73.Sh HISTORY 74The TCP and UDP 75.Nm 76MIBs 77first appeared in 78.Fx 4.0 . 79.Sh AUTHORS 80.An Geoffrey M. Rehmet 81