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 September 6, 2015 16.Dt BLACKHOLE 4 17.Os 18.Sh NAME 19.Nm blackhole 20.Nd a 21.Xr sysctl 8 22MIB for manipulating behaviour in respect of refused SCTP, TCP, or UDP connection 23attempts 24.Sh SYNOPSIS 25.Cd sysctl net.inet.sctp.blackhole Ns Op = Ns Brq "0 | 1 | 2" 26.Cd sysctl net.inet.tcp.blackhole Ns Op = Ns Brq "0 | 1 | 2" 27.Cd sysctl net.inet.udp.blackhole Ns Op = Ns Brq "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 SCTP, TCP, or UDP ports where there is no socket listening. 34.Pp 35The blackhole behaviour is useful to slow down an attacker who is port-scanning 36a system in an attempt to detect vulnerable services. 37It might also slow down an attempted denial of service attack. 38.Ss SCTP 39Setting the SCTP blackhole MIB to a numeric value of one 40will prevent sending an ABORT packet in response to an incoming INIT. 41A MIB value of two will do the same, but will also prevent sending an ABORT packet 42when unexpected packets are received. 43.Ss TCP 44Normal behaviour, when a TCP SYN segment is received on a port where 45there is no socket accepting connections, is for the system to return 46a RST segment, and drop the connection. 47The connecting system will 48see this as a 49.Dq Connection refused . 50By setting the TCP blackhole 51MIB to a numeric value of one, the incoming SYN segment 52is merely dropped, and no RST is sent, making the system appear 53as a blackhole. 54By setting the MIB value to two, any segment arriving 55on a closed port is dropped without returning a RST. 56This provides some degree of protection against stealth port scans. 57.Ss UDP 58Enabling blackhole behaviour turns off the sending 59of an ICMP port unreachable message in response to a UDP datagram which 60arrives on a port where there is no socket listening. 61It must be noted that this behaviour will prevent remote systems from running 62.Xr traceroute 8 63to a system. 64.Sh WARNING 65The SCTP, TCP, and UDP blackhole features should not be regarded as a replacement 66for firewall solutions. 67Better security would consist of the 68.Nm 69.Xr sysctl 8 70MIB used in conjunction with one of the available firewall packages. 71.Pp 72This mechanism is not a substitute for securing a system. 73It should be used together with other security mechanisms. 74.Sh SEE ALSO 75.Xr ip 4 , 76.Xr sctp 4 , 77.Xr tcp 4 , 78.Xr udp 4 , 79.Xr ipf 8 , 80.Xr ipfw 8 , 81.Xr pfctl 8 , 82.Xr sysctl 8 83.Sh HISTORY 84The TCP and UDP 85.Nm 86MIBs 87first appeared in 88.Fx 4.0 . 89.Pp 90The SCTP 91.Nm 92MIB first appeared in 93.Fx 9.1 . 94.Sh AUTHORS 95.An Geoffrey M. Rehmet 96